V. SUPPLEMENTARY PAPERS

1. Preliminary Results of Lysimeter Studies on the Dynamics of Calcium in the Irrigated Calcareous Soils of South Lebanon
2. Some Studies on the Calcareous Soils of Egypt
3. Contribution to the Study of Calcareous Soils of Hodna - Central Algeria
4. Calcareous Soils of Bulgaria, their Characteristics, Problems and Improvement

1. Preliminary Results of Lysimeter Studies on the Dynamics of Calcium in the Irrigated Calcareous Soils of South Lebanon

by

P. Gras
ORSTOM Mission to the
National Agricultural Research Institute
Tel Amara, Lebanon

1.1. Introduction

The concept of active calcium carbonate generally used to define the quantity of CaCO₃ which can be mobile in the soil, allows the taking into account of neither the calcium concentration in the soil solution (therefore the amount of calcium absorbable by the plant roots at a given moment), nor the movements to which this calcium can be subjected either by drainage or by being precipitated through heavy evaporation.

In a highly calcareous soil, the concentration of calcium in the liquid phase depends little on exchangeable calcium of an absorbant complex. The principal source is the carbonate of calcium existing in the solid state (mainly the CaCO₃ called 'active') and the principal factor of solubility is the partial pressure of the carbon dioxide contained in the soil's atmosphere.

In this report are presented the experimental results concerning the calcium content of water which has passed through calcareous soil subjected to various treatments and these results are discussed in relation to the theoretical concentration of a saturated calcium solution.

The object of such work is to define the nature of the different phases in the calcareous environment in which the vegetation grows during the course of the year and to study some elements leading to a knowledge of the evolution of this environment under the effect of intensive irrigation and the application of fertilizers.

1.2. Experimental Material

The material which served to support our efforts came from the experimental station of the National Litani Office at Lebaa near Saïda. This material had been previously set in the soil enabling it to be linked with one of the inventory series at the time of the pedological study of the joint FAO-NARI(IRAL) project in the Daoudiyé trials. Its chief characteristics are: uniformly deep brown colour, differentiation of weakly defined horizons, depth varying between 50 and 80 cm, percentage of coarse material (before clearing of stones) of about 15%; the main analytical data are the following:

content of total CaCO3	=	50%
content of active CaCO3	=	22%
organic matter	=	1,5%
clay	=	48%
fine silt	=	33%

The clay fraction is composed of 21.8% calcium carbonate and 6.05% magnesium carbonate. In the rest (that is 72.2% there are found mainly clay minerals of the type 2/1 (montmorillonite), a little kaolinite and some iron.

The distribution of calcium carbonate has also been studied and of magnesium among the different granular fractions: 53% calcium carbonate was found in the silt fraction, against only 24% in the clay fraction and 23% in the sandy fraction. It follows that the active CaCO₃ comes more from the calcareous grains contained in the fine silt than from those contained in the clay. The silt seems to play an important role as well with regard to the hydraulic properties of the soil.

1.3. Experimental Apparatus

This consists of a battery of six lysimeters: small barrels of 200 litre capacity pierced underneath to allow the collection of drained water (Figure 1). In addition three copper tubes were introduced laterally at depths of 10, 40 and about 60 cm for the removal and the titration of the carbonic gas in the soil atmosphere.

Figure -1 - Cross section of a Lysimeter (f = 57 cm, S = 25.5 cm²)

The following treatments were carried out on each lysimeter:

Lysimeters	Pebbles		Irrigation		Minimum Fertilizer
No.	with	without	with	without	with	without
1	+		+			+
2		+	+			+
3	+		+		+
4		+	+		+
5		+		+		+
6	+			+		+

Fescue grass (Festuca arundinacea) was sown in June 1970 on lysimeters 1, 2, 3 and 4, and lysimeters 5 and 6 were kept permanently without any vegetative cover.

The four irrigated lysimeters each received considerable and identical quantities of water: 1 400 mm in 1970 and 1 600 mm in 1971. The water came from the Karaoun dam and contained very little chlorine and sodium; on the other hand it was always supersaturated with calcium carbonate (between 51 and 65 mg/l of calcium).

Two applications of fertilizers were made on lysimeters 3 and 4, one in May 1970 (per hectare: ammonium nitrate 400 kg, triple superphosphate 2 000 kg and potassium chlorate 480 kg) and the other in November of the same year (ammonium nitrate 600 kg).

The amounts of water drained from each lysimeter were measured daily. Average samples of this water were taken and analysed about once a month. The CO₂ content of the soil atmosphere was determined every month from December 1970 onward with the Drager gas detection technique.

1.4. Results obtained in 1970 and 1971

a) CO₂ content in the atmosphere of the lysimeters in 1971 (Figure 2)

A first fact is recognized which is valid for all the lysimeters except sometimes for No. 1: the CO₂ content increased with depth whatever the season.

In the two non-irrigated trials (5 and 6), the CO₂ content was 0,03% from June to November. Then it increased slowly until in the month of April it reached a maximum which was no greater than 0.08% in 1971, but which rose to 0.35% at 60 cm in 1972.

In the four irrigated lysimeters, on the other hand, the quantity of CO₂ was slightly greater between June and November than between November and May. The curves had summarily the same form as the curve for the mean monthly temperatures. So the sharp decline in the CO₂ content observed between the 17 November and 7 December corresponds to a sudden coldness in the climate (from 20°C to 6°C).

Two maximums in the CO₂ content appeared at the same time in the 4 lysimeters, one in June and the other more noticeable in September.

The minimums came on the one hand in February-March and on the other in December. There was also a reduction, temporary but none the less obvious, in July.

Figure 2 Content in CO₂ in the atmosphere of the lysimeters at 10 cm, 40 cm, 60 cm

It could be judged therefore that the application of fertilizers had the effect of increasing the CO₂ content, by stimulating bacterial activity and respiration of the roots,

Moreover, it seems that in lysimeters 3 and 4 the release of CO₂ linked with the biological activity is felt from the end of March. On the contrary, the cutting carried out in May would cause better diffusion of the carbonic gas in the outside atmosphere, therefore a lowering of the CO₂ content in lysimeters 3 and 4 in May and June.

Finally it should be noted that the May to September CO₂ contents at 40 and 60 cm are always higher in lysimeters 2 and 4 containing soil without pebbles, than in lysimeters 1 and 3 where the stones have not been removed.

b) Concentration of calcium in drainage water

Lysimeters with non irrigated trials

The only data obtained during the winter of 1970-1971 having proved insufficient we wished to complete them with data gathered during the course of the winter of 1971-1972. At the time of the first rain it was ascertained that the drained water and above all that which came from lysimeter 6 was turbid and very rich in solid matter in suspension. At the same time the strength of calcium was relatively high (50 and 70 mg/l). Then very rapidly the particles in suspension disappeared while the calcium concentrations were stable until February (60 to 70 mg/l).

In March there was a fall in the calcium concentration, the minimum reached at the end of March was between 35 and 50 mg/l.

Lastly, in April at the time when the contents of CO₂ are highest, the calcium concentrations again became identical to those observed in January-February. Thus in uncultivated and non irrigated calcareous soils the leaching of CaCO₃ during the first rains should be considered in relation to the drying that the earth has undergone during the summer and with the settling which has resulted.

This is the mechanism of pelicular alteration to calcareous stones which happens in this medium which is porous, inflated and poor in organic matter. The calcium carbonate goes into suspension rather than dissolves in the water.

In April, on the contrary, it is the presence of carbonic gas which causes a dissolution of the "carbonate reserve" which was extant in the soil.

Irrigated lysimeters (Figures 3 and 4)

The influence of irrigation and the application of fertilizers on the concentration of calcium in the drainage water is illustrated by the curves in Figures 3 and 4.

No matter what the material (earth with or without pebbles) and whether fertilizer has or has not been added, periodical variations exist in the calcium concentration. These differences are based in the first estimate in the same way as the variations in the CO₂ content of the atmosphere.

Although the minimums were observed at the same time (April) and bad appreciably the same value of nearly 10 mg/l for the two years, in contrast there was a clear enough difference between the maximums of 1970 and those of 1971. In 1970 the maximums in lysimeters 1 and 2 were observed in June (87 and 100 mg/l) and September-October (78 and 94 mg/l). In 1971 in the same lysimeters the maximums were respectively no more than 60 to 65 mg/l and 70 to 75 mg/l during a period stretching from August to December. The measurements taken in the first seven months of 1972 confirm that there is from one year to another a progressive decrease in the concentration of calcium in the drained water especially during the irrigation period. It was also noted that the first rain in November-December seemed to elicit a more intense hut ephemeral enough dissolution of the calcium carbonate rather as in the test lysimeters, although this period corresponds to a sharp decrease in the CO₂ content.

Figure 3 Average content of CO₂ in the atmosphere of the Lysimeters (at 40 cm)

Figure 4 Average content of CO₂ in the atmosphere of the Lysimeters (at 40 cm)

In May 1970 fertilizer application to Nos. 3 and 4 caused a strong rise in the concentration of calcium in the drained water in July-August, The same thing happened, although not so severely, after the second application of nitrate in December 1970. This influence from the fertilizers was still to be seen in 1971, but it was much less obvious. The absence of data concerning CO₂ in 1970 allowed us to make only two observations regarding the action of the fertilizers. These occurred in two ways:

- indirectly by stimulating bacterial activity and root growth. The resultant carbonic gas caused intense solubility of the calcium carbonate.

- directly with regard to the monocalcium phosphate (triple superphosphate). In effect this reacted rapidly on the calcium carbonate in the following pattern (ARVIEU 1972):

monocalcium phosphate + CaCO₃ ® bicalcium phosphate + CO₂

In a second stage the bicalcium phosphate reacted slowly with calcium carbonate to create octocalcium phosphate at first and then apatite calcium, both of them insoluble therefore unavailable to plants. During the course of this second stage, which was slower than the first, there was also a release of CO₂. According to ARVIEU the two stages happen far more quickly and with a far greater speed when the temperature is higher and the quantity of CaCO₃ is greater.

At the limit the two stages occur simultaneously, the bicalcium phosphate reacting on the calcium carbonate at the same time as it forms.

c) Comparison between the concentrations of calcium measured and the theoretic concentrations of saturated calcium (Figures 5 and 6)

The concentration of theoretic calcium in a saturated solution in the presence of pure calcite is linked with the partial pressure of CO₂ by the following relation:

log [Ca++] = 0.33 log PCO2 + b

(1)

the value of b varies in terms of the temperature according to the following relation established by T. Stchouskoy-Muxart

log b = - 0.055 log t + 0.4605

if it is admitted that the atmospheric pressure is constant and equal to 1 atmosphere, the following is given:

VCO₂ being the volume of CO₂ in the soil atmosphere expressed in percent; the equation (1) becomes:

log [Ca⁺⁺] = 0.33 log VCO₂ + (b - 0.66)

to simplify the calculations the variable log 1000 VCO₂ has been used in the place of log VCO₂. Therefore the relation obtained is:

log [Ca++] = 0.33 log VCO2 + b - 1.32

(2)

the right side terms in equation (2) for different temperatures have been shown in Figure 5.

Figure 5 Concentration of calcium in the drained water in relation to the CO₂ content at 60 cm

Figure 6 Excess of deficit of calcium in the water in relation to the saturation

One can therefore, knowing t and VCO₂ calculate [Ca⁺⁺] = S: concentration of theoretical calcium from the water of the saturated soil.

The difference C-S between the concentration measured and the theoretical concentration, for the sane content in CO₂ of the atmosphere and for the same temperature, allows the estimation of the excess or deficit of calcium in the water having passed through the lysimeters and the comparison of this to the values obtained for the rainwater and for the irrigation water. All these values collected in 1971 are given in Figure 6. It can be calculated that in the four irrigated lysimeters the values of C-S were at a minimum in April and at a maximum in November. The drained water from lysimeters 1 and 2 was constantly supersaturated with calcium whilst in lysimeters 3 and 4, which had received fertilizers in 1970, this water showed in April a loss of calcium in relation to the saturation; on the contrary from June to December the excess of calcium was higher than in the water coming from lysimeters 1 and 2. It seems, therefore that nearly always the percolation water is supersaturated with calcium; in other words, only part of the calcium was found in solution in the form of bicarbonate of calcium whilst the other part was in the form of very fine calcareous particles in suspension in the water. It is moreover the same thing for the water used for irrigation which is, as shown by the I curves, extremely supersaturated with regard to an equilibrium content of calcium in natural water. From this fact irrigation water has a very different action on calcareous soil to that excercised by rain water which has a stronger dissolving action.

One can interpret the curves in Figure 6 in the following way. In the autumn the soil encloses a high reserve "of easily soluble calcium carbonate" (calcium carbonate carried by the irrigation water or coming from the alteration of the pre-existing CaCO₃,). Rain water carries away during the course of its passage through the soil not only calcium bicarbonate but also some very fine particles in suspension. Gradually as the winter passes, the reserve of CaCO₃ diminishes and the quantities of calcium carried away show a noticeable reduction. In lysimeters 3 and 4, the content in CO₂ being higher, the reserve of "easily soluble calcium carbonate" is more rapidly reduced so that in April the drained water is under-saturated. From May onwards there is not only a carrying away of calcium but also a depositing in the soil of a part of the calcium contained in the irrigation water. The difference between the saturation curves of the irrigation water and those of the drained water represented on Figure 6 illustrates the quantities of calcium which have remained in the soil. One can estimate that these quantities are higher in lysimeters 1 and 2 than in lysimeters 3 and 4.

d) Deposits and losses of calcium between June 1970 and October 1971

Table 1 Additions (+) and losses (-) of the quantity of calcium in the irrigated and non irrigated lysimeters in 1970 and 1971

Lys. No.	from 1.6.70 to 10.11.70	from 11.11.70 to 19. 4.71	from 20.4.71 to 19.10.71	Total
1	+ 14.7	- 8.9	+ 17.0	+ 22.8
2	+ 13.8	- 9.3	+ 14.8	+ 19.3
3	+ 23.0	- 14.4	+ 15.4	+ 24.0
4	+ 21.6	- 15.0	+ 14.0	+ 20.6
5	- - -	- 9.7	- - -	- 9.7
6	- - -	- 8.5	- - -	- 8.5

The figures in the preceding table represent the difference between the quantities of calcium supplied (with the water and with the mineral fertilizers) and the quantities carried away by lixiviating and in the harvest.

During the winter of 1970-71 lysimeters 1, 2, 5 and 6 released into the drained water about the same quantity of calcium (9 to 11 g), while Nos. 3 and 4 lost a lot more (15 g about).

During the two summers, lysimeters 1, 2, 3 and 4 were watered with the following significant quantities: 355 and 426 litres. Owing to the intense evapotranspiration the volume of drained water only represents about a third of the irrigation water. For this reason and also because the drained water is clearly less saturated than the irrigation water, the quantities of calcium lost are hardly significant (13 to 17 g) when considered against the quantities of calcium added (45 g). Irrigation for lysimeters 1 and 2 definitely indicates an increase in calcium of 31.7 and 28.6 g and for lysimeters 3 and 4 an increase of 28,6 and 26.2 g. But to these last two figures should be added also 9.4 g of calcium coming from the phosphate fertilizer in 1970 (the triple superphosphate used included 18.7% Ca). In relation to the calcium phosphate ammonium phosphate would introduce the double advantage of not increasing the calcium content of the soil on one side and on the other of lasting much longer in an assimilable form (Arvieu 1972).

Finally the overall balance (last column in the table) demonstrates, and it could not be clearer, that the irrigation of calcareous soils with water supersaturated with calcium bicarbonate, causes an enrichment of calcium which is not compensated by the lixivation in winter under the influence of rain water.

1.5. Conclusion

It appears from the preliminary studies undertaken in the lysimeters that the carbonic gas contained in the soil body, the content of which varies considerably during the course of the year as a result of the processes undergone by the soil, is an important factor in the solubility or precipitation of calcium. But this is not the only factor which interferes with the dynamics of the calcium. Account must be taken of the existence of very fine calcareous particles in suspension in the gravitational water.

From the practical point of view it confirms that irrigation of calcareous soils with water containing high percentages of calcium carbonate shows a clear enough increase in the quantity of CaCO₃ in the soil, an increase which in the long term could have a harmful effect on the plants. Furthermore, the use of calcium phosphate as a fertilizer must be reconsidered in the light of its contribution to the increase in calcium and the rapidity with which this fertilizer is rendered unavailable in a calcareous soil.

REFERENCES

Arvieu, J.C. 1972, Etude cinétique des réactions entre orthophosphates solubles et CaCO₃. Propriétés physico-chimiques des apatites phospho-calciques formées. Thèse faculté des Sciences de Nice.

Bachelier, G. 1968, Problèmes relatifs á l’atmosphère du sol et utilisation possible d'un détecteur de gaz pour la mesure de sa teneur en gaz carbonique. Cah. ORSTOM sér. Pédol. Vol. VI No. 1

Crahet, M. Le pH des sols calcaires. Bulletin AFES July-August No. 4 1967

Drouineau, G. 1963, La chlorose calcaire. Bulletin de la Société française de physiologie végétale. T9 No. 4. December

Lamouroux, M. 1972, Etude des sols formés sur roches carbonatée-pédogenèse fersiallitique au Liban. Collection mémoires ORSTOM No. 56. Paris

Stchouzkoy-Muxart, T. 1971, Contribution á l'étude de la solubilité de la calcite dans l'eau en présence d’anhydride carbonique á 20 et 30 C. Bulletin de l'association de géographes français No. 389-390 May-June

Thiebault, M., Sayouni, M. et Lamouroux, M. 1968, Analyses quantitatives de roches silicatées, calcaires ou dolomitiques et de leurs produits d'altération. Rapport Ronéo IRAL-ORSTOM

Yaalon, D.H. Physico-chemical relationship of CaCO₃, pH and CO₂ in calcareous soil. 1954 Trans, intern. Cong. SoilSci. Leopoldville. No. 2:356-369

Yaalon, D.H. 1957, Problems of soil testing on calcareous soils. Plant and Soil VII No. 3 March

Enquête Pédologique et programmes d'irrigation connexes. 1969, Rapport final Volume II Pédologie FAO/SF: 51/LEB 10, FAO Rome

2. Some Studies on the Calcareous Soils of Egypt

Contribution from the Soils Department
University of Ein Shams, Cairo, Egypt

2.1. Definition

Contrary to saline and alkali soils, there is no accepted definition for calcareous soils. The available definitions are thought to be inadequate when plant growth and production are considered. Since several phenomena such as micronutrient deficiencies lime induced chlorosis and phosphate fixation are associated with the presence of certain levels of CaCO₃ in soil, they could be used as criteria for defining calcareous soils. In a study by Hilal et al, (4) on a chemical and biological approach toward the definition of calcareous soils, phosphorus retention was investigated as a criterion.

A set of soil mixtures, ranging in CaCO₃ content from 0 to 96%, was prepared and used in a column study to determine the level at which the CaCO₃ fraction becomes a dominant factor controlling P3² movement and distribution.

Detection of P in the displaced soil columns' solutions revealed that the downward movement of P³² in fine sand columns was much higher than in loam columns and was very low in oolitic sand. Increasing the percentage of oolitic sand in the soil mixture from 1 to 10% caused a sharp drop in P³² recovery with the displaced soil solutions. Any increase in CaCO₃ content from 10 to 96% did not show any further drop in P³² movement. A picture that is thought to be similar to that of solubility and saturation in simple solutions. Application of CaCO₃ in the clay fraction also brought out the saturation effect on P³² movement at 2% only.

The amount of P³² removed with the soil solutions was generally low compared to that retained in the soil columns. Counting the activity of P3² in soil columns' sections, after five displacements, indicated that P³² movement from the top soil increased by increasing CaCO₃ from 1 and 2% to 6%. The amount of P32 removed from the top soil was however retained in lower sections. A very sharp decrease in the amount of P³² migrating from the top soil was observed when CaCO₃ content was raised from 8% to 10%. More than 70% of added P³² was retained in the top section of the 10% column, compared to less than 10% retained in top sections of columns containing lower levels of CaCO₃. Increasing CaCO₃ above this level was far less effective. A similar picture was shown when the CaCO₃ material used was in a clay size fraction. However the sharp increase in phosphate retention in the top section took place at 8% CaCO₃ rather than at 10%.

The results of this study indicate that a proposal to consider 8 to 10% CaCO₃ as a limit for defining calcareous soils is worth more consideration.

As a follow up, a pot culture experiment was conducted to determine as a criterion for the definition of calcareous soils the CaCO₃ percentage at which it exerts its drastic effect on plant growth and P³² and Fe uptake. Soil mixtures were prepared by mixing different calculated amounts of Nile loam, fine sand and fine oolitic sand to give soils 1, 2, 4, 6, 8, 10, 20, 30, 40 and 50% CaCO₃, The mixtures were so prepared that their clay contents were equal and had the same texture. Corn plants were selected for this study.

The results of plant growth and P³² and Fe uptake support each other and clearly indicate that When CaCO₃ reaches 8% of the soil components, it controls the biological and chemical characteristics of the soil. A conclusion was drawn that 8% CaCO₃ could be the margin at which the soil can be considered calcareous.

2.2. Nutrient status

Research carried out concerning the status, behaviour and availability of nutrients in the calcareous soils of Egypt revealed the following:

a) Phosphorus

Laboratory, pot and field experiments by El-Damaty et al. (1) showed that calcium carbonate is the main compound limiting the phosphate availability to plants, the effect being dependent on the particle size of carbonates. Retention of phosphate opposite to its release was proved to be favoured by the existence of calcium carbonate particularly if the latter is in fine particles.

It may be worth mentioning that the so-called "P-maximum release" obtained from certain leaching techniques, was found to be a good estimate for phosphate availability to plants. The study revealed that calcareous soils are, sometimes, higher in their supplying power than those of a silicate nature possibly due to the energy of a desorption on the surfaces of soil particles.

Experiments also indicated that certain processes seemed to be encouraging for phosphate retention in calcareous soils. Relatively high temperature along with successive wetting and drying cycles of soil proved to favour the depressing of the phosphorus availability. Retention of phosphate was found to be increased with incubation as well as when soil was subjected to interchanging cycles of wetting and drying.

Studies by Metwally and El-Damaty (5) showed that values obtained of the water soluble, exchangeable, and non-exchangeable K and total K were comparatively lower than those of the alluvial soils. The last three forms correlated significantly with the clay content and negatively with the CaCO₃ per cent. The non-exchangeable form of K, determined by heating at 500°C for two hours and subsequent extraction with 1.0 NH₄OAc, was found to give the highest correlation with the potassium uptake in a continuous experiment.

was found to control the uptake of K by maize and soybean from sand cultures, more than the K activity. Increasing the Mg concentration from 1 × 10^-5 mol/l to 5 × 10^-5 mol/l largely decreased K uptake at constant K-activity and K-activity ratio.

Potassium bonding energy as estimated from Langmuir adsorption isotherms and K-fixation were found to be much lower in calcareous soils than in alluvial soils.

The total amount of iron was found by Elgala (2) to be about 5,5 × 10³ ppm with only about 13 ppm as available iron. With respect to the behaviour of added iron compounds, results show that the application of iron sulphate did not cause a pronounced change in the amount of water, and NH₄OAC soluble iron but that the Fe EDDHA addition resulted in a significant increase in the amount of water soluble iron.

A positive response in plant growth, dry matter and protein nitrogen content was obtained When iron was applied in the form of Fe EDDHA to the soil or as a foliage spray on corn plants grown in a calcareous soil at Burg El-Arab, west of Alexandria. The ineffective use of iron sulphate as compared with the iron chelate was found and was related to the rapid oxidation or precipitation of Fe SO₄.

Plants grown on a calcareous soil showed chlorotic symptoms and severity of chlorosis was related to the level of phosphorus fertilizers and soil moisture content. The percentage of "active iron" decreased by increasing phosphorus level and soil moisture content.

The total amount of Mn in Egyptian calcareous soils was found by Elgala (2) to vary between 125 and 220 ppm while the available amount ranges from 45 to 100 ppm. Metwally and El Damaty (5) using the isotopic exchange technique showed that the equilibrium Mn in calcareous soils includes, beside the water soluble and the exchangeable Mn, the chelated Mn as extracted by 0.1 N pyrophosphate zinc sulphate in 1N NH₄OAC for two hours and the portion of easily reducible Mn extracted by 0.025% hydroquinon. The conventional extractant, namely 0.2% H.Q. used to extract what was agreed upon as easily reducible Mn, extracted non-equilibrium Mn even after 30 minutes of extraction. Manganese extracted by 0.025% H.Q. was found to be mainly the trivalent form as extracted by 0.1 M pyrophosphate (Na-pyrophosphate).

Manganese extracted by both 0.1 M Na pyrophosphate and 0.025% H.Q. gave the highest significant correlation with the Mn uptake by soybean. The two extractants proved to be more reliable than other extractants of H.Q. of higher concentrations which extracted non-equilibrium Mn and resulted in lower correlation coefficients with the Mn uptake. They were also superior to the other extractants used, i.e. 0.1 M H₃PO₃, 0.05 M Na-EDTA, NaOAc of pH 5.5, as far as estimating plant available Mn was concerned.

When Mn was added to calcareous soils under normal moisture conditions, chemical and biological oxidation proceeded much more rapidly than under waterlogging conditions. The Mn⁺⁺ added undergoes rapid oxidation and may be precipitated as hydrated Mn(OH)₂ or MnCO₃ which is immediately oxidised. These hydrated freshly precipitated oxides are considered to constitute part of the equilibrium Mn, but they become inert, more ordered and less active upon dehydration and therefore transformed to non-equilibrium manganese.

The calcareous soils due to their lower adsorption capacity and microbial activity can maintain higher Mn⁺⁺ concentration in the active form for a longer period than alluvial soils when it is added to the soil.

The total amount of Zn in Egyptian calcareous soils as determined by Elgala (2) varies between 18 and 28 ppm, while the available amount ranges from 1.1 to 1.8 ppm.

Barley plants showed a higher content of Zn when Zn fertilizers were applied to various calcareous soils containing from 20% to 70% CaCO₃, which indicates the deficiency of Zn in such soils. However, the chelate form of Zn EDDHA was more highly efficient than the inorganic form in supplying available Zn. There was a noticed decrease in plant uptake of Zn as a function of increasing phosphorus or CaCO₃ content in the soil.

Metwally and El Damaty (5) found that when the chemically available zinc extracted by 1.0 N, 0.5 N, 0.1 N HCl, 2.ON MgCl₂, Na₂-EDTA, and dithiozon, was correlated with the plant uptake of zinc, only dithiozon and Na₂-EDTA gave significant correlation.

The eight calcareous soils used responded to zinc fertilization at a rate of 3,0 mg/kg soil, and were believed to be zinc deficient compared to the alluvial soil used which gave no response to such fertilization.

Metwally and El Damaty (5) in their study of boron adsorption by calcareous soils found that the adsorption capacity of calcareous soils was much lower than alluvial soils owing to their lower content of amorphous oxides that are the main soil constituents contributing to boron adsorption and fixation.

The effect of cropping with alfalfa for different periods on the organic matter content and some physical properties of the calcareous soils in the northern region of Tahrir Province was studied by El Maghrabi et al. (3). The results are given in the following table.

Sampling date	Cropping period (month)	Organic matter %	Bulk density (g/cm3)	Porosity %	Hydraulic conductivity (cm/hour)	Infiltration rate (cm/hour)	Moisture equivalent %
22.5.1971	0	0.15	1.74	36.9	-	1.45	14.9
	13	0.26	1.64	39.0	-	2.41	18.0
	18	0.30	1.55	42.0	-	2.95	18.3
	31	0.39	1.44	46.0	-	5.83	20.9
22.5.1972	0	0.16	1.75	37.0	0.24	1.36	-
	25	0.34	1.62	39.3	0.41	3.48	-
	30	0.37	1.51	42.8	0.66	6.36	-
	43	0.65	1.27	51.0	2.5	12.4	-

Contributors

(1) El-Damaty, A.H., El-Leboudi, A. and Robishy, A.

(2) Elgala, Abdel Monem.

(3) El Maghrabi, T., Elgala, A.M. and El-Damaty, A.H.

(4) Hilal, M.H., Antar, P. (National Research Centre) and El-Damaty, A.H. (Ein Shams University).

(5) Metwally, A. and El-Damaty, A.H.

3. Contribution to the Study of Calcareous Soils of Hodna - Central Algeria

by

T.G. Boyadgiev

3.1. Introduction

The configuration of the relief of the arid and semi-arid regions of North Africa is characterized by extensive plains with gentle slopes belonging to the late Tertiary (Villafranchien) and to the recent Quaternary Age, called slopes (glacis). The old and middle slopes are gravelly marl; the more recent ones consist of fine clay material. These plains extend to the foot of the mountain chains.

The Hodna Basin is a typical example of this relief. Situated some 200 km due south-east of Algiers, separated from the Mediterranean by 150 km of mountains, it is surrounded by the mountains of Hodna to the north, the massif of Aurès to the east, the hills of Zab to the south-east and those of Ouled Naaîl to the south-west; it rises gradually to the west to the high Algerian-Moroccan plateaux.

Thus confined the Hodna Basin is subdivided (8) from north to south in the following physiographic regions:

- mountainous regions
- plains of Hodna
- Chott and the Sebkha
- the region of R'Uel (sand)

The mountainous landscape, formed of limestone, marl and sandstone, of which the highest point is 1 745 m, descends to the foothills and to the plains (altitude around 800 m) made up of a succession of slopes, old, middle aged and more recent ones, to join the Chott and the Sebkha with deltaic deposits at an altitude of 391 m. In the region of R'Uel the landscape rises again to 850 m consisting of sand dunes and slopes associated with isolated hills and then it joins the Saharan Atlas mountains.

This upper semi arid climate has cold winters with an annual rainfall of 400 to 600 mm in the mountains of the north and becomes arid upper with temperate to cold winters and an annual rainfall of 200 to 400 mm in the plains. In the Chott and Sebkha and R'Uel regions the climate is lower, arid, with temperate winters and an annual precipitation of less than 200 mm.

The vegetation passes successively from forested to calcicole, gypso-calciferous, silty gypso saline, silty saline and hyper saline groups and disappears completely in the Sebkha. In the south vegetation is psammophilous in association with other groups,

Groundwater is found at a depth of more than 40 m in the high parts of the Hodna basin and is almost at surface level in the Sebkha. The mineral content of the water increases and salinization of the sulphate-chloride type becomes chloride-sulphate and chloride. In the N'Gaous valley the groundwater is not very deep and the water is rich in bicarbonate.

3.2. The soils of the Hodna

The major pedological processes in the Hodna basin are those responsible for calcimorphic, steppe, vertisol, gypsomorphic, halomorphic and desert formations. Erosion and alkalinity are equally noticeable.

The distribution of the Hodna soils is in relation to these phenomena and reflect the pedogenetic factors and conditions. Nevertheless, the accumulation and particular attributes of the gypsic limestones and soluble salts as well as the dune formations give the specific characteristics of the respective soils and landscape in the Hodna plains, the Chott, the Sebkha and the region of R'Uel.

The accumulation of CaCO₃ in the soils of the Hodna plains and in the R'Uel is closely connected to its distinct formation,

It has been established that the form of CaCO₃ depends as much on the content as on the soil texture. This relationship is shown in Fig. 1. The following forms have been observed:

Figure 1 The relation between the calcium carbonate content and the clay content for different forms of lime

- calcareous crust	(croûte calcaire)
- calcareous encrustation	(encroûtement calcaire)
- calcareous concretions and/or nodules	(amas et/ou nodules calcaires)
- diffused CaCO3	(calcaire diffus)
- groundwater calcareous layer accumulation	(accumulation du calcaire de nappe phréatique)

Each of these formations is linked with specific soil characteristics.

3.3. Soils with a calcareous crust

The calcareous crust is characteristic of medium textured Moulouyens and coarse textured Amiriens slopes. The CaCO₃ content is about 80-90 percent; it is found in the form of lamellar or non lamellar hard crusts. The thickness of the crust is from a few centimetres to more than a metre. The crust becomes less hard at depth and is sometimes overlaid by powdered lime.

The soils have an AC profile. The humous horizon is thin (from 8 to 26 cm), dark brown (7.5 YR 5/6) with a silt or loamy sand texture. The CaCO₃ content from 20 to 40% in the upper part increases sharply (80-90% in the crust; sometimes between the A horizon and the crust there can be seen a horizon with calcareous nodules or with a crust of debris or encrusted debris, the calcareous content of which is 50-70%. The total of active CaCO₃ varies between 12 and 16% in the upper horizon and from 16 to 19% in the lower. The level of gypsum is negligible (<1%). The conductivity is normally below 2 mmhos/cm; certain profiles are lightly to moderately saline (from 2 to 10 mmhos/cm).

The A horizon contains about 1.5% organic matter with a C/N value of 9; the exchange capacity varies between 2 and 18 meg/100 g of Boil and that of available P₂O₅ is 30 to 120 ppm.

The soils have a capability index (7) of lower than 0.4 and are left for sheep grazing; in some areas they are suitable for cereal growing.

3.4. Soils with calcareous encrustation

CaCO₃ encrustations are linked with old and middle slopes (saletiens, amiriens et polygéniques). They are found in the form of a thin crust, non zonal and broken up, of tuff hardened or calcareous powder. The total CaCO₃ content varies between 50 and 80%.

It should be mentioned that the CaCO₃ is of a much more complex form. Accordingly a powdered horizon with tuffs can be found and sometimes a horizon with concretions and/or calcareous nodules above or below the encrusted horizon, sometimes the nodules fill the length of the profile, and sometimes there is a layer of narrow compacted bands and nodules; layers of gypsum have also been observed.

This great variety could be explained by the varying geomorphological positions as well as by the chemical and granular composition of the weathered products.

The soils have an AC profile. Their upper horizon is thin, about 12 cm, dark brown in colour (7.5 YR 5/6), with a polyhedral subangular structure that is fine or very fine grained and of a medium texture. The humous horizon lies over a gravelly one, which is encrusted and of a bluish colour (7.5 YR 8/2).

The total CaCO₃ content on the surface is an average 30-40% of which the active fraction is from 9 to 15%; the calcareous accumulation is between 40 and 100 cm deep. Occasionally, a second calcareous horizon can he seen about a metre lower and separated by a gypseous horizon. The content of active CaCO₃, in highly calcareous horizons is 11-1856.

The soils are either non gypseous across the profile or with one or two gypseous horizons in which the gypsum content is about 76%. The conductivity is normally less than 2 mmhos/cm in the upper part and from 2 to 7 mmhos/cm when deeper; the maximum conductivity observed is 32 mmhos/cm. The salinisation is the sulphate or chloride type with respectively Ca > Mg > Na and Na > Mg > Ca,

The surface horizon has the following physico-chemical and chemical compositions: organic matter of about 0.9%, C/N of 8 to 12; T of 3 to 18 meq/100 g of soil; exchangeable K⁺ of 0,2 to 0.8 meq/100 g; available P₂O₅ about 50 ppm.

Encrusted soils have a capability index below 0.4 and are used for sheep grazing. The surfaces which can benefit from shallow flood waters will grow cereal crops and give satisfactory results.

3.5. Soils with calcareous concretions and/or nodules

Concretions and/or nodules typify above all the middle slopes (tensiftiens). In terms of the texture, a CaCO₃ content of 10 to 60% gives rise to soft concretions often with a central core and isolated or composite nodules.

The soils have an ABC profile. The upper horizon is seldom thick (10 to 30 cm, rarely 60 cm), brown yellowish (7.5-10 YR 5/6), medium textured (mainly loam or loamy-sand) and with a polyhedric structure subangular medium to fine, lamellar on the surface. This horizon is calcareous (CaCO₃ of 15 to 40%) with an active lime content of 7 to 18% The organic matter content is on the average 0.6 to 0,9% with a C/N ratio of 8 to 11; the exchange capacity oscillates between 13 and 23 meq/100 g of soil. The exchangeable K⁺ is 0.4 to 0.8 meq/100 g, available P₂O₅ is 30 to 70 ppm, total P₂O₅ is 0,5 to 0.9%, Na/T is 5 to 20% and the pH is 7.7 to 8.1 which also typifies the A horizon.

The lime accumulation horizon, found immediately below the A horizon, goes down to 180 cm deep if it does not rest on a horizon of accumulated gypsum and ends at about 45-115 cm deep if it is overlaid by a gypseous horizon.

The CaCO₃ in the B horizon, of which the total is on average 40-60%, causes a hardening of the whole horizon. The numerous galleries and animal fosils which are typical of this horizon are also hardened, cemented by the limestone. The total active CaCO₃, is 14-17%.

The soils are either non gypseous across the horizon, or strongly gypseous (28-69) below the horizons of accumulated CaCO₃. They are sometimes saline with a conductivity which reaches 14 mmhos/cm.

These soils are suitable for the growing of cereals and fodder. Their capability index is 0.4 to 0.6. The dryness of the soil, the hardened horizon and the high active CaCO₃ content are the limiting factors for plants.

3.6. Diffused CaCO₃ soils

This form of limestone is typical of the recent piedmont slopes (soltanien-rharbien) with a groundwater depth greater than 7m. These slopes are intersected lay watercourses with a depth of 5 m upstream which lessens progressively downstream. The CaCO₃ content is about 30%.

The soils are browny yellow (10 YR 5/4) with a polyhedral structure subangular medium to heavy tortured and with a fine, lamellar layer on the surface. The CaCO₃ appears sometimes in the form of mycelium or as a few concretions; efflorescence, stains and gypso-saline channels are also noted.

The horizons are hardly distinguishable. The texture is medium to heavy with a stratification of grain size distribution (lithologic) at depth.

The rate of total CaCO₃ is constant across the profile or shows a slight rise at depth. The percentage of active CaCO₃ is from 11 to 14%. The gypsum content is negligible; conductivity is less than 8 mmhos/cm. Salinisation is of the sulphate-chloride type with a rise relative to the chloride in the upper part of the profile.

It is notable that on these piedmont slopes there have also developed soils with a horizon of accumulation of gypsum at depth as well as soils affected by soluble salts, which too have a diffused distribution of CaCO₃.

Diffused calcareous soils have the following chemical and physio-chemical characteristics: organic matter from 0.5 to 1.3% regularly or irregularly distributed over the whole depth; C/N 8 to 11, pH 7.8-8.0; T of 8 to 28 meq/100 g of soil, exchangeable K 0.5 to 1,0 meq/100 g; total K₂O of 3.7 to 7.1%, available P₂O₅ of 20 to 70 ppm, total P₂O₅ of 0.3 to 1.9% and Na/T of 5 to 15%, These soils are very good to good for dry farming and for irrigation with a capability index of 0.6 to 1.0. Their productivity can be raised by the practice of rational agriculture and all the more if they benefit from shallow flood-waters and from irrigation.

3.7. Soils with a groundwater calcareous layer accumulation

The accumulation of layered CaCO₃ has been observed on the recent slopes in the region of N'Gaous where the groundwater level is quite shallow (about 1 m) and the water is rich in bicarbonate.

The soils are of a greyish colour associated with reddish marks; the texture is medium to heavy and the structure polyhedral subangular.

These soils are heavily calcareous, of non distinct form or with the inclusion of nodules; the CaCO₃ content is 60-70% equally distributed across the profile; the rate of active CaCO₃ is 14-19%. They contain no gypsum. The conductivity is below 2 mmhos/cm. The organic matter content is 2-3% in the upper part and diminishes evenly with depth. The C/N ratio is 7-10. There is a rise in exchangeable Na + K in the part over 75-90 cm. Their T oscillates between 8 and 18 meq/100 g of soil. The mineral amounts are: available P₂O₅ from 80 to 100 ppm; total P₂O₅ about 2%; exchangeable K⁺ 0.3-0.7 meq/100 g and total K₂O 3.7-4.2%. These soils have a capability index of about 0.6 and are now used for apricot orchards. Drainage would improve their capability index.

3.8. The place of the Hodna calcareous soils in the different classification systems

Without recalling the following classification concepts: U.S.A., French, Russian and FAO/Unesco for the soils of the semi-arid and arid region, we will attempt to classify the soils of Hodna according to the norms of these classifications.

The approximate correlation among the different classifications is given in Table 1, Concerning this table one can make the following remarks:

U.S.A. classification; they classify in the same group soils with a calcareous encrustation and with concretions and/or calcareous nodules. To stress the importance of the horizon strongly enriched with CaCO₃, it is necessary to introduce a new group which could be called Hypercalcic Calciorthods. These are the soils which are up to one metre deep, have a highly calcareous horizon which is typified by:

- a thickness of 15 cm or more

- CaCO₃ content above 40% if the clay content is below 10%, or is 60% and more if the clay content is 35% or more, or if the intermediate content of the clay has a proportionate percentage of CaCO₃, (see Figure 1)

- The form of the CaCO₃ - thin crust, non lamellar, broken up hardened tuff; powdery

If a Gypsiorthod has this horizon the group would be a Hypercalcic Gypsiorthod.

A new group could perhaps also be proposed for the soils with an accumulation of layer limestone, to be known as Calcic Haplaquept.

Russian classification; the individualization of the limestone is not taken into consideration; a subdivision at least at the level of the brown-grey arid type of soils and of the sierozems is to be recommended.

French classification; in the present state, the French classification needs to be made precise concerning the difference between the brown calcareous soils and the sierozems, as well as the subdivision at the level of the sub - groups; new sub-groups corresponding to the different forms and characteristics of limestone should be recognized.

FAO/Unesco classification; the definitions of Yermosols and Xerosols need to be more precise. A precise definition could be based on the presence of:

- an accumulated horizon of gypsum in the part above 150 cm deep;

- conductivity above 2 mmhos/cm. in the part at least one metre from the surface;

- aggregates with an extremely fine structure in the upper horizon.

The coarseness of the texture as well as the annual rainfall must also be taken into consideration.

Table 1 Classification of the Calcareous Soils of Hodna

Form of the CaCO3 in the part above one metre deep	American classification	Russian classification	French classification	FAO/Unesco classification	Zonal soils of Hodna
Crust	Xerollic and Typic Paleorthids	Brown-grey arid soils on hard limestone	Brown calcareous soils with CaCO3 encrustment	Calcic Xerosols and Calcic Yermosols petrocalcic phase	Sierozems with a calcareous crust
Encrustation	Typic and Xerollic Calciorthids Calcic Gypsiorthids	Sierozems on pebbles Brown-grey arid soils developed on gypsum1/	Brown calcareous soils with a calcareous encrustment	Calcic and Gypsic Xerosols Calcic Yermosols	Sierozems with a calcareous encrustment
Concretions	Typic and Xerollic Calciorthids Calcic Gypsiorthids	Pale Sierozems Brown-grey arid soils developed on gypsum1/	Encrusted Sierozems	Calcic and Gypsic Yermosols	Sierozems with concretions and/ or calcareous nodules
Diffused	Xerollic and Fluventic Camborthids	Alluvial soils Sierozems with layers2/	Weakly developed soils with humic alluvial deposits	Haplic Xerosol	Modal Sierozems weakly developed soils. Steppic
Accumulation of calcareous layers	Typic Haplaquepts	Layered Sierozems Sazovie3/	Hydromorphic soils with a redistribution of calcareous nodules	Calcic Gleysols	Hydromorphic soils with an accumulation of calcareous layers

^1/ in Russian "podstilaemie gypsom"
^2/ in Russian "lougovato sierozemnie"
^3/ in Russian "sazovie lougovie sierozemi"

A hypercalcic phase corresponding to the petrocalcic could also be suggested. These are the soils having a highly calcareous horizon, defined as the Hypercalcic Calciorthids.

To classify the zonal soils of Hodna at group levels, the following observations have been taken into account:

- the content and form of the CaCO₃ in the soils as it reflects the evolution of the country during the Quaternary age

- the presence of a crust the formation of which is continuing

- the presence of different forms of limestone in the soils, most of which are in association.

- the differentiation of the lime in soils with a coarse texture is more intense than in heavy soils

- the presence of different types of lime in hydromorphic environments.

On this basis the zonal soils of Hodna are classified using the French nomenclature on a level with the sub-class of Sierozems; the subdivision in groups is according to the degree of differentiation of the lime (see Table 1). The groups could be subdivided into the following sub-groups: typical, saline, gypsifereous with depth, vertic and wind eroded (1).

3.9. Conclusion

In the plains of Hodna it can be seen that there is a very close relationship between the soils, the piedmont slopes and the form of accumulation of the limestone. The nature of the weathered products, the depth of the groundwater, the chemical composition of the water, the texture as well as the thermo-dynamic conditions have played and are playing an important role in the semi-arid and arid conditions concerning the accumulation of calcium carbonate in the soils and its particular form.

Considering that the different stages of evolution of the soils are determined by the form of the limestone, the sierozems are classified at the level of a sub-class instead of a group.

The U.S.A., Russian, French and FAO/Unesco classifications should be perfected and/ or made precise.

REFERENCES

FAO. 1968, Definitions of Soil Units for the Soil Map of the World, World Soil Resources Reports No. 33. FAO, Rome.

FAO. 1970, Key to Soil Units for the Soil Map of the World. Working Document AGL:SM/70/2, FAO, Rome.

FAO. Carte pédologique du Hodna 1:100 000 - Notice explicative, Algiers, (Boyadgiev) (in press)

FAO. Les sols du Hodna. Rapport préparé pour le Gouvernement de la République Algérienne Démocratique et Populaire par le FAO/PNUD. Technical Report (in preparation). FAO, Rome.

Ruellan, A. 1970, Contribution a la connaissance des sols des régions méditerranéennes: les sols a profil calcaire différencié des plaines de la Basse Moulouya (Maroc oriental). These Fac. Sci. Strasbourg

Soil Conservation Service. 1970, Soil Taxonomy of the National Cooperative Soil Survey. USDA Soil Conservation Service, Washington, D.C. (unedited text)

Sys, C. and Verheye, W. 1972, Working Group on the Principles of Land Classification in Arid and Semi-arid Regions. International Training Centre for Postgraduate Soil Scientists, State Univ. Ghent, Belgium.

Travaux, C.P.C.S. 1967, Classification des sols, édition 1967. Document diffusé par le Laboratoire de Géologie-Pédologie de l'E.N.S.A. de Grignon (1963-1967)

4. Calcareous Soils of Bulgaria, their Characteristics, Problems and Improvement

by

L. Raikov
N. Poushkarov Institute of Soil Science
Sofia, Bulgaria

4.1. Introduction

There are three different climatic influences on the small territory of Bulgaria (111 thousand km²) situated in the eastern part of the Balkan peninsula: continental from the north, oceanic from the west and Mediterranean from the south. The high mountains (over 2 000 m) also affect the climate of the country. Stara Planina (the Balkans) divides the country into a northern and southern part respectively; it also restricts the cold air coming from the north, while Rila and the Rhodopes restrict the Mediterranean influence from the south. Thus there exist several climatic regions in the country with respective prevailing influences: moderate continental, transitory continental, continental Mediterranean, mountain climatic, etc.

Both the orography and geological structure in the country are rather complex. There are sedimentary, magmatic and metamorphic rocks. The most widespread sedimentary ones, are: loess, limestones, marls, sandstones, clays, etc, of the magmatic rocks the granites and andesites are the most widespread, and of the metamorphic ones, gneisses, schists, phyllites, etc. The variegated geological basis, the complex orography, the different climatic influences, the rich flora (over 3 000 plant species), the ancient agricultural management and other factors all contribute to the formation of a comparatively great soil diversity.

Calcareous soils are mostly represented by calcareous chernozems and rendzinas (humic carbonate) covering 10 percent of the country's territory, as well as by smaller areas of calcareous and typical smolnitsas, cinnamonic soils with rendzinas, brown forest soils with rendzinas, eroded chernozems, etc. The area covered by calcareous soils is smaller than that of the carbonate parent rocks, which is due to the fact that the leaching processes in the country are widely spread and therefore leached soils are very-common. As regards arable land calcareous soils cover about one fifth of the country's territory.

4.2. Calcareous Chernozems

Calcareous chernozems are widespread in the northern part of the Danube plain; they are developed on loess, loess-like clays and marls, and cover about 0.72 million ha, being divided into two regions: along the Danube and around the towns of Shouman and Varna. They are differentiated, moderately thick on loess, slightly thick on loess-like soil-forming material, and skeleton ones on hard carbonate rocks and eluvium. The moderately thick ones with 40-80 cm deep humic horizon are the most widespread. No compaction of the transitory horizon is observed in these soils. The carbonate micelle is between 20 and 80 cm, but there are cases with the micelle found to a depth of 170 cm. Most of the calcareous chernozems are of a moderate sandy loamy soil texture. The content of particles smaller than 0.001 mm is about 22 percent, while in the loess, on which the soils are formed, these particles are 12%. The amount of physical clay (particles smaller than 0.01 mm) is from 26-44% as regards the parent rock. A tendency is observed toward an increase Of clay particles from west to east, which is related to the carrying away of soil material by the Danube river. The hydromicas, montmorillonites and kaolinites are the most widespread clay minerals prevailing in the calcareous chernozems. Humus content is not very high, about 2-3% in the soils cultivated for a long time. Humus of 25-35% is encountered in the one metre deep soil layer containing the arable soil layer. The humic acids and humins prevail in the humus. CaCO₃ content is not very high; it is usually from 1-3% and pH is about 7.5-8.0. Sorption capacity is mostly 20-30 meq/100 g of Soil; exchangeable calcium cations prevail strongly (78-86%), while those of magnesium are 10-20% only, of potassium and sodium less than 5%. The total amount of P₂O₅ is 0.17%; the calcium phosphates also prevail strongly. The content of water soluble phosphates is very low. As a whole, the phosphorus status of calcareous chernozems is unfavourable because of the concentration of phosphorus in the coarse fraction of the soil and weak weathering of the phosphate minerals. These Soils are well supplied with potassium. They have a positive reaction with nitrogen, phosphorus and zinc (e.g. with maize).

Calcareous chernozems have favourable physical properties, such as porosity and air permeability; they are not compacted along the depth of the whole profile. Water permeability and filtration are both comparatively high, which leads to an insufficient water balance. As a result of the continental steppe effect in the region there are often droughts irrespective of the fact that the mean annual amount of precipitation is about 600 mm.

4.3. Rendzinas (humic calcareous soils)

These soils are usually developed on hard limestone rocks in the four mountain and mountain regions of the country, while in the flat and hilly regions they are developed on diluvial, limestone clays, etc. They cover a total area of 0,304 million ha. The shallow rendzinas, 20-30 cm thick, that are often subjected to erosion are the most widespread, The rendzinas developed on ancient flood cones are of a thicker humus horizon. Rendzinas are connected with transitions with the zonal soils of the type of chernozems and cinnamonic forest soils. They are often dark brown and redbrown in colour,

Rendzinas are sandy loamy soils with a different skeleton.

The partial studies carried out on the composition of clay minerals have shown a certain dominance of montmorillonites, micas and kaolinite. They are loose, structured, aerated, with unfavourable water relationships. The content of humus in the rendzinas from the mountain regions is from 5-7%, while in those from the plains, from 2-3% only is accumulated mostly in the arable soil layer. The fulvoacids are less than the humic acids. The carbonates vary from several percent only to 4.0% in the arable layer, while in the subsoil arable layer they can even reach up to 75%? pH is usually about 7.5-8.0. Sorption capacity reaches, with some differences for a heavier soil texture, up to 35-45 meq/100 g of soil (the rendzinas from all regions in the country have not yet been studied).

The studies made on the forms of iron and alluminium in some of the rendzinas have shown an increased content of amorphous forms of iron and a decrease of silicates and especially their oxycrystal forms, and the high mobility of aluminium compounds. The last phenomenon differentiates the rendzinas from the cinnamonic forest and brown forest soils.

Rendzinas are poorly supplied by available forms of nitrogen and phosphorus and well supplied by potassium. They are poor in zinc and available iron. They also show a good response to nitrogen and phosphorus fertilizer application; residual effect of phosphorus is particularly good.

4.4. Some Problems in the Use of Calcareous Soils in Agriculture

Calcareous soils are intensively used in agriculture. Most of the main arable crops in the country (wheat, maize, sunflower, beets, vines, etc.) are grown on those soils in North Bulgaria. Hence the importance of all the practices that can improve fertility of these soils or restrict some of the unfavourable properties.

4.4.1. Calcareous Chernozems

Low water holding capacity and comparatively high soil permeability are among the unfavourable properties of the calcareous chernozems. With non irrigated lands very often the yields from the crops grown on these soils, particularly in the summer, are lower when compared to those from the leached chernozems situated in the same neighbourhood. The effect of fertilizers applied is also smaller, particularly for maize grown without irrigation. Thus, irrigation of calcareous chernozems is necessary to ensure the required water supply for the development of the crops, as well as for the increase of nutrient uptake. This has also been proved by the results obtained from field experiments as well as by agricultural practice. In the course of the last few years irrigation has been widely introduced with calcareous chernozems.

In order to improve the water holding capacity of calcareous chernozems experiments are being carried out with conditioners (polymers) with perlite, organic and other materials. Positive results have been obtained from experiments on small plots with the above quoted substances, though no recommendations can be made for practical purposes at this stage.

Improvement of the phosphate status of calcareous chernozems is an important problem. It has been established that when higher rates of superphosphates (the so-called "store fertilizer application") are applied to soils with a lower content of carbonates, a higher level of available phosphates is created which are not fixed irreversibly and can be used by plants. The increase in phosphate levels has raised the problem of the low supply of available zinc and boron with some crops (maize, alfalfa, fruit trees, etc.). To this end zinc fertilizers were widely applied recently; they have proved very efficient with the above mentioned crops.

Soil erosion control is another very important problem to be solved with calcareous chernozems in these regions.

4.4.2. Rendzinas

Rendzinas have been studied to a lesser extent than the chernozems. The data available show the requirement of nitrogen and phosphorus fertilizers to be introduced, as well as trace elements such as zinc and iron. Chlorosis is often met in these soils too. Vines, tobacco, arable crops, etc. are mostly grown on them.

Since rendzinas usually occupy the fore-mountain and hilly regions they are subjected to erosion To combat this terraces can be made on small spaces, where the soils are thicker, e.g. on some deluvial deposits. Erosion control practices (grass buffers, re-seeding, etc.) are of great importance and can help to increase fertility, since they improve the nitrogen balance and production of protein.

Irrigation of rendzinas in flat regions has also proved efficient but has given rise to some difficulties due to relief, some technical problems and the hazards of erosion.

The following conclusions can be drawn from this brief review of the problems of calcareous soils and the improvement of their fertility. Studies should be conducted on:

(i) the role of carbonates and their dynamics along the depth of the profile under the conditions of the country. Both type and size of the particles of the carbonate minerals should be studied in detail

(ii) the effect of carbonates on mobility and availability to plants of phosphorus, zinc and other important major and trace elements

(iii) efficient ways of improving the water holding capacity of calcareous soils

(iv) the erodibility of calcareous soils and determination of the most important appropriate hydrotechnical and management practices as regards erosion control.

Alexiev, A., 1972, Disertatsiya (Institut po pochvoznanie)

Boyadzhiev, T., Pochvoznanie i agrohimiya, kn.2 1966, kn.3 1968, Sofia

Pochvite na Balgaria, 1960, Avtorski kolektiv, Sofia

Pochvena karta na Balgaria, 1968, 1:400 000, Sofia

Prognoza - poddarzhane i povishavane na pochvenoto plodorodie 1971 v NRB, Sofia