4. EFFECT OF GYPSUM AND CALCIUM CARBONATE ON PLANTS

4.1 Introduction
4.2 Winter Crops
4.3 Summer Crops
4.4 Classification of Field Crops Based on Their Tolerance to Gypsum
4.5 Fruit and Forest Trees

4.1 Introduction

Gypsiferous soils are very variable and there are many factors that affect their properties in relation to plant growth. Gypsiferous soils can be productive and managed profitably if they are first studied properly. The effect of the chemical properties of gypsiferous and calcareous soils on the growth of plants, both natural vegetation and crops, and their mineral contents have been investigated by numerous authors.

Before further discussing the effects of gypsum on plants it is worth noting that measurements of total gypsum in soils are unreliable and do not reflect the actual amount present as proved by Sayegh et al. (1978). Figures quoted in the literature for the gypsum content of soils are commonly lower than the actual amount present.

Van Alphen and de los Rios Romero (1971) conclude that up to 2 percent gypsum in the soil favours plant growth, between 2 and 25 percent has little or no adverse effect if in powdery form, but more than 25 percent can cause substantial reduction in yields. They suggest that reductions are due in part to imbalanced ion ratios, particularly K:Ca and Mg:Ca ratios. Hernando et al. (1963, 1965) studied the effect of gypsum on the growth of corn and wheat by varying the gypsum level in the soil up to 75 percent. They show that high levels of gypsum caused poor growth of corn, especially as the soil moisture was maintained at 80 percent of field capacity. However, wheat showed minimum growth where the soil contained 25 percent gypsum at all soil moisture levels ranging from 15 to 100 percent of field capacity. Akhvlediani (1962) concludes in general, that agricultural production on gypsiferous chernozem and chestnut soils is not affected when the gypsum content is between 15 and 30 percent. Bureau and Roederer (1960), report that 30 percent gypsum content in soils of Tunisia is toxic to plant growth. Van Alphen and de los Rios Romero (1971) state, from field observations in the Ebro Valley of Spain, that plant growth is reduced where the gypsum content exceeds 20 to 25 percent.

Soil gypsum affects the mineral contents of plants. Hernando et al. (1965) showed, using water culture, that increasing the concentration of SO₄ mixture (K₂SO₄ + MgSO₄) increased the uptake of NO₃, K, Mg and sulphur by corn but decreased the uptake of Ca and P. Boukhris and Lossaint (1970, 1972) studied the mineral contents of 52 natural species growing on gypsiferous soils in Tunisia, and reported that various species growing under the same ecosystem responded differently to the excess of Ca and SO₄ present in the soils, depending upon their biogeochemical properties. In general, the chemical composition of the leaves or aerial parts of plants is influenced by the plant family. A comparison of field observations, with water culture studies, confirms the behaviour of plant species toward nutrient adsorption. A high level of SO₄ in the soil can raise the SO₄, level in the gypsum tolerant species (called gypsocline); but to a lesser extent in the species (so-called gypsophytes) found on natural gypsum soils. However, there are some gypsophytes called thiophores which have great ability to accumulate high levels of S in their leaves. The SO₄ concentration in some thiophores is thirty times greater than that in other species living in the same environment (Table 4.1).

Table 4.1 CHEMICAL COMPOSITION OF WILD PLANTS ON GYPSIFEROUS SOILS

Species	% Dry matter
	Ca	Mg	Na	K	Cl	SO4
Atriplex halimus	6.29	2.68	1.95	3.56	1.82	19.05
Salsola tetrandra	1.77	1.40	15.86	3, 60	4.82	21.59
Salsola vermiculata	2-3.5	1-1.5	6.9-10	1.2-3.5	1.1-4.2	4.2-91
Anabassis oropediorum	4.8-10.9	1.5-1.6	2.0-2.2	1.1-1.8	0.6	0.6-0.7

In the literature, it is reported that the chemical as well as the physical properties of the gypsiferous soils affect plant growth and mineral composition of plants.

From intensive field observations of gypsiferous soils in Iraq, Smith and Robertson (1962) found that root growth was inhibited where the gypsum content of soil was over 10 percent. This is apparently because of the poor transmission of air and water caused by poor structure. They also found that soils containing more than 25 percent gypsum in the rooting zone give poor growth. In the spring, wheat crops wilt on shallow gypsiferous soils when other crops on deeper soils show no signs of distress. Roots do not penetrate the gypsum layer, even when it is quite wet. Kovda (1954) and other workers observe that plant roots do not penetrate a soil layer containing 25 percent of gypsum or more. Mardoud (1980) observes that pine roots cannot penetrate a soil layer with 60 percent of gypsum. The roots extend horizontally and the trees show signs of poor growth compared with trees on soils with gypsum horizons at greater depth. Boyadgiev (1974) shows that the presence of well-crystallized gypsum within the first metre of soil affects the performance of cotton crops significantly. He found it difficult to evaluate the adverse effect of gypsum content on the cotton yields because of its link to other variables which also cause these soils to be less than ideal for cotton production. Boyadgiev (1974) also noted that crops such as alfalfa could grow very well and give high yields even in soils containing up to 50 percent of powdery gypsum as long as no gypsic layer impeding root elongation and extension is present in the soil profile at shallow depth. Similar effects have been noted by Amami et al. (1967) in the oasis at Tozeur in Tunisia, where good yields of alfalfa and date palms were obtained in the highly gypsiferous soils. Similar results were obtained in the Ebro Valley of Spain with crops such as alfalfa, wheat and apricots.

It appears from the above results that the gypsum content of soils is only one of several factors which affect plant growth and yield of crops. The other factors are:

i. the depth of the topsoil over a gypsic layer
ii. the hardness and degree of crystallization of the gypsic layer
iii. the total and active calcium carbonate contents
iv. the availability of plant nutrients and moisture content in the root zone
v. the type of crops grown and their relative tolerance to gypsum
vi. the drainage conditions and salinity of the soil.

4.2 Winter Crops

Wheat

Wheat grows least well when the gypsum content in soils is around 25 percent (Hernando et al. 1963). Smith and Robertson (1962) observe that wheat grown on shallow gypsiferous soils shows signs of wilting in the spring and the roots were unable to utilize moisture from the wet gypsic layer below. Van Alphen and de los Rios Romero (1971) record high yields of wheat on gypsiferous soils in the Ebro Valley, Spain. Mardoud (1980) found that Mexican wheat cultivars yielded an average of 4 tonnes per hectare on the shallow gypsiferous soils of the Euphrates Valley (gypsum content less than 25 percent in the 0-15 cm layer and 25-35 percent in the 15-30 cm layer). This yield is considered very satisfactory under the conditions in the Valley.

Barley

Barley (H. vulgare) is an important crop well adapted on gypsiferous soils under dry farming agriculture, and is widely used by farmers in Northern Syria and Iraq. A two-course rotation is practised. Fallow is extensively used after wheat or barley, in localities where the average annual rainfall is 200-300 mm. All the evidence suggests that barley like wheat is tolerant to gypsum. Yields of 1.5 to 3 tonnes per hectare were obtained under rainfed agriculture depending upon total seasonal rainfall and distribution. However, Mardoud (1980) obtained up to 4 tonnes per hectare in the Euphrates Valley with supplementary irrigation.

Vetches

American vetch (Vicia dassicarpa) was tested under irrigation on various types of gypsiferous soils of the Euphrates Valley. The active nodules were very limited in number in the first cropping season and yield, on the very shallow gypsiferous soils, ranged between 1 to 2 tonnes of seeds or 8.4 tonnes of green fodder per hectare. The yield during the second season improved significantly averaging 2.5 tonnes of seeds or 25 tonnes of green fodder per hectare. Lower yields were obtained on sandy gypsiferous soils. Under rainfed farming agriculture and with rainfall exceeding 250 to 300 mm, vetches could be a good crop to follow wheat or barley in the rotation.

Lentils

Lentils (Lens esculenta) is a dry farming crop, which is usually alternated with wheat or barley in a two-course rotation. Lentils are used as well as vetches on the rainfed, gypsum-affected soils of Iraq and Syria as a break crop. Lentils under irrigation gave, in shallow gypsiferous soils, a moderate yield of 1.3 to 1.5 tonnes per hectare in the first year. An average yield of 0.5 to 1 tonnes per hectare was obtained on the sandy gypsiferous soils of the Euphrates Valley. As in the case of vetches, it was noticed that few nodules were present on the root system in the first year of cropping with obvious signs of nitrogen deficiency on the plots receiving no nitrogen fertilizer (Mardoud 1980).

Broad beans

Broad beans (Vicia faba) are usually planted, under dry farming conditions, in the higher rainfall zone (350 to 500 mm as annual rainfall). Supplementary irrigation is required to obtain higher yields under lower rainfall conditions. Broad beans grown in irrigated fields of shallow and very shallow gypsiferous soils of the Euphrates Valley gave a poor yield of 0.3 to 0.5 tonnes per hectare at first. Data on the performance of broad beans on deep gypsiferous soils are not available.

Trifolium

Trifolium (Trifolium alexandrinum) is an important fodder crop under irrigated agriculture. Matar (private communication) tested its tolerance to gypsum in pots, under greenhouse conditions with ample K and P fertilization. Four successive cuts were harvested at the flowering stage. The effect of gypsum content was significant in the first cut, reducing the yield by more than 50 percent where the gypsum content exceeded 20 percent (Table 4.2). The effect of gypsum becomes much less in the following cuts. This significant reduction in yield could reflect the delay in the seed emergence and stand establishment caused by the gypsum and calcium carbonate contents. The total fresh weights of the four successive cuts demonstrates that trifolium is quite tolerant to gypsum content in soils. Data on its performance on a field scale are not available.

Table 4.2 EFFECT OF GYPSUM CONTENT OF SOILS ON YIELDS (FRESH WEIGHT IN GRAMS) OF FOUR CONSECUTIVE CUTS OF TRIFOLIUM GROWN IN POTS (Matar, unpublished work)

Gypsum (%)	No. of cuts				Total weight
	1	2	3	4
0	281.3	133.2	363.0	409.8	1187.2
5	281.6	161.8	316.1	414.5	1174.0
10	260.0	154.9	322.2	432.5	1169.7
20	233.8	143.9	292.6	375.6	1045.9
40	129.6	123.2	282.4	389.1	924.3

Alfalfa

The performance of alfalfa (Medicago sativa) as a major crop on gypsiferous soils is quite impressive. High yields of irrigated alfalfa are recorded on the highly gypsiferous soils of the Ebro Valley of Spain (Van Alphen and de los Rios Romero 1971). Amami et al. (1967) reported yields of 10 tonnes per hectare of alfalfa on light textured gypsiferous soils containing 20 to 25 percent gypsum in the root zone. Mardoud (1980) harvested 20 to 60 tonnes per hectare of fresh fodder in the first and the second year of cropping. The yields were obtained where alfalfa was grown in medium deep soils (less than 25 percent gypsum content in the first 45 cm of the soil profile); and similar yields, 14 and 61 tonnes per hectare were also obtained in shallower gypsiferous soils (15 to 20 cm depth with 25 percent gypsum content). The performance of various alfalfa cultivars tested on the shallow gypsiferous soils of the Euphrates Valley was significantly different. The result of variety trials is reported in Table 4.3.

Table 4.3 YIELDS OF ALFALFA GROWN IN SHALLOW GYPSIFEROUS SOILS¹

¹ 15-30 cm deep with less than 25 percent gypsum

Other Winter Crops

Other cereal crops can be grown on gypsiferous soils with success. Loomis (1944) shows that the yields of oats (Avena sativa) improved slightly in the presence of gypsum in a pot experiment.

4.3 Summer Crops

Many summer crops can be grown on gypsiferous soils with various degrees of success. These include cotton, sugar beet, potato, groundnut, soybean, sesame, tomato and sunflower.

Cotton

Cotton is an important cash crop. Boyadgiev (1974) notes that crystallized gypsum particles in soils depressed the yield of cotton in the Euphrates Valley. Mardoud (1980) obtained good yields of cotton (4 tonnes per hectare) in moderately deep gypsiferous soils (with 45 cm of soil, with less than 25 percent gypsum content), 1.94 tonnes per hectare in medium deep sandy soils with 25 to 50 percent gypsum content in the root zone. Vieillefon (1976) found little effect of gypsum on the performance of cotton in soils with medium calcium sulphate content at Ain Zerig. Minashina et al. (1983) observes that yields of cotton grown on grey-brown gypsiferous soils fell by 16 percent in soils with 10 percent gypsum. A larger decrease in yield was observed in soils with higher gypsum levels. They also observe that the quality of the cotton fibre deteriorates as the gypsum content in the soil increases. Although the field observations in the Euphrates Valley gave promising yields of cotton, greenhouse and recent field studies by Minashina suggest that cotton is not always sufficiently tolerant to gypsum. More research is needed to study the effect of gypsum on cotton cultivars in the various gypsiferous soil types.

Sugar beet

Limited work has been done on the suitability and tolerance of sugar beet (Beta vulgaris) in gypsiferous soils. Mardoud (1980) obtained 17.5 to 22.5 tonnes per hectare of autumn-grown sugar beet and 35 tonnes per hectare of spring-cultivated sugar beet in moderately deep gypsiferous soils in the Euphrates Valley. These yield levels are considered moderate.

Corn

Corn (Zea mays) is one of the main crops in the irrigated areas of the arid zone. The effects of soil gypsum content on corn growth and nutrient composition has been studied by several workers, for example

Hernando et al. (1963, 1965). Growth of corn was reduced with the high gypsum levels. The interaction between gypsum content and soil moisture stress was found significant in its effect on corn growth and performance. Mardoud (1980) obtained 2 tonnes per hectare of corn seeds grown in moderately deep gypsiferous soils of the Euphrates Valley. Some foliar symptoms of micro-element deficiences were noticed, however.

Soybean

Soybean (Glycine max) is strongly affected by the gypsum content of soils. On the sandy gypsum soils of the Euphrates Valley (25 to 50 percent gypsum content) it gave very low yields of about 0.4 tonnes per hectare of grain seeds (Mardoud 1980). Soybean, grown in pots, however, showed a marked tolerance to gypsum. The effects of Rhizobium innoculation, of fertilization and of the gypsum content in different gypsiferous soils on seed production were not studied.

Groundnut

Groundnut (Arachis hypogaea) is a good leguminous crop for light to medium textured soils. Yields of 1.25 to 1.42 tonnes per hectare of seeds have been obtained in the Euphrates Valley on sandy gypsiferous soils, with 25 to 50 percent gypsum content (Mardoud 1980). In spite of adequate P and K fertilization, groundnut grown in pots with 6 kg soil showed a gradual decrease in fresh weight of tops as the gypsum content increased. The average yield of fresh tops dropped by 35 percent at a gypsum content of 40 percent. The effect of gypsum content on the yield of seed was more pronounced (Figure 4.1). There were no visual signs of nutrient deficiencies on the plants. There were no nodules on the roots of plants grown in the gypsum treatments.

Walker et al. (1976) found that application of gypsum to soils low in calcium increased the percentage of oil in all peanut cultivars; while the nitrogen content of the seed was reduced. Davidson et al. (1983) reports that application of gypsum to groundnuts grown in Georgia increased germination and reduced aflatoxin contents by 40 percent.

Figure 4.1 Effect of gypsum content on yield of groundnut tops and seed in a pot experiment

Tomato

Field tomatoes (Lycopersicum esculentum) showed significant tolerance to gypsum. A yield of 17 tonnes per hectare of fruit was obtained when tomato was grown in moderately deep gypsiferous soils in the Euphrates Valley (Mardoud 1980). In recent years, irrigation with sulphate-rich water has been of interest to many scientists studying the growth and yield of tomato crops. Papadopulos (1984) found that the weight of fresh tomato fruit was decreased by about 50 percent when irrigated with sulphate-rich water. Similarly, Russo (1983) found that tomatoes grown in gypsiferous-sodic soils gave low yields because they had a small fruit weight. Martinez et al. (1984) report that the total sulphur content in leaves and roots of tomato plants is significantly increased as the S04 levels in the substrate increase. Increasing the level o f NO₃ in the growing medium increases tomato yields and the total sulphur accumulated in the leaves and roots decreases. However, at high levels of SO₄ in the growing medium the addition of NO₃ decreases the yield. That leads to the reasonable conclusion that the effect of SO₄ is not ion specific but is mainly an osmotic effect.

Potato

Potato (Solarium tuberosum) planted in the moderately deep gypsiferous soils of the Euphrates Valley with less than 25 percent gypsum content in the top 45 cm gave a poor yield of 7.3 tonnes per hectare of tuber. More research is needed to determine the tolerance of potatoes to gypsum.

Sesame

Sesame (Sesasum orientale) crop grown in the gypsiferous soils of the Euphrates Valley gave an average yield of 1.8 tonnes per hectare (Mardoud 1980).

Sunflower

The only published work on the effect of gypsum content on sunflower (Helianthus annuus) performance is that of Mardoud (1980) who obtained 1.27 to 1.7 tonnes per hectare of seed grains on sandy gypsiferous soils, containing 25 to 50 percent gypsum.

Other Summer Crops

Other summer crops for which data are available include:

Sorghum, an important crop for animal feed and human consumption yielded between 2.25 and 3.9 tonnes per hectare in the medium deep gypsiferous soils of the Euphrates Valley. However, the yield dropped to less than 0.65 to 1.85 tonnes per hectare when grown in shallower soils (15 to 30 cm thickness with less than 25 percent gypsum content).

Onion, a sulphur-loving plant, yields about 24 tonnes/ha in the sandy gypsiferous soils of the Euphrates Valley. The cooking quality of the bulbs is, however, unsatisfactory and the taste is very strong.

Figure 4.2 Effect of soil gypsum content on relative yields of leaves and roots of Burley tobacco

American Burley, the irrigated broad-leaved tobacco, grown in pots with gypsum content ranging between 0 and 40 percent, is extremely sensitive to gypsum. The yield of leaves dropped by more than 95 percent when grown in soil containing 5 percent gypsum or more (Figure 4.2). Complex symptoms of various nutrient deficiencies appeared on the tobacco leaves. Ryding (1978) found in greenhouse trials, that application of gypsum decreased the yield of flue-cured tobacco although the Ca content of leaves was increased.

4.4 Classification of Field Crops Based on Their Tolerance to Gypsum

The tolerance, yield and product qualities of many agricultural crops grown on gypsiferous soils are not yet well known. As a first approximation, the main agricultural crops are classified below into four main groups in relation to gypsum tolerance based on the available data. This preliminary classification should be considered a guideline only.

Group I: Tolerant to gypsum - agricultural crops which show tolerance to 40 percent of gypsum in soil without a significant decrease in yield: alfalfa, trifolium, wheat, barley, lentil, oats, tomato and onions.

Group II: Semi-tolerant to gypsum - agricultural crops which show tolerance to 20 percent of gypsum in soil without a significant decrease in yield. The yield may drop by about 50 percent at higher levels of gypsum (say 40 percent gypsum). This group includes: broad beans, sugar beet, sorghum, corn, soybean and sesame.

Group III: Semi-sensitive to gypsum - agricultural crops which show tolerance of up to 10 percent of gypsum without a significant drop in yield. Yields fall at higher levels of gypsum. This group includes: cotton, groundnut, potato and sunflower.

Group IV: Sensitive to gypsum - among the test crops, tobacco was sensitive to gypsum.

4.5 Fruit and Forest Trees

In addition to the gypsum content of the surface layer and its distribution in the profile, many workers have found that the hardness and degree of cementation of the gypsic layer is of great importance to the success of fruit orchards and forest trees. A cemented layer at shallow depth impedes the extension of the root system by mechanical resistance and consequently limits the growth and production. Kalashnikov and Romanov (1949) found, from some afforestation experiments conducted on dark chestnut soils overlying gypsum, that the greater the soil depth above the gypsum layer the more suitable the soils are for afforestation. They also found that oak (Quercus) and pear trees, 13 to 15 years old, grew 3.5 to 5.0 metres high when the gypsum layer was at 170 cm from the surface and 2 to 4 metres only when the gypsum layer was at 1.10 to 1.25 metres depth. Mardoud (1980) noticed that the roots of forest trees, such as pines (Pinus halepensis) and eucalyptus, grown in the Euphrates Valley of Syria could not penetrate any gypsic horizon containing more than 60 percent of gypsum. Their roots extend horizontally and the trees showed signs of weak growth compared with trees grown in soils with no gypsic horizon. Other researchers report that several species were found resistant to gypsum and gave good yields in highly gypsiferous soils. Van Alphen and de los Rios Romero (1971) cite that high yields of apricots (Amermeniaca vulgaris) were obtained in the Ebro Valley of Spain when grown on gypsiferous soils with a gypsic layer at a depth of 30 to 60 cm. The average apricot yield obtained in the El Burgo de Ebro was in the order of 8 tonnes per hectare.

A good yield of palms was obtained in gypsiferous soils containing 50 percent gypsum in the oasis of Tozeur, Tunisia (Amami et al. 1967).

Minashina (1956) reported excellent growth of grapevines grown on gypsiferous soils of the Kirovabad Massif, even in soils with a gypsic layer at shallow depth. But work carried out in the Euphrates Valley in Syria, has shown that grapevine tolerance to gypsum depends on the variety grown. Except Muscat and Cardinal, all varieties tested gave a poor growth on the moderately deep gypsiferous soils of the Euphrates Valley (Mardoud 1980). More research is needed to determine grapevine varieties and rootstocks tolerant to gypsum. In Spain, wine produced from grapes grown on gypsiferous soils is of poor to medium quality, but table grapes are of good quality (Prof. Roquero, personal communication). In the Murcia area of Spain, where the soils are highly gypsiferous with a marked petrogypsic horizon, apricots, peaches, pears, olives and grapes are extensively planted. The farmers are very satisfied with the high yields obtained. The peach varieties are grafted onto rootstocks tolerant to soils with a large calcium content.

Observations on other fruit trees grown in the moderately deep gypsiferous soils of the Euphrates Valley show that many species tolerate gypsum, including pomegranate (Punica granatum), peaches (Amygdalis persica), plums and figs (Ficus carica) and apricots. Pistachio (Pistacia vera) however shows poor adaptability to gypsiferous soil conditions. Only three of 46 trees of local pistachio cultivars remained alive after four years.

Several papers have been published recently on the effect of high sulphate-rich waters on ion adsorption and fruit quality of lemon trees. Cerdá et al. (1982) and Fernandez et al. (1983) found that sulphate ions are not taken up as readily as other soluble ions such as chloride or boron; and never exceeded 240 mmole/kg of plant. These authors believe that some of the sulphate effects reported in the literature might not be the result of sulphate uptake but due to the salinity of the soil solution. Rind thickness and rugosity were the only fruit quality characteristics affected by the sulphate application. The effect of the gypsum content in soils on the yield of lemon trees and other citrus varieties is not yet known.

Boukhris and Lossaint (1970, 1972) in a study conducted on the gypsiferous soils of Tunisia, found that resinous trees including Pinus halepensis and other oligophore trees absorb few of the ions present in gypsiferous soils and control their uptake of ions very efficiently. They are poor in all mineral nutrients especially potassium and calcium. Constant Mg, S, N and P contents were observed in pines and other trees grown under the various climatic conditions of the gypsiferous soils of Tunisia. The low levels of nutrients in pines make them quite successful and adaptable to all types of soil environments including gypsiferous ones. Wild (1974) found dwarfing of woody species, particularly of Colophospermum mopane, grown on a gypsum deposit in Botswana. He attributed the dwarfing partly to the poor physical properties of these compacted and clayey soils.

The available information on the tolerance of various species of fruit trees and their rootstocks to the gypsum content of soils is still inadequate. Much more information is needed on the performance and adaptation of various tree species to the gypsiferous soils, and on the effect of gypsum on fruit yield and quality.