5. IMPROVEMENT OF PRODUCTIVITY OF GYPSIFEROUS SOILS

5.1 Introduction
5.2 Rainfed Agriculture
5.3 Irrigated Agriculture
5.4 Fertilization of Crops

5.1 Introduction

The effects of gypsum on the growth and mineral content of various field crops, fruit, and forest trees is discussed in the previous chapters. Most research on nutrition and performance of plants grown in gypsiferous soils has been done under greenhouse and laboratory conditions and there has been little field experimentation. Gypsiferous soils have been cultivated under dry farming systems for centuries mainly with cereal crops and small-grain legumes. Because of population pressure and recent technological developments in the use of underground waters some gypsiferous soils are now irrigated. Under irrigation, new problems have arisen through the introduction of high-yielding crops especially those least tolerant of gypsum. The intensive leaching of nutrients, calcium solubilization from gypsum, and the removal of exchangeable potassium and magnesium affect the productivity of gypsiferous soils.

When considering soil management all aspects of the soil should be taken into account otherwise the main limiting factor may be overlooked. The management of gypsiferous soils is discussed below in three sections.

5.2 Rainfed Agriculture

Wheat and barley have been cultivated since the earliest times on the gypsiferous soils of the northeastern plains of Syria and north of Iraq (Smith and Robertson 1962). Here, traditionally the land has been dry farmed, i.e. left fallow once every two years. Farmers assumed that fallowing improved the fertility status of the soil and the storage of soil moisture. The same practice was applied on the hilly and eroded uplands of Spain (Van Alphen and de los Rios Romero 1971). On the Kirovabad massif of Azerbaidzhan (Minashina 1956, 1958) and in Spain, grapes are cultivated successfully giving good yields, especially of table varieties.

The improvement in the productivity of gypsiferous soils under rainfed conditions is currently approached by several methods depending upon the soil properties. Some of the techniques are outlined below.

Soil terracing has been practised for many centuries on the deep hilly soils of the Murcia area of Spain to prevent erosion. Fruit orchards have been planted including peaches, pears, olives and other crops. Supplementary irrigation has been used to increase productivity where water resources are available.

Harrowing the land after harvesting and before the rainy season is a common practice to improve the infiltration of water and conserve soil moisture.

The organic matter of soils can be increased by replacing fallow by small-grain leguminous crops in the wheat-fallow rotations. This was practised in Syria and Iraq, especially in areas where the annual precipitation ranges between 250 to 450 mm. Akramov (1981) discusses the positive effect of manure on converting unproductive gypsiferous soils into productive ones.

Subsoiling can be undertaken to break the cemented gypsic subsoil. This improves root penetration and reduces susceptibility to drought, especially in the case of fruit and forest trees. It improves crop establishment and has been practised by many farmers in Algeria, Syria and other countries for planting pistachio, almond, etc., in soils with a hard calcareous crust. Caution should be exercised not to mix the topsoil and subsoil. The former usually contains less gypsum and has a higher organic matter than the latter.

Fertilization is very beneficial in increasing productivity. It has become a general practice under rainfed conditions to apply nitrogen and phosphorus to cereals. The rate of applied nitrogen fertilizers depends on the annual precipitation; N is generally applied where rainfall exceeds 260 mm annually. Phosphorus fertilizers are very effective in increasing the yield of cereals, especially under low rainfall.

Cereals are grown satisfactorily on soils with less than 30 cm depth and less than 25 percent gypsum content, especially if precipitation is adequate, ranging between 250 and 350 mm. Under higher rainfall these types of soils are satisfactory for many varieties of grape vines.

5.3 Irrigated Agriculture

5.3.1 Leaching gypsum from soils
5.3.2 Irrigability of gypsiferous soils
5.3.3 Salinity and leaching requirements

Under dry farming, the yields obtained depend to a large extent on precipitation. Yields of wheat and barley and some other crops are doubled by introducing irrigation to the farms. Yields of 3 to 4 tonnes of wheat grain per hectare are obtained using irrigation on gypsiferous soils in Spain, Syria and Iraq.

The application of water is the main factor in increasing the productivity of the soils of arid climates. Where soils contain gypsum in powdery form, either in the surface layer or deeper in the soil profile, and where no mechanical resistance impedes root extension, the application of irrigation water has been found beneficial improving the productivity of the soils. Crops such as alfalfa, grapes, apricots, and others give good yields in such soils with high gypsum content.

5.3.1 Leaching gypsum from soils

Because of the sensitivity of many crops to gypsum, attempts have been made to leach some of the gypsum from the soil profile when irrigation water was available. Albareda et al. (1962) studied the effect of leaching on gypsiferous soils using various solutions in pot and lysimeter experiments. They reported the following:

1. The use of (NH₄)₂CO₃ solution was found to cause the optimum elimination of gypsum in gypsiferous soils with both low and high levels of gypsum.

2. The presence of urea in soil helped to increase the leaching rate of gypsum.

3. With the addition of some soil conditioners, leaching of gypsum was improved but it accumulated in deeper horizons. However, when (NH₄)₂HPO₄ was added to the soils the amount of gypsum leached was increased.

4. The application of KCl solution resulted in more efficient gypsum leaching from soil surface horizons. But, it was less effective in leaching gypsum from subsurface horizons.

5. In field experiments, the application of ammonium phosphate/carbonate and small amounts of manures led to an efficient removal of gypsum down to 20 cm depth. But, only (NH₄)₂CO₃ caused no accumulation below that zone.

5.3.2 Irrigability of gypsiferous soils

The pressing need for increased agricultural production in arid and semi-arid regions can be met by understanding the soil characteristics of the marginal lands which are dominant in such areas where there are significant areas under irrigation. Irrigation development usually requires costly investment so mistakes are expensive and are to be avoided. All land needs to be surveyed and evaluated to ascertain its suitability for irrigation before investment takes place. An initial soil survey provides a basis for land evaluation as well as a guide to the potential agricultural value of the land.

Most systems of land classification are derived from the system of the US Bureau of Reclamation, and are adapted to local conditions. The land classes are mainly defined by the limitations and the intensity of these limitations. FAO has developed methods of assessing the suitability of land for specific uses so that reliable predictions and recommendations can be made (FAO 1985). Since most of the Middle Eastern countries are situated in the arid and semi-arid regions, reference should be made to the work of Sys and Verheye (1972), Barzanji (1973), and the observations of other agricultural scientists who have worked in these regions. They suggested systems to indicate the suitability of the land for irrigation and the main irrigated crops to be cultivated.

The suitability of soils for irrigation in arid and semi-arid regions depends mainly on the following factors: soil texture, structure, depth, total and active calcium carbonate contents, gypsum content, salinity and sodicity, drainage, and slope. The relative importance of each of these depends on many factors such as climate, type of crops and soil management. However, the following discussion will deal mainly with the gypsum content of the soil, its index value, and its effect on suitability for irrigation.

Sys and Verheye (1972) and Barzanji (1973) provide a scheme to evaluate suitability for irrigation based on the standard particle-size analysis and physico-chemical characteristics of soils. They take into consideration soil texture, soil depth, calcium carbonate and gypsum contents, salinity and sodicity, drainage, and slope as factors influencing the suitability of a soil. For each of these factors a range of indices is suggested according to their relative importance in the water-holding capacity and the water intake of the soil profile. An index 1.0 indicates optimum conditions; lower values, suboptimal conditions for crop production. For example, the influence of the gypsum concentration in soils is comparable to that of calcium carbonate, except gypsum is more soluble and may cause dissolution pockets and successive development of a characteristic microrelief if the soils are irrigated. The general indices in Table 5.1 are suggested where soils and crops information are very limited.

Table 5.1 RATING INDEX ON THE BASIS OF AVERAGE GYPSUM CONTENT IN THE UPPER 100 cm OF THE SOIL OR TO A LIMITING LAYER (Sys and Verheye 1972, Barzanji 1973)

Gypsum content(%)	Gypsum indices
Up to 0.3	0.9
0.3-10	1.0
10-25	0.85
25-50	0.60

Smith and Robertson (1962) observe that yields of annual and perennial crops are depressed when grown on soils where the gypsum content of the root zone is higher than 25 percent. Thus, Barzanji (1973) suggested the following parameters for annual crops where the indices were calculated according to the weighted average of the gypsum content of the upper 40 cm of the soil profile (Table 5.2). For perennial crops the weighted average of the gypsum content was calculated for the upper 100 cm of the soil, if no gypsic layer is present. If there is a gypsic layer the weighted average is calculated for soil above the gypsic layer only (Table 5.3).

Table 5.2 GYPSUM INDICES FOR ANNUAL CROPS WITH SHALLOW ROOT SYSTEM ACCORDING TO THEIR GYPSUM TOLERANCE (Barzanji 1973)

	I	II	III
Gypsum(%)	Crops that tolerate a high level of CaSO4	Crops that tolerate some CaSO4	Crops sensitive to CaSO4
up to 0.3	1	1	1
0.3-10	1.1	1	0.75
10-25	0.8	0.7	0.5
>25	0.5	0.4	0.3

The above classification adopted by Barzanji was based on limited data, field observations, and information given by Smith and Robertson (1962). In more recent studies, many annual and perennial crops are found to perform well in highly gypsiferous soils; and the depth of the gypsic layer and the degree of its cementation determine to a large extent the irrigability of that type of soil.

Table 5.3 GYPSUM INDICES FOR PERENNIAL CROPS WITH A DEEP ROOT SYSTEM ACCORDING TO THE GYPSUM CONTENT OF THE SOIL (Barzanji 1973)

Gypsum class	Index
	I	II	III
1. >25%, with or without a gypsic horizon	0.5	0.4	0.3
2. 10-25%, without a gypsic horizon	0.8	0.7	0.5
3. 10-25%, with a gypsic horizon with in 50 cm	0.5	0.4	0.3
4. 10-25%, with a gypsic horizon at 50-100 cm	0.65	0.55	0.4
5. 0.3-10%, without a gypsic horizon	1.1	1	0.75
6. 0.3-10%, with a gypsic horizon (10-25%) within 50 cm	0.8	0.7	0.5
7. 0.3-10%, with a gypsic horizon (10-25%) at 50-100 cm	0.95	0.85	0.7
8. 0.3-10%, with a gypsic horizon (>25%) within 50 cm	0.5	0.4	0.3
9. 0.3-10%, with a gypsic horizon (>25%) at 50-100 cm	0.8	0.7	0.5
10. <0.3%, without a gypsic horizon	1	1	1
11. <0.3%, with a gypsic horizon (<10%) within 50 cm	1.1	1	0.75
12. <0.3%, with a gypsic horizon (<10%) at 50-100 cm	1.05	1	0.9
13. <0.3%, with a gypsic horizon (10-25%) within 50 cm	0.8	0.7	0.5
14. <0.3%, with a gypsic horizon (10-25%) at 50-100 cm	0.9	0.85	0.75
15. <0.3%, with a gypsic horizon (>25%) within 50 cm	0.5	0.4	0.3

The five classes outlined below are based on the observations of agricultural scientists on the irrigability of various types of gypsiferous soils in the Euphrates Valley. The main characteristics used to distinguish these classes are:

i. the presence and depth of the gypsic, petrogypsic or crust horizon
ii. the gypsum content in the topsoil and throughout the soil profile
iii. the type of plants to be grown

1. The depth to a gypsic horizon is greater than 200 cm

2. The gypsum content of the top 80 cm is less than 5 percent

3. The gypsum content is less than 25 percent in the 80-120 cm layer and less than 30 percent in the 120 to 150 layer.

Second Class

1. The depth to a gypsic horizon is greater than 200 cm

2. The gypsum content is less than 15 percent in the top 60 cm

3. The gypsum content is less than 30 percent in the 60-100 cm layer and greater than 40 percent in the deeper layers.

Third Class

1. A petrogypsic horizon at a depth greater than 150 cm

2. The gypsum content is less than 30 percent in the first 60 cm

3. The gypsum content is less than 20 percent in the top 30 cm, less than 40 percent in the 60-100 cm layer, and the gypsum content can exceed 40 percent in the layers beyond 100 cm.

Fourth Class

1 A petrogypsic horizon at a depth of 75 cm or more

2. Gypsum content is less than 30 percent in the top 30 cm

3. Gypsum content is less than 40 percent in the 30-60 cm layer and up to 40 percent in the following layer

4. The total CaCO₃ and gypsum content is less than 60 percent in the top 60 cm.

Fifth Class

Soils of this class are not suitable for irrigation or agricultural use and should be left mainly as natural pastures. They are marked by:

i. bad topographical conditions
ii. presence of a petrogypsic horizon at a depth of less than 75 cm from the surface
iii. gypsum content of more than 30 percent in the top 30 cm
iv. total gypsum and CaCO₃ contents exceeding 60 percent in the lower horizon

5.3.3 Salinity and leaching requirements

Most natural gypsiferous soils are also saline. For example, presence of a compact gypsic or carbonate-gypsic layer in the soils of the Golodnaya steppe impedes the leaching of salts from certain solonchaks. In the presence of a cemented gypsic layer, the application of excess irrigation water could lead to the formation of a perched water-table and to gypsum accumulation in the surface layer of the soil profile. Where there is insufficient irrigation water available for leaching of salts there is a possibility of surface crust formation.

Leaching is necessary to keep the salt content low. An effective drainage system is required to maintain a relatively low water-table and keep salinity under control. Soils with a cemented gypsic layer (with petrogypsic horizon) impede the installation of drainage systems. The effects of solution channels can cause displacement of tiles and consequent breakdown of the drainage systems. Similar difficulties can also occur in open drains because of the lateral solution of the gypsum beds and the slumping of overlying material. Smith and Robertson (1962) recommended that drainage systems are installed well above any cemented or impervious gypsic layer. Thus, a soil with a gypsic layer at a depth of less than 1.5 metres is not very suitable for drainage installation.

5.4 Fertilization of Crops

The fertility of gypsiferous soils and the use of fertilizer is discussed in Chapter 3.

In general, there is need for ample nitrogen fertilizers for crops grown on gypsiferous soils, especially under irrigated agriculture. There are no significant differences between the requirements of crops for nitrogen on gypsiferous and non-gypsiferous soils.

The need for phosphorus applications to crops grown in gypsiferous soils is higher than in non-gypsiferous soils because there are more calcium ions in the soil solution.

Molybdenum applications might benefit many plants, especially legumes. Recently, the application of potassium and micronutrients has been shown to be of value in many North Africa and Middle East countries. In practice, it is rare to find only one micronutrient limiting yield.