3. FERTILITY OF GYPSIFEROUS SOILS

3.1 Introduction
3.2 Fertility and Use of Fertilizers

3.1 Introduction

Surface layers of gypsiferous soils in many parts of the world are poor in nitrogen and phosphorus (Minashina 1956, Van Alphen and de los Rios Romero 1971). Total N as determined by the Kjeldahl method, ranges between 50 and 260 mg/100 g of soil; and total P extracted by 20 percent HCl ranges between 13 and 52 mg/100 g of soil. However, the extractable K determined using 20 percent hot concentrated HCl ranges between 200 and 700 mg/100 g of soil; and the water extractable K is generally between 4 and 5 mg/100 g, sometimes rising to 10 mg/100 g, a level considered by many workers as adequate for the growth of most crops. There is a wide difference of opinion about the need for potassium fertilizer on arid and semi-arid soils. The generally accepted concept, using soil analysis as a basis, is that the soils of the arid and semi-arid zones are rich in potassium. Soils of the arid and semi-arid zones are however mostly alluvial and heterogeneous, so it is not possible to make general statements. The existing routine laboratory tests for determining available potassium for plants do not always reflect the true situation under field conditions, because of the variations in the clay mineralogy of the soils. Furthermore the soils of the arid and semi-arid regions may contain minerals with a high K-content extractable under the routine laboratory tests but not extractable by plants under field conditions. Dobroval’skiy (1965) found that gypsiferous soils are poor in manganese, copper, zinc and molybdenum.

Rozanov (1961), Kurmangaliyev (1966b) and Van Alphen and de los Rios Romero (1971), using the limited available data, state that there is no essential difference between the soil fertility of gypsiferous and non-gypsiferous soils when found under the same pedogenic conditions. They however excluded soils with a gypsic layer at shallow depth, in which the volume of soil containing essential elements is limited.

Kovda (1954) considers that the accumulation of gypsum in soils results in very low fertility, and that their productivity remains low under irrigation, even with applications of fertilizers and organic manures. He found the potential productivity of gypsiferous soils to be related to the depth of the gypsic layer. In soils with a gypsic layer below 60 cm depth, the plant roots penetrate freely and the soil volume for nutrients is adequate. Fertilization of these soils improves plant growth and increases yield. In shallow soils, with a gypsic layer near the surface, the soil volume is limited and fertilization becomes of special importance for plant growth.

3.2 Fertility and Use of Fertilizers

3.2.1 Phosphorus availability and immobilization
3.2.2. Nitrogen and organic matter
3.2.3 Calcium, magnesium and potassium
3.2.4 Micronutrients availability and problems

There is limited data available on the fertilization of gypsiferous soils under field conditions. Van Alphen and de los Rios Romero (1971) describe experiments in which nitrogen fertilizers in the form of ammonium sulphate and nitrate were added to irrigated cereals, cotton, sugar beet, fruit trees and other crops. Similarly, superphosphates and potassium were applied to cultivated crops under both rainfed and irrigated farming. Little attention was given to micronutrient deficiences in these experiments. Sayegh (1979) points out that soils in the Middle East are deficient mostly in nitrogen, phosphorus and micronutrients and to some extent in potassium.

Sayegh et al. (1981), discussing their fertility trials on both calcareous and gypsiferous soils, point out that the farmers in the Middle East face other limiting factors such as inadequate soil preparation, lack of irrigation, and weed control, which are prerequisites for crops to respond to fertilizer applications. Their experiments show that intensive cropping creates a demand for plant nutrients.

Hernando et al. (1963, 1965) who studied the interaction between gypsum content and available soil moisture on the growth of corn plants and their mineral uptake, note that water consumption by plants decreased with increasing gypsum content and that the uptake of phosphorus, potassium and magnesium decreased while nitrate, sulphate and calcium increased, with increasing gypsum content.

In general, plants react differently to the different gypsum content of soils. Plants tolerant to gypsum, for example alfalfa, usually maintain their ionic composition whatever the gypsum content of the soil (Matar, unpublished work).

Gypsiferous soils have been cultivated for centuries under traditional rotational rainfed farming systems in which wheat or barley is followed by leguminous grain crops or by fallow. Under rainfed farming conditions, yields depend mainly on rainfall and are usually low to moderate. Soil chemical properties are in a dynamic equilibrium. Gypsum and other salts are leached in the rainy season to deeper horizons and returned to the surface horizons during summer by capillary rise. When gypsiferous soils are irrigated changes in their chemical properties take place involving further movement of gypsum salts and nutrients.

3.2.1 Phosphorus availability and immobilization

The relative importance of organic and inorganic forms of phosphorus in soils depends upon the soil organic matter content. Organic phosphorus is usually low in the subsoil of temperate soils, as it is also in arid soils. Calcium, aluminium, iron and sometimes magnesium are the main metal ions that bind phosphorus in soils (Novozamsky and Beek 1978).

Phosphorus solubility in calcareous and gypsiferous soils

The solubility of phosphorus in soils depends upon the presence of other ions in the soil solutions which exist in equilibrium with their solid phases. The following is an illustration of a simple system (Novozamsky and Beek 1978):

Ca4H(PO4)3. 3H2O 4Ca2+ + H+ + 3PO43-+ 3H2O	log Ksp = -46.91
Octacalcium phosphate
Ca10(OH)2(PO4)6 10Ca2+ + 6PO43- + 2OH-	log Ksp = -113.7
Hydroxyapatite
Ca10F2(PO4)6 10Ca2+ + 6PO43- + 2F-	log Ksp = -120.86
Fluoroapatite
AlPO4. 2H20 Al3+ + H2PO4-+ 2OH-	log Ksp = -30.5
FePO4. 2H2O Fe3+ + H2PO4-+ 2OH-	log Ksp = -34.9
CaHPO4 Ca2+ + HPO42-	log Ksp = -6.66

The activity of phosphates in soil solution at equilibrium with one or more of the solids mentioned above depends upon the activities of Ca²⁺, A1³⁺ and Fe³⁺ and on the pH of the soil system. The formation of phosphate-metal complexes may even affect further the concentration of total soluble phosphorus in soils. On the other hand, many factors can affect the solubility of the compounds containing Ca²⁺, A1³⁺ and Fe³⁺ ions, such as calcite - CaCO₃, gypsum-CaSO_4. 2H₂O, gibbsite-Al(OH)₃, amorphous Fe(OH)₃ and others. The activity of the metal ions depends on the soil pH, the redox potential and the partial pressure of CO₂, and on the purity, the degree of crystallization and the particle-size distribution of minerals present. The solubility of calcium phosphates increases as a function of CO₂ levels (Harmsen 1984).

Simple phosphate-solubility diagrams could be developed whenever the solubilities of cation in soils form solid compounds with phosphates which are in equilibrium with the soil phases. When gypsum co-exists with calcite in soils, as is the case in most gypsiferous soils, the calcium activity in the soil solution is much higher when compared with soil in equilibrium with calcite only. The solubilities of calcium phosphate compounds in soils in the presence of gypsum are therefore lower, compared to that in the presence of CaCO₃ alone. Figure 3.1 shows the solubility of tricalcium phosphate in the presence of CaCO₃ or CaCO₃ and gypsum combined at various levels of CO₂ pressure. Consequently, it is expected that levels of soluble or available phosphorus are lower in gypsiferous soils than in calcareous soils.

When phosphate fertilizer is applied to land the immobilization rate of P varies significantly between soils. Larsen et al. (1965) suggested that the decrease in labile phosphate could be the result of the formation of a crystalline basic calcium phosphate at a rate that increases with soil pH (Figure 3.2). Matar (unpublished data) finds that, in general, the rate of P immobilization is greater in gypsiferous soils than in calcareous soils, as predicted from theoretical derivations (Figure 3.3). The P immobilization was significantly greater for soils with 40 percent of gypsum when compared with soils with 5 percent of gypsum, especially in the first period following the P application (Figure 3.4).

Figure 3.1 The solubility of calcium phosphate (calcium and hydrogen ion activities and inorganic phosphate concentrations) in the presence of calcite or calcite plus gypsum in an aqueous system at various levels of CO₂ (Harmsen 1984)

Figure 3.2 Relationship between half-life of labile phosphate and hydrogen-ion concentration of soils (Larsen et al. 1965)

Figure 3.3 Average P-immobilization curve for calcareous versus gypsiferous soils

Figure 3.4 P-immobilization curve in soils with 5 percent and 40 percent of gypsum by weight

In general, gypsiferous soils are very low in phosphorus and need to be fertilized. They are buffered to a pH level of 7.5 to 8.4 resulting in low availability of natural soil phosphorus. Applied phosphorus quickly reverts to insoluble forms. Sayegh and Abdul Majid (1969) studied the fate of applied phosphorus in alkaline, calcareous and non-calcareous soils (Tables 3.1 and 3.2). It was found that, in the highly calcareous Bazourye soil with the effect of cropping, the applied P over a period of one year was mainly converted to forms of Ca-P and water soluble and easily replaceable P. In the slightly calcareous Innsar soils, the distribution of the applied P was mainly in the forms of Ca-P and Al-P, followed by organic P and soluble P. In the alkaline non-calcareous Zaouter soil, the applied P was distributed in a variety of forms in the following decreasing order: residual-P, Fe-P, Al-P, Ca-P, organic P. The retention of P was more rapid in the alkaline non-calcareous Zaouter soil which contained larger amounts of iron and aluminium oxides. The increase of Fe-P over a short period suggests that the chemical activity of hydrous oxides is appreciable, although the pH is alkaline. The above work has been carried out mainly in calcareous and non-calcareous soils. Phosphate retention would be more severe in gypsiferous soils, because of their higher calcium activity. Applied phosphorus fertilizer should be about 50 percent water-soluble to be effective on gypsiferous-calcareous soils. Atanasiu et al. (1978) found that combinations of water-soluble and citrate-soluble phosphates proved to be the most advantageous for the P supply for plants. Phosphorus in ordinary and concentrated superphosphates is about 100 percent water-soluble, while phosphorus in compound fertilizers varies in water solubility. Fertilizers that are wholly soluble have very reactive forms of phosphorus which in perfectly buffered soils, with a near neutral reaction, are not readily transformed into forms of soil phosphorus. Basically, the amount of the various discrete chemical fractions of P which are found in the soil determines the relative effectiveness of phosphate fertilizers on crop growth (Lindsay and De Ment 1961). The retention of P in soils is the result of chemical precipitation and physico-chemical sorption, and the subsequent availability to crops depends on the rate at which the P is released to the soil solution. In fact, soils do not have a well-defined capacity to retain phosphate. The extent and rate of immobilization are subject to several factors including the following:

i. the concentration of phosphate in the soil solution: an increase in phosphate concentration is accompanied by an increase in phosphate immobilization

ii. the time of reaction of phosphate with the soil: some of the reactions between the phosphate and the soil surfaces are completed quickly. In some tropical soils 95 percent of the applied P is immobilized within five minutes. In calcareous and gypsiferous soils, some reaction products are formed slowly so that the initial rapid stage in the reaction is followed by a slow decline in P solubility

iii. The mineralogical nature of the soil: the amount and forms of iron and aluminium oxides and the amount and types of CaCO₃ present influence phosphorus retention.

Phosphorus retained
Soil series	Clay	Phosphorus	After 1 day		After 4 days		After 10 days		After 30 days
	(%)	added
		(as ppm of soil)	ppm1	%2	ppm1	%2	ppm1	%2	ppm1	%2
Bazourye	50.8	100	84	84	88	88	89	89	93	93
		200	148	74	153	76	164	82	180	90
		400	251	63	261	65	293	73	305	76
Zaoutar	46.8	100	94	94	96	96	97	97	99	99
		200	190	95	191	95	195	97	198	99
		400	349	87	364	91	378	94	386	96
Innsar	42.7	100	73	73	83	83	88	88	92	92
		200	138	69	145	72	161	80	175	87
		400	168	42	236	59	281	70	296	74

¹ P retained in ppm of soil
² P retained as percent of P added

i. the plant species and its tolerance to gypsum
ii. the amount of phosphorus fertilizers added
iii. the percentage of gypsum and calcium carbonate in soils.

¹ 25 mg P per 2 kg of soil
² 400 mg P per 2 kg of soil

³ Water soluble and easily replaceable P

From this overview, it can be concluded that, in gypsiferous soils, higher rates of phosphorus applications than normal are essential because of their higher rate of P immobilization.

Table 3.3 EFFECT OF LEVEL OF GYPSUM ON UPTAKE OF P BY CORN AND ALFALFA PLANTED IN GYPSIFEROUS SOILS

Gypsum(%)		P Concentration in dry matter
		Corn leaves	Alfalfa tops
		(% × 10)	(% x 10)
0		6.36	3.55
5		3.23	2.91
10		3.48	2.60
20		3.20	2.80
40		1.91	2.99
LSD	.05	.72	1.42
	.01	1.05	2.00

3.2.2. Nitrogen and organic matter

The organic matter content of most gypsiferous soils is relatively low and ranges between 0.4 and 1 percent for surface layers to less than 0.2 percent in the subsurface layers. The type of organic compounds and humus are affected by the presence of gypsum in soil. Velasco et al. (1980) found, from study of humification under different forest ecosystems in the province of Toledo, that gypsum in soils affects the humification process. The humus formed under Quercus woodlands on gypsum marls and limestones is a calcic mull with insoluble fulvic acids. Singh and Taneja (1977), report that the rate of nitrogen mineralization in soils is usually stimulated by the addition of gypsum to saline non-gypsiferous soils at a rate of 2.5 to 5 t/ha. However, the addition of higher rates of gypsum (7.5 to 10 t/ha) led to a lower level of N mineralization (Fig. 3.5). It is believed that the addition of gypsum at a low rate stimulates soil micro-organisms responsible for mineralization. Gupta and Salaran (1971) found that the addition of gypsum stimulates the fungus population in saline and sodic soils and decreases the actinomycetes count. The above discussion of the rate of nitrogen mineralization is mostly relevant to saline-sodic or sodic soils where gypsum is added as a soil ameliorant and to counteract the effect of toxic salts on plant growth. There is little published on the effect of gypsum on N mineralization and the activities of micro-organisms in gypsiferous soils.

Figure 3.5 Changes in mineral nitrogen status in gypsum-amended soil

Van Alphen and de los Rios Romero (1971) considering the nitrogen contents of gypsiferous soils note that the surface layers of soils in Spain contained generally less than 0.25 percent of total nitrogen (as determined by Kjeldahl); and that nitrogen contents in soils of the Kirovadad Massif, USSR, range between 0.07 and 0.26 percent. The surface layers of gypsiferous soils in the Euphrates Valley are relatively poor in total N, ranging between 0.07 and 0.15 percent.

Under dryland farming (average annual rainfall 200-300 mm), it is expected that significant and economic responses to nitrogen fertilization are unlikely, especially when wheat or barley crops are preceded by fallow. However, a significant response to N fertilizer is expected when wheat-wheat or barley-barley rotations are practised.

Under irrigated agriculture on gypsiferous soils with a low level of organic matter and total nitrogen, the regular application of N fertilizers is essential to secure adequate yields of most crops.

The problems associated with the application of nitrogen fertilizers are shown in Figure 3.6. To prevent the loss of ammonia, the nitrogen fertilizers should be incorporated well into soils with pH higher than 7. Little is known about the ammonium-fixing capacity of the soils and the possible economic implications where nitrogen fertilizer is added in ammonium forms. Sayegh and Rehman (1969) found that some soils with alkaline reaction can fix up to 46 percent of applied ammonium fertilizer. This corresponds to about 200 kg of nitrogen per hectare which is equivalent to the amount added by local farmers (Table 3.4). This property of fixing large quantities of ammonium may decrease yield.

Figure 3.6 Problems associated with the application of N-fertilizers

Table 3.4 AMMONIUM FIXATION IN DIFFERENT SOILS

Soil properties	Soil series
	Innsar	Bazourye	Zaoutar
NH4 fixed (%)	46	16	41
pH	8.2	8.4	7.4
CEC (mEq/100 g)	68.27	25.72	39.66
Nitrogen (%)	0.03	0.01	0.02
CaCO3>3 (%)	3.7	71.0	0
Clay (%)	50.8	42.8	46.8
Silt (%)	34.2	41.0	40.0
Sand (%)	15.0	16.2	13.2
Textural class	Clay	Silty clay	Clay
Soil clay minerals
Montmorillonite	Dominant	Moderate	Small
Illite	Small	Moderate	Small
Kaolinite	Small	Trace	Dominant

Three main factors should be considered in the management of nitrogen fertilizer applications

1. The form of the N (Figure 3.6).

2. The clay content and clay mineralogy of the soil.

3. The amount and type of CaCO₃ present.

As a rule of thumb, nitrogen needs to be applied at rates from 1.2 to 1.5 times the anticipated removal by the crop. Where generous amounts of N have been applied and where soils contain pans that impede leaching, large amounts of nitrate may accumulate in the soil. Some fields in Texas, USA, were found to have more than 600 kilograms per hectare of nitrogen in the form of nitrate, an amount far in excess of that needed for sugar beet production and resulting in a low sucrose percentage and high impurities. Where furrow irrigation is used, soil nitrate may move to the top of the ridges and accumulate as water evaporates, becoming unavailable to plants unless it is washed down into the root zone.

Where crops, for example wheat, are sown in the cold season when nitrification is inhibited, it is more profitable to apply complex fertilizers containing about half of their nitrogen content in the form of nitrate rather than applying ammonium or urea fertilizers, since the nitrate-N is the preferred form for uptake by wheat before the soil warms up.

The published results of field experiments conducted on gypsiferous soils to determine the optimum level of nitrogen fertilizers needed by various crops are very limited. The rate of nitrogen application adopted by most farmers for Mexican wheat, sugar beet and corn ranges between 70 and 140 kilograms per hectare. Cotton is fertilized with 50 to 120 kilograms of nitrogen per hectare, depending on the type of gypsiferous soils and the expected yields (Van Alphen and de los Rios Romero 1971, Mousli 1980). Usually one-third to one-half of N fertilizers is being applied before sowing, the rest is applied in one or two splits. Other crops such as sorghum (Sorghum vulgare), onion (Allium cepa), tomato (Lycopersicum esculentum) are fertilized with 100 to 120 kilograms of N per hectare (Bennett and Adams 1972). These applied rates of N fertilizer do not reflect the optimum rate for optimum yield.

It can be concluded that almost without exception, all calcareous and gypsiferous soils are low in nitrogen and need to be fertilized.

3.2.3 Calcium, magnesium and potassium

The inter-relation between the three macronutrients, calcium, magnesium and potassium are well known in plant nutrition studies (Ologunde and Sorensen 1982; Agboola and Corey 1973; Terman et al. 1975). The increase in calcium concentration in solution leads usually to a drop in potassium uptake by plants. Similarly, the increase in calcium concentration could lead to a drop in magnesium uptake. In general, the relationship between Ca, Mg and K is related to texture, organic matter and the type of clay in the soil.

Hernando et al. (1965) in a study on the mineral nutrition of corn on soils rich in gypsum, found in water culture, that increasing the concentration of Ca²⁺ and SO₄^2- ions, by the addition of CaSO₄, increased the uptake of Ca and S and decreased the uptake of K and Mg. Prasad and Paliwal (1976) report that application of gypsum to soils irrigated with water which has a high Mg: Ca ratio, equivalent to 16: 1, improves yields of crops and increases the Ca: Mg ratio in the plant tissues.

Poonia and Bhumbia (1973) show, on a sodic soil, that the application of labelled-gypsum (⁴⁵CaSO₄. 2H₂O) to soils in pots, grown with barley, caused an increase in potassium content in tested plant tops. But, the application of labelled CaCO₃ (⁴⁵CaCO₃) caused a slight increase in K uptake by crops. The application of either CaCO₃ or gypsum increased the Ca content of the barley plants, but had no definite effect on the Mg content. The availability of Ca from applied gypsum was considerably more than that from applied calcium carbonate (Table 3.5).

Table 3.5 EFFECT OF DIFFERENT LEVELS OF ⁴⁵CaSO₄ AND CaCO₃ ON THE YIELD AND CHEMICAL COMPOSITION OF BARLEY GROWN IN A NON-SALINE SODIC SOIL (ESP = 48.4)

Treatment % of gypsum requirement	Dry matter yield (g/pot)	mEq 100 g-1				%
		Ca	Mg	Na	K	N	P
45CaSO4
0	2.96	13.9	13.3	188.0	37.0	2.17	0.250
25	8.16	20.0	9.5	103.7	66.0	2.38	0.164
50	9.60	20.4	11.4	94.7	69.7	2.35	0.135
75	9.13	21.7	12.6	80.0	72.7	2.43	0.133
100	11.08	23.2	10.9	73.3	69.7	2.52	0.130
LSD (5%)	1.29	1.1	1.9	6.7	3.6	0.15	0.013
45CaCO3
0	3.16	13.9	12.7	166.7	45.7	2.98	0.238
25	2.76	16.9	12.9	168.7	52.7	3.23	0.224
50	3.76	22.4	16.0	165.3	42.3	3.13	0.229
75	4.26	21.6	14.1	176.3	41.8	3.10	0.219
100	4.06	22.0	17.9	193.3	53.5	3.03	0.217
LSD (5%)	1.31	1.2	2.2	12.1	4.4	0.14	0.006

Carlson et al. (1974) found that the application of gypsum to soils with high potassium-fixing capacity led to a release of potassium to the soil solution by a cation exchange process. This continued release under irrigation might deplete the potassium and other similar exchangeable cations such as magnesium from ordinary soils. From a theoretical point of view and from experimental work (Matar and Jabbour 1982) it is expected that the exchangeable potassium and magnesium in irrigated gypsiferous soils will be leached (Pavan et al. 1984). Consequently, the available potassium and magnesium in gypsiferous soils will become deficient under these conditions. There is limited information on the changes in available K and Mg occurring in soils irrigated for a long period of time. Most gypsiferous soils occur in arid climates and are subjected to salinization when put under irrigation unless leaching is practised (Doorenbos and Pruitt 1976). Under irrigation the extra water added to soils in excess of the evapotranspiration by the crop could lead to an extensive loss of nutrients.

Accordingly, it becomes obvious that K and possibly Mg fertilization is of major importance to plant growth and for the maintenance of productivity of irrigated gypsiferous soils. Their application rates depend on local field conditions, the cropping system, the soil management practices used, and the degree of crop intensification.

The results of analyses of major cation concentrations on the adsorption complex and some other characteristics of some gypsiferous soils of the Balikh Valley in Syria are shown in Table 3.6.

Table 3.6 CHARACTERISTICS OF SOME GYPSIFEROUS SOILS IN THE BALIKH BASIN OF SYRIA (after Gibb et al. 1967)

Characteristic	Shallow soils		Medium deep soils
	Topsoil	Subsoil	Topsoil	Subsoil
% Gypsum	10	54	0.5	16
% CaCO3	-1	-	24	8
pH	7.8-8.3	7.0-8.2	7.0-8.2	-
% Organic matter	0.8-1	0.2-0.8	0.6-1.2	0.4-0.5
Cation exchange capacity (mEq 100 g)	9-11	4-7	10-14	-
% Ca on the adsorption complex	60-90	-	15-16	-
% Mg on the adsorption complex	-	-	20-45	-
% K on the adsorption complex	5-10	-	8-12	-
% Na on the adsorption complex	2-3	-	-	-

¹ Not measured

The debate on the need for potassium fertilizer on irrigated or non-irrigated calcareous and gypsiferous soils has been underway for some time. The generally accepted concept from soil analysis is that the soils of arid and semi-arid zones are rich in potassium. This is however a misconception because:

i. the gypsiferous soils of the arid and semi-arid zones are mostly alluvial and extremely heterogeneous, thus one cannot make general statements;

ii. the routine, laboratory tests for determining available potassium do not reflect the true situation under field conditions;

iii. there are marked variations in the type and amount of clay minerals, CaCO₃, gypsum and the kind and amount of potassium minerals present in the soils.

Table 3.7 EXTRACTION OF K UNDER DIFFERENT MOISTURE CONDITIONS USING DIFFERENT EXTRACTING SOLUTIONS

Soil	Solution	Field	50% Field Capacity		Air drying
		Capacity1	Field drying2	Laboratory drying	Field3	Laboratory
				mEq/100 g of soil
1. 5Y 7/2	NH4OAC	0.77	0.40	0.53	0.65	0.96
Light grey	NaOAC	0.47	0.44	0.71	0.71	1.34
3. 10 YR 3/2	NH4OAC	2.64	2.60	2.55	2.43	4.27
Very dark grey-brown	NaOAC	1.91	2.01	2.45	1.95	2.22
7. 10 YR 3/4	NH4OAC	0.53	1.2	0.42	1.96	0.71
Dark brown	NaOAC	0.08	1.13	0.29	1.46	0.62
8. 10 YR 3/4	NH4OAC	1.35	0.24	0.95	0.46	1.37
Dark brown	NaOAC	0.68	0.08	0.80	0.35	0.86
14. 5 YR 4/6	NH4OAC	1.56	2.57	1.71	2.78	2.79
Yellowish red	NaOAC	0.47	0.64	0.41	1.03	0.82

¹ Collected 25-26 February 1969
² Collected 8-9 April 1969
³ Collected 17-18 June 1969

This shows that the existing routine laboratory tests do not reflect the true availability of potassium for plant growth under field conditions. In most cases the extractable potassium is much exaggerated, that is, the extracted value corresponds to a "high" category yet the soil is K-deficient, while in other cases the value is "low", but the soil does not respond to applied potassium fertilizer. In general, at an equal content of exchangeable K, the concentration of potassium in the soil solution varies considerably depending on the pH, amount and type of CaCO₃, amount and type of clay and amount and form of gypsum present. The K: Ca and Mg: Ca ratios in the soil solution are very low when the gypsum content is high resulting in a very low uptake of K and Mg from the soil solution which accounts for low crop yields (Van Alphen and de los Rios Romero 1971).

In conclusion, the application of potassium fertilizer is necessary on gypsiferous and on calcareous soils where vegetables, fruit trees and grasses are grown and where they are intensively cropped. Potassium increases the resistance to certain diseases, helps to overcome water stress, increases potassium uptake in clayey calcareous soils where aeration is poor and improves the quality of crops.

Under rainfed conditions, plants grown in gypsiferous soils may not suffer from any K or Mg deficiencies. Potassium and magnesium cations leached during the wet season, can return to the surface during the drier part of the year and become available to plants. However, the Mg levels on the adsorption complex of the soil and in plants should be monitored to prevent its deficiency.

3.2.4 Micronutrients availability and problems

In the past 25 years, the need for application of micronutrients has not been recognized except in special cases (Fritz et al. 1984). During this period however, substantial information has accumulated concerning the content of micronutrients in soils and plants, their availability to plants, the needs of the crop for micro-nutrients and the effects of their application on crop yields (Shorrocks 1984, Katyal and Randhawa 1983, Sillanpää 1982). This has been especially so in North Africa and the Middle East (Sillanpää and Vlek 1985, El-Fouly 1980, 1983). Further information and research however, are required to identify crop requirements and the need for micronutrients on various soil types and the varied agro-ecological conditions.

Availability of micronutrients to plants grown in soils is one of the major factors determining productivity. The high concentration of soluble calcium in gypsiferous soils, affects the availability of some macronutrients such as phosphorus, potassium, magnesium. The availability of micronutrients is also affected, on the one hand, by the presence of gypsum and its level and, on the other hand, by the effect of phosphorus and possibly potassium fertilizers added in large doses. The effect of gypsum on the solubility and availability of various micronutrients to plants is discussed below.

Molybdenum

Stout et al. (1951) found that, in contrast to phosphorus, the uptake of molybdenum by plants is reduced by sulphur application in the form of gypsum, as illustrated in Table 3.8. They explain that the effect of is possibly due to direct competition between two divalent anions of the same size SO₄^2- and MoO₄^2-.

Table 3.8 REDUCTION OF MOLYBDENUM CONTENT IN TOMATO AND PEA PLANTS BY APPLICATION OF CaSO₄ 2H₂O TO SOIL

CaSO4. 2H2O application	Mo concentration
(ppm)	Tomato	Peas
0	5.25	12.80
100	3.52	8.05
400	2.45	5.70

Gupta and Munro (1969) observe that sulphur added as gypsum to soils drastically reduces molybdenum in tops and roots of Brussels sprouts. Their results indicate that the major effect of S on Mo content of plants appears to be at the site of uptake on the root surfaces. Consequently they conclude that because of the relationship between S and Mo care must be exercised in using fertilizers when Mo is present in limiting concentrations in the plants. Optimum fertilization practice on gypsiferous soils may require the application of Mo.

Reisenauer (1963) also shows that S applied as gypsum reduces the yield and N concentration in leguminous crops such as peas on a soil low in molybdenum. The application of Mo overcame the depressive effect of S. Reisenauer suggests that S, in addition to its competitive effect on the adsorption of Mo, has an apparent inhibitive effect on Mo utilization within the plant at the low levels of Mo in plant tissues.

Molybdenum has long been known to be required for the normal assimilation of N in plants. Of the four enzymes found to contain Mo, nitrogenase and nitrate reductase are found in plants (Price et al. 1972). The Mo in these two enzymes appears to have similar catalytic functions judged from electromagnetic resonance studies. Nitrate reductase enzyme is essential for the assimilation of nitrates, since it is the catalyst in the first step in the reduction of NO₃ to NH₃. The other major molybdo-protein enzyme in plants is nitrogenase which reduces N₂ to NH₃, which is then assimilated by the plant.

The availability of Mo in gypsiferous soils should be considered when determining a suitable fertilization programme. Deficiency in Mo limits crop growth and especially that of legumes which require Mo for the functioning of nitrogenase. Berry and Reisenauer (1967) show that accumulation of Fe by tomato tops depends on the levels of Mo in the nutrient solution. Plants deficient in Mo show least uptake of Fe. However, Olsen and Watanabe (1979) found that the addition of gypsum to six sodic soils, planted with sorghum plants, decreased the Mo concentration from 2.33 to 1.26 ppm in the plants, while the Fe content was increased from 56 to 65 ppm.

Iron, Manganese and Copper

The effect of gypsum on the other micronutrients has not been so thoroughly investigated. Van Alphen and de los Rios Romero (1971) mention that chlorosis appears on many crops grown on gypsiferous soils in Spain, such as apricots and peaches. Recent work in Syria has shown that the effect of gypsum on the micro- nutrient contents of crops depends to a large extent on the plant species. Crops tolerant to gypsum, such as alfalfa, resist any significant changes in their macro- and micronutrient contents. However, for semi-tolerant crops such as corn, high levels of gypsum in soils (greater than 20 percent by weight) lead to a significant drop in Mn content (at the 1 percent level) from 79 to 57 ppm; while the same amount of CaCO₃ in soils did not have any significant effect on the Mn content (Matar, unpublished work). Dobroval’skiy (1965) found that samples from gypsiferous layers contain low amounts of Mn, Cu, Zn and Mo. Soils from Spain showed a deficient level of Fe but Mn and Cu levels were found satisfactory (see Appendix 4).

Boron

Chaudhary et al. (1976) found that application of gypsum to sodic soil lowered the concentration of boron in oats and corn plants and the Ca:B ratio was increased several fold. They found in a laboratory experiment that the application of gypsum lowered the soluble B concentration in the soil solution. The change in the solubility of B compounds could be either due to the added gypsum which will increase the calcium concentration in the soil solution, or to the effect of gypsum on the soil pH. More investigations are needed on the chemistry of B in gypsiferous soils.

Zinc

Since phosphorus and potassium fertilizers are essential to maintain the productivity of gypsiferous soils, discussion of their effect on the availability of micronutrients to plants is very relevant. Though, it is now generally accepted that P and Zn interact in plants, the effect of the level of P in soils on Zn uptake remains controversial (Olsen 1972). In some cases, application of phosphorus decreases the total uptake of Zn in plants. Loneragan et al. (1979) consider that the effect of P in inducing Zn deficiency may be attributed to the effect of P in promoting growth and thus diluting the available Zn to values below those required for plant growth. Loneragan et al. (1979) consider that the effect of P cannot be attributed to any effect of the soil or the root in suppressing Zn adsorption or Zn transport since the plants grown in P-treated soils had more Zn in their tops than non-P fertilized plants.

The application of Zn compounds, in addition to P, to gypsiferous soils, improves the yield of corn tops and raises the Zn content of plants as well. It slightly reduces manganese uptake by corn. Safaya (1976) reports that the application of Zn to soils reduces the rates of Cu and Mn uptake more than that of Fe. In gypsiferous soils in Spain, Zn deficiences are more likely to appear on Zn-sensitive crops (Appendix 4).

In conclusion, the availability of micronutrients in gypsiferous soils could be the limiting factor for the growth of many plant species. Gypsum and calcium carbonate in soils, as well as other macronutrients added as fertilizers, could have significant effects on the availability of micronutrients to various plant species. More research is needed on the macro-and micronutrient status of gypsiferous soils.

The maintenance of the productivity of gypsiferous soils under irrigation, requires balanced use of both macro and micronutrients. Crops under irrigation show different fertilizer requirements to those under rainfed agriculture. Wallace (1980) makes the following recommendations for the use of micronutrients on arid soils to avoid toxicity:

1. The micronutrients should be applied after the need for them has been established by the following criteria: by visual symptoms; by leaf analysis with critical levels; by soil analysis with critical levels or by yield responses in test plots.

2. The correction of micronutrient deficiencies is of little value if other factors are more limiting. For example, soil water management, pest and weed control and others. The effect of micronutrient applications on all crops in the rotation should be considered, and a written record kept of the quantities applied so a build up of toxicity can be avoided.

3. Nematodes, a high water-table, and imbalanced fertilizer application contribute to micronutrient deficiencies. Correction of such causes help avoid deficiencies.

4. Finally, the need to fertilize gypsiferous soils to maintain their productivity requires extensive field research programmes to determine the optimum level and kind of nutrients needed by different crops. It is rare that one micronutrient is the only factor limiting yield. The following are the major factors influencing the micronutrient status and needs: pH, calcium carbonate content, gypsum content, irrigation, water quality, amount and application system (Devaux 1980), soil texture, increasing use of NPK (Finck 1984, Fritz et al. 1984) use of high yielding varieties (Fritz et al. 1984, El-Fouly 1983) and climatic conditions (Sillanpää and Vlek 1985). Much more information is needed.