A Study of the Effect of Salts on the Solubility and Mobilization of Soil Nutrients

Many of the soils of Iraq have salinity problems. The causes of soil salinity were considered. These are principally evapontranspiration of irrigation water containing large amounts of salts, and a rising ground water table through saline soil profiles, A secondary cause is the effect of large amounts of fertilizer. The work reported in this thesis is concerned with an examination of saline soils. The investigation set out to measure the concentrations of the soluble ions and their relationship with each other. Thus ions studied are those involved in saline soils and which affect plant growth. The soil samples used in this work were taken from the banks of the Forth estuary, which is periodically flooded by estuarine water. This is one of the few situations where saline soils are found in temperate climates. The soils were analysed, as was the plant cover and standing water. It was found that Na and Cl were the dominant ions in these soils, while Mg and SO4 were present in lesser amounts. Similar amounts of Ca were found in both the flooded (gleyed and muck soil) and non flooded soils (brown earth soil). In the brown earth, Ca and Mg were the predominate cations with lesser amounts of Na and K. The analysis of standing water indicated that the predominate ions were Cl and Na, and the order of cation concentrations were Na > Mg > Ca > K The cation exchange capacity of the gleyed soil was higher than muck and brown earth soils. Physical analysis showed that these soils had a very fine texture and had a very low permeability under saturation conditions (less than 1.4 x 10e-4 cm/sec). In experiments carried out to measure the effects of different salts (NaCl, Na2SO4, KCl,K2SO4, MgCl2, MgS04, CaCl2, CaSO4) on the solubility of soil cations two concentrations were used - low concentration (0, 2,5,10 meq/1 and high concentration (20, 60, 80 meq/1). It was found that with a few exceptions at dilute concentrations the order of displacement of cations followed the lyotropic series Ca > Mg > K > Na while with anions the order was SO4 > Cl. However at high concentrations conflicing results were obtained due to interferences,(such as insolubility, ion pairs etc.) which changed the order of efficiency of displacement of the cations. Since phosphate is an essential plant nutrient, the effect of salts on its solubility was studied. Some experiments were carried out on the native phosphate in the soil. It was clearly shown that the type of salt had more effect on decreasing the native phosphate in gleyed soil than in brown soil, and also with the same soil the salt had a greater effect on PO4 with top soil than with sub soil. In most cases the addition of Cl reduced the solubility of native phosphate more than SO4. The relative effect of different cations on the solubility of phosphate was not in the same order as the lyotropic series. The effects differ with the type of salt added and with soil depth. In other experiments phosphate solution was added to the soil in different amounts. In general, the presence of salts reduced the solubility of phosphate. Sulphate had a less marked effect on the solubility of phosphate than chloride, while in the case of salts with the same anion, the decrease in the solubility of phosphate was in the order Na > K > Mg > Ca This can be accounted for by interactions between salt solution and soil. This work suggested that soil salinity restricts the amount of soluble phosphate in soil solution. This could explain why in saline soils the amount of phosphate available to the plant might be lower than that available in a non-saline soil. The effect of salt on calcareous soil was also studied. To simulate calcareous soils, powdered calcium carbonate was added to soil samples. The solubility of cations when salt solutions were applied was found to be lower in the treated soil than in the controls except in the case of calcium. The effect of salt on the dissociation of Ca from calcium carbonate was found to be greater in the case of SO4 than Cl, while with the same anion the difference between cations can be explained by a valency effect. The effect of NaCl and MgCl2 for individual and mixed samples of these salts showed a marked increase in the solubility of Ca in the case of the mixture, compared with the individual salts. This suggested that the leaching of calcareous soil is less effective in removing salts from the soil than the leaching of a non-calcareous soil