Cation type and ionic strength effects on the solution composition of an acidic subsoil

Abstract

Sodium, potassium, magnesium and calcium chloride solutions of four concentrations (0.4, 1.0, 4.0, 1O.Omeq dm-3) and distilled water were equilibrated with the highly weathered, acidic subsoil of a Plinthic Paleudult from Natal at a soi1:solution ratio of 2.2: 1, then separated by centrifugation with an immiscible liquid and analysed for inorganic solutes. With each salt, increasing ionic strength resulted in lower solution-pH (the maximum pH was 4.95 in the distilled water equilibration) and higher aluminium concentration and activity. These effects were much less marked for sodium (maximum ApH of 0.47) than for the other cations (maximum ApH of 0.83) and both the concentration and activity of aluminium were correspondingly lower (by tenfold at the highest chloride concentration) in the sodium solutions.Irrespective of the nature or concentration of the electrolyte added, pH and the activities of AI3+ and silica in solution were consistently interrelated in a way which suggests that equilibrium with the gibbsite and poorly crystalline kaolinite in this soil had been closely approached. The results provide a basis for anticipating the effect of infiltrating solutions of fertilizer salts on subsoil acidity and suggest that a beneficial effect may accrue from the presence of sodium in the cation suite of acid soils.

Organic Amendments Increase Soil Solution Phosphate Concentrations in an Acid Soil

“…The addition of organic amendments increased the concentrations of cations present in solution, with compost addition resulting in the most significant increases in Mg, Na, K, Ca, and Al. Such an increase in the EC and associated ionic strength of solution is likely to have contributed to the lack of pH change measured in the compost-amended treatments between the 3-and 26-day incubation times compared with the untreated soil and ligniteamended treatments (Manson and Fey, 1989). The elevated compost pH at the commencement of the experiment also indicates that the compost may have still been immature, with the high levels of CO 2 production likely to swamp any predicted alkalinization effects associated with the incubation process (Bernal et al, 1998).…”

Soil Chemical Factors Limiting Plant Root Growth

“…Potassium (Rb86) applied to subsoils was transferred from alfalfa to maize grown in a sand culture (surface medium) within 40 days and to maize within 10 days (Habben and Blevins, 1989). Manson and Fey (1989) found evidence that the addition of low levels of N a (less than 2 meg kg -1) to low N a soils may alleviate acid soil toxicity. Additions of NaCl decreased the activity of Al by lO-fold and increased rooting in acid subsoil.…”

The effects of sulphate and fluoride on the sorption of aluminium by an oxisol

“…Sodium ions are less strongly Initial F/AI ratio (Manson & Fey, 1989). Chloride is generally considered not to form complexes with aluminium; no chromatographic peak attributable to any chloro-aluminium species was observed.…”

“…The ionic strength of the soil suspensions, both in the presence and in the absence of co-ordinating anions, was kept near constant by preparing them in 0.1 M NaCI. Sodium ions are less strongly Initial F/AI ratio sorbed than other species such as K +, Ca2+ and Mg2+, and thus should interfere with the sorption of aluminium t o a lesser extent than the other ions (Manson & Fey, 1989). Chloride is generally considered not to form complexes with aluminium; no chromatographic peak attributable to any chloro-aluminium species was observed.…”

“…This process has been implicated in the retention of sulphate in soils (Prenzel, 1983;Courchesne & Hendershot, 1990). Alternatively, treatment with sulphate has been found to increase the cation exchange capacity of Oxisols due to the sorption of the ion, which increases the negative surface charge of the soil (Marcano-Martinez & McBride, 1989). Aluminium sorption could therefore be expected to increase, as the sorption of this ion is thought to be largely electrostatic at low pH (Ritchie, 1989).…”