The surface charge of several natural kaolinites was measured in the pH range 3-10 using an exchange technique. The positive charge was found to increase with decreasing pH and sometimes to increase with increasing ionic strength; it occurred on the kaolinites at pH values as high as 9 and 10 and was particularly evident at high ionic strengths. The positive surface charge on kaolinites is thought to be due to exposed alumina such as is found on oxide surfaces. Aluminium was found to dissolve from kaolinite at pH values beiow about 6.5. Aluminium dissolution increased with decreasing pH and time. When the proportion of dissolved aluminium ions balancing negative surface charge was taken into account, the negative and net negative surface charge on kaolinite was concluded to be largely due to pH independent charge resulting from isomorphous substitution, together with some pH dependent charge due to exposed SiOH sites. If Na+ was the index cation, dissolved aluminium ions from the clay replaced some of the Na+ balancing the negative surface charge. However, when Cs+ was the index cation, less Cs+ balancing the negative surface charge on the clay was replaced by dissolved aluminium. As the concentration of either Na+ or Cs+ was increased, less dissolved aluminium replaced the index cation as a counteraction to the negative surface charge.

Abstract--Model cation-exchange curves are presented for an idealized kaolinite surface where the charge on the surface (1) has its origin in cation substitution in the structure, and hence, is pH independent; and (2) is produced by protonation/deprotonation reactions of oxide-like sites, and hence, depends on the acid and base strengths of the surface sites, as welt as the ionic strength. Two pH-independent situations are considered: one where the exchanging ions have no selectivity for the surface and are all in the diffuse layer; and one where selectivity exists for one ion and where that ion is partly in a Langmuir-Stern layer and partly in the diffuse layer. If one of the exchanging ions is a proton, the shape of the curves and their position on the pH scale depend on ionic strength and ionic selectivity.The model curves are compared with data for actual kaolinites. Under most conditions exchangeable AI is released from the structure, and the shape of the charging curves becomes similar to that of an oxidelike surface. However, if titration is carried out rapidly, or account is taken of the presence of AI, the proton binding curves are similar in shape to those expected for sites resulting from cation substitution in the structure of kaolinite, either near the surface or at the edge of the crystal.

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In glasshouse experiments with low levels of soil applied phosphorus (P), yields of four annual pasture legumes (Medicago polymorpha, Trifolium subterraneum, T. balansae, Ornithopus compressus) increased with increasing P concentration in the seed.In a further experiment, M. polymorpha cv. Serena was grown at the same plant density from seed of two P concentrations and two seed sizes when two levels of finely ground superphosphate were applied to the soil. Higher P concentrations in the seed increased yields of dried tops by about 30% for the first harvest (21 days), 20% for the second harvest (52 days), and 9% at maturity (103 days), and seed yields by 11%. Larger seeds increased yields of dried tops by between 6-46% for the first two harvests but at maturity yields of dried tops and seed were unaffected by seed size. None of the interactions were statistically significant (P > 0.05), except for the first harvest when two interactions (P concentration in the seed x seed size (i.e. P content in seed), and P applied to the soil x P concentration in the seed x seed size) were significant at P < 0.05 level.In a field experiment, Trifolium subterraneum clover seed (two cultivars) of the same size but with two different P concentrations was sown at the same plant density and two levels of granulated (0.2-5 mm) superphosphate were applied to the soil surface. The higher level of superphosphate increased dried herbage yields of the dense clover swards by three-to four-fold 90 and 120 days after sowing. The higher P concentration in the seed increased yields of dried herbage by between 50 to 25%, depending on the level of P applied to the soil and the harvest date.

The yield response of canola (also known as rape, Brassica napus) and wheat (Triticum aestivum) to applications of phosphorus (P) as single superphosphate was measured in three field experiments in south-western Australia. The P was banded with the seed while sowing at 4 cm depth. The P requirements of the species was determined from the amount of P required to produce 90% of the maximum yield. Canola consistently required less P than wheat, from 50 to 55% less P for dried tops, and 30 to 58% less P for seed. The P concentration and P content (P concentration multiplied by yield) in dried tops or seed was consistently larger for canola than wheat.