INTERIM EVALUATION OF PHOSPHOGYPSUM AS AN AMELIORANT FOR SOIL ACIDITY IN SUGARCANE
Results are reported of a 3x3x3 factorial trial in which phosphogypsum (PG), dolomitic limestone (DL) and single superphosphate (SS)were applied to sugarcane. The trial was situated on a Kranskop form soil with a moderately toxic aluminium (AI) content. Based on the combined yield responses from the plant and first ratoon crops, treatments with SS were superior to those with PG, and little response was obtained from DL. The high P in-furrow treatment (90 kg P/ha) gave a highly significant response of 3 t/ha sucrose compared with 2,1 t/ha from the best PG treatment (5 t/ ha). Leaf P and Ca also increased with both PG and SS application. Chemical changes in the soil following PG application resulted in considerable leaching of Mg below the 400 mm soil depth. Improved chemical changes included increased pH and Ca levels in the subsoil together with a reduction in exchangeable AI. The SS/PG interaction, evidence from third leaf P analyses, and lack of any marked response to liming coupled with good root development to depth indicated that yield response to PG was due mainly to its P content. Enhanced Ca content and diminished acid saturation in the subsoil may be beneficial.
The Results of P Fertilizer Trials Conducted in the Natal Midlands
The results are reported of six field experiments which were condu~cted in the Natal Midlands to determine the P requirement (sf strongly P-fixing soils and to provide data for calibrating the phosphorus desorption index (PDI) procedure for routint advisory purposes. Meswred responses to P treatments were related to soil Psorption properties. The findings indicated that supplementing the tratditional soil Truog P test with the rapid PDI screening technipue for measuring P fixation was warranted. The revised procedure for making P fertilizer recommendations is described and indicates that soils with the same Truog extractable P level but different P retention properties will not necessarily receive the same P recommendations. A survey suggeqts that about 30% of the soils in the Natal Midlands will bitnefit from fertilization with high rates of superphosphate.
Cation exchange capacity and degree of saturation with metal cations of highly weathered soils
The soils were found to develop a large but variable pH-dependent negative charge which shows a slight tendency to increase with increasing concentration of the measuring salt at similar pH values.