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.
EMPIRICAL MODELLING AND PREDICTION OF SUGARCANE YIELDS FROM FIELD RECORDS
Commercial sugarcane records for 19 seasons from 146 fields were obtained from five estates in the KwaZulu-Natal Midlands. Extensive editing and cleaning of the agronomic records was required. Regression models were developed, and depending on the predictor variables selected, the best model accounted for 55% of the observed yield variation, based on 535 crop cycles. The final models were validated using an independent data set of 47 observations and satisfactory performances by the models were confirmed. The 95% confidence limits of yield predictions for the population mean were within 10% of long-term mean yields and could be useful for resource allocation and harvest planning at the estate level. Reliable yield predictions for individual fields could not be made and within-field resource variations could not be adequately accounted for. Key physical field attributes associated with sugarcane yield were locality, aspect, altitude, soil type and effective rooting depth. Season and rainfall were important climatic variables. Of the factors influenced by management, sugarcane variety, plant-ratoon status, crop cycle, N and K nutrition and the topsoil Ca:Mg ratio were important yield predictors. The relative importance of individual predictors varied with the specific combination of resources for a particular observation.
A comparison of methods of phosphate application to ratoon cane.
Using a virgin Table Mountain Sandstone (Inanda series) soil with a low P status and high sorption capacity, a comparison of methods of superphosphate application to ratoon cane was carried out. By means of P-32 labelled superphosphate theuptake of fertilizer P by the plant was studied in first ratoon cane at intervals f rob 24 to 18 weeks after the date of treatment application. Uptake of labelled P was greater when the fertilizer was applied over the top of a trash blanket as compared with application to bare soil. Broadcasting was superior to banding irrespective of the presence or absence of a trash blanket, whilst deep placement was no more effective than surface banding on bare soil. Total P uptake showed the same trends but the treatment differences were not as great. Differences between treatment effects on cane yields did not attain a level of statistical significance in the first ratoon. In the second ratoon, however, when residual effects were measured, the surface-applied treatments were significantly (P<0.01) better than the deep placement treatment, and broadcasting was superior (P <0.05) to surface banding. Treatment effects on the crop's growth rate, harvested crop characteristics and soil P status were determined. The results are discussed in the context of the movement of P in the soil, P sorption characteristics of different soils and methods of fertilizer placement which might minimize P fixation. Introduction It has generally been accepted that fertilizer P moves from its zone of placement in very small amounts. Mass flow of water carries to the roots of the plant all the nitrogen, calcium, magnesium and some of the potassium needed by the plant but, because the concentration of P in the soil solution, is generally very low, the amount of movement of P in this way is generally insignificant (Cook, 19663). Barber (1964') calculated that only 25% of the plant's requirements reached the plant in this way. The remainder of the supply of P occurs by diffusion to the root zone. As an illustration of the slow rate of P diffusion, Russell (19687) records how, after 15 years of basic slag application to grassland in England, the P content of the soil below 7.6 cm was barely affected. * Present address: Research Laboratories, Shell and B.P. S.A. Petroleum Refmery (Pty.) Ltd., Island View Laboratory, P.O. Box 400, Durban. It is also accepted -that for good plant growth it is essential that the roots of a young plant should reach the phosphate zone in the soil as soon as possible. In this regard there is no doubt that placement of fertilizer P for the plant crop of sugarcane should be in the bottom of the furrow at planting. "Where the P status of the soil is very low or the sorption capacity is high it is necessary, by fertilizer placement, to attempt to restrict the amount of soil that the fertilizer actually contacts in order for part of the growing medium at least to reach a satisfactory P level" (Larsen, 19676). To test the applicability of this statement to sugarcane production in Natal an experiment was established in 1966 to study (i) the uptake of phosphorus by ratoon cane and (ii) the response in terms of cane growth and ultimate yield, using different methods of applying single superphosphate (8.3% P).