Fertilizer Research 39: 77-82, 1994. © 1994 Kluwer Academic Publishers. Printed in the Netherlands. Wheat growth from phosphorus fertilizers as affected by time and method of application in soil with an acidic subsurface layer E. Purnomo & A. S. Black School of Agriculture, Charles Sturt University, P.O. Box 588 Wagga Wagga, NSW 2678, Australia Received 7 April 1993; accepted in revised form 4 May 1994 Key words: Acidic subsurface layer, application rate, North Carolina phosphate rock, placement method, time of application, triple superphosphate, wheat Abstract Using soils with an acidic subsurface layer, three glasshouse experiments were carried out to evaluate the effect of placement method and application rate of triple superphosphate (TSP) and North Carolina phosphate rock (NCPR) on dry matter (DM) yields. Time of application of NCPR on DM yield response of wheat was also studied. For Experiment 1, soil was collected in depth intervals of 0-2; 4-6; 6-8; and 8-10 cm from a red earth (chromic luvisol). The treatments included two P sources (TSP and NCPR), three placement methods (broadcast, banded or mixed into the subsurface layer, 6-8 cm), and six application rates. In this P deficient soil with an acidic subsurface layer, there was relatively little effect of application method of TSP on wheat yield responses. The maximum dry matter yield responses for broadcast, band and mix application methods was 30, 42 and 50 %, respectively. Responses to NCPR broadcast, band and mix methods were 20, 9 and 44 %, respectively. Mixing NCPR into to acidic subsurface layer produced yields similar to those from TSP although a higher application rate of P as NCPR was needed to achieve this outcome. Treatments for Experiments 2 and 3 were time of application of NCPR (0 and 30 days before sowing) and rate of application of NCPR (0 and 40 mg P/pot). In Experiment 2 (same soil as Experiment 1) application of NCPR prior to sowing, resulted in higher Colwell P concentration than when applied at sowing, but time of application had no effect on final DM yields. Experiment 3 used a red podzolic (chromic luvisol) soil which had a lower P-status, was more acid and had a lower exchangeable Ca 2+ concentration than the red earth. Application of NCPR prior to sowing resulted in lower DM yield than when it was applied prior to sowing. Introduction The most effective economic procedures for fertilizer application need to be adopted in Australia, because the soils are P deficient. Normally P deficiency is over- come using water soluble single superphosphate fertil- izer (SSP). This fertilizer is applied to crops by drilling at a depth between 2 and 5 cm. The combination of drilling in bands and application at sowing generally have given the best yield responses from SSP (Rudd and Barrow, 1973). Many soils in south eastern Australia have become acid (Helyar and Porter, 1989). Within the acid surface soil an acidic subsurface layer developed (Conyers and Scott, 1989). Soil in the 2 to 10 cm depth interval (subsurface layer) was 0.63 to 1.18 pH units lower than that in the 0 to 2 cm depth. Direct drilling results in fertilizer being placed into this acidic subsurface layer. Under acidic conditions, availability of soluble P fertilizer decreases rapidly. Application of SSP in the less acidic surface (0 to 2 cm) may be a desirable practice. An alternative P source for acidic soils is reactive phosphate rock (PR). Best agronomic responses have been obtained when PR was incorporated throughout the soil with pH(H20) of less than 6 (Boltand and Gilkes, 1990). Our hypothesis is that incorporation of PR in the acidic subsurface layers may increase P availability compared with mixing throughout the soil.