Phosphorus and water management in soil under no-till agriculture

Under no-till conservation agricultural practices, crop residues are returned to the soil's surface whereas under conventional tillage they are mixed within the top ploughed layer (5-20 cm depth). Moreover, P fertilizer drilled in the soil at sowing of the previous crop is also mixed in ploughed soil while in no-till it concentrates in rows or slits, usually near the soil surface. Generally, this results in the stratification of phosphorus with depth under no-till more than under CT. In a semi-arid region, where topsoil layer remains dry for prolonged periods during crop growth, the possibility exists that plant roots may not be able to access the fertiliser P stranded in the dry layer. For example, Strong et al. (1997) showed that residual fertiliser value of P applied in the previous year was only 20-40% under low soil water regime to that of well-watered soil. However, water use efficiency increases as the P application increases in P responsive soils. Water use efficiency is generally higher under no-till than CT practice in semi-arid regions (Gibson et al. 1992; Norwood 1999). In spite of these findings, with adequate P fertilisation of crops, the drying of the topsoil layer does not appear to be a limiting factor for crop production (Weil et al. 1988). On the other hand, repeated P application in a limited area of soil and without further mixing with soil under NT results in enhanced P uptake and greater grain yields (Hargrove 1985). Controlled traffic under NT practice may prove to be even more beneficial in soils with high P sorption capacity since only the limited soil volume is fertilized