Years of no-till farming, plant nutrient redistribution from sub- to top soils and continuous topsoil fertiliser applications of relatively immobile nutrients like phosphorus (P) have caused a stratified soil nutrient profile. In conjunction with a dry start or early sowing this can cause limited P uptake from 10-30cm below the surface and thereby limited yield potential. CSBP tested the effect of low subsoil nutrition on yield responses last season.
CSBP’s phosphorus (P) trials were located in 5 locations across the grain belt. All of those locations revealed a sharp decline in subsoil Colwell P values from topsoil levels. Topsoil Colwell P on those sites suggested that maintenance fertiliser rates may have been sufficient, but wheat responded to much higher P rates to produce more profitable returns.
The pH was generally good across all sites, except Dandaragan, where sub soil compaction was a constraint. Two of the sites were gravelly (Williams, Kojonup).
Site locations, placed on the background of soil test results from the last 3 seasons:
Fig. 1 visualises the decline in subsoil P values. If only topsoil analysis is used for recommendations then this can be misleading and potentially costing
Figure 1: Colwell P readings from 0-10cm (top), 10-20cm (middle) and 20-30cm (bottom). Trial locations are indicated by white circle. Coloured dots are soil samples analysed by CSBP laboratory within the last 3 seasons (red = < 16, orange = 16-30, green = 31-45, blue = > 45).
Across the sites, a P application of 20 kg/ha returned about 95% (or higher) relative yield. It indicates the importance of knowing and then integrating subsoil nutrition into an improved fertiliser recommendation. Maintenance rates for P would have been insufficient on most sites and would have left up to 20% of yield potential unrealised, costing gross margin (assumptions: wheat price 300 $/t, P cost = 3.2 $/kg).