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.

Soil tests:

Soil tests

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

Fig 1

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).

Yield responses:
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). 

Yield responses

Andreas Neuhaus
By Andreas Neuhaus
- Agronomist (Data Analysis & Modelling)

Andreas joined CSBP in 2008 and brings 30 years’ experience in technical and scientific agricultural research and development to the role. Working closely with the research and agronomy teams his data analysis and modelling expertise has been an integral part of the development of NuLogic.

Andreas is actively involved in national and regional agronomy research projects, CSBP innovation projects and represents CSBP as a subject matter expert at industry conferences as well as writing technical articles for scientific publications.

Agronomic insights

Want to stay up to date? Have our latest research findings and news delivered to your inbox