Soils naturally form layers, and when nutrients concentrate in an area it’s called stratification. Soil type and agronomic practices affect stratification. Stratified nutrients can be a problem when nutrients are concentrated somewhere crops can’t access when they need them.
Mobile nutrients such as Nitrogen and Sulphur can form bulges below the root zone. In duplex soils, mobile nutrients can also accumulate in the subsoil. Immobile nutrients such as phosphorus (P) and zinc (Zn) can remain stranded near where they are applied in the topsoil.
No-till paddocks have less soil mixing than tilled paddocks. This exacerbates stratification of P. Wider row spacings used to accommodate stubble retention also increases P stratification. Future crops planted between the bands may not have access to residual soil P.
New research on stratification
Nutrient stratification can affect crop yield. New GRDC research will help growers understand:
- if stratification can be predicted based on soil types and properties
- how stratified nutrient bands potentially affect crop yield
- where different nutrients (N, P, K, S) are likely to accumulate
- how fertiliser management programs might be adapted to work with stratified nutrients.
Meeting nutrient demand in stratified soils
Understanding where nutrients are in the soil can help match nutrient supply from fertilisers to crop demand. It is important to test topsoil and subsoil.
For example, a topsoil test indicates K deficiency, while levels in the subsoil test are adequate. The topsoil deficiency may not limit the crop. By the time the plant grows to the stage it needs more potassium, its roots should have grown into the subsoil. If starter K is required, the rate only needs to satisfy the early crop. Testing the subsoil means a more accurate amount of fertiliser (in this case less) can be applied.
Subsoil testing can also identify soil properties restricting root growth and grain yields e.g. salinity, sodicity, compaction, waterlogging.