Testing deep phosphorus (P) can improve fertiliser recommendations with a more accurate look at soil P stocks. It can help to know how much P is in the profile, and where it is.
The topsoil is usually the most nutrient dense part of the soil. Maturing crops also rely on subsoil nutrients, especially if that is where the moisture is. P is not very mobile. Fertiliser P tends to stay close to where it is applied, usually at or near the soil surface. Over time, P can become stratified in the soil. No-till paddocks can have even more P stratification as there is less soil mixing.
Phosphorus buffering index (PBI) and fertiliser history also affect P stratification. In soils with a low PBI, the applied P will leach deeper into the soil. In soils with a high PBI, the applied P accumulates over the years and stratifies in the topsoil. Very little will move deeper, unless soil mixing or cultivation occurs.
Over time, successful cropping removes nutrients from the subsoil. When subsoil P runs down, crop yields can drop. Deep banding can be needed to supply the P for maturing crops.
One trial by CSBP in Western Australia showed Colwell P levels in a high PBI gravelly loam were nearly six times higher in the topsoil than the subsoil. Colwell P levels were:
- 35 mg/kg in 0 – 10 cm
- 5 mg/kg from 10 – 20 cm
- 6 mg/kg from 20 – 30 cm.
In WA, subsoil sampling is more common to test for pH, aluminium, and potassium (K). Getting more information about other nutrients is also important. Ideally, get deep nitrogen (N), P, and sulphur (S) tested.
Under no-till, K is also often stratified in the soil. A topsoil test could indicate K deficiency, while levels in the subsoil are adequate. You might be able to apply less K if deeper K reserves can supply the crop later in the season. The starter K rate only needs to satisfy the early crop.
Still use starter P
Whatever the P status of the soil profile, using starter P is good practice. Young plants can struggle to scavenge P from the soil, even when background levels are good. Young crop roots won’t access deeper P until later in the season. Crops need enough P to start to develop a good root system to take up other nutrients.
Photo: CSBP trial showing nil P (left), and 17kgP/ha (right)