A poor or failed crop last season will affect nutrient budgets this season. Cereal hay removes about 1–2 kg of phosphorus (P) per tonne of dry matter. Canola removes about 3 kg of P per tonne (see Table 1).
Table 1. Comparison of nutrients removed in grain, straw, canola hay, cereal hay, and Lucerne
|Nutrients removed (kg/tonne)||Grain||Straw||Canola Hay||Cereal Hay (GS70)||Lucerne|
Phosphorus is a relatively immobile nutrient. If it is not removed in grain or hay, P tends to remain in the soil. Soil type will determine its availability to the next crop.
Adjusting your phosphorus budget
Soil test to check P stocks. If the soil tests are above critical values, the crop is unlikely to respond to additional P, so consider reducing P fertiliser rates. This only applies to moderately alkaline, neutral and acid soils where most P should still be available. On these soils, less P is immobilised during a dry year. Calcareous soils continue to fix P, even during a dry season. If soil stocks are low, do not reduce rates.
The general ‘rules of thumb’ after a bad year are:
- apply half your normal P if yield was less than 0.5 t/ha AND soil test values are moderate to good
- use two-thirds your normal rate on non-calcareous soils if yield was more than 0.5t/ha, or the crop had healthy growth
- use near normal rates on calcareous soils.
Still apply starter P as young crops aren’t good at getting residual P out of the soil. Apply at least 3 kg/ha to support young crops. Any less will be unevenly distributed and give poor results.
Photo:GRDC-Phosphorus needs can change after a dry season.