How to adjust phosphorus after a dry year

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
P 3 0.5 3 2.1 3
N 20 5 30 16 28
K 4 10 3 12 22
S 3 0.5 3.5 1.5 3

Source: Bill Bowden, WADA

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.

Starter P

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.

More

What’s the minimum Phosphorus rate if you are following a poor crop?

Phosphorus after the dry – experience from the past

Phosphorus – how much can we reduce P

Tips to tweak your crop nutrition program following drought

Deep placement of fertiliser delivers results

 

Photo:GRDC-Phosphorus needs can change after a dry season.

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