What to do about fertiliser after a tough year

After a year with failed crops due to drought, it’s not ‘business as usual’ for the next season’s inputs. Some fertiliser nutrients can remain in the soil if they weren’t taken up by the crop. Demand for other nutrients can increase after drought.

Key nutrients after a tough year

Nitrogen (N)

Soil nitrogen (N) is likely to be higher than usual:

  • Much of the fertiliser N will still be present if the crop was not grazed or cut for hay. 
  • Low soil moisture means the conversion of organic N to plant available forms is slow. When it rains, there’s likely to be a spike in available N.

If it looks like it’s going to be another dry season, minimise N inputs at seeding. Plan to apply N during the season. This works best with winter dominant rainfall and long-term average cereal yields over 2.5 t/ha.

Phosphorus (P)

Cutting back on P rates can be sensible. Be wary of zero inputs as seedlings still need enough P for healthy growth. General ‘rules of thumb’ after a bad year are to apply:

  • half your normal P rate if the yield was under 0.5 t/Ha AND soil test values support maintaining soil P.
  • two-thirds of your normal rate on non-calcareous soils if crops yielded over 0.5 t/Ha or with healthy growth;
  • near normal rates on calcareous soils.

Potassium (K)

If you’re not on sandy soils you could expect most K you’ve applied to still be there. As a guide, removing 2 t/ha straw will remove about 20 kg K/ha from the paddock. But whether you need to apply it can be difficult to say. Applied K only had an economic benefit in WA with K levels below 40 mg/kg in the top 10 cm of soil. In NSW responses to K fertiliser are not always seen in paddocks with K deficient topsoils.

Watch for acid soils

Soils tend to become more acid (pH < 5.5) after drought. Watch paddocks with a borderline pH more carefully than usual. Acid soil can cause manganese and aluminium toxicity in some crops. Canola does not tolerate acid soils.

Long Fallow Disorder

Long fallow disorder (LFD) is when crops fail to thrive despite adequate moisture. Having little plant life in a paddock for a long time affects soil biology. Vesicular-arbuscular mycorrhizae (VAM) are symbiotic fungi that help crops access nutrients. VAM live on plant roots. They cope by going dormant when plants aren’t present. It can take a long time for VAM activity to recover. Crops with LFD:

  • seem to remain in their seedling stages for weeks
  • development is very slow
  • yields are lower than expected.

If LFD is likely, using starter fertilisers containing P and Zn is the best option. Even though it does not always correct the symptoms. Place fertiliser close to seed to ensure young plants can access nutrients. Maize, sunflower and mung beans are more sensitive to LFD.

Check with soil tests

Always soil test to check soil nutrient levels. Deep soil testing is the best way to check if the crop will respond to N and K.

This post is a revision of 2016’s Tips to tweak your crop nutrition program following drought.

Photo courtesy of the GRDC.

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