Following drought growers look to recover production and protect vulnerable soils. When it comes, the break may present opportunities to cut back on some usual nutrient inputs while moving back into production. Soils should be managed with an eye to their long term fertility.
Soil phosphate levels may increase a little during droughts where input rates exceed removal by crops. Soil testing confirms if paddocks are likely to be responsive to Phosphorus (P). Reduction of phosphorus fertiliser rates is often reasonable in the recovery year, but zero P inputs carry a risk of reduced seedling vigour. Growers should seek specific advice but general rules of thumb suggest:
Apply half your normal P rate following failed crops (crops with little growth and yielding less than 0.5t/ha);
Use two thirds normal rate on non-calcareous soils following crops yielding more than 0.5t/ha or which had healthy growth;
Use near normal rates on calcareous soils.
In paddocks that were sown but failed to produce a crop, soil nitrogen (N) is likely to be higher than usual because:
Most N incorporated before sowing the failed crop will still be available if the crop was not grazed, or cut for hay.
Mineralisation of N (conversion to plant available forms) continues slowly during drought, and increases strongly once it rains.
Reduce up-front cost and risk by minimising N inputs at seeding and plan to apply N in response to how the growing season develops. This is most feasible in areas where rainfall is winter dominant and long term average cereal yield exceeds 2.5 – 3 t/ha.
Deep soil sampling is the most reliable method for determining soil-nitrate levels. Heavy pre crop and early incrop rains can move soil nitrogen down the soil profile where it may not be readily available to young winter crops. Consider the need for starter N if soil tests suggest much of the nitrate is deeper in the profile, even if total levels are good.
Long Fallow Disorder
Soils naturally contain beneficial fungi that help a crop access nutrients like phosphorus and zinc. A group of fungi called vesicular arbuscular mycorrhizae (VAM) are particularly good at helping crops access soil Phosphorus and Zinc.
VAM can die out in a paddock from a lack of host plant roots during long clean fallow periods or drought. The severe reduction or lack of VAM can show up as a syndrome known as long fallow disorder (LFD) – the failure of crops to thrive despite adequate moisture. Plants seem to remain in their seedling stages for weeks, development is very slow and yields are lower than expected.
If LFD is likely, using starter fertilisers containing Phosphorus and Zinc is the best option, though does not always correct the symptoms. Fertiliser containing P and Zn must be placed in close proximity to seed to ensure these nutrients are available until the root volume expansion or VAM can assist in finding more nutrients. Consult an experienced agronomist to establish appropriate fertiliser rates for your situation. Sorghum is not as sensitive to LFD as maize, sunflower or mung beans.
Avoid canola in acid soils
Manganese can cause toxic effects in some crops, and soil levels tend to increase in acid soils (pH below 5.5) following droughts. Avoid sowing susceptible crops including canola on problem acid soil paddocks.
Michael Wurst, PIRSA, Chris Dowling, Back Paddock Co. & Mark Conyers, NSW DPI
Photo courtesy of the GRDC.