Lab tests help advisors with fertiliser and soil amendment planning. Without lab tests, crop nutrition can be a ‘best guess’. Returns from fertiliser inputs might not be optimal. Yields could suffer.
Soil, plant tissue, and grain testing can guide crop nutrition throughout the season. Soil tests indicate crop available nutrient levels in the paddock. Grain and tissue tests confirm that:
- crops access soil nutrients
- nutrients move into the grain.
In the long-term, soil test results are useful to calculate and track nutrient performance indicators.
Soil test before planting to:
- check nutrient availability
- identify soil chemical constraints
- refine fertiliser rates
- watch pH and soil carbon levels (cropping can lower soil pH and carbon stocks).
Test both the top and subsoil to gauge soil nutrient stocks. Nitrogen (N), potassium (K) and sulfur (S) are mobile and can move deeper into the profile. A 0–10 cm test level can seem insufficient when really there are deeper reserves the crop can use. You might be able to use less fertiliser than you would based on the 0–10 cm results. Sometimes N, K & S levels can be high near the surface, and low in the subsoil. Then you need more fertilser to achieve potential.
Tissue tests are better than soil tests at picking up trace element deficiencies. Tissue test the youngest fully expanded leaf blade. Test when the crop is young, preferably before visual symptoms appear. Yield has already declined by the time you can see symptoms. Nutrient treatments are less effective in older crops.
It is difficult to diagnose a deficiency only from visual symptoms. These can look like:
- crop diseases
- the effect of weather events
- different nutrient deficiencies.
Test grain at harvest to see:
- if the nutrient content is high enough to maximise seed vigour
- if any nutrient levels were low and could have restricted crop yield
- nutrient removal rates
How often should you test?
Test soil nitrogen (N) every year if you want to refine nitrogen budgets. Test other nutrients ever 2 – 3 years to monitor nutrient levels.
While it’s not common to test soil through the growing season, in-crop tests can:
- better assess the contribution from summer and autumn nitrogen mineralisation
- help refine in-crop N rates.
Leaf tests are a diagnostic tool. There’s no need for regular testing of healthy looking crops.
- to monitor when significant changes are made to a seasonal nutrient program
- in young crops where soil tests or paddock history suggest risk of a trace element deficiency
- to troubleshoot crops with symptoms, particularly where you can compare with healthy crop zones in the paddock.
Test grain after:
- high yielding crops to factor in nutrient removal for next seasons nutrient budget
- poor crops not explained by other factors
- normal crops to track long term nutrient budgets.