Soil testing is a valuable tool for crop nutrition decision making. The standard 0 – 10cm soil test doesn’t always tell us enough about the crop availability of more mobile nutrients. Nitrogen and Sulphur, as anions (negatively charged), tend to move deeper into the profile with rainfall. On sandy acid soils, Potassium (a positively charged cation) can also be mobile and leach into the lower root zone, but it is not as mobile as sulfur or nitrogen.
Complementing the traditional soil sampling technique by going deeper into the root zone allows better assessment of the nutrients available to the crop. Deeper soil tests, where well calibrated, enable better prediction of requirements across the life of the plant. Deep soil reserves do not necessarily preclude starter fertiliser applications.
Deeper soil samples are often used for monitoring N, and sometimes soil moisture. Adding K and S to these deeper soil tests can inform more efficient application of these nutrients. More information can enhance the rate recommendations as either a reduction in the fertiliser rate that was based on a 0 – 10 cm sample, or to increase the rate to meet a measured need.
Nigel Wilhelm, SARDI, Rob Norton, ANZ IPNI, Abby Jenkins, NSW DPI.