Nutrient Removal and Soil Testing (September 2018)


Where droughted crops are harvested and yields are low, the removal of major nutrients such as N, P & S will also be low. This presents an opportunity to reduce costs in 2019, by reducing inputs in line with removal by the previous crop. Previous on-farm demonstrations by Rural Management Strategies, have found that where soil tests indicate that Phosphorus levels are at or above critical values, very low rates of starter fertiliser can be applied, with little to no effect on final crop yield. In some cases, cereal crops sown with low starter fertiliser rates had reduced early biomass in comparison to higher rate treatments, but grain yield was not affected.

With respect to Sulphur, paddocks which have had a history of Gypsum application leading to reasonable S soil test levels, are less likely to respond to further Sulphur application.

Deep soil Nitrogen tests taken in the past two years by RMS clients have generally indicated that soil N levels are high. This is a function of conserved soil moisture over the last two to three summers, plus high application rates of N fertilisers in excess of crop removal. This is unlikely to change in 2019 as removal this year will also be low.

There are significant savings to be made in 2019 by reducing starter fertiliser rates, postponing gypsum applications and optimising Urea application rates. However, accurate soil test data is required to provide the confidence that rates can be reduced without affecting yields. During late spring 2018, a representative cross section of paddocks should be soil tested so that fertiliser rate decisions can be made early and orders adjusted accordingly.

Other factors to consider when setting starter fertiliser rates include:

  • Sowing time. Early sown crops have greater root growth and improved ability to extract nutrients from the soil profile. Hence later sown crops are more likely to be responsive to applied Phosphorus.

  • Crop Type. Cereals are more responsive to banded Phosphorus than canola and pulse crops. Also, canola tends to perform better when Nitrogen is applied early (or continuously from previous legume Nitrogen), whereas cereals can recover / respond to later application

  • Choice of product. Gypsum is the cheapest source of Sulphur, especially after a pasture or grain legume where the Nitrogen component of Sulphate of Ammonia is not required.