Replacement Nutrient Quantities Strongly Influenced By Fertiliser Management.

Replacement nutrient estimates can range over two-fold depending on fertiliser management

Standardised grain and stubble nutrient concentrations are often used to calculate the quantity of fertiliser to replace nutrient removed in grain. However, grain nutrient concentrations are variable and stubble concentrations rarely reported. For example, a survey of 114 grain samples delivered to silos in south-eastern Australia found a coefficient of variation of between 6 and 30% depending on nutrient. We used an experimental approach to quantify the influence of fertiliser management on grain and stubble nutrient concentrations, and replacement nutrient quantities. Measurements were made on the “nil” and “all” treatments of nutrient omission experiments conducted on canola (3 sites) and wheat (2 sites) conducted in south-eastern Australia in 2016.

Canola grain in the “all” treatment had an N concentration up to 63% higher than the “nil” treatment, and S up to 59% higher, while P and K ranged from a 39% decrease to a 15% increase. Wheat grain in the “all” treatment had an N concentration up to 125% higher than the “nil” treatment, P up to 9% higher and S up to 22% higher. Estimates of nutrient replacement per tonne of grain, which included an allowance for stubble burning, varied by up to 2-fold for N depending on fertiliser treatment, and up to 13% for P, 84% for K and over 3-fold for S. It was concluded that a combination of either grain and stubble testing, or regular soil testing is needed to avoid either over-fertilisation or under-fertilisation.