Harvest Nutrient Sampling

June 2023

With combinable crop harvest fast approaching (or already underway for some areas) it may be worth considering options for monitoring success or limitations with input decisions and soil management practices to close the feedback loop. It is always difficult in agriculture, particularly in an annual cropping cycle, to successfully achieve this, due to the timespan involved and the number of variables that are at play during the process. However, this does not mean it is not worth attempting it to help influence decisions for the coming season.

The last newsletter in the spring covered the merits of sampling the harvested produce, grain or silage, to measure the nutritional offtake to interpret the impacts of the management of each nutrient measured. There has been some good evidence to show that, particularly for phosphate, crops grown on low P index soils, or those that have not been able to access sufficient phosphate during the growing season, are likely to show low levels in the grain. This may not be quite so clear cut for potassium however, as the results of experiments have shown relatively stable levels of grain potassium, despite big variations in yield and soil K levels.

Although for potassium this may mean it is difficult to use these results to influence decision making for future management, it does mean that growers can more easily budget K removals and therefore replacement levels, something that is harder to achieve with other nutrients where both yield and grain concentration vary in response to differences in soil supply.

Response of grain yield and K concentration of wheat to soil K supply grown in a strongly K-limiting soil or a soil with excess K supply.

Crop	Strongly K-limited		Excess soil K supply
	Yield (t/ha)	Grain K (g/kg)	Yield (t/ha)	Grain K (g/kg)
Wheat	3.7	3.9	5.71	3.7

From a long-term experiment on an Oxisol near Queensland, Australia, described in Bell et al.(2009)