Soil nutrients, in particular phosphorus (P) and potassium (K), are being depleted in the clay soils supporting the grains and cotton industries of northern NSW and Queensland, especially in the deeper layers of the soil profile. Current commercial soil tests used to measure these reserves do not detect some of these changes because previously unrecognised pools of nutrients have been replenishing those measured by the extractants. Even when enough fertiliser is applied to maximise yields, inorganic pools of P and K which are not measured by commercial tests are decreasing. Across soil types these backup pools of nutrients range in size and their rate of supply, and in the case of K, the availability to plants can be moderated by high levels of other elements like Na. It is therefore important to quantify the size of these pools, their interaction with applied nutrients in fertiliser and their capacity to supply plant nutrient requirements in our current farming systems.     

In conditions where nutrient limitations are identified or where nutrient replacement strategies have been adopted for long term sustainability, effective fertilisation strategies need to be developed to ensure efficient use of those nutrients by plants. The experiments needed to make these assessments are expensive and time consuming, and so certainty of selecting responsive sites becomes important. The uncertainty with current P and K diagnostic tests, due mainly to the existence of variable sized pools of nutrient reserves, has complicated this selection process.

This small preliminary project was conducted to assess the potential of new P and K diagnostic tests to clearly identify sites with low levels of available P and K on which future fertiliser trials could be conducted. It was also part of an initial assessment of the variability in P and K reserves in different soils and cropping systems across the region. Findings from this project will feed directly into a joint GRDC and CRDC project in which laboratory, field and glasshouse studies will develop new guidelines to ensure profitable use of P and K fertilisers as well as long term sustainability of grains and cotton farming systems.

Key outcomes from this project included (i) evidence of strongly stratified reserves of P and K within the 0-30cm sampling depth in cotton growing soils across the industry, which may have significant implications for nutrient access by cotton crops; (ii) evidence that there is substantial variability in the P and K reserves in soils across the regions, and that the size of these reserves is often not related to existing soil test methods; and (iii) this variability is being used, in combination with in-crop tissue testing, to improve understanding of the relationship between soil tests and plant nutrient status, and to ultimately guide selection of sites for longer term experimentation to develop P and K fertiliser guidelines for the grains and cotton industries.