Using Rb Relative Abundance to Determine Root Activity in K Fertilizer Bands

Agricultural systems are increasingly reliant on fertilizer inputs to achieve productivity targets and maintain soil fertility. Efficient nutrient capture by roots is critical in developing effective fertilizer application strategies. However, differentiating nutrient derived from fertilizers versus that obtained from background soil sources can be difficult, especially when one application may deliver residual nutrient benefits across a sequence of crops grown in rotation.

Fertilizer nutrient recovery is commonly estimated Differences in nutrient uptake by crops grown in fertilized and unfertilized treatments are often used to estimate, but this often leads to underestimation - especially when root proliferation in and around fertilizer bands results in preferential uptake from the concentrated nutrient source rather than the bulk soil volume. Fertilizer source isotopes (e.g., isotopes of P and N) or non-source tracers (e.g. Rb for K fertilizers) added to fertilizer bands can more specifically identify fertilizer-derived nutrients However, these techniques based on the enhanced abundance of the tracer isotope or tracer element can be expensive in field-scale experimentation.

We report from glasshouse studies how differences in the ratios of K and Rb in fertilized and unfertilized plants relate to the natural abundance of Rb in soils and KCl fertilizers. These differences were used to quantify root responses to K fertilizer applications in sequences of maize followed by cotton, and vice versa, in successive glasshouse experiments using 13 clay soils collected from the NE Australian grains region. The KCl fertilizer was either banded or dispersed through the soil volume and in a subset of soils, the impact of adding P, or N and P, to a KCl band was also investigated. Results suggest this technique can be used to quantify fertilizer K recovery, and to explore the impacts of fertilizer blends on root activity in and around fertilizer bands.