Validation of soil testing guidelines and critical values for nutrients for the management of modern cotton cultivars

Australian cotton yields are three times the world’s average. This success is partly due to ensuring high levels of soil fertility for maximum crop growth. An important strategy has been the use of critical soil test values to assess the likelihood of a cotton response to the addition of fertiliser. However, there is much uncertainty around these critical soil test values for cotton, particularly for the two most limiting nutrients, nitrogen (N) and phosphorus (P).This is important because of the high cost of fertiliser and, in the case of N, associated greenhouse gas emissions.Critical soil test(N and P)values for cotton are unreliable. Reasons include(i)differences in the nutrient requirements of cotton varieties (historic vs modern), and(ii)differences in the pools of soil nutrients currently tested for,and those which are accessed by the crop. Furthermore, understanding remains limited on the link between plant tissue N values and cotton responsiveness to N topdressing. Overall, this has caused reduced confidence in soil/plant testing among cotton producers, yield deficits, and/or fertiliser in efficiencies.The overall aim of this project is to assess the critical soil and tissue test values for important nutrients (N and P), which are required for maximum cotton growth. Most notably, we will: 1) identify the critical soil P concentrations for old and new cotton varieties in a stratified and mixed soil profile; 2) identify the critical profile N content above which N responses are not expected for old and new cotton varieties; 3) assess the cotton response to a N topdressing with increasing plant tissue N values for old and new cotton varieties, and; 4) assess the opportunity to manipulate N topdressing rate on the basis of varying tissue N concentrations for old and new cotton varieties using different starting soil N fertility.This project will help improve the nutrient guidelines and lead to improved cotton productivity/sustainability.