In Australia, premature senescence (PS) is occurring with increasing frequency in cotton crops. By the time PS is noticed, it is generally too late to take corrective measures and the use of plant tissue testing as a diagnostic tool has not been widely successful in predicting K deficiency (Kerby and Adams, 1985). Therefore, it is important to detect beforehand susceptible fields, so that timely action can be taken to avoid substantial yield losses, as insufficient K is believed to be one of the main contributing factors of PS.