Impact of time of day and time since irrigation on cotton leaf blade and petiole nutrient concentrations

Abstract

Petioles are used in weekly nutrient monitoring programs because they are more responsive to short‐term changes than leaf blades. However, this responsiveness may allow other factors, such as sampling time, to mask genuine changes in nutrient status. The objectives of this study were to measure the influence of time of day and time since irrigation on petiole and blade phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg) concentrations and to compare petiole and blade samples in their stability and usefulness in evaluation of cotton nutrient status. Cotton was grown on a sandy soil in the Coastal Plain region of Georgia. Treatments were K application rates of 0, 55, and 110 kg K₂O/ha. In early bloom, leaf blades and petioles were sampled from the first fully expanded leaf immediately following irrigation at 13: 00 Monday through Friday and on Tuesday at 7: 00, 9: 00, 11: 00, 15: 00, 17: 00, and 19: 00. Blades were separated from petioles, and P, K, Ca, and Mg were measured. Petiole and blade nutrient levels were positively correlated for P, K, Ca, and Mg. The difference in K rates was not as well reflected in the blade K levels as in the petiole K levels. As time since irrigation increased and the soil dried out, the petiole K levels of the 55 and 110 kg K₂O/ha treatments became significantly different. Petiole and blade P and K levels declined earlier in the day than Ca and Mg levels as the moisture content of the leaves declined. Blade moisture content dropped earlier than petiole moisture content which led to increased nutrient concentrations. The large daily variation in petiole nutrient levels could result in altered fertilizer recommendations. Therefore, it is of critical importance to sample cotton petioles consistently at the same time of day and the same time in relation to irrigation scheduling in order to differentiate between these timing effects and the timing effect due to crop growth stage.