Effect of environment and plant phenology on prediction of plant nutrient deficiency using leaf analysis in Leucaena leucocephala

Abstract

Plant analysis is an important tool for predicting plant nutrient imbalances associated with variable soil fertility and it is usually based on analysis of index plant parts such as the youngest fully expanded leaf (YFEL). Recent use of the YFEL to diagnose plant nutrient status of Leucaena leucocephala subsp. glabrata (leucaena) pastures has given unreliable results. Two field trials, one irrigated and one dryland, were conducted in subtropical Queensland to investigate the effect of index leaf selection, plant phenology and environmental factors (ambient temperature and water stress) on leaf nutrient concentrations. The YFEL was identified as the best plant part to sample because it was readily identifiable and had consistent concentrations of most nutrients compared to older and younger leaves provided specific conditions were met when sampling. At both sites there was significant (P < 0.05) seasonal variation in nutrient concentrations in leucaena YFEL, which was poorly correlated with ambient temperature but strongly correlated with rainfall in the preceding 28 days and chronological age of YFEL. Advancing plant phenological stage of development increased the chronological age of YFEL from 12 to 73 days under irrigation since no new leaves were produced for prolonged periods during pod filling and maturation. Similarly, YFEL could be 146 days old on plants in vegetative stages of growth under prolonged drought in dryland conditions. YFEL of ~21 days of age or less were found to be optimal for analysis. Furthermore, as the calcium (Ca) concentration of YFEL was strongly correlated with leaf chronological age, this parameter could be used to determine the age of the leaves sampled. YFEL with Ca concentrations >0.75% DM were likely to be >21 days in age and should not be used for the diagnosis of plant nutrient status. It was concluded that leaf analysis could be used to confidently assess leucaena plant nutrient status provided the YFEL were sampled from actively growing plants in vegetative development that had received rainfall/irrigation in the preceding 28 days and were <21 days of age.