Mineral nutrition of cacao (Theobroma cacao)l. iii. the effects of different levels of nitrogen and calcium on the growth and nutrient content of seedlings grown in sand culture

The levels of nitrogen which produced the optimum growth of cacao plants were found to be 80 ppm from germination until the seedlings were four months old and then 140 ppm up to eight months from planting. Eight-month-old plants receiving 140 ppm nitrogen were still increasing in weight compared to those receiving 110 ppm or less. The levels of calcium which produced optimum growth were 10ppm at two months, 50 ppm at three months, and 75 ppm at four and eight months from seeding. When the plants were eight months old, it was found that increasing levels of nitrogen had increased the percentage of root dry weight but had had little effect on the stems. They had also increased the total plant dry weight, the leaf area, and the weight of leaf per unit area (gm.jdm.J). At eight months, increasing levels of calcium had tended to decrease the percentage weight of leaves and increase the percentage weight of stem of the total plant, while the percentage root remained unchanged. There was no interaction on dry weight of plants between the nitrogen and calcium treatments. Chemical analysis of eight-month-old plants showed that increasing levels of nitrogen in the nutrient medium had increased the concentrations of nitrogen and sodium and reduced the concentrations of potassium and molybdenum in the leaves. The higher levels of calcium in the nutrient medium increased the concentrations of calcium, iron, zinc and molybdenum and decreased the concentrations of nitrogen, manganese and sodium in the leaves. It is of interest that higher levels of nitrogen were accumulated in the leaves when the nutrient medium was deficient in calcium. Work in progress indicates that such accumulation of nitrogen may have a bearing on leaf-edge scorch when the source of nitrogen is nitrate. Successively higher levels of nitrate nitrogen in the range 14-110 ppm in the nutrient medium usually produce marked increases in the growth of cacao (Lockard, Vamathevan and Thamboo, 1959; Lout\\ 1962), while a level of 220 ppm tends to retard growth and produce characteristic symptoms ofleaf scorch reminiscent of calcium deficiency(unpublished data). These observations suggest a possible interaction between nitrogen and calcium in the growth of cacao. The response of grasses and some legumes to increasing levels of calcium has been found to vary for different varieties (Bradshaw, Lodge, Jowett, and Chadwick, 1958; Snaydon and Bradshaw, 1961; Snaydon, 1962). Levels of calcium in the nutrient medium above 20 ppm gave little increase in dry weight in most varieties. The level of calcium supplied to cacao seedlings in sand-culture is normally much higher than this. However, there is little published evidence to show what levels are optimum, above which little or no increase in growth occurs, or whether the optimum level varies with age. Olsen (1942) investigated four different species of plants and showed that as the calcium content of the medium increased, three of them (Sinapis alba, Dianthus barbatus, and Hordeum distichum) absorbed more calcium but less magnesium and potassium. Tussilago farfara differed from these three species in that the potassium content increased with increasing levels of calcium in the nutrient medium. Data on the nutrient concentrations in the leaves of plants grown in control nutrients and those deficient in nitrogen or calcium are fairly extensive (Murray, 1957; Lockardet ai, 1959; Loue, 1962; Lockard and Asomaning, 1964). There is, however, no information on nutrient concentration in the leaves of cacao plants receiving factorially applied levels of nitrogen and calcium. The data presented here are the results of an experiment designed to investigate the relationship between these two elements and their effects on the growth and nutrient content of cacao seedlings.