Cation-Anion Balance of Field-Grown Crops. I. Effect of Nitrogen Fertilization

Plant species differ in their cation-anion balance, but there are rather substantial variations in the contents of organic anions (C–A) and excess–base (EB), and in the EB/N ratio of plants of a given species when grown under widely different conditions. The causes of such variation within a species are of interest because of the relationship of the C–A in plants to crop growth and the effect of the EB/N ratio of harvested crops in modifying the acidity produced by N fertilizers. The primary objective of this study was to determine the effect of a widerange in amounts of N fertilizer on the C–A and EB/N of several crop species grown under field conditions. Leaf samples of corn (Zea mays L.) and above-ground plant samples of corn, oats (Avena sativa L.), and orchardgrass (Dactylis glomeruta L.) were taken for analysis from plots of field experiments on several soil types that received different amounts of N fertilizer. Nitrogen fertilization increased the C–A, EB, and N contents of corn leaves, oats, and orchardgrass. For example, the addition of 358 kg/ha N in one experiment increased the average N content of corn leaves 89 meq/ 100 g or 106% and the EB and C–A contents 36 and 37 meq/100 g or 50 and 52%, respectively (Table 3). Likewise, in an experiment with oats, the addition of N up to 101 kg/ha increased the average EB and C–A 17 and 15 meq/100 g, respectively. The increase in C–A normally associated with an increase in N was much less where large amounts of K fertilizers were applied and was counteracted when moderate amounts of nitrate accumulated in the plants. No consistent effect on the C–A and EB of corn plants was obtained from N fertilization. Nitrogen fertilizers markedly lowered the EB/N ratio of the three crops studied. In one experiment, the EB/N of corn plants was decreased from an average of 0.91 to 0.53. An EB/N of 0.91 means that 91% of the theoretical acidity from nitrification (of that N removed in the crop) would be expressed, whereas an EB/N of 0.53 means that only 53% of the acidity from nitrification would result.