Effects of Exchangeable Soil Calcium, Magnesium and Calcium/Magnesium Ratios on Plant Nutrition and Growth of Lettuce on an Ultisol
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1999
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Two greenhouse pot experiments were conducted to study the effects of exchangeable soil calcium, magnesium and calcium/magnesium ratios on nutrition and growth of lettuce (Lactuca sativa L.) on an Ultisol (Manana soil series) with low pH (4.35), Ca (0.57 cmolc kg-1) and Mg (0.60 cmoh kg-1) in Hawaii to obtain calibration data for Ca and Mg in Hawaii soils and test the current sufficiency recommendations for Ca and Mg for making fertilizer recommendations. The objectives of this study were; to identify the sufficiency levels of exchangeable soil Ca and Mg for growth of lettuce; to investigate the validity of an ideal Ca/Mg ratio for growth of lettuce and to determine the effects of soil Ca and Mg levels as well as Ca/Mg ratios on soil nutrients and the nutrition and growth of lettuce. Lettuce yield increased as soil Ca increased and also as plant Ca level increased. The Cate- Nelson method was applied to determine the critical levels of Ca, Mg and the Ca/Mg ratio in the soil and plant. A critical soil Ca level for lettuce was determined to be 1.9 cmolc kg-1 and is more reasonable and lower than the value of 5 cmok kg-1 that is currently recommended in Hawaii. Lettuce in the zero Ca treatment with 0.57 cmoh kg-1 soil Ca exhibited Ca deficiency symptoms in the Ca experiment. A critical plant Ca concentration for lettuce at maturity was also determined to be 4 g kg-1. Exchangeable soil cations interact with each other and application of a large amount of liming material can cause cation imbalance. In the Ca experiment, soil Mg, K and Na decreased as soil Ca increased. Application of Ca increased the soil Ca level, increased Ca uptake by the plant and reduced the uptake of Mg and Na but had no effect on the uptake of P. Soil Ca restricted K uptake at low Ca levels due to decreased ion selectivity and leakiness of membranes membranes when Ca was deficient. Lettuce growth was normal with all soil Mg levels in the Mg experiment. Lettuce yield also was not related to plant Mg level. A critical soil Mg level for lettuce could not be established, however, the soil Mg level of the zero Mg treatment, 0.67 cmoU kg’\ was apparently adequate for normal lettuce growth. Therefore, the sufficiency range for soil Mg recommended in Hawaii (2.5 to 3.3 cmok kg'^) appears too high. Lettuce in the zero Mg treatment did not show any Mg deficiency symptoms. A critical plant Mg concentration for lettuce at maturity also could not be determined, however, the plant Mg concentration of the zero Mg treatment, 4 g k g '\ was apparently sufficient for normal growth of lettuce. Interactions between soil cations also occurred in the Mg experiment where soil Ca, K, and Na decreased as soil Mg increased. Increased levels of soil Mg increased the uptake of Mg by the plant, and reduced the uptake of Ca and Na, but had no effect on the uptake of K and P. In the soil Ca/Mg ratios ranged from 0.11 to 7.70, lettuce growth was limited by a Ca/Mg ratio of around 0.11 and no yield reduction was observed in the Ca/Mg ratio range from 0.50 to 7.70, which is within the optimal range. This study provides evidence for the conclusion that plants can grow normally within a broad range of Ca/Mg ratios. Lettuce yield was related to both soil Ca/Mg ratio and plant Ca/Mg ratio. The lower critical level of the soil Ca/Mg ratio for lettuce was determined to be 0.5. However, caution should be used in interpreting yield response to the soil Ca/Mg ratio because soil Ca or Mg levels can also affect plant growth. The critical plant Ca/Mg concentration ratio for lettuce at maturity was also determined to be 0.5. Plant Ca/Mg ratios in lettuce were significantly related to soil Ca/Mg ratios.
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