DIFFERENTIAL ACIDITY TOLERANCE OF TROPICAL LEGUMES GROWN FOR GREEN MANURE IN ACID SULFATE SOILS

The growth of four tropical legumes (Cajanus cajan, Sesbania aculeata, S. rostrata, and S. speciosa) used as green manures in the tropics wa s studied in a glasshouse experiment. Two acid sulfate soils (Typic Su lfaquept, Bang Pakong Series; and Sulfic Tropaquept, Rangsit Series) w ere adjusted to four pH levels: 3.8 or 4.0 (original soil pH), 4.5, 5. 5, and 6.5 (amended with lime). Dry weight was determined 49 days afte r sowing. Concentrations of N, P, K, Ca, Mg, Fe, Mn, and Al were also determined in aerial plant parts at harvest. The legumes responded dif ferently to soil acidity and liming, but not to soil type. Cajanus caj an had the highest biomass production, followed by S. aculeata, S. ros trata and S. speciosa, in this order. The N concentration closely para lleled biomass production, suggesting that the growth of symbiotic rhi zobia and nodulation were perhaps more susceptible to soil acidity tha n were the host plants. Liming to pH 5.5-6.0 was recommended for the l egumes' growth based on the quadratic relationships between dry-matter yield and soil pH, In the unlimed soils, the Ca concentration in C. c ajan and S. aculeata (0.32%) was twice as high as that in the two low- yielding legumes (0.15%). Furthermore, plant Ca increased exponentiall y (or quadratically in case of S. speciosa) as lime additions increase d. It was estimated that for adequate growth, the Ca requirement in th e shoot dry matter was approximately: C. cajan 1.2% Ca, S. aculeata 0. 8%, S. rostrata 0.6%, and S. speciosa 0.4% In contrast with Ca, the co ncentration of Fe, and to a lesser extent Mn, was significantly lower in C. cajan and S. aculeata than in S. rostrata and S. speciosa. The r atio of Ca to Al in plant tops was used to characterize plant toleranc e to soil acidity, and to quantify the critical Al concentration in th e plants. It appears that greater than or equal to 90% maximum growth was attained only when Ca/Al was greater than or equal to 150 for C. c ajan and S. speciosa, greater than or equal to 200 for S. rostrata, an d greater than or equal to 300 for S. aculeata. Cajanus cajan tolerate d up to 80 mg Al kg(-1) in the shoot dry matter, whereas significant g rowth reduction occurred in the Sesbania species at levels > 30 mg Al kg(-1).