Differential acidity tolerance of tropical legumes grown for green manure in acid sulfate soils

Abstract

The growth of four tropical legumes (Cajanus cajan, Sesbania aculeata, S. rostrata, and S. speciosa) used as green manures in the tropics was studied in a glasshouse experiment. Two acid sulfate soils (Typic Sulfaquept, Bang Pakong Series; and Sulfic Tropaquept, Rangsit Series) were 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 after sowing. Concentrations of N, P, K, Ca, Mg, Fe, Mn, and Al were also determined in aerial plant parts at harvest.

The legumes responded differently to soil acidity and liming, but not to soil type. Cajanus cajan had the highest biomass production, followed by S. aculeata, S. rostrata and S. speciosa, in this order. The N concentration closely paralleled biomass production, suggesting that the growth of symbiotic rhizobia and nodulation were perhaps more susceptible to soil acidity than were the host plants. Liming to pH 5.5–6.0 was recommended for the legumes' growth based on the quadratic relationships between dry-matter yield and soil pH. In the unlimed soils, the Ca concentration in C. cajan and S. aculeata (0.32%) was twice as high as that in the two low-yielding legumes (0.15%). Furthermore, plant Ca increased exponentially (or quadratically in case of S. speciosa) as lime additions increased. It was estimated that for adequate growth, the Ca requirement in the 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 concentration 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 ratio of Ca to Al in plant tops was used to characterize plant tolerance to soil acidity, and to quantify the critical Al concentration in the plants. It appears that ≥ 90% maximum growth was attained only when Ca/Al was ≥ 150 for C. cajan and S. speciosa, ≥ 200 for S. rostrata, and ≥ 300 for S. aculeata. Cajanus cajan tolerated up to 80 mg Al kg^-1 in the shoot dry matter, whereas significant growth reduction occurred in the Sesbania species at levels > 30 mg Al kg^-1.