"lime" Acid Soils with Green Manures
Aluminum phytotoxicity severely limits the productivity of many acid soils around the world. This condition can be treated with lime, but lime is often costly or unavailable to farmers in developing countries. However, a greenhouse study has shown that freshly added organic materials can, on a short-term basis, replace lime in reducing soluble aluminum (Al) and increasing crop yields.
The Trials
A greenhouse study was conducted to evaluate the potential of three green manures to detoxify strongly acidic soils, and to identify the process responsible for the Al detoxification. Cowpea (Vigna unguiculata), leucaena (Leucaena leucocephala) and guinea grass (Panicum maximum) were compared. These tropical green manures are known for their acid tolerance. The soil, an Ultisol (Humoxic Tropohumult, Kaneohe series), had a pH of 4 and soluble Al saturation of 50%.
Treatments were in 1kg pots with two plants per pot, they included: conventional lime Ca(OH)2 at four doses, 1.0, 1.8, 3.6, and 7.2 cmol(OH)/kg soil; and the green manures, each at 0, 5, 10, and 20 g/kg soil. All treatments received basal fertilizers to insure that Al toxicity was the sole limiting factor.
Sesbania cochinchensis , an Al sensitive leguminous tree, was grown as a test crop for four weeks. Plant dry matter weight and chemical composition were used to determine the liming effectiveness of the manures.
Soil chemical analysis was done prior to planting. Since all treatments had adequate fertilizer before planting, plant growth was used as an indicator of Al toxicity. The manures increased pH and reduced soluble Al at varying levels.
Results
Using plant growth as a measure of Al detoxification, cowpea and leucaena were quite effective as ÒlimingÓ sources. Leucaena was the most effective overall; However, cowpea was more effective at the lowest dose (5 g/kg yielded 2.24 g biomass/pot). Guinea grass had a negative effect on the sesbania, probably because the nutrient-poor grass tied up available soil nutrients. Treatments amended with the guinea grass at the two lower rates reduced growth compared to the unamended treatments. At the highest application, 20 g/kg, guinea grass produced a very small growth increase.
Discussion
The growth increases in the sesbania can be attributed to the increase in soil pH and the consequent reduction in soluble Al. The discrepancy between the effectiveness of the different green manures suggests that the relationship between pH and soluble Al in manure-amended soils varies significantly with the manure source. Sesbania seedlings produced more biomass when grown in the leucaena or cowpea treatments that received 20 g/kg than they did in the lime treatment that received 7.2 cmol(OH)/kg, even though the green manure treatments had higher soluble Al concentrations. This means that the forms of soluble Al probably differed among the trials amended with green manure and those amended with lime, and that Al phytotoxicity also probably differed. Soil analysis revealed that the cowpea and the leucaena amended soil contained nearly 5 to 10 times more soluble carbon than the limed soil. Also found were several organically complex Al species which are not phytotoxic.
Conclusion
The results of this study show that green manures can replace lime in detoxifying soils at least on a short-term basis, which is enough time to allow young seedlings to establish. Freshly added organic materials can reduce soluble Al and increase crop yields.
N.V. Hue, I. Amien. Aluminum detoxification with green manures. (1989). Commun. In Soil Sci. Plant Anal., 20(15&16), 1499-1511.
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N.V. Hue
Department of Agronomy and Soil Science
College of Tropical Agriculture and Human Resources
University of Hawaii
Honolulu, Hawaii 96822 USA