A joint research project between the Government of Japan and the International Crop Research Institute of Semi Arid Tropics, ICRISAT, in India began in 1985 to investigate the interaction between these legumes and the soil chemistry of Indian Alfisols and Vertisols. The project analyzed the root activities of pigeonpea and chickpea, and their effect on the growth of maize, sorghum and soybean. The experiments took place at the ICRISAT Center in India.
In the semi arid tropics Vertisol, Alfisols, and soils related to Alfisols account for 14.54 million km2, or 69% of the land area. These soils are usually phosphate deficient. The phosphate that does exist is chemically bound with iron, calcium or aluminum. Iron bound forms of phosphate predominate in Alfisols whereas calcium phosphates are abundant in Vertisols. Pigeonpea and chickpea appear to have developed mechanisms to release plant available phosphate from these chemically bound forms. Phosphorus Uptake.
It has long been recognized that chickpea and pigeonpea, when grown on phosphorus deficient soils, are less responsive than other crops to P fertilizer. An experiment was conducted to confirm this observation. Chickpea, pigeonpea, and sorghum were grown on P deficient Alfisol and Vertisol fields. Sorghum responded to P applications on both soil types, chickpea showed no response to P application on the Vertisol but did respond in the Alfisol, and pigeonpea responded moderately only in Vertisol.
Chickpea oozes several acids, mallic acid in particular, from its roots and shoots. These acids lower the rhizosphere pH, which dissolves calcium bound phosphate to release P for plant uptake. Pigeonpea on the other hand, exudes piscidic acid from its roots which reacts with iron-bound phosphate to release P.
Phosphorus uptake by subsequent maize in Alfisols was highest following a pigeonpea-sorghum combination even though this combination absorbed the most P over the four growth periods. The P absorbed by maize may have been derived from the iron-bound P solubilized by pigeonpea. In the Vertisol, P uptake by maize was highest following chickpea, presumably due to the solubilization of calcium-bound P. Data suggest that the available P pool increases with cultivation of pigeonpea in Alfisol, and with chickpea in Vertisol.
In the Vertisol, sorghum grain yield following pigeonpea was about the same as (rainfed) or lower than (irrigated) the yield following the fallow. But in the Alfisol, yield after pigeonpea exceeded 2t/ha after cultivation of pigeonpea as compared to sorghum which yielded about 1t/ha. The beneficial effect of pigeonpea on Alfisol could be ascribed to its ability to absorb Fe-P.
Compared to sorghum, growth of a chickpea crop was found to improve grain production of subsequent pigeonpea. This response was not attributable to residual N, but may be attributable to the ability of chickpea to solubilize calcium-bound P.
Further quantification of the beneficial effects of these legumes is required to more comprehensively demonstrate the importance of legumes to low input cropping systems in the semi-arid tropics.
Pigeonpea Releases Phosphate. 1992. International Agricultural Development. March/April.
Contact: Chris Johansen, ICRISAT, Patancheru, Andhra Pradesh, 502 324, INDIA. Fax: 91(842)241239.