Two Legumes Unbind Phosphate

In India, pigeonpea (Cajanus cajun) and chickpea (Cicer arietinum L.) have long been an integral component of traditional cropping systems, despite their low and inconsistent yields. Recent research reveals justification for this tradition. Pigeonpea and chickpea have the unique ability to access insoluble phosphates in phosphate-poor soil, and increase the pool of available phosphate for succeeding crops.

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.

Accessible P for Successors

Both legumes were shown to increase the pool of P available to a subsequent maize crop. Pigeonpea, chickpea, sorghum and a sorghum-pigeonpea combination were grown four times in succession in pots filled with Alfisols and Vertisols with low P availability.

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.

The Root of the Matter

Like many legumes, pigeonpea and chickpea are deep rooting plants with strong tap root systems that allow them to access moisture and nutrients. Pigeonpea is able to root deeply in Alfisols, which may have a murrum layer consisting of iron nodules at 40-60 cm depth. This layer usually forms a barrier and prevents maize and sorghum roots whereas pigeonpea roots can readily penetrate it. This penetration may be enhanced by the pigeonpea's P uptake mechanism. Similarly, root penetration of chickpea in calcareous Vertisols may be enhanced by the roots acidification mechanism.

The Residual Effects

The residual effect of pigeonpea on sorghum were examined in Vertisols and Alfisols. Pigeonpea was grown under rainfed and irrigated conditions, or the field was left fallow. Nitrogen fertilizer was applied to mask the effect of fixed nitrogen.

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.

Effect on Soil Moisture

Soils in the semiarid tropics, particularly Alfisols, are often characterized by low water infiltration. Water infiltration in plots planted to sorghum from pigeonpea was greater than in plots with continuous sorghum. Water infiltration in plots planted to pigeonpea was greatest in plots where chickpea was previously grown. The cultivation of pigeonpea and chickpea was shown to improve water infiltration, decrease crust formation, and improve soil aeration. Improved P status, soil physical conditions and water infiltration due to the cultivation of pigeonpea in Alfisols manifested themselves in better root development of succeeding crops.


The study revealed many beneficial effects of the two legumes, chickpeas and pigeonpeas, and a singular ability to accommodate poor growing conditions. In summary those effects were:

  • Chickpea and pigeonpea have a unique mechanism that allows them to access soluble phosphate in phosphate poor soils.

  • Both crops increase the P pool for subsequent crops.

  • The deep rooting ability of chickpea in Vertisols, and pigeonpea in Alfisols, allows for increased productivity through water extraction, and increased access to nutrients, particularly phosphorus.

  • Chickpea and pigeonpea may also increase productivity by improving soil quality in crop rotations and through intercropping.

    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.

    Joji J., N. Ae, K. Okada, C. Johansen. 1990. Improvement of Soil Productivity Through Legume-Based Cropping Systems in Indian Alfisols and Vertisols under Semi-arid Environments in Soil Constraints on Sustainable Plant Production in the Tropics. Tropical Agriculture Research Series. Japan.

    Pigeonpea Releases Phosphate. 1992. International Agricultural Development. March/April.

    Contact: Chris Johansen, ICRISAT, Patancheru, Andhra Pradesh, 502 324, INDIA. Fax: 91(842)241239.