0290-B5

Fertilizer Phosphorus Use Efficiency in Cowpea under Agroforestry

A. Raja Rajan, M. Paramathma and J. Balamurugan ¹

Abstract

A field experiment was conducted in a 17-month-old agroforestry plantation with three tree species (Graviella robusta, Tectona grandis, and Gmelina arborea) to determine the effect of different levels of P on the biomass yield and per cent utilization of P applied at four levels (0, 11, 22 and 33 kg P ha ^-1 ) to cowpea (Vigna unguiculata (L) Walp.). Radioactive ³² P-labeled super phosphate was applied in 1 x 1 m microplots made out in the centre of plots of 8 x 8 m in which the three tree species had been grown with a spacing of 2 x 2 m. Cowpea plants raised with a spacing of 45 x 15 cm were harvested at flowering stage and dry matter yields were recorded. The ³² P activity was determined in the samples and the per cent phosphorus derived from fertilizer and the phosphorus utilization efficiency were worked out.

The response of cowpea plants to the applied super phosphate was significant. The dry matter yield and the %Pdff were significantly enhanced with the level of P application, while the PUE decreased. The PUE in cowpea ranged from 14.59 to 20.50 per cent, depending on the P level and from 16.89 to 19.80, depending on the tree species under which it is grown. Among the three tree species, Graviella robusta contributed to the highest dry matter yield, %Pdff and PUE.

Introduction

The concept of agroforestry implies the integration of farming with forestry practices on the farm to the benefit of agriculture. Properly distributed tree growth acts as a foster mother to agriculture. This is particularly true in dry climates. Tree growth in such cases conserves soil moisture, increases atmospheric humidity, improves soil fertility, protects field crops against scorching and desiccating effects of winds and generally makes the climate more equable and pleasant, there by stopping up agricultural yields (Pritchett & Fisher 1987).

Growing field crops like pearl millet, legumes and oilseeds in a field predominated by Prosopis cineraria and Zizyphus sp. is an example of traditional agroforestry followed by the farmers of India since ages (Agarwala 1988).

Out of the reports that are available on agroforestry only a few deal with soil-plant relationships. Intensive research efforts are needed to device appropriate soil and fertilizer management practices for different agroforestry systems. In agroforestry the inter crops are usually fertilized with normal doses and as such no information is available on the different proportions of applied nutrients taken up by the intercrops. Hence this investigation was taken up to determine the effect of different levels of P on the biomass yield and per cent utilization of applied super phosphate in cowpea (Vigna unguiculata (L) Walp.) grown under three different tree species.

Materials and Methods

The experiment was conducted in the 17-month-old agroforestry plantation at Forest College and Research Institute, Mettupalayam. The plantation consisted of three species of trees, viz., Tectona grandis, Graviella robusta and Gmelina arborea raised with a spacing of 2 x 2 m. The soil at the experimental site was a sandy loam with pH 6.6 and low in available N and P. Microplots of dimensions 1 x 1m were made out in between two rows of the trees raised in larger plots of size 8 x 8 m. Radioactive ³² P tagged super phosphate fertilizer carrying a specific activity of 14.8 MBq g ^-1 of P was applied at three levels, viz., 11, 22 and 33 kg P ha ^-1 in the microplots. The rest of the macroplots received ordinary super phosphate at stipulated levels. There were 36 microplots in all, comprising four P levels (including a P check), three tree species and three replications. Common applications of 20 kg N ha ^-1 as urea and 40 kg K ha ^-1 as KCl were made to all the plots. The plots were sown with cowpea seeds with a spacing of 45 cm between rows and 15 cm between plants. Within each microplot there were 12 plants. Routine cultural practices were followed and the cowpea plants were harvested at flowering stage and radioassay for ³² P activity was carried out using a Geiger Muller counter as per standard procedure (IAEA 1976).

Results and Discussion

The dry mater yield of cowpea plants was significantly influenced by the application of P (Table 1) as compared to control. With the increase in the level of P applied, the dry matter yield also increased, from 51.9 g m ^-2 in control to 87.1 g m ^-2 at the highest level of 33 kg P ha ^-1 . This showed the response of cowpea to P. This was only to be expected as the experimental soil was poor in available P (Olsen P kg ha ^-1 ).

The dry matter yield of cowpea was also influenced significantly by the tree species. Several workers have reported enhanced nutrient and water availability under woody perennials (Sanchez 1988). Increased soil organic matter status due to the trees may also have added to the synergism, as we observed in our earlier studies (Balamurugan et al. 1999; Balamurugan et al. 2000). Reports are available from various parts of the world to prove this point. The highest dry matter yield was under Graviella robusta (77.7 g m ^-2 ) and the lowest under Gmelina arborea(61.0 g m ^-2 ). The effect of Tectona grandis was intermediate (70.7 m ^-2 ). Such differences among the tree species could be attributed to the nature of the canopy and the consequent shading effect. Among the three species, Graviella robusta exerted the least shading effect on cowpea. Cowpea is a shade-sensitive crop (George 1982) and, therefore, had put forth comparatively lesser biomass under Tectona grandis and Gmelina arborea

The per cent phosphorus derived from fertilizer (% Pdff) in cowpea plants increased significantly with the level of application of P. The increase was from 21.07 per cent at 11 kg P ha ^-1 to 37.74 per cent at 33 kg p ha ^-1 . This was obviously due to the increase in the availability of P in the soil consequent to super phosphate application. Since the experimental soil had initially low available P status, its response to applied phosphate was on the expected line. The three tree species differed among themselves significantly with respect to the %Pdff. The values ranged from 26.84 to 3.83 per cent. The effect of Graviella robusta was the most pronounced. This was evidently due to the comparatively better growth and P uptake under this tree species than that under the other two species.

The phosphorus utilization efficiency (PUE) was also significantly influenced by the levels of applied P and the tree species. The PUE ranged from14.59 to 20.50 per cent, depending on the level of P applied (Table 2). However, the relationship between the two was negative. The PUE decreased significantly with the increase in the level of application of P. This because the uptake of P by cowpea did not increase in the same magnitude as the increase in the level of P applied. Comparing the effect of tree components, it was observed that the highest PUE was under Graviella robusta as compared to the other two species. This was because the dry matter yield, the P uptake and the %Pdff were all highest under Graviella robusta.

Conclusions

The results of the study indicate that response of cowpea plants to the applied super phosphate was significant. The dry matter yield and the %Pdff significantly enhanced with level of P application, while the PUE decreased. The PUE in cowpea ranged from 14.59 to 20.50 per cent, depending on the P level and from 16.89 to 19.80, depending on the tree species under which it is grown. Among the three tree species, Graviella robusta contributed to the highest dry matter yield, %Pdff and PUE.

Literature Cited

Agarwala, V. P., 1988. Forests in India. Oxford & IBH Publishing Co. (P) Ltd., New Delhi.

Balamurugan, J., K. Kumaraswamy and A. Raja Rajan, 1999. Effect of natural forest on the chemical properties of soils. Madras Agric. J. 86: 549-551.

Balamurugan, J., K. Kumaraswamy and A. Raja Rajan, 2000. Effect of Eucalyptus citriodora on the physical and chemical properties of soils. J. Indian Soc. Soil Sci. 48: 491-495.

IAEA, 1976. Tracer Manual on Crops and Soils. Tech. Report Series 172, IAEA, Vienna.

Pritchett, W. L. and R.F. Fisher, 1987. Properties and Management of Forest Soils. John Wiley & Sons, New York, 494 p.

Sanchez, P.A., 1987. Soil productivity and sustainability in agroforestry systems. In: Agroforestry: A Decade of Development. H.A. Steppler and P.K.R. Nair (Eds), ICRAF, Nairobi, Kenya, p. 205-223.

Thampan, P.K., 1994. Trees and Tree Farming. Peekay Tree Crops Development Foundation, Cochin, 447 p.

Table 1. Dry matter yield of cowpea (g m ^-2 ) at flowering stage

Table 2. Per cent phosphorus derived from fertilizer and phosphorus utilization efficiency (%) in cowpea under agroforestry

¹ Radioisotope (Tracer) Laboratory,
Tamil Nadu Agricultural University, Coimbatore 641 003, India
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