Saturday, 15 July 2006
155-156

Rock Phosphate a Cheap Source of P' for Crops in Acidic and Neutral Soils of Karnataka.

M.V Ravi, Sujata F Harlapur, G.N. Thippeshappa, C.A. Srinvasamurthy, A.S. Kumaraswamy, V.R. Ramakrishna Parama, R. Siddaramappa, S. Bhaskar, M.V. Bhargavi, N.G. Basavaraju, G. Ravi, M.N. Ravi Kumar, B. Gayathi, and K.G. Ankegowda. Univ of Agricultural Sciences, Gandhi Krishi Vignana Kendra, Bangalore, 560 065, India

Rock Phosphate (RP) or Phosphate Rock (PR) is the trade name for mineral phosphate having useful attributes for its direct application in acidic soils as source of phosphorus. India is reported to have 145 million tonnes of RP deposits in different parts of the country but most of it is said to be of low grade and cannot be utilized for production of water-soluble phosphatic fertilizers economically. More than 3 lakh ha of cultivable land in Karnataka fall under low pH soils. Such soils are low in phosphorus and exhibit high P fixation capacity, thus difficult to manage with respect to phosphorus nutrition. The field experimentation in different soil situation in Karnataka using major cereals, pulses and oil seed crops reveal the advantage of using PR with SSP at various combinations. But the compaction of water soluble SSP or MAP with RP are of recent origin, their use as P fertilizers to crops, dissolution, transformation, electro-chemical changes in soil which influence the available P in flooded soil conditions and enzymes activity in the rhizosphere soils is a study of first of its kind. The results obtained from these investigation and their practical utility have been published in various journals and the same is highlighted in the synopsis. The performance of compacted P fertilizers on crop production was studied with rice-groundnut, rice-cowpea, rice-rice and ragi-groundnut cropping sequence in acidic and neutral soils. Rice and ragi crops were grown as the main crop and groundnut and cowpea were raised on residual P. The results revealed that phosphorus as SSP was comparable with most of the compacted fertilizers tested. Direct application of rock phosphate hold promise in acidic to neutral soils in terms of the crop yield. Compacted P fertilizers further increased the yield of grain and straw as compared to non-compacted fertilizers. Residual effect of compacted fertilizers was noticeable in the second crop. The RP dissolution rate was higher in acidic soils compared to neutral soils and increased upto 30 days and declined gradually thereafter. Among the non-compacted sources, Gafsaphos registered higher PR dissolution (0.5 M NaOH) and NCRP by BaCl2-TEA method. Among the compacted sources, JPR (A) or (B) compacted with MAP + S gave higher PR dissolution compared to other phosphate rocks. Olson's method recorded higher plant extractable P than Bray's at all the intervals of incubation. The transformation of P fertilizers have revealed that, the soil treated with water soluble SSP or MAP recorded maximum Fe-P and Al-P as compared to other phosphate rocks. Gafsaphos, among non-compacted sources, recorded maximum Ca-P. The different P fraction in acidic soil followed the order: Fe-P > Al-P > Ca-P and that in neutral soil: Fe-P > Ca-P > Al-P. The enzymes activity revealed that there was no much change in acid phosphatase occurred during the growth stages of crop (ragi), while urease activity decreased with crop growth, the dehydrogenase activity in the neutral soil increased. The addition of PRs increased the enzymes activity in soil where as water soluble sources reduced the same. Among non-compacted sources, gafsaphos and among compacted sources, JPR (A) + MAP + S recorded higher activity compared to other phosphate rocks. The electro-chemical changes in flooded soils particularly those of pH and Eh, influence the available P in the soil. The pH of the soil increased towards neutral range and drop in the Eh was evident in all the treatments upon submergence resulted in the increase in the available P2O5 content of the soil at different intervals of crop growth. It may be concluded that RP can be used as P fertilizer in acidic soils that resulted in higher P availability, uptake and yield. Application of gafsaphos would be better P source in neutral soils. Apart from this, compaction of water soluble SSP or MAP and S with RP hold promise in terms of agronomic production irrespective of soil and cropping sequence followed.

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