Amelioration of iron deficiency in rice and transformations of soil iron in coarse textured soils of Punjab, India

The productivity of coarse textured soils is generally low due to their poo r inherent fertility in particular due to the low water holding capacity an d nutrient availability. With the adoption of irrigated upland rice-wheat c ropping system on these soils where water cannot be impounded even after pu ddling, widespread deficiency of Fe has been reported in rice. Iron deficie ncy is most severe when the coarse textured soils are brought under rice cu ltivation for the first time. Iron deficiency in rice grown on such soils r esults from inherent low Fe content of soils and unfavorable conditions for reduction of Fe3+ to Fe2+. In field experiments, soil application of even 200 kg FeSO(4)7H(2)O ha(-1) proved inferior compared with 3-sprays of 1% Fe SO(4)7H(2)O solution for remediation of Fe deficiency in rice. Incorporatio n of Sesbania aculeata green manure before transplanting rice was very effe ctive in ameliorating Fe deficiency and significantly increasing rice yield and DTPA-extractable Fe in soils. Results of greenhouse experiments demons trated that incorporation of Sesbania green manure markedly decreased pH an d redox potential (Eh) and increased ionic strength, partial pressure of CO 2 and Fe concentration in soil solution. Chemical equilibria studies showed that Fe solubility was controlled by Fe(OH)(3)-Fe2+ system throughout the periods of submergence and after the peak of Fe2+ in the soil solution Fe-3 (OH)(8)-Fe2+ and Fe(OH)(3)-Fe-3(OH)(8) systems were predominant. Under fiel d conditions transformations of a part of Fe from the crystal lattice and/o r cristalline oxides of Fe to amorphous oxides and bound by carbonates over the years increased the availability of Fe in the soils.