Characterization of acid soils of Mizoram.

Thiry-four surface soil samples from different altitnde representing Entisols, Inceptisols and Ultisols and one representative profile from each of the soil orders and samples of another Ultisol profiles from Jhum area collected before and after burning of the slash were studied. All the soils were acidic and about 57 per cent of Entisols, 42 per cent of Inceptisols and 77 per cent of Ultisols were high in available N. About 71 per cent of Entisols, 100 per cent of Inceptisols and 77 per cent of Ultisols were high in available K, rest being medium in K. All the soils were low to very low in available P and fixed on an average 82.6 to 96 per cent of added P (1000 ppm). Phosphate fixation was positively related with clay, organic carbon and Fe and Al oxide contents of soil and negatively with pH. Phosphorus appears to be the most limiting nutrient for plant growth. These soils have adequate supply of plant available Mn, en and Fe but, 71 per cent of Entisols, 33 per cent of Inceptisols and 20 per cent of Ultisols were deficient in zinc (<0.8 ppm DTPA extractable Zn). Total soil acidity varied from 8.6 to 16.3 cmol(p+)kg−1. The mean contribution of pH-dependent and exchange acidity to total acidity was 88 per cent and 12 per cent, respectively and exchangeable A13+ constituted 81 per cent of exchange acidity fraction. The Al saturation of soil based on effective CEC was 43.0 per cent at pH 4.5 and was essentially zero at pH 5.6. These soils have low permanent negative charge (σp) and most of the CEC of soil is due to pH dependent charge. Burning of slash under Jhum cultivation significantly increased the pH, available P, exchangeable bases and ECEC but decreased exchange acidity, organic carbon, mineralisable-N and NO3-N content of soil. The SMP-single buffer method was found to be the best for predicting the lime requirement of soils of Mizoram. Highest soybean yield was obtained when lime at the rate of 5.4 t CaCO3 ha−1 (in the form of dolomite) was applied. Highest benefit to cost ratio of 1.69 was obtained at this level of lime application.