Quantification of nutrients recycled by tank silt and its impact on soil and crop

Abstract

Tanks were an integral part of rural life in India traditionally. However, with decreasing collective action by the community inappropriate soil and water management practices adopted by the farmers, encroachments of tanks and waterway by the individuals resulted in neglect of the tanks in villages. Good practices such as desilting and application of silt to agricultural fields were abandoned. Continued mining by crops and reduced application of organic manures have resulted in deficiency of several nutrients particularly that of micronutrients. ICRISAT in association with Modern Architects of Rural India (MARI), an NGO conducted a pilot project and quantified major and micro-nutrients present in the tank silt and also its impact on soil health and crop yields. The depth of silt in 12 tanks de-silted ranged from 1.2 m to 3.0 m. The pH of the tank silt ranged from 6.5 to 8.5, while the organic carbon content was found to be low (0.5% to 0.8%). The available N content of tank silt ranged from 328 mg kg-1 to 748 mg kg-1, available P 5 to 35 mg kg-1 and K 271 to 522 mg kg-1 silt. Similarly, available S ranged from 12 mg kg-1 to 30 mg kg-1 zinc from1.2 mg kg-1 to 5.6 mg kg-1 and boron 0.4 to 0.8 mg kg-1 silt. Microbial population was found to be low and it could be due to excessive use of pesticides for cash crops like cotton and chilli grown in the catchment area. Textural analysis indicated 70 to 80% clay, while the silt ranged from 15 to 25%. Addition of tank silt at 50, 100, 150 and 375 tractor loads per hectare improved the available water content by 0.002, 0.007, 0.012 and 0.032 g g-1 of soil, respectively in the plough layer and enhanced the tolerance of rain-fed crops to moisture stress by three to five days. The farmers could recover the investment made on transport of the silt through increased net profit in cotton and chilli compared to turmeric and maize. Further, the saving on pesticides alone was to the tune of Rs. 2500 ha-1 in cotton and chilli crops, which has indirect beneficial impact on the ecosystem. De-silting was found to be an economically viable activity both in terms of farmers’ and project’s perspective to create more storage capacity as well as to return the silt back to the fields. De-silting activity needs greater support from the government and non-governmental agencies for achieving multiple outputs like employment generation for landless, rejuvenating of the tanks and for enhanced productivity of dryland crops.