Dwindling soil fertility has become an increasing and utmost urgent problem that has to be looked in the right perspective in tropical agriculture. For understanding the role of different processes of nutrient stocks and flows in a soil system, a budgetary approach is a good tool for analyzing the turnover of nutrients in the soil. This approach offers a good scope to monitor the trends in nutrient mining/enrichment in any agricultural production system and to provide early warning of adverse trends to formulate policy interventions. Keeping this fact in view, the present study was carried out to calculate nutrient balance at micro (plot) and meso (farm) levels in irrigated farms of Coimbatore district of Tamil Nadu state of India, by employing the decision support model "NUTMON-Toolbox" which is a computer software that allows quantification of nutrient balances in a system based on the assessments made on nutrient inflows and outflows. NUTrient MONitoring (NUTMON) is a multiscale approach that assess the stocks and flows of N, P and K in an well defined geographical unit based on the inputs viz., mineral fertilizers (IN 1), manures (IN 2), atmospheric deposition (IN 3), biological nitrogen fixation (IN 4) and sedimentation (IN 5) and outputs viz., harvested crop produces (OUT 1), crop residues (OUT 2), leaching (OUT 3), denitrification (OUT 4) and erosion losses (OUT 5). To determine nutrient flows, a stepwise approach was adopted in which the different determinants of IN 1-5 and OUT 1-5 are calculated, estimated or assumed. The calculated nutrient balances at crop activity level indicated a negative balance for cereals and pulses while positive for vegetable crops. Similarly at farm level the nutrient balance was negative for N and K and positive for P. The farm level nutrient balance studies in irrigated farms revealed a mixed trend despite the size of the farm. The farmers of marginal and medium sized farms managed the N and P fertility efficiently, rather than the farmers of small and large sized farms, where nutrient depletion was observed. In all the farm size holdings, the K balance was highly negative. The positive balance of P is attributed to the accumulation of P over years due to P fertilizer application and minimized losses from the soils. The negative balance of N and K implies that a net depletion of these nutrients from the soil reserves occured. N is mobile in the soil system and is also lost from the system by leaching, volatilization of NH3 in soils whose pH is above neutral and denitrification in soils where submergence is a practice. All these three channels operate in the study area. Yet the wider negative balance obtained may be due to suboptimal use of inputs in the study area. Continued nutrient mining process proceeds at the expense of nutrients from the mineral and organic matter reserves and that limits the crop yield and renders the land chemically degraded. This process if left unchecked might lead to an irreversible loss of soil fertility and eventually jeopardize the production in the years to come and leaving the soils unfertile for the posterity. To mitigate the undesirable trends in N, P and K balances, DSSIFER (Decision Support System for Integrated Fertilizer Recommendations) computer software was used to derive site, crop and situation specific fertilizer recommendations. The results of NUTMON-Toolbox generated nutrient balances showed that in all the farms the N, P and K balances were positive when the fertilizer prescriptions were DSSIFER based. Thus, the NUTMON-Toolbox employed in the present investigation can effectively play a role in assessing the problems and help in developing strategies and practices for enabling policy interventions.

Key words: NUTMON, Nutrient budgeting, DSSIFER