The nutrient budget for a rice field (Figure 2)
can be estimated as follows (all components
measured in kg elemental nutrient ha -1 ):
B = M + A + W + N 2 - C - PS - G
where
Inputs: M is the nutrient source added
(inorganic and organic); A is the atmospheric
deposition (rainfall and dust); W is the
irrigation, floodwater, and sediment (dissolved
and suspended nutrients); and N 2 is the
biological N 2 fixation (N only).
Outputs: C is the net crop removal with grain
and straw (total uptake less nutrients in crop
residues returned); PS is the total loss due to
percolation and seepage; and G is the total
gaseous loss due to denitrification and NH 3
volatilization.
The overall nutrient budget at a particular site
varies widely depending on the cropping