Can smarter nitrogen fertilizers be designed?

There are numerous difficulties in modeling the uptake of nitrogen by roots. To help make progress a very simple model based on experimental results was constructed. The uptake of nitrogen was described by a first-order equation in which the uptake parameter combined the effects of root size and the uptake per unit root size. The loss of nitrogen was modeled using a similar equation. Using the model, the hypothesis that fertilizer nitrogen losses from high-yielding irrigated rice could be almost eliminated by delivering the precise amounts of nitrogen required to support growth at any given time was demonstrated to be false. The model was used to compute the effects of applying a constant amount of fertilizer nitrogen in eight ways (splits), and also the effects of a slow-release and of an 'ideal' fertilizer. Counter to intuition, the practicable limit to recovery of fertilizer nitrogen was about 57%, for the high rates of nitrogen considered here and with the values for the model parameters used here. This arises because it is necessary to have a certain concentration of nitrogen in the available soil pool (freely-exchangeable plus solution nitrogen) from which the crop can take up the nitrogen it needs, and concurrently the available soil pool will lose nitrogen at a rate controlled by various biophysical processes.