Nitrogen in Agriculture: An Overview and Definitions of Nitrogen Use Efficiency

Keywords: : Nitrogen, N-use efficiency, NUE, N recovery, soil P, soil K, soil organic matter, soil structure.

The increased use of nitrogen (N) fertilisers in agriculture in the last five decades is justified by the increased yield potential of the crops grown and the judicious use of agrochemicals to control weeds, pests and diseases to protect that potential. Nitrogen use-efficiency can be assessed in a number of ways, discussed here with examples, but is usually determined as percent recovery of added N in the harvested product when calculated by the difference method. Assessed in this way, N-use efficiency is frequently only about 50%. But N-use efficiency of applied N should include that taken up by roots to produce the above-ground plant, where N plays a vital role in photosynthesis, the fixation of carbon dioxide to produce sugars, and the root system, which takes up nutrients and water. The fate of this N after harvest is important, but the total amount in the crop should be included in any estimate of percent recovery of applied N. Within the plant-soil system, the soil microbial population competes with the plant for fertiliser N applied to increase plant growth. Hence any fertiliser N remaining in the soil should also be included in an estimate of the fate of applied N. The use of ¹⁵N-labelled fertiliser allows the fate of the applied N to be determined. At harvest up to 70%, and sometimes more than this, of the applied N can be accounted for in the above-ground crop and in the soil to 100 cm. The fate of the N which is not accounted for should be a major research topic.

Optimising the use and recovery of N fertiliser applied at the recommended amount for the expected yield requires that no factor which is within the control of the farmer or grower should adversely affect yield. These include the control of weeds, pests and diseases of both the above-ground plant and the root system wherever possible. It is equally important to optimise soil conditions so that all crops grown in sequence benefit. Root growth and function should not be jeopardised by poor soil structure. Soil organic matter plays an important role in maintaining and improving soil structure. Inefficient use of N fertilisers can arise when there is a too little plant-available phosphorus (P) and potash (K) in the soil. One function of P is to promote root growth to increase the uptake of water and nutrients. Applied fertiliser N increases the number and size of individual plant cells and consequently the water content of the plant. To maintain cell turgor increased quantities of K are required and if not available dry matter is not produced. Examples of these important factors in ensuring N-use efficiency are given in the paper.

A E Johnston and P R Poulton, Soil Science Department, Rothamsted Research, Harpenden, Herts. AL5 2JQ, UK.

48pp, 21 tables, 10 figures, 51 references