On crop production and the balance of available resources

One of the main insights achieved in the early days of agricultural science is that each environment has a specific balance of resources, which is available to the crop. This balance determines crop production, the effect of resource addition and the effect of agronomic operations. However, attempts to quantify this balance are scarce. It is normally taken into account indirectly by a general description of soil, climate, topography, land use history, etc. This paper advocates quantifying this balance by quantification of the degree of limitation of resources. A coefficient (between zero and one) is developed which implements this idea. The paper shows that under a moderate assumption, the sum of the limitation coefficients of all resources equals one. This makes the deduction possible of non-limiting resources. The original binary concept of limitation can be regarded as a special case of this coefficient. The paper shows that crop response to addition of a resource can be viewed as the product of: the limitation coefficient, the use efficiency, and the amount of the dose. General crop production principles as the law of diminishing returns and the law of the optimum can be interpreted easily this way. Methods to estimate experimentally the limitation coefficients are discussed. The methods are illustrated by estimating the degree of limitation of nitrogen and phosphorus in southwest Niger. These two elements account for more than 70% of the total limitation (of carbon dioxide, radiation, water, and all nutrients), which is in agreement with other scientists in this region who indicate these two elements as the ‘principal’ limiting factors. © 2000 Elsevier Science B.V. All rights reserved.