In general, if irrigation is an optional practice, yields are given with and without irrigation. Productivity can be expressed in quantity of a product per unit land area, such as kilograms or metric tons per hectare. Productivity indices have the advantage of being less vulnerable to changes in technology than are expressions of productivity based on yields. Productivity ratings express the predicted yields of specified crops under defined management as percentages of standard yields. Such a rating provides a scale for comparing the productivity of different kinds of soils over large areas.
Ratings lend themselves to numerical treatment.
Productivity ratings permit comparison of the productivity of crops having yields that differ markedly in numerical values. These entities represent similar levels of production so the productivity ratings would be similar. Selection of the standard yield of a crop depends on the purpose of the rating. For national comparison, the standard yields should be for a high level of management on the best soils of the region for the crop. If potential production is of interest, yields under the best combination of practices are used.