NUTRIENT CYCLING: THE ESTIMATION OF ORCHARD NUTRIENT UPTAKE

Increasing concern with resources of raw materials and fossil fuels on the one hand, and with causes of pollution on the other hand, has led to the widespread realization that excessive use of fertilizers is undesirable. Apart from such general considerations, the cost of fertilizers is an important factor in the economic production of crops. Moreover, the use of too much fertilizer may not only be wasteful but may actually be harmful to the crop, often by adversely affecting quality.

Some understanding of the movement of nutrients in the soil-plant system is an essential basis for assessing what rates of fertilizer addition are desirable. Cooke (1970) has shown how the calculation of nutrient balance sheets can supplement the results of field experiments and provide useful general indications of the fertilizer requirements of crops. Examples of the value of this approach for perennial plantation crops have been given for rubber (Shorrocks, 1965; Watson, 1964) and coffee (Cannell and Kimeu, 1971).

Of the many examples of nutrient balance sheet in the proceedings of a recent symposium on the cycling of mineral nutrients in agricultural ecosystems (Frissel, 1978), the only one concerned with fruit crops is a single set of figures representing the arithmetic mean of values for citrus, apple, pear, peach, grape and persimmon obtained from a Japanese handbook.

Very little attempt has been made to construct nutrient balance sheets for orchards, in the absence of adequate information on soil components such as rates of leaching and fixation of nutrients. However, estimates of uptake by fruit trees have been made by weighing, sampling and analysing the crop and various parts of the trees. This paper is mainly concerned with such estimates.