Nitrogen Nutrition in Relation to Yield and Quality of Grapefruit.

Investigations at this and other stations have shown generally that nitrogenous fertilizers exert considerable influence in increasing the yields of grapefruit and other citrus fruits. The factors affecting quality of grapefruit and the relation of nitrogen nutrition to quality have not yet been fully studied with the result that orchard management practices, particularly in Arizona, cannot be made with the assurance of modifying both yield and quality. A study of the physiology of fruiting of Marsh Seedless grapefruit was begun at this station in the autumn of 1937. Some results have been obtained which may suggest methods of rationalizing the cultural program in terms of treatments affecting both the yield and the commercial quality of the fruit produced. These -experiments indicate that a relatively high nitrogen content of the tissue at the time of blossoming is followed by a relatively large set of fruit. Present evidence suggests that high quality fruit at harvest follows a relatively low nitrogen content of the tree during the summer and fall and may be materially affected by cultural practices carried out during the growing season. It is suggested that such practices as heavy nitrogen fertilization, with clean cultivation during the summer bring about a vegetative tree condition leading to the production of coarse-textured thick-skinned fruit largely of the lower commercial grades. On the other hand starvation practices such as the growing of competing cover crops, and the withholding of nitrogenous fertilizers during the summer months tend to bring about a low nitrogen composition of the tree, and a condition favorable to the production of smooth-textured, thin-skinned fruit of relatively superior market grades. Data leading to this view were obtained by grading experimental lots of Marsh Seedless grapefruit at a commercial citrus packing house. The commercial grading of grapefruit is based to a considerable extent upon external appearance. Smooth, symmetrical, thin-skinned fruit are classified into superior grades, while rough, thick-skinned fruit fall into the lower commercial grades or are discarded as culls. A summary of the experimental treatments, yields, and the proportion of fruit of each of the commercial grades is presented in table I. Yields have been calculated as percentages of the previous year's production. The accompanying table shows that the trees in plots 1, 2, and 3 which had been given treatments to increase the nitrogen content of the tree prior