Fertility of soil and moisture supply determine in large measure both the growth made by the tree and the size of nuts. The nuts borne on trees growing on fertile soils adequately supplied with moisture are generally much larger in size than those borne by trees on infertile soil or soil poorly supplied with soil moisture. Deficiency of either nitrogen, or moisture, or both is particularly effective in limiting the size of nuts produced. Pecans grown under soil conditions in which both nitrogen and moisture were deficient have been known to attain only about one-fourth the size of nuts of the same varieties grown in the same orchard but under conditions of clean cultivation and supplementary nitrogen applications. A prolonged drought during the time that the nuts are increasing in size very frequently causes them to be much smaller than they would have been had the moisture supply been adequate. The size of the crop borne by a tree determines in a very large measure the size that the nuts attain at maturity. There is generally an inverse relationship also between the number of nuts borne in a cluster on a shoot and the size they attain. In this respect nut crops are little different from apples and peaches, which, too, are sold on the basis of size. In order to produce fruits of large size having a high market value, the crops are thinned in years of a heavy set of fruit. In the case of pecans, for example, thinning the crop at the time the nuts are growing in size on heavily producing trees is a very effective method of increasing the average size of the nuts allowed to remain on the trees. The earlier the thinning is done the more effective it is; however, it will increase the size of the nuts even when done as late as when the shells have started to become hard. No practical and economical method of thinning the crop of nuts has as yet been found; nevertheless it is well to bear in mind that a large crop borne by a tree generally means reduced average size of the nuts at harvest. +Filling or Development of the Kernels+ In general, the fruits (nuts) of a nut-bearing tree are what might be termed storage organs. In them are stored mineral elements and such elaborated food materials as carbohydrates (sugars and starch), oil, amino acids, and proteins that have been removed from the leaves and wood of the tree. These materials are stored for future use of the embryo in the nut to sustain respiration, to permit germination, and to maintain the seedling until it has produced enough leaf area to become self-sufficient.