The degree to which nuts are filled or how well the kernels are developed at harvest is determined by a rather large number of interrelated factors: (1) Size of crop, or ratio of number of leaves per nut; (2) average size of nuts; (3) condition of leaves; (4) amount of second growth of the trees; (5) size of preceding crop and how well the nuts produced were filled; (6) disease and insect injury to the nuts; (7) weather conditions; (8) heterosis or effect of cross-pollination on embryo size.
+Size of crop:+ Nut growers want their trees to bear large annual crops of nuts. It is very seldom that one hears a nut grower express the opinion that a certain tree is carrying too many nuts for the crop to attain proper size and fill well, yet this is very often the case. Furthermore, the production of a large crop of poorly filled nuts one year is almost certain to result in a light crop or none at all the following year. There is a very close inverse relation between the size of the crop produced and the degree to which the nuts are filled at harvest, namely, the larger the crop the less the nuts will be filled. It has been pointed out above that nuts are storage organs, and the food materials required to grow and fill them must be made in the leaves. When too many nuts are set and carried through to the filling period, in proportion to the number of leaves or the leaf area of the tree, it is not possible for the leaves to synthesize the large amount of food materials required to fill the nuts. In pecans, for example, it has been shown that six to eight leaves are required normally to fill a nut properly and 10 or more leaves per nut if the tree is to flower and set a crop the following year. Other ratios for number of leaves or leaf area exist with other kinds of nuts. It is general experience that large crops of nuts remove such large amounts of food materials and minerals from the trees that a light crop or no crop at all is produced the following year. This is especially true if the nuts are not especially well filled in the "on crop year."
+Size of nuts:+ Almost everyone prefers large nuts to small ones, and that is one reason, why the larger sizes command a higher price on the markets. Many remember how popular the McCallister hican was a number of years ago because of its extremely large size. Such varieties of the pecan as Nelson and Mahan were very popular because the nuts produced were generally much larger than those of other varieties. These varieties remained popular until experience in growing them showed that they were very often poorly filled at harvest. As a general rule, large nuts are more difficult to fill properly than small nuts. This is obvious, because much more food material must be made by the leaves and transported to fill the kernels of large nuts than is required to fill an equal number of nuts of smaller size. In seasons with conditions favorable for both tree growth and growth in size of the nuts, it is often the experience that the nuts are poorly filled at harvest. On the other hand, if the weather is dry during the period in which the nuts are growing in size, they are much more likely to be well filled at maturity. In fact, the writer has seen several instances in which, because of severe drought in the spring, pecans were undersized, yet the kernels developed and filled so well that the shells of the nuts cracked at maturity.