+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.
The question may be asked, why is it so important that nuts be well filled? The answer is very simple, because the quality of the oily nuts is determined by how well the kernels are filled. All but one of our most important nuts—almonds, filberts, hickory nuts, pecans, and walnuts—are oily nuts; and well filled kernels contain from 50 to 75 percent or more of oil, depending upon the species. Chestnuts are starchy nuts and contain less than one percent of oil. The relationship between the degree of filling and the composition of the kernel in oily nuts is outstanding, in that the better filled nuts have a higher content of oil and a lower content of protein, carbohydrates, water, and undetermined constituents than do poorly filled nuts. Highest quality of the kernels is directly associated with highest oil content and highest degree of filling. Nut kernels that are poorly filled are often hollow, shrunken, shriveled, and chaffy. When eaten they may taste sweet, but are lacking in the oily flavor characteristic of the particular species of nut eaten. It is only in the best filled nuts that highest quality, flavor, and oil content are found.
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.
+Condition of leaves:+ To produce well filled nuts, nut trees must bear a large leaf area and the leaves must be in good health and vigor. If they are to produce annual crops, the trees must carry their leaves until cold weather in the late fall, undamaged by insects or diseases. The importance of a large leaf area free from injury or abnormal condition is so great that it can hardly be overemphasized in connection with nut production. It can be definitely stated that under normal conditions the size of the crop produced and the degree to which the nuts are filled is directly related to the leaf area and the length of time it is carried by the tree.
If the leaf area is to be large, the trees must make good, strong, vigorous shoot growth, and this means that proper attention must be given to fertilization to insure that the trees have adequate amounts of nitrogen, potassium, magnesium, zinc, and boron, as well as all other essential elements. The elements mentioned have been found most likely to be deficient in the soils of eastern and southern United States. In those regions their lack may be expected most frequently to limit tree growth or the filling of the nuts because of their effects on the leaves and the consequent inability of the leaves to make food materials. Deficiency of one or more of these elements results in leaves that are not able to make food materials in anywhere near such amounts as do normal leaves well supplied with all essential elements. In severe cases, deficiency of one or more of these elements results in chlorosis of the leaves, still later in leaf scorch, and finally in premature leaf fall. Trees having leaves in such condition cannot be expected to fill the nuts borne by them.
Most nut trees grown about home or farmstead are deficient in nitrogen, as the trees must compete with grass, weeds, shrubbery, or other trees. Frequently there is not enough plant food for all. A deficiency of nitrogen limits the growth and the leaf area carried by a tree. A deficiency of potassium or magnesium very greatly limits the amount of food material made by the leaves and hence greatly decreases the filling and the oil content of the kernels. Zinc or boron deficiency has a similar effect.
Hence, to insure the production of well filled nuts, one must be certain that the trees are well fertilized and that the fertilizer elements applied are in proper balance one with the others.