Nitrogen promotes the growth of new wood and leaves, giving the latter a dark green color. In fact the color of the leaves and the amount of the wood growth are usually good indicators of the need of nitrogen. Nitrogen in excess develops over vigorous growth and prevents the maturity of wood and buds. It always has a tendency to delay the maturity of the fruit by keeping it growing late. On many varieties it tends to produce poorly colored fruits.
When trees are making a normal amount of growth in a year—say a foot to three feet or more—and when the leaves are of good size and a dark green in color, there is little need of nitrogen. But when trees are not growing satisfactorily and the leaves have a sickly yellow color, then the need of nitrogen is evident. On early soils and in long growing seasons nitrogen may be more freely and safely used than under other conditions.
The effect of phosphoric acid and potash on the tree and fruit is much more uncertain. They are supposed to influence the quality and the flavor of the fruit, giving better color and flavor, and this they undoubtedly do to some extent. Potash probably gives the leaves a darker green color. The precise effect of these two elements is at present a subject of much discussion, one set of investigators maintaining after a long and careful investigation that these effects are too small to be worth while, and the other claiming that they have a marked effect in the ways above indicated. The only safe guide is the actual local result. If the fruit is satisfactory in every way it will be of little use to try fertilizers. On the other hand, if it is not, then it will pay to experiment with them. The needs of and the results on different soils are so variable that it is always wise to experiment on a small scale before using fertilizers extensively.
Stable Manure.—The necessary plant food is best supplied by stable manure applied at the rate of ten loads per acre for a light application to twenty loads per acre for a heavy application. This amounts to a load for from two to five mature trees. Such an application will not only go far toward supplying the necessary nitrogen, phosphoric acid, and potash, but especially if coarse will add considerable humus and improve the physical condition of the soil.
Except on land which washes badly, manure should be applied in the fall and winter. It should not be piled near the trunk of the tree but spread uniformly over the entire surface of the ground. It is particularly important to spread the manure under and beyond the farthest extent of the branches as this is the most important feeding root area of the tree.
Commercial Fertilizers.—Where manure is not available or where it cannot be applied in sufficient amounts, commercial fertilizers may be resorted to, after they have been experimentally tested out. Leguminous cover crops are the best source of nitrogen, as has been indicated, but where these do not grow well, or in seasons when they have for some reason failed, nitrate of soda or dried blood are good substitutes. From two hundred to three hundred pounds of one or the other of these may be applied broadcast in the spring soon after growth is well started and all danger of its being checked by frost or cold weather is past. It is well to apply the nitrate of soda in two applications a few weeks apart, especially on soils which are leachy and in wet seasons, as part of the nitrogen may leach away if all is applied at once. These should be thoroughly worked into the soil with a spring-tooth harrow.
To supply the other two elements, from two hundred to four hundred pounds of treated rock phosphate or basic slag for the phosphoric acid, and the same amount of sulphate of potash for the potash, should be applied at any time in the early part of the season, preferably just before a light rain, and worked into the soil as before. Home-made wood ashes are a good source of both these elements, and especially of the potash. They cannot be purchased economically in any quantity, but on the general farm there could be no better way to utilize the wood ashes made around the place than by applying them two or three bushels to a full grown tree every year or two. Wood ashes are also a good source of lime, being about one-third calcium oxide. Thus a large amount of available plant food will be supplied to the tree, and where it is needed should result not only in better wood growth but in the formation of vigorous leaf and fruit buds for the following year.
Lime is not usually considered as a fertilizer except on soils actually deficient in it. But it will usually be advisable to apply from one thousand five hundred to two thousand pounds of fresh burned lime or its equivalent, in order to correct any natural soil acidity, to hasten the decay of organic material, to increase the activity of the soil bacteria, and to improve the physical condition of the soil by floculating the soil particles and helping to break up lumpy soils. Lime also helps to liberate plant food by recombining it with certain other elements in the soil. All these effects make a more congenial medium for the leguminous crops to grow in, and it is frequently advisable to use lime for this purpose alone. After this first heavy application about 800 pounds of lime should be applied per acre every four or five years.