Most of the experiments in this orchard have now completed 25 years; there have been few changes and these minor ones. Certain lessons may be drawn from this quarter century of research:
1. The fertility of an orchard soil is more than its plant food content. It involves the nature of the soil, its depth and topography, its previous treatment, the use of fertilizers and manures, the amount and nature of the cultivation and the covers or sods grown. Fertilizers are only part of the problem of soil fertility.
2. In this orchard any treatment that has influenced the trees at all has done so in the following order: first, the cover crops; perhaps several years later, leaf color; shortly after, branch growth and circumference increase; and last of all, yield.
3. The reason for this sequence of results is that the treatments, whether chemical fertilizers, manure, or cover crops, have influenced yields chiefly by changing the organic matter content of the soil; that is, those treatments which have resulted in the production of larger cover crops have ultimately resulted in the production of more fruit.
4. The organic content of the soil has been a considerable factor in determining the amount of water in this soil. Those treatments which have built up the organic content have kept the soil in condition to soak up rainfall rather than to lose it by surface run-off. A larger water supply, in turn, has produced more cover crops.
5. The site of this orchard seems nearly level to the casual eye; yet slope, with its accompanying erosion, together with differences in depth of soil, have created nearly as large differences in growth and yield as any treatment. Good treatments have nearly offset the initial disadvantage of poor soil; but it is more economical to plant the orchard on good soil than to attempt the improvement of a poor soil.
6. A short, non-legume sod rotation is an efficient means of building up a depleted orchard soil. After a sod of any kind becomes thick tree growth is checked and yields decline. Orchard sods should be turned under or partially broken, frequently.
7. Moisture conditions often are more favorable in the sod orchard than in the cultivated orchard. Runoff is checked by a sod and less water is used by a sod in mid-summer, after it has been mowed, than by a heavy cover crop.
8. Under a non-legume sod the soil nitrate supply becomes very low in late May or early June, necessitating early applications of nitrogenous fertilizers. Annual applications of 10 pounds of nitrate of soda per tree, or its equivalent in sulphate of ammonia or other forms, have proved profitable in this orchard. Superphosphate, in light applications, has increased sod and cover crop growth.
9. Trees receiving annual tillage with July seeding of cover crops have not done as well as those under sod rotations. If the cover crops are seeded in early June, as has been practiced since 1929, the difference may not be marked.