In a year when there is a full crop of fruit on the trees cultivation should be continued as late as possible as all the stimulus that can thus be secured will be necessary to help the fruit attain good size and maturity, and at the same time enable the tree properly to mature its fruit and leaf buds for the following year. On the other hand, in a year when there is not a full crop of fruit cultivation should be stopped early so as to avoid forcing a too rank growth of wood and foliage and continuing the growth of the next season's buds so late that they may not mature and therefore may be in danger of winter killing. The different kinds of cover crops which may be used in the apple orchard will be considered in the next chapter as they are so closely associated with fertilization. Strictly speaking, however, a cover crop is used principally to secure its mulching and physical effects on the soil in the intervals between the seasons of tillage. In addition to its physical and feeding effects the cover crop serves to check the growth of trees in the latter part of the season by taking up the nitrates and a part of the moisture, thus helping to ripen the wood. SOD MULCH.--The ordinary sod culture which is practiced in so many orchards should not be confused with the sod mulch system. The one is a system of neglect, the other of intention. In the sod mulch system the grass sod is stimulated and encouraged and when the grass dies or is cut, it is left on the ground to decay, forming a soil mulch meanwhile. The removal of grass from the orchard as hay is poor practice and should be discouraged. The grass mulch may well be supplemented by the addition of other grass, straw, leaves, coarse manure, or other similar materials. Sometimes this mulch is put on to the depth of six inches or even a foot around the tree. Thus practiced it is very effective in conserving moisture and in adding the humus which is so necessary to the soil. Sod and tillage have somewhat different effects on the tree and on the fruit. Let us see what these effects are. It is common knowledge that fruit is more highly colored when grown in sod than when grown under a tillage system. This is probably largely due to the fact that tillage keeps the fruit growing so late that it does not mature so well or so early. Fruit is usually two or three weeks later in tilled than in sod orchards. It has been shown that fruit grown under tillage keeps from two to four weeks longer than that grown in sod. It is claimed also--but this is a disputed point--that tilled fruit has a better quality and flavor. Certain it is that fruit grown in sod is drier and less crisp and juicy.