UNIVERSITY OF CALIFORNIA AGRICULTURAL EXPERIMENT STATION COLLEGE OF AGRICULTURE Benj. Ide Wheeler, President BERKELEY, CALIFORNIA Thomas Forsyth Hunt, Dean and Director CIRCULAR No. 98. PLOWING AND CULTIVATING SOILS IN CALIFORNIA. By ClTAS. B. LlPMAN. OBJECTS OF PLOWING AND CULTIVATING. Careful observation and experiment have amply' shown that plant roots must have air to develop successfully. They have also shown air to be essential for the life of all of the beneficial soil bacteria upon which we depend for the solution of plant food in the soil, and for the addition of nitrogen from the air. In other words, the most vital factors in successful plant growth are dependent on a good supply of air in the soil. Such a supply of air can best be furnished and maintained through plowing and cultivating. Besides accomplishing the objects above named tillage operations are important, because they make pos- sible the turning under for humus formation of organic manures, and for turning under other forms of organic matter and other fertilizers. They keep down weeds, they make a larger reservoir for water, and they prevent evaporation of water from soils. REASONS FOR THE NEED OF DEEP PLOWING IN CALIFORNIA. California's celebrated deep soils, and the need for deeply plowing them, have become understood through a careful study of the climatic conditions of this and other arid regions, or those in which every year has a long period of several months of drouth. The most striking fact learned in these studies is that clay forms very slowly under arid con- ditions. It is produced from feldspar minerals when certain elements (like sodium and potassium) are leached out of them. It is easy to reason, therefore, that if clay can only be formed from the leaching action of water on feldspar minerals through which certain elements are washed out, that where there is not enough water to leach the minerals, the elements spoken of will not be washed out, and therefore clay will not be formed at all or only very slowly. But if clay forms very slowly there will not be anything in the soil to exercise a cementing action on the sand and silt particles. Further, if the soil particles are not cemented they must remain loosely scattered, and, therefore, air will circulate freely between them. Because of this fact it is possible for air to reach down to greater depths in the soil. This means that proper conditions are produced at greater depths in the soil for plant root development and bacterial growth. The deeper penetration of roots and soil bacteria again means that organic matter and nitrogen will be found at greater depths in the soil, from the decay of dead roots. All these agencies, together with air, will work toward making the same (1) kind of soil at several feet below the surface as it is at the surface ; the same in color, fineness or coarseness, amount of soluble plant food, and bacterial activity. These are the reasons for the depth of California soils in their important bearings, as shown by scientific investigations. What has been said above has an important bearing on our methods of tilling soils in California. For if the soil is just the same one foot in depth as it is at the surface, why not make use of the fertility of it instead of confining the roots of our plants to a thin layer of soil? We have nothing to fear from turning the deep soil up, because it supports plant growth as well as the surface soil, as is evidenced by newly graded soil in alfalfa fields and orchards in California where at times two and three feet of the surface soil are removed in grading. This is not true in soils of the East, because clay forms rapidly there under the heavy leaching action of continuous rainfall on feldspar minerals, and pro- duces a cold, light colored and inactive subsoil on which plants will not grow until it has been weathered for a year or two. It is, therefore, not only safe to plow deeply in California, but it is necessary for the maintenance of fertility, because a large and deep root system, the formation of which it encourages, will cover a large surface of soil particles and thus have a larger amount of food and water at its disposal. Deep plowing, therefore, is the first requisite to successful agriculture in California from the standpoint of soil management. HOW DEEP TO PLOW. By deep plowing we do not mean the same plow depth for every crop and soil. The following are general rules, which will serve as an efficient guide. For orchard crops plow eight or nine inches ; for grain, 8 to 10 inches ; in preparing land for alfalfa 12 to 14, or even 16, inches. For vegetables at least 10 to 12 inches are necessary, and for lima beans and sugar beets 12 to 14 inches will be better. In general, a shallower plowing will do for the crops above mentioned on sandy soils, but it is important to plow deeply silty clays, clay adobes, and, in general, soils which "run together" easily and bake. WHEN TO PLOW. Plow as early as possible in the fall preceding the planting, for potatoes, lima beans, beets ; also in preparing land for alfalfa and for grain. If the soil is not too heavy and will allow it, prior to early rains, plow in summer — especially in "summer-fallowing" for grain. In orchard practice it is best to plow in spring, and the plowing here will serve also for turning under the cover crop where that is used. Plowing may also be done in summer or early fall in the orchard if the soil is very heavy, and can then serve as a preparation for seeding the cover crop. On most soils, however, the spring plowing is sufficient for all purposes of aerating the soil and of turning under various fertilizing materials and weeds. CHANGE THE PLOW DEPTH EVERY YEAR. One of the causes which helps to form plow sole, plowpan, or irriga- tion hardpan, is plowing to the same depth every year. This is ex- plained in another circular, but it suffices to say here that we must therefore keep the subsoil layers from becoming hard and impervious. This can be done by breaking up the compacted layer formed by the sole of the plow in plowing. Such breaking up can be accomplished by changing the plow depth every year. Thus, if plowing to a depth of eight inches one year, plow to a depth of nine inches the next year, and go back again to eight the third year. In most cases this plan will prevent plow sole from forming. In very fine grained soils a subsoil plow may have to be used following in the furrow made by the ordi- nary plow to accomplish the same end. WHY WE CULTIVATE. Saving moisture in California soils is one of the most important factors to success in farming. Keeping good air conditions in the soil is another. Both of these things can be accomplished by covering the soil with a blanket or mulch. The latter cools the soil, holds the water in, and prevents the soil from packing, thus keeping up good air con- ditions. Anything that covers the soil will serve as a mulch. But while straw and manure, which are the most effective mulches, can be used on small acreages, they are too expensive and involve too much labor for large acreages. For the latter, therefore, we have found that we can make a very efficient blanket or mulch out of the upper layer of the plowed soil, by constantly cultivating it. This, frequently stirred, soil layer, which is loose and dry, acts like a layer of straw or manures. It is known as a dust mulch. Constant stirring is absolutely necessary to maintain a useful mulch. Wetting of the upper soil, especially by rain, compacts it, and the water evaporates from it very rapidly. Therefore, a soil surface wetted by rain or irrigation water must be cultivated as soon as it will not be puddled when stirred by the cultivator. But even when soil is not wetted it tends to become more and more compacted from the weight of its own particles. Therefore, whether dry or wet constant stirring of the surface soil in summer is imperative. WHEN AND HOW TO CULTIVATE. In orchards where heavy crops of fruit are borne, the writer would recommend cultivating every two weeks between the plowing under of the cover crop and the next sowing of it. Where no cover crop is growing, cultivating should be done with the same frequency through- out the spring and summer. This practice must be particularly empha- sized in orchards grown without irrigation. In cultivating vegetables more frequent cultivation than that employed in orchards is strongly recommended. It is always to be remembered that the cost of cultivation in different districts may vary — 4 — enough to make it advisable in some eases to cultivate more frequently than above recommended, and in other cases less frequently. The more frequently cultivation is carried out on summer-fallowed land consistent with the income from the same, the better. The writer would recom- mend at least one cultivation every three or four weeks. With reference to the depth of cultivating California soils it must be said that careful experiments have shown a 5-inch depth to be about the best. Six inches may be used where more than a 10-inch plow depth is employed. IMPLEMENTS TO USE IN PLOWING AND CULTIVATING. The best tool for plowing^ so far as the type is concerned, is the mold- board plow. Any form of it, simple or compound, as suited to the size of the ranches or the flat, rolling, or hilly" nature of the land will accom- plish the work better than other forms of plows. For some purposes the disc plow will serve best, however, and especially for very deep plowing, as in the case of preparing land for alfalfa, a disc plow of the Spaulding type will be particularly efficient. For cultivators use the lightest implements that will accomplish the task consistent with strength. The disc cultivators and harrows serve very well, but in fine grained soils the chisel cultivators will be found very efficient aids to the discs because they penetrate very deeply and serve to aerate the soil and keep it from packing. Light tools are recommended because they will not pack the soil so readily and help to form plow sole. For the same reason as few animals as possible should be used in plowing and cultivating as will efficiently and easily do the work. SUMMARY. 1. Proper plowing and cultivating are essential factors in successful soil management. Reasons are given above. 2. Deep plowing is a prime necessity in California soils. Eight to ten inches in depth is right for ordinary crops. Fourteen to sixteen inches in preparing land for alfalfa. Details given above. 3. The plow depth must be varied slightly every year to help prevent plow sole formation. 4. Constant cultivation is important on California soils. Once in two weeks in orchards or oftener. About once a month on summer- fallowed land. 5. The ordinary mold-board plow is best for plowing. Its shape must be suited to the topography and heaviness or lightness of soil. 6. Disc harrows and cultivators serve best for our silt and adobe soil. The ordinary tooth cultivator will do for very light soils. See details above.