Manure on Potatoes.—There are excellent cash crops that may get more than their fair share of the farm supply of fertility, and against the interest of fields in the farm not adapted to cash crops. The justification is found in the farm ledger. In some regions potatoes are the best crop in point of net income per acre, where the acreage is kept restricted so that there may be plenty of organic matter to help in conserving moisture. It is not good practice to use fresh manure, and especially that from horse-stables, for potatoes. A heavy application makes an excessive growth of vine, and the yield of tubers suffers. A stronger deterrent is the effect that fresh manure has on the development of the spores that produce the disease known as potato-scab. Rotted manure is less dangerous, and few crops repay its use in higher degree than the potato. Some growers prefer to make heavy application of fresh manure to grass for corn, and follow with potatoes so that they can profit by the rotted organic matter that remains. In this way the physical condition is made excellent, moisture is well held in a dry season, and commercial fertilizers can supplement the plant-food left in the manure.
When to plow Down.—Excellent farmers differ regarding the relative efficiencies of manure plowed down and that mixed with the top soil. Both classes may be right for their individual instances. The plowing down of manure helps to deepen the soil, and that always is desirable. It causes plants to root deeply, and that is a distinct benefit in a drouthy season, and always desirable. When a soil is in such tilth that the breaking-plow always brings fertile soil to the surface, the plowing down of manure gives excellent results, though it should be permitted to leach at the surface for a few weeks before being turned under. When land is being prepared for a seeding to grass or clover, the supply of manure should not be plowed down wherever the breaking-plow brings soil to the surface that is deficient in humus. In the latter case the manure always should be used as a top-dressing, and should be evenly spread and well mixed with the surface soil. It is needed there far more than it can be needed farther down. The surface soil always should have a high content of organic matter.
Heavy Applications.—When the farm supply of manure is small, applications should be light. The manure should not be the dependence for plant-food on a part of a field, or a single field of the farm, under such circumstances. It is more profitable to give a light dressing to a larger area. The manure is needed to make a fertilizing crop grow, and a very few tons per acre can assist greatly, when rightly used. The manure is needed to furnish bacteria to the soil, and a small amount per acre is useful for this purpose. Always there is temptation to use all the manure on a field convenient to the barn, and to concentrate it on a sufficiently small area to make a good yield sure. The loss to the farm in this method is heavy. The thin spots and the thin fields have first right to the manure as a top-dressing, and six tons per acre will bring larger returns per ton than twelve tons per acre. At the Pennsylvania experiment station the land receiving ten tons of manure per acre in the common four years' rotation of corn, oats, wheat, and mixed clover and grass gives added returns of $1.63 a ton, while an application of eight tons pays $1.85 a ton, and a six-ton application brings the value per ton up to $2.41. These applications are made twice in the four years.
Reënforcement with Minerals.—A ton of mixed manure in the stable contains about ten pounds of nitrogen, five pounds of phosphoric acid, and ten pounds of potash. This makes the percentage of nitrogen and potash the same, while the percentage of phosphoric acid is only half as high. A commercial fertilizer of such percentages would be esteemed a badly balanced one. Certainly the phosphoric acid should be relatively high, as this constituent of plant-food runs low in the soil. If 50 pounds of 14 per cent acid phosphate were added to each ton of manure while it is being made in the stable, seven pounds of phosphoric acid would be added, making the percentage in the manure a little higher than that of the nitrogen and the potash. A better balance is given to the fertility. There cannot be any loss in this purchased plant-food, if the stable floor is tight. Fermentation cannot drive it off, and when applied to the soil it is tightly held. Practically no phosphoric acid is found in drainage waters. Eight tons of manure thus reënforced would contain the same amount of plant-food as a ton of fertilizer having 4 per cent nitrogen, 5 per cent phosphoric acid, and 4 per cent potash. The addition of the 50 pounds of acid phosphate per ton does not bring the phosphoric acid content up as high relatively as in most commercial fertilizers, but it helps. The total amount in the eight tons manure may be sufficient, and the greater part of the total has sufficiently immediate availability, while the manure must undergo decomposition, and some of the nitrogen and potash does not become available within the year.
Durability of Manure.—Tests of the durability of manure in the soil involve some uncertain factors, but we are interested only in the effects of applications. These effects may continue for a long term of years, and an example will illustrate. Land may be too infertile to make a good clover sod. If a good dressing of manure be given half the land, affording proper conditions for making a sod, the result will be a heavy growth of clover, while the seeding on the unmanured half will be nearly a failure. If no manure or fertilizer be used in the crop-rotation, the probability is the manured portion of the field will again make a fairly good sod. How much this success may be due to the remains of the manure, and how much is attributable to the effect of the clover and to better bacterial life introduced and favored by the manure, no one knows. Probably the greater part of the benefit comes only indirectly from the manure applied three or four years previously. Half of the field may thus be lifted out of a helpless state and remain out of it for a long term of years, while the other half grows only poorer. A probable illustration of this lasting indirect effect may be seen in one of the plats in the soil fertility experiments on the Pennsylvania experiment station farm.
Experiments at the Rothamstead station, England, show some lasting results from applications of manure. Director Hall cites the case of one plat of grass land which was highly manured each year from 1856 to 1863, and has since been left unmanured. In 1864 this plat gave double the yield of an adjoining plat which had been left unmanured during the eight years. In 1865 the plat, last manured in 1863, gave over double the yield of the unmanured. In the following ten years its yield was a half more than that of the unmanured. In the next ten years the yield was a quarter more. In the next ten years it fell to 6 per cent more than the plat that had received no manure in the beginning of the experiment. In the following ten years it rose to 15 per cent. Here is a lasting effect of manure for over forty years where grass was grown continuously.
CHAPTER XV
CROP-ROTATIONS
The Farm Scheme.—Notwithstanding some of the theorizing that does not commend itself to the practical man, farm management is taking on the form of a science. It involves the organization of a farm for best results, and in the scheme that should be worked out for any particular farm the most important feature is the crop-rotation. The selection of crops is controlled by so many local considerations, including the personal likes and dislikes of the farmer, that very rightly the kinds of rotation are innumerable. The order in which crops may be grown with most profit is less variable, and yet even here local conditions may quickly derange the scheme of a theorist. There is, however, such right relation of facts to each other that we are getting a working philosophy, and the individual farmer can bend practice to his own liking in considerable degree, and yet not compel plants to do their part at a disadvantage. He has much liberty in the order of their growing, without endangering profits materially. Theoretically, this is not true, and the factors of production on any farm are such that the largest return is obtainable in only one scheme of farming. Practically there is rather wide liberty.