The following information on manures comes from Five Acres and Independence by M. G. Kains. Five Acres and Independence is also available to purchase in print.
Unquestionably, the best of all fertilizers is stable manure because as it decays it supplies not only all its mineral elements but nitrogen and humus. It is in partially digested form and is full of micro-organisms which help to unlock other plant food held in the mineral compounds of the soil. Unfortunately it is scarcer and more costly than before the days of autos and tractors, though, for small garden work, it may be had in dried, pulverized form through garden supply stores or direct from the manufacturers.
Though drying concentrates these manures and kills the weed seeds most of them contain, it also destroys the bacteria contained in the fresh product, but this loss is offset by the almost odorlessness and convenience of applying the dried product.
Soils fertilized by manures and other natural products will grow better crops than those enriched with chemical fertilizers, unless these latter are supplemented by humus supplied in some way. But this remark must not be taken as condemning chemical fertilizers. (Chapter 25.)
Whether manure shall be applied while fresh, after rotting or in compost will depend upon various considerations. When the aim is to improve the physical condition of a heavy clay soil fresh manure will give best results, especially when plowed or dug under during autumn. It generally decays before spring and as it does its products of decomposition dissolve hitherto insoluble soil minerals. When the aim is to improve the physical condition of light, sandy soils well decayed manure applied in spring will give best results because its plant foods are more quickly available than those of fresh manure.
Decomposition is slow in heavy soils because the entrance of air and water is slower than in light ones. Hence in heavy soil no effect may be noticed for a year but the effects will last longer when they do start because there is less loss of plant food by leaching than in light sandy soils. Unless the season is dry and the supply of moisture scanty the plant food in fresh manures applied to sandy soils becomes available about as fast as the plants can take it up. If well decayed manure is applied to them its nitrogenous compounds may be washed out of the soil before the plants can utilize them. Clay soils do not waste food in this way but absorb and retain it. So though they require more work to get them in condition they are usually well worth the efforts made toward their improvement.
For ordinary vegetable crops a 2-horse load of manure to 2,500 square feet (50′ x 50′) is a liberal amount, though half this quantity will give fair results and twice as much will be best for crops grown for their foliage or stems—celery, spinach, cabbage, cauliflower, etc.
Always fresh and rotted manure should be applied to the surface before digging or plowing; dried, pulverized manures afterwards and then thoroughly rake or harrowed in the surface inch or so of soil. Liberal dressings of dried manures are: Sheep, 100 pounds to 1,000 square feet; poultry and pigeon, 75 pounds; horse, 10 to 150, cow, 150 to 200.
The crude materials upon which bacteria work consist of dead organic matter such as plant roots, leaves, stems, animal wastes and the bodies of dead animals. Higher plants cannot use these materials in their crude form; only after they have been converted into nitrates and other compounds as the final products of decay or their demolition by worms, bugs, molds, yeasts, bacteria and other forms of life, all of which play their parts in the process. When the quantity of crude material is abundant nitrate formation may be so great as to exceed the need of the growing crop; when deficient the crop may use it more rapidly than it is made and may even suffer because of this lack. To prevent such unfavorable results either nitrates must be supplied as fertilizer or the quantity of crude material in the soil must be increased.
As nitrates are developed from ammonia in the soil, applications of ammonium salts such as sulfate are often made—at the risk of making the soil more or less acid, unless lime is also added to neutralize the acid. When rotted manure is available it is better than ammonium salts because it supplies not only ammonia but considerable organic matter for the bacteria to work upon and for a much longer time. But as manure is scarce and often costly, green manures, especially legumes, are more available, cheaper and, when abundant, even more lasting in their effects. (Chapter 26.)
Unless soil conditions are favorable to decay—warm, moist and aerated—action will be slow. The physical condition of the soil must, therefore, be made favorable by such processes as plowing, harrowing and, where feasible and necessary, by irrigation. Organic matter such as manure and green manure plowed under when mature (as in straw and corn stalks) or when the soil is dry will decay much less quickly than if it is succulent and when the soil is warm and moist.
Until recently market gardeners near large cities usually considered 10 to 20 tons of stable manure to the acre annually necessary to produce good vegetables. Whether more or less would be profitable has been mainly guesswork, especially since the auto is steadily lessening supplies and increasing prices. Experience has suggested that though yields were increased they did not always pay, also that under certain conditions moderate applications of commercial fertilizer paid better than did larger ones. However, as such cases were unrelated they were unconvincing; so the Maryland Experiment Station undertook a series of comparative experiments with six leading truck crops (cabbage, potatoes, spinach, corn, tomatoes and peas) on a silt-loam soil of fairly good fertility underlaid with clay to determine if possible the profitable amounts of manure and fertilizers necessary to maintain fertility. After 13 years T. H. White and V. R. Boswell have tabulated and detailed results in bulletin 309 from which the following specially significant statements are quoted:
All treatments were beneficial insofar as yield is concerned, varying from a mean percentage increase of 123% for 500 pounds of fertilizer up to 312% for 6 tons of manure plus 750 pounds of fertilizer. Eight tons of manure yielded 19.3% more than four tons; 12 tons yielded 13.9% more than 8 tons and 37.08% more than 4 tons, the lower yield being taken as 100% in each case of these comparisons. Considering the increasing applications…successively larger applications gave successively, although not proportionately, larger yields. But such increases are not always accompanied by greater profits.
Applications tended not only to maintain but increase productivity. Though chemical fertilizers alone, without addition of organic matter, seemed to maintain or even increase productivity, actually considerable amounts of crop residues and green manure crops of weeds were plowed under. It is questionable whether fertilizer alone would have given such results on a lighter soil or under management in which no appreciable amount of organic matter was turned under.
Because of the scarcity and high price of manure it is necessary to depend on other materials to a large extent to maintain high yields. The experiments show that there was no significant difference in yield between 4 tons of manure vs 500 pounds of fertilizer; between 8 tons of manure vs 1,000 pounds of fertilizer; between 12 tons of manure vs 1,500 pounds of fertilizer; 2 tons plus 250 pounds fertilizer vs 8 tons manure; 2 tons plus 250 pounds vs 12 tons manure; 2 tons plus 250 pounds vs 1,000 pounds fertilizer; 4 tons plus 500 pounds vs 12 tons manure; or 4 tons plus 500 pounds vs 1,500 pounds fertilizer.
These results strongly emphasize the fact that under the conditions of the experiment: 1, commercial fertilizers maintain yields as well as light to medium applications of manure and, 2, light applications of manure plus fertilizer are as effective as medium applications of manure alone or of fairly heavy applications of fertilizer alone. Furthermore, it was proved that certain manure-fertilizer combinations are superior to single treatments; for instance, 2 tons manure plus 250 pounds fertilizer yielded 33.19% more than 4 tons manure alone; 2 tons plus 250 pounds fertilizer yielded 38.50% more than 500 pounds fertilizer alone; 4 tons plus 500 pounds yielded 36.4% more than 8 tons alone; 4 tons plus 500 pounds yielded 19.17% more than 1,000 pounds fertilizer alone; 6 tons plus 750 pounds fertilizer yielded 32.8% more than 12 tons alone; 6 tons plus 750 pounds yielded 19.36% more than 1,500 pounds of fertilizer alone.
In emphasizing these results it must be remembered that the soil is a silty loam of fairly good native fertility and that the crop residues and some weed growth were turned under annually on all plots. The same amounts of manure might prove relatively more important on soils especially low in organic matter to which no plant residues were added.
After all, the experimenters conclude, the grower is interested in yields insofar as they increase his money (net) returns. They remark that the data show that manure produces desired yields, but does so at high cost if it must be bought. If manure is available at lost cost, or produced on the farm, there is no question as to its value. If it must be bought at a high price and hauled a considerable distance, it appears that under conditions similar to those described, it can be replaced more profitably by crop residues and commercial fertilizers. To these may also be added green manures and cover crops. (See Chapter 26.)