The three essential elements (nitrogen, phosphorus, and potash) together with the “agents” are secured by plants from the soil, and these are termed collectively “plant foods.”
Manures. Manures may be included under this heading, and, strictly speaking, they furnish a measure of plant foods to the plants. However, their chief value lies in the supply of better texture, more water-holding capacity, greater aeration, fermentation and its action upon soil elements, and the introduction of the necessary minute organisms which are instrumental in changing unavailable compounds into usable forms. The actual plant-food value of manure is low unless it is used in large quantities, and even then because of the leaching and the “burning,” many of the nutrients are lost.
Peat. Domestic peats may be added to the category of plant foods, since it has been proven frequently that the nitrogen they contain becomes available to the plants providing the fiber-decomposing bacteria are introduced either into the soil or the peat. This is easily accomplished without any inoculation cultures, thru the addition of small quantities of stable manure (1 part to 20), which contains many of these bacteria. Decomposed straw, leaves, and litter may also be used as plant food supplying the necessary elements in doses similar to those of manure. These artificial manures can be made readily thru the use of complete fertilizers and lime incorporated in the straw, which is kept wet frequently enough to secure rapid decomposition.
Balanced Rations. Strictly speaking, the term “plant food” applies to the balanced commercial fertilizers which are being sold by many dealers.
These commercial plant foods are composed of nitrogen, phosphorus, potash, and some of the lesser elements in proportions which have been determined experimentally and practically to be of greatest general application. The forms in which the prime elements are carried may be of mineral or organic origin.
Altho all plants do not require soils of the same composition, nor are they satisfied with the applications of a well-balanced plant food, to save confusion in most instances it is best to use these balanced rations.
No sane person would endeavor to consume enough food at one sitting to last for a week, and yet we hear frequently of plant food applications of astonishing proportions and invariably disastrous results.
A DISCUSSION of plant foods would not be complete without mention of the carriers of the important elements and their desirability. Nevertheless, complete plant foods are easier to get and simpler to use.
Nitrogen. Nitrogen is obtainable in the form of chemical compounds, such as nitrate of soda, ammonium sulphate, calcium nitrate, and such strictly organic substances as blood, tankage, cottonseed meal, and soot. Pulverized sheep, hen, and rabbit manures are also often advocated for the same purpose. The strictly chemical compounds are very quickly available and should be applied lightly or in liquid form. The others are more slowly soluble and may be used in heavier doses.
The first group may be applied at the rate of 1 pound to 100 square feet if in dry form, with the exception of the extremely strong synthetic nitrogen compounds which should be dissolved at the rate of 1 ounce to 7 gallons of water. Most others can be used at the rate of 1 ounce to 2 gallons of water. The organic substances may be used at 3 to 5 pounds per 100 square feet, one application being sufficient during the season, whereas the others (nitrate of soda, ammonium sulphate, and so on) need be applied several times during the season. All plant foods, particularly those of nitrogenous nature, should not come in direct contact with the foliage of the plants, as burning is likely to follow. This danger is reduced if the foliage is washed off immediately after the application.
Phosphorus. We hear and read so much about bonemeal that many have come to believe that this is the only plant food which is safe and worth using. Its factor of safety cannot be denied, since it is almost impossible to overdose plants with it. The reason is not far to seek bonemeal is extremely slowly available, giving off its main constituent phosphorus only after a prolonged sojourn in the soil. We know, however, that phosphorus alone does not constitute a balanced ration, no more than the eating of fats alone can be construed as a scientific human diet. Bonemeal does contain a small amount of nitrogen. We find that where phosphorus is the limiting factor of growth, that is, where plants will languish without it, the use of superphosphate is to be preferred to bonemeal. It is more quickly available and is much cheaper. However, phosphorus is present in fairly high amounts in a balanced plant food, and additional applications may be needed only in specific instances. Phosphorus is also found in tankage and other slaughter house byproducts. The usual rate of phosphorus application is 5 pounds to 100 square feet.
Potash. Potash comes to us usually in three common forms potassium sulphate, potassium chloride, and wood ashes. The first two are extremely soluble and should be used when the plant is in a vigorous state of growth. The ordinary amounts recommended are 2 pounds to 100 square feet. Wood ashes if unleached are safer to use, but fully four or five times the foregoing amount is necessary to equal the action of the chemical ingredients. Potash is frequently lacking in light soils and applications of the material are necessary in addition to the balanced plant foods which may have been used.
Lime. Little has been said about lime and its effects. This mineral (calcium) has a very important function in the soil by changing its texture and especially by reducing acidity and by making nitrogenous materials quickly available. The last factor often makes applications of lime undesirable. It should not be mixed or used together with animal manures, ammonium sulphate, tankage, blood, and others. It should not be used when it is mixed with superphosphate, making this material less available. We find also that most of our garden plants are either indifferent to a slightly acid (sour) condition of the soil or they actually prefer this acidity. As a consequence, our worries about liming the soil should be limited only to conditions where great acidity exists. This acidity may have to be corrected thru the application of lime in one of several forms. But before attempting this correction it would be well to test for acidity by means of a soil-testing outfit. If the need for lime is unquestioned and the reduction of the acidity desired by the plant, then lime should be used.
Lime comes to us in the form of quicklime, water-slaked lime, air-slaked lime, and ground limestone. Quicklime and water-slacked lime produce a very quick reaction and cause rapid liberation of nitrogenous materials and at the same time are helpful in changing the texture of clay soils. Air-slaked lime and ground limestone are less active and are usually preferred, altho they do not become as effective in clayey soils as the first two mentioned. No rules can be given for the amounts of applications, since they depend entirely upon the degree of acidity of the soil.