Nitrogen may be supplied in various other forms: Raw or steamed bone may furnish 3 to 6 per cent of nitrogen, although the nitrogen in this form becomes available very slowly; dried blood, which contains from 6 to 14 per cent of nitrogen, decomposes rapidly; ground fish, which contains 7 to 8 per cent of nitrogen, is used extensively in some trucking regions. Tankage varies greatly in composition, ranging from 4 to 12 per cent of nitrogen; cottonseed meal decays rapidly and ranks with blood in the availability of its nitrogen, but its high value as a cattle food is rapidly reducing its use as a fertilizer; natural or Peruvian guanos were formerly used very extensively, but the supplies are practically exhausted, so the material is now of little commercial importance. Nitrogen for all classes of vegetables should be derived from at least two different sources, usually including nitrate of soda.

106. The Use Of Phosphoric Acid

Phosphoric acid is most essential in growing cereal crops, but it is scarcely less important in vegetable gardening. Soils deficient in phosphorus fail to give large yields and the crops are also slow in maturing. In vegetable gardening, more importance is attached now to the use of phosphoric acid than ever before, and the gardener should not lose sight of the fact that most soils are lacking in this plant food.

Rock phosphates are the chief sources of supply. They vary from 12 to 18 per cent in available phosphoric acid. Raw and steamed animal bone are also in common use, and bone tankage is employed by some vegetable growers. Thomas slag, also known as iron phosphate and Thomas phosphate, which contains 15 to 20 per cent of phosphoric acid, is another desirable form.

107. The Use Of Potash

Potash is especially important for the root crops, as beets, carrots, turnips, radishes and parsnips. It also enters largely into the composition of many other vegetables. Applications are particularly important for sandy soils and for muck lands.

Muriate of potash is the most common form used by vegetable growers; but sulphate of potash is preferred for potatoes, and kainit is used sometimes by asparagus growers.

108. Rate Of Application

The rate of application of chemical fertilizers depends upon such factors as (1) the character of the soil, (2) the previous crop grown and the manner in which it was fertilized, (3) the moisture conditions, (4) the importance of early maturity, (5) the possibility of irrigation, (6) the amount of land available and (7) the character of the crop to be grown. Applications vary from a few hundred pounds to two or more tons to the acre. One ton to the acre for a single crop is considered liberal, although this amount is often exceeded. At Norfolk, Virginia, 3000 pounds are frequently applied during the season. While dangers attend the free use of fertilizers (109), the only sound business policy is to apply the various elements in fully sufficient quantities. Regarding this matter, Dr. Edward B. Voorhees says (Voorhees, Fertilizers, p. 264) : "Apply a reasonable excess of all the essential fertilizer constituents to all of the crops. Nevertheless, because of the peculiarities of growth of the different plants, as well as the different objects of their growth, distinctions should be made in reference to the kinds and amounts of plant food applied, and these conditions should be borne in mind, in order that the most profitable results may be secured." (See notes on fertilizers in Chapter XXI (Cultural Directions. Artichoke, Asparagus).)

109. Dangers Of Excessive Applications

When approved methods of soil management are followed, practically no danger attends the use of large amounts of commercial fertilizers. If the amount of vegetable matter decreases and the soil acidity increases, several difficulties may develop. The soil may become hard and unproductive, not because of large applications of fertilizers, but because of small additions of humus. With the continued application of large quantities of mineral fertilizers the soil may become extremely acid, when leguminous cover or green manurial crops cannot be grown successfully. Malnutrition diseases sometimes develop in strongly acid soils, especially if humus is deficient.

The causes and the control of malnutrition disease have been studied by the Virginia Truck Experiment Station. When the disease is present the following symptoms may be observed (Va. Truck Expt. Sta. Bul. I, p. 5): (1) "The plants stop growing when they should be making their most rapid development. In many cases they slowly weaken and die, while in others growth is resumed later in the season after rains have occurred. (2) There is a change of leaf color to a lighter green, especially in the spaces between the veins, which turn yellowish-green or even brown. In cabbage the margins of the leaves are frequently of a uniform yellow color. (3) The roots of the affected plants are poorly developed. Many of the lateral feeders are killed back repeatedly, until the root system becomes stubby. (4) No fungi or bacteria can be connected with the disease. In most cases none is present."

In the Norfolk region 3,000 pounds of fertilizer to the acre are often applied during the season's operations and this amount is used year after year, resulting in a strongly acid soil. The Virginia Truck Experiment Station states also (Va. Truck Expt. Sta. Bul. 1, p. 5) that "acid soils are less favorable for the production of most truck crops than neutral soils. A slight amount of acidity is not ordinarily injurious, but examinations made at our request by the Bureau of Soils of samples from fields where cabbage suffered from malnutrition, showed these soils to be abnormally acid, so much so that 3,500 to 6,300 pounds of lime would be required to neutralize an acre to a depth of 1 foot. This condition is apparently the result of many years of intensive trucking, involving the use of repeated heavy applications of commercial fertilizers made up in large part of chemicals which leave the soil more acid.

"Only a portion of the fertilizer applied is actually taken up by the plants, the remainder being left in a different form, which will have an influence on the soil reaction. For example, sulphate of ammonia, muriate and sulphate of potash, and acid phosphate tend to leave the soil more acid, while nitrate of soda, carbonate of potash and Thomas phosphate tend to make the soil alkaline. In the brands of fertilizers most used in this section, the acid-forming ingredients largely predominate.

"One of the most important factors contributing to malnutrition is the exhaustion of the organic material in the soil. Fields where this disease occurs are found to contain only 1.65 per cent organic matter, while normally 3 to 5 per cent should be present.

"This deficiency is to be expected from such complete dependence on commercial fertilizers, which cannot take the place of stable manures and green manures in a permanent system of agriculture.

"Remedial and preventive measures recommended for malnutrition diseases are: (1) Limitation of the amounts of fertilizer used; (2) adjustment of the composition of the fertilizer to suit the crop requirements; (3) the rational use of lime; (4) the maintenance of the organic matter of the soil."