Of the three elements supplied by commercial fertilizers to garden crops, nitrogen is more frequently the limiting factor than either potash or phosphoric acid, because in cultivated soils it is lost more quickly and, also, because it is more expensive to buy, and therefore likely to be supplied in less abundance. Nitrogen plays the most important part in the growth of leaves and succulent.stems, and therefore it is particularly valuable in the production of such crops as cabbage, lettuce, brussels sprouts, spinach, kale, swiss chard, endive, celery, sweet corn and asparagus. Also, large amounts of nitrogen are very necessary in growing onions.
Nitrogen is valued largely according to its availability. For this reason nitrate of soda is considered by commercial growers the most valuable nitrogenous fertilizer. It is available to the plant as soon as dissolved, while stable manures, dried blood, tankage, bone meal and other organic materials must rot or decay before they are of any benefit as plant foods. For the money invested probably no other fertilizing material is capable of giving such large net returns, provided all conditions are satisfactory for its use. Many experiments have been made at the New Jersey station, and one of the bulletins (N. J. Sta. Bul. 172, p. 11), which reports the results, contains the following statement: "It is quite possible to have a return of $50 an acre from the use of $5 worth of nitrate of soda on crops of high value, as, for example, early tomatoes, beets, cabbage, etc. This is an extraordinary return for the money invested and labor involved; still, if the value of the increased crop from its use was but $10, or even $8, it should be regarded as a profitable investment, since no more land and but little more capital were required in order to obtain the extra $5 or $3 an acre. It is the accumulation of these little extras that oftentimes change an unprofitable into a profitable practice".
The amount of nitrate of soda applied to the acre at any one time may vary from 100 to 250 pounds. Larger applications are sometimes made, but they are of doubtful economy. The better and safer practice is to make frequent applications of smaller quantities.
There are no rules concerning the frequency of applications, but it depends upon the fertility of the soil, the character of the crop and the time of planting. Nitrate of soda is especially valuable for early spring applications before soil nitrification becomes active. If used when the ground is cool it may be the means of encouraging a rapid growth when all other agencies fail. A common practice is to see that some nitrate is contained in the fertilizer applied before planting. After two or three weeks a second application of the nitrate can often be made with profit, and additional ones are frequently advantageous. For more specific information on this question see Chapter XXI on the cultivation of different classes of vegetables.
The following methods may be employed in applying nitrate of soda: (1) It may be applied alone or mixed with other fertilizers before sowing or transplanting. If the soil does not contain a large percentage of sand, the loss from leaching is not likely to be serious. (2) As a top dressing around the plants or along the rows. This may be done by hand or when space will permit with a side-delivery fertilizer distributor. (3) In dry weather it is an advantage to open furrows along the rows, distribute the nitrate in the furrows and then close them with a small shovel or hiller; the same purpose may be accomplished by the use of a drill; or even by cultivation, after an application on the surface, the fertilizer may be mixed thoroughly with the moist soil. (4) The quickest and easiest way to apply this salt is to sow by hand with a full swing of the arm, as when sowing clover seed, letting the fertilizer fall where it will. Many gardeners who have adopted this labor-saving plan claim that it is safe in fertilizing plants, even those which have tender leaves, provided the foliage is perfectly dry, for every crystal that strikes the leaves naturally rebounds or glances to the ground. The broadcasting method is safe for cabbage, even if the salt lodges in the axils of the leaves. (5) Nitrate may also be dissolved in water and then applied by means of a hose; or better, by the Skinner system of irrigation. One ounce of nitrate to one gallon of water is the proper proportion for most purposes. If there is fear of burning the foliage, clear water may be sprayed on the plants after the solution of nitrate has been applied.
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.