A chemical analysis of a soil consists in finding the amounts of nitrogen, phosphoric acid, potash, lime, magnesia, and humus that it contains. It may be carried further, and the other constituents determined. These materials, except the humus and nitrogen, are extracted from the soil by strong acids. The action of these acids is many times stronger than is ever brought to bear on the soil in its normal condition in the field. It is therefore impossible at present to draw any certain conclusions from the results of such an analysis that are applicable to field conditions.

If, however, an analysis shows only a very small amount of nitrogen, then one may conclude that the soil is deficient in this element and will probably be benefited by its application. But this may be as easily told by a simple inspection of the field while plants are growing. A soil deficient in nitrogen is constantly showing its condition in the plants. Short growth of straw and vine, failure to develop a full, dark-green color, and the growth of sorrel and ox-eye daisy, all tell as accurately as the chemist with all his skill that the soil lacks nitrogen. And it is the same with the other constituents. It is only when a soil is extremely deficient in certain plant-foods that an analysis shows the cause of the trouble.

The great majority of all soils, good and poor agriculturally, differ only in narrow limits as to their composition. Every soil that yields well does not contain more plant-food than one that yields less; on the other hand, many soils that give poor yields are often rich in plant-food.

Two samples of soil were recently examined in the chemical laboratory. On one of the soils alfalfa grows readily, on the other it has failed. It might seem that the cause could be discovered by analyzing the two samples. Following are the results: —

No. 1, that does not grow alfalfa

Nitrogen (N)        . . . 0.07 per cent

Phosphoric acid (P2O5) . 0.12 per cent Potash (K2O) . . . .0.14 per cent Lime (CaO) .. . .0.17 per cent Magnesia (MgO) . . 0.24 per cent Organic matter (humus) 3.45 per cent

No. 2, that grows alfalfa

Nitrogen (N) .... 0.07 per cent Phosphoric acid (P2O5) . 0.12 per cent Potash (K2O) . . . .0.13 per cent Lime (CaO) . . . .0.20 per cent Magnesia (MgO) . . . 0.22 per cent Organic matter (humus) 3.15 per cent

Soils have an average weight of 2,000,000 lb. per acre for a depth of eight inches, and the composition of the two soils by weight is as follows: —

No. 1

.07 N          = 1,400 lb. per acre.

.12 P2O5 = 2,400 lb. per acre. .14 K2O= 2,800 lb. per acre. .17 CaO = 3,400 lb. per acre. .24 MgO = 4,800 lb. per acre. 3.45 humus = 69,000 lb. per acre.

No. 2

0.12 P2O5 = 2,400 lb. per acre. 0.13 K2O = 2,600 lb. per acre. 0.20 CaO = 4,000 lb. per acre. 0.22 MgO = 4,400 lb. per acre. 3.15 humus = 63,000 lb. per acre.

It will be seen that in chemical composition these soils are practically-identical, and yet one grows good alfalfa and one does not.

This shows that the chemical composition is not always the deciding factor in fertility. As a matter of fact, it is rarely the deciding factor. A soil that showed higher amounts of plant-food than in the cases cited above gave very low yields. A good system of tile drains was put in this field, and three years later the crops were very large. The draining produced no differences in the chemical content, but it brought success. Failure may be due in other cases to poor tilth, acidity, bad rotations, and various physical causes.

Chemical analyses of soils are valuable mainly to assist in conducting investigations of a scientific character. With the present methods they are of little use as a means of deciding what fertilizer should be applied. The farmer should experiment with different fertilizers, and not depend on a chemical examination of his soil, unless he has reason to think that he has a very special problem. The widespread notion that chemical analyses of soil and of plant will tell what fertilizers to add and what crops to grow is erroneous.

Field tests to determine fertilizer needs may be made as follows: —

The field should be plowed before the plats are laid out. Then use substantial stakes at the corners of the plats and mark them well. It would be well to leave a space of 4 feet between each two plats, to be sure that the plants on one plat cannot feed on the fertilizer each side of it.

Do not lay out the plats on land that has been manured within one year. If you made fertilizer experiments last year, do not use the same set of plats again this season.

The following diagram shows the arrangement of the plats, with the spaces between, each plat containing 1/20 of an acre: —

1. PlatK.

15 lb. Muriate potash

100 lb. lime on this half

2. Plat N.

15 lb. nitrate soda

100 lb. lime on this half

3. Plat P.

30 lb. super phosphate

100 lb. lime on this half