Loss of Plant-food. - The soil is composed chiefly of material that never will enter into the structure of plants, but that serves us by affording a congenial place for plant-roots. It anchors the plants, holds moisture for them, and offers opportunity for all the processes necessary to the preparation of plant-food and to its use. In this material are the abundant supplies of such plant-food as silica, but, as has been previously stated, their very abundance leads us rightly to disregard them in our thinking. Our interest is only in the very small percentage of material that is composed of the four constituents which may be lacking in available form in the soil: nitrogen, phosphoric acid, potash, and lime. We believe that the only consideration that now need be given lime is as a soil-corrective and, when there is no acidity, we may assume that there is plenty of lime present. When yields of crops tend to decrease, the only plant-foods with which we are concerned are nitrogen, phosphoric acid, and potash.

The materials were stored in all agricultural land, and much of the supply is in inert forms. They help to make what we call the natural strength of the land. The rotting of organic matter, tillage, and many other agencies bring about some availability. The removal of crops, leaching, etc., reduce the supply. The right use of commercial fertilizers involves the addition of some plant-food when the available supply in a particular soil is inadequate.

Prejudice against Commercial Fertilizers. - The owner of land that was made very fertile by nature, and that has not been cropped long enough to reduce the supply of available fertility to the danger-point, rarely fails to entertain a prejudice against commercial fertilizers. It is the rule that he refuses to consider their use until the decrease in crop yields becomes so serious that necessity drives. If his land is not contributing its fair share of grain, vegetables, etc., to the markets, but has all its products converted into meat or milk, the supply of available plant-food may remain sufficient for so long a time that the matter cannot have any interest for him. If the land is producing some crops for market, there is reduction in its mineral store. It is the rule that the boundary of profitable use of commercial fertilizers pushes westward from the older and naturally poorer seaboard states about one generation after need shows in the crop yields. Lack of knowledge, the association of the use of commercial fertilizers with poor land, and some observation of the unwise use of fertilizers, combine to create a lively prejudice. They are viewed as stimulants only, and costly ones at that.

Are Fertilizers Stimulants? - Some words carry with them their own popular condemnation. We are accustomed to draw a sharp line between foods and stimulants, and to condemn the latter. To stimulate is to rouse to activity. Tillage does not add one pound of plant-food to the soil, and its office is to enable plants to draw material out of the soil. It makes activities possible that convert soil material into crops. Fertilizers add plant-food directly to the soil, and it is also to their credit that their judicious use favors increased availability in some of the compounds already in the soil. The greater part of the labor put on land is designed to make plant-food available, either by providing moisture, or ease of penetration of plant-roots, or activity of bacteria, or other means that will permit plants to remove what they need for growth. Fertilizers supply fertility directly and indirectly, but it is their direct service in meeting a deficiency in plant-food that affords all needed justification for their use by practical farmers.

Referring to the thirty years’ soil fertility experiments of the Pennsylvania station, Hunt says that they “show that there is nothing injurious about commercial fertilizers. For thirty years certain plats in this experiment have received no stable manures. No organic matter has been added to the soil except that which was furnished by the roots and stubble of plants grown. These plats are not only as fertile as they were thirty years ago, but they have yielded, and continue to yield, as good crops as adjacent plats which have received yard manure every two years in place of commercial fertilizer.”

Soil Analysis. - There is wide misconception regarding the value of chemical analysis of the soil as an aid in making choice of a fertilizer. Analysis has shown that some soil types are relatively richer in plant-constituents than are others, and it has shown abnormal deficiency in some types of limited area. It has given us more knowledge of soils, but as a guide to fertilization in particular instances it usually has no value. The samples used by an analyst are so small that the inaccuracy in his determination may easily be greater than the total amount of plant-food in a very heavy application of commercial fertilizer. A field that has been reduced to temporarily low productive power by heavy cropping or bad farming methods may show a greater content of plant-food than another field that is in a highly productive condition. This is a fact difficult of acceptance by some who want the aid of science, but such are the present limitations. The weight of a fertilizer application is so small in comparison with the weight of the surface part of an acre of land that the use of a ton of fertilizer may not be detected in the analyst’s determinations, and moreover his determinations of actual availability in the soil’s supplies are not serviceable in the selection of a fertilizer for any particular field and crop.

Physical Analysis. - Chemical analysis is costly and unsatisfactory as a guide to fertilization. Physical analysis by a competent man may have distinct value, and especially to one lacking experience with his soil. The mapping of soils by national and state authorities has given pretty accurate knowledge of hundreds of soil types, their location and characteristics, and when a soil expert obtains a sample of soil and the history of its past treatment, he can assign it to its type and give to its owner dependable advice regarding its crop-adaptation and probable fertilizer requirements.

The Use of Nitrogen. - There is no fully satisfactory way of determining the kind and amount of fertilizer that should be used at any particular time for any one crop. Perfection in this respect is no easier in attainment than in other matters. There are, however, means of arriving at conclusions that are a valuable guide.

In a general way, nitrogen is in scant supply in all worn soils. Wherever the cropping has been hard, and manure has not gone back to the land, the growth in stalk and leaves of the plant is deficient. The color is light. Inability of a soil to produce a strong growth of corn, a large amount of straw, or a heavy hay crop, is indicative of lack of nitrogen in nearly every instance.

The legumes, such as clover, and the stable manures are rich in nitrogen, and when the scheme of farming involves their use on all the land of the farm, no need of purchased nitrogen may arise in the production of staple crops. In the black corn soils the nitrogen content originally was high.

Lands that naturally are not very fertile rarely have enough available nitrogen. Where timothy is a leading crop, the demand for nitrogen is heavy. A cold spring or summer, checking nature’s processes in the soil, may cause a temporary deficiency in available nitrogen in land that usually has a sufficient supply. Associating a rank growth of stalk and leaf with an abundance of nitrogen, the experienced man can form a pretty safe opinion regarding the probable profitableness of an investment in this element. It costs nearly four times as much per pound as either of the two other constituents of a fertilizer, and so far as is feasible it should be obtained through the legumes and stable manure.

Phosphoric-acid Requirements. - Soil analyses show that the content of phosphoric acid in most soils of this country is relatively small. The results of experiments with the various constituents of fertilizers are in accord with this fact. Fertilizer experiments at the various stations and on farms are nearly a unit in showing that if any need in plant-food exists, phosphoric acid is deficient. When crop-producing power decreases, and the farmer begins to seek a commercial fertilizer to repair the loss, he finds that bone-dust or acid phosphate is serviceable. The resulting increase in yield often leads to such sole dependence upon this fertilizer that clover and manure are disregarded, the percentage of humus is allowed to drop, and finally the fertilizer is brought into disrepute. The need of phosphoric acid is so common that it is the sole plant-food in much fertilizer, and the dominant element in practically all the remainder on the market.

The Need of Potash. - Land which is deficient in organic matter ordinarily is lacking in available potash, and responds with profit to applications, provided the nitrogen and phosphoric-acid requirements have been met. Clay soils contain far more potash than sandy soils, and in a farming scheme for them that permits the use of manure and clover, it may not become necessary to buy much potash. The liberal use of straw in the stables, and the saving of all the liquid manure, are helps. Farms from which the hay and straw have been sold for a long period of time develop an urgent need of potash. Much muck land is very deficient in this constituent.

Fertilizer Tests. - Every farmer should conduct some fertilizer tests for himself. It is only the soil itself that can make an adequate reply to a question regarding its needs. The test should be made under conditions furnishing evenness in the soil, and it should be continued for years. There is pleasure to an intelligent farmer in such questioning of his soil, and only in this way can assurance be obtained that the investment in fertilizers is the wisest that can be planned for the farm.

There are only three plant constituents to be tested, but they must be used in combination as well as singly. A soil that is deficient in the three may not give any return from potash alone, and usually does not, although it may give a marked increase from use of phosphoric acid alone. The plats may be eight rods long and one rod wide, containing each one twentieth of an acre, and having strips two feet wide separating them. The following chart suggests quantities of fertilizers to be used on the one-twentieth acre plats, 10 in number:

Variation in Soil. - The difficulty in determining the character of fertilizer for a field, due to variation in the soil, is overestimated. Very often a land-owner says, “I have a dozen kinds of soil in every field.” This is true in a way, it may be, but if all the field has had the same treatment in the past, the probability is that the fertilizer which is best for one part of the field will be quite good for the other parts. The likeness in characteristics that permits the land to be cropped as one field gives some assurance of likeness in plant-food needs, even where the proportion of clay and sand varies and the color is not the same.

There may be wide variation in the productive power of the fields of a farm, due to the treatments they have received. The land that grows heavy clover in a close rotation, or receives all the stable manure, may need neither nitrogen nor potash, while another field, hard-run by timothy and corn, may need a complete fertilizer. When a careful fertilizer test on land of only average productive power has been made, the owner has some definite knowledge of his soil that enables him to give more intelligent treatment to all his fields than was possible before the test had been made. He observes the appearance and yield of plants where the plant-food requirement was fully met, and makes allowance in other fields for gains or losses in the soil due to different treatment. It is out of the question to become discouraged before a beginning has been made. If yields are limited by absence of plant-food, fertilizers must be used. If money must be expended for fertilizers, it is only good business to know that the money is expended to the best advantage.