Talks on Manures / A Series of Familiar and Practical Talks Between the Author and the Deacon, the Doctor, and Other Neighbors, on the Whole Subject

by Joseph Harris

There was here and there, a good field of wheat. As a rule, it was on naturally moist land, or after a good summer-fallow, sown early. I know of but one exception. A neighboring nursery firm had a very promising field of wheat, which was sown late. But their land is rich and unusually well worked. It is, in fact, in the very highest condition, and, though sown late, the young plants were enabled to make a good strong growth in the autumn.

In such a dry season, the great point is, to get the seed to germinate, and to furnish sufficient moisture and food to enable the young plants to make a strong, vigorous growth of roots in the autumn. I do not say that two hundred pounds of superphosphate per acre, drilled in with the seed, will always accomplish this object. But it is undoubtedly a great help. It does not furnish the nitrogen which the wheat requires, but if it will stimulate the production of roots in the early autumn, the plants will be much more likely to find a sufficient supply of nitrogen in the soil than plants with fewer and smaller roots.

In a season like the past, therefore, an application of two hundred pounds of superphosphate per acre, costing three dollars, instead of giving an increase of five or six bushels per acre, may give us an increase of fifteen or twenty bushels per acre. That is to say, owing to the dry weather in the autumn, followed by severe weather in the winter, the weak plants on the unmanured land may either be killed out altogether, or injured to such an extent that the crop is hardly worth harvesting, while the wheat where the phosphate was sown may give us almost an average crop.

Sir John B. Lawes has somewhere compared the owner of land to the owner of a coal mine. The owner of the coal digs it and gets it to market in the best way he can. The farmer’s coal mine consists of plant food, and the object of the farmer is to get this food into such plants, or such parts of plants, as his customers require. It is hardly worth while for the owner of the coal mine to trouble his head about the exhaustion of the supply of coal. His true plan is to dig it as economically as he can, and get it into market. There is a good deal of coal in the world, and there is a good deal of plant food in the earth. As long as the plant food lies dormant in the soil, it is of no value to man. The object of the farmer is to convert it into products which man and animals require.

Mining for coal is a very simple matter, but how best to get the greatest quantity of plant food out of the soil, with the least waste and the greatest profit, is a much more complex and difficult task. Plant food consists of a dozen or more different substances. We have talked about them in the pages of this book, and all I wish to say here is that some of them are much more abundant, and more readily obtained, than others. The three substances most difficult to get at are: nitric acid, phosphoric acid and potash. All these substances are in the soil, but some soils contain much more than others, and their relative proportion varies considerably. The substance which is of the greatest importance, is nitric acid. As a rule, the fertility of a soil is in proportion to the amount of nitric acid which becomes available for the use of plants during the growing season. Many of our soils contain large quantities of nitrogen, united with carbon, but the plants do not take it up in this form. It has to be converted into nitric acid. Nitric acid consists of seven pounds of nitrogen and twenty pounds of oxygen. It is produced by the combustion of nitrogen. Since these “Talks” were published, several important facts have been discovered in regard to how plants take up nitrogen, and especially in regard to how organic nitrogen is converted into nitric acid. It is brought about through the action of a minute fungoid plant. There are several things necessary for the growth of this plant. We must have some nitrogenous substance, a moderate degree of heat, say from seventy to one hundred and twenty degrees, a moderate amount of moisture, and plenty of oxygen. Shade is also favorable. If too hot or too cold, or too wet or too dry, the growth of the plant is checked, and the formation of nitric acid suspended. The presence of lime, or of some alkali, is also necessary for the growth of this fungus and the production of nitric acid. The nitric acid unites with the lime, and forms nitrate of lime, or with soda to form nitrate of soda, or with potash to form nitrate of potash, or salt-petre. A water-logged soil, by excluding the oxygen, destroys this plant, hence one of the advantages of underdraining. I have said that shade is favorable to the growth of this fungus, and this fact explains and confirms the common idea that shade is manure.

The great object of agriculture is to convert the nitrogen of our soils, or of green crops plowed under, or of manure, into nitric acid, and then to convert this nitric acid into profitable products with as little loss as possible. Nitrogen, or rather nitric acid, is the most costly ingredient in plant food, and unfortunately it is very easily washed out of the soil and lost. Perhaps it is absolutely impossible to entirely prevent all loss from leaching; but it is certainly well worth our while to understand the subject, and to know exactly what we are doing. In a new country, where land is cheap, it may be more profitable to raise as large crops as possible without any regard to the loss of nitric acid. But this condition of things does not last long, and it very soon becomes desirable to adopt less wasteful processes.

In Lawes and Gilbert’s experiments, there is a great loss of nitric acid from drainage. In no case has as much nitrogen been obtained in the increased crop as was applied in the manure. There is always a loss and probably always will be. But we should do all we can to make this loss as small as possible, consistent with the production of profitable crops.

There are many ways of lessening this loss of nitric acid. Our farmers sow superphosphate with their wheat in the autumn, and this stimulates, we think, the growth of roots, which ramify in all directions through the soil. This increased growth of root brings the plant in contact with a larger feeding surface, and enables it to take up more nitric acid from its solution in the soil. Such is also the case during the winter and early spring, when a good deal of water permeates through the soil. The application of superphosphate, unquestionably in many cases, prevents much loss of nitric acid. It does this by giving us a much greater growth of wheat.

I was at Rothamsted in 1879, and witnessed the injurious effect of an excessive rainfall, in washing out of the soil nitrate of soda and salts of ammonia, which were sown with the wheat in the autumn. It was an exceedingly wet season, and the loss of nitrates on all the different plots was very great. But where the nitrates or salts of ammonia were sown in the spring, while the crops were growing, the loss was not nearly so great as when sown in the autumn.

The sight of that wheat field impressed me, as nothing else could, with the importance of guarding against the loss of available nitrogen from leaching, and it has changed my practice in two or three important respects. I realize, as never before, the importance of applying manure to crops, rather than to the land. I mean by this, that the object of applying manure is, not simply to make land rich, but to make crops grow. Manure is a costly and valuable article, and we want to convert it into plants, with as little delay as possible, which will, directly or indirectly, bring in some money.