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

At Rothamsted, among my experiments on the growth of continuous wheat, at the end of forty years, the soil supplied with salts of ammonia has yielded, during the whole time, and still continues to yield, a larger produce than is obtained by a liberal supply of phosphates and alkaline salts without ammonia.

When we consider that every one hundred pounds of wheat crop, as carted to the stack, contains about five per cent. of mineral matter, and one per cent. of nitrogen, it is impossible to avoid the conclusion that my soil has a large available balance of mineral substances which the crop could not make use of for want of nitrogen. The crop which has received these mineral manures now amounts to from twelve to thirteen bushels per acre, and removes from the land about sixteen pounds of nitrogen every year.

Analyses of the soil show that, even after the removal of more than thirty crops in succession, without any application of manure containing ammonia, the soil still contains some thousands of pounds of nitrogen. This nitrogen is in combination with carbon; it is very insoluble in water, and until it becomes separated from the carbon, and enters into combination with oxygen, does not appear to be of any use to the crop.

The combination of nitrogen with oxygen, is known as nitric acid. The nitric acid enters into combination with the lime of the soil, and in this form becomes the food of plants.

From its great importance in regard to the growth of plants, nitric acid might be called the main spring of agriculture, but being perfectly soluble in water, it is constantly liable to be washed out of the soil. In the experiment to which I have referred above—where wheat is grown by mineral manures alone—we estimate that, of the amount of nitric acid liberated each year, not much more than one-half is taken up by the crop.

The wheat is ripe in July, at which time the land is tolerably free from weeds; several months, therefore, occur during which there is no vegetation to take up the nitric acid; and even when the wheat is sown at the end of October, much nitric acid is liable to be washed away, as the power of the plant to take up food from the soil is very limited until the spring.

The formation of nitric acid, from the organic nitrogen in the soil, is due to the action of a minute plant, and goes on quite independent of the growth of our crops. We get, however, in the fact an explanation of the extremely different results obtained by the use of different manures. One farmer applies lime, or even ground limestone to a soil, and obtains an increase in his crops; probably he has supplied the very substance which has enabled the nitrification of the organic nitrogen to increase; another applies potash, a third phosphates; if either of these are absent, the crops cannot make use of the nitric acid, however great may be the amount diffused through the soil.

It may possibly be said that the use of mineral manures tends to exhaust the soil of its nitrogen; this may, or may not, be true; but even if the minerals enable the crop to take up a larger amount of the nitric acid found in the soil year by year, this does not increase the exhaustion, as the minerals only tend to arrest that which otherwise might be washed away.

We must look upon the organic nitrogen in the soil, as the main source of the nitrogen which grows our crops. Whatever may be the amount derived from the atmosphere, whether in rain, or dew; or from condensation by the soil, or plants, it is probable that, where the land is in arable cultivation, the nitrogen so obtained, is less than the amount washed out of the soil in nitric acid. Upon land which is never stirred by the plow, there is much less waste and much less activity.

The large increase in the area of land laid down to permanent pasture in England, is not due alone to the fall in the price of grain. The reduction of fertility in many of the soils, which have been long under the plow, is beginning to be apparent. Under these circumstances a less exhausting course of treatment becomes necessary, and pasture, with the production of meat, milk, and butter, takes the place of grain fields.