Let us look at the question from two points of view: first, in regard to the cost of the ingredients; and, secondly, in regard to the growth of the crop.
We will begin with wheat. A crop of wheat, machine-reaped, contains, as carted to the stack, about six pounds of soil ingredients in every one hundred pounds; that is to say, each five pounds of mineral matter, and rather less than one pound of nitrogen, which the plant takes from the soil, will enable it to obtain ninety-four pounds of other substances from the atmosphere. To grow a crop of twenty bushels of grain and two thousand pounds of straw, would require one hundred and sixty pounds of minerals, and about thirty-two pounds of nitrogen; of the one hundred and sixty pounds of minerals, one-half would be silica, of which the soil possesses already more than enough; the remainder, consisting of about eighty pounds of potash and phosphate, could be furnished for from three to four dollars, and the thirty-two pounds of nitrogen could be purchased in nitrate of soda for six or eight dollars. The actual cost of the ingredients, therefore, in the crop of twenty bushels of wheat, would be about ten to twelve dollars. But as this manure would furnish the ingredients for the growth of both straw and grain, and it is customary to return the straw to the land, after the first crop, fully one-third of the cost of the manure might, in consequence, be deducted, which would make the ingredients of the twenty bushels amount to six dollars. Twenty bushels of wheat in England would sell for twenty-eight dollars; therefore, there would be twenty-two dollars left for the cost of cultivation and profit.
A French writer on scientific agriculture has employed figures very similar to the above, to show how French farmers may grow wheat at less than one dollar per bushel. At this price they might certainly defy the competition of the United States. It is one thing, however, to grow crops in a lecture room, and quite another to grow them in a field. In dealing with artificial manures, furnishing phosphoric acid, potash, and nitrogen, we have substances which act upon the soil in very different ways. Phosphate of lime is a very insoluble substance, and requires an enormous amount of water to dissolve it. Salts of potash, on the other hand, are very soluble in water, but form very insoluble compounds with the soil. Salts of ammonia and nitrate of soda are perfectly soluble in water. When applied to the land, the ammonia of the former substance forms an insoluble compound with the soil, but in a very short time is converted into nitrate of lime; and with this salt and nitrate of soda, remains in solution in the soil water until they are either taken up by the plant or are washed away into the drains or rivers.
Crops evaporate a very large amount of water, and with this water they attract the soluble nitrate from all parts of the soil. Very favorable seasons are therefore those in which the soil is neither too dry nor too wet; as in one case the solution of nitrate becomes dried up in the soil, in the other it is either washed away, or the soil remains so wet that the plant cannot evaporate the water sufficiently to draw up the nitrates which it contains.
The amount of potash and phosphoric acid dissolved in the water is far too small to supply the requirements of the plant, and it is probable that what is required for this purpose is dissolved by some direct action of the roots of the plant on coming in contact with the insoluble phosphoric acid and potash in the soil.
In support of this view, I may mention that we have clear evidence in some of our experiments of the wheat crop taking up both phosphates and potash that were applied to the land thirty years ago.
To suppose, therefore, that, if the ingredients which exist in twenty bushels of wheat and its straw, are simply applied to a barren soil, the crop will be able to come in contact with, and take up these substances, is to assume what certainly will not take place.
I have often expressed an opinion that arable land, could not be cultivated profitably by means of artificial manures, unless the soil was capable of producing, from its own resources, a considerable amount of produce; still the question had never up to this time come before me in a distinct form as one upon which I had to decide one way or the other. I had, however, no hesitation in coming to the conclusion, that grain crops could never be grown at a profit upon my relation’s land, and that consequently, for some years, it would be better to give up the attempt, and try to improve the pasture.
After what I have said about the insolubility of potash and phosphoric acid, it may possibly be asked—why not give a good dose of these substances at once, as they do not wash out of the soil—say enough to grow sixty crops of grain, and apply the nitrate, or ammonia every year in just sufficient amounts to supply the wants of the crop?
The objections to this plan are as follows: assuming the most favorable conditions of climate, and the largest possible produce, the wheat could certainly not take up the whole of the thirty-two pounds of nitrogen applied, and the crop which requires nearly one pound of nitrogen in every one hundred pounds of gross produce, would be certainly less than three thousand two hundred pounds, if supplied with only thirty-two pounds of nitrogen. If we take the total produce of the best and worst wheat crop, grown during the forty years of our experiments, we shall arrive at a better understanding in the matter. The following are the figures: