The Story of the Soil Part 9

CHAPTER XIV

A LESSON IN FARM SCIENCE

"THE subject is somewhat complicated," Percy replied, "yet it involves no more difficult problems than have been solved in many other lines. The chief trouble is that we have done too little thinking about our own real problems. Even in the country schools we have learned something of banking and various other lines of business, something of the history and politics of this and other countries, something of the great achievements in war, in discovery and exploration, in art, literature, and invention; but we have not learned what our soils contain nor what our crops require. Not one farmer in a hundred knows what chemical elements are absolutely required for the production of our agricultural plants, and one may work hard on the farm from four o'clock in the morning till nine o'clock at night for forty years and still not learn what corn is made of.

"All agricultural plants are composed of ten chemical elements, and the growth of any crop is absolutely dependent upon the supply of these plant food elements. If the supply of any one of these plant food elements is limited, the crop yield will also be limited. The grain and gra.s.s crops, such as corn, oats, wheat, and timothy, also the root crops and potatoes, secure two elements from the air, one from water, and seven from the soil.

"The supply of some elements is constantly renewed by natural processes, and iron, one of the ten, is contained in all normal soils in absolutely inexhaustible amount; while other elements become deficient and the supply must be renewed by man, or crop yields decrease and farming becomes unprofitable.

"Matter is absolutely indestructible. It may change its form, but not a pound of material substance can be destroyed. Matter moves in cycles, and the key to the problem of successful permanent agriculture is the circulation of plant food. While some elements have a natural cycle which is amply sufficient to meet all requirements for these elements as plant food, other elements have no such cycle, and it is the chief business of the farmer to make these elements circulate.

"Take carbon, for example. This element is well represented by hard coal. Soft coal and charcoal are chiefly carbon. The diamond is pure crystallized carbon, and charcoal made from pure sugar is pure, uncrystallized carbon. This can easily be made by heating a lump of sugar on a red hot stove until only a black coal remains. Now these different solid materials represent carbon in the elemental form or free state. But carbon may unite with other elements to form chemical compounds, and these may be solids, liquids, gases.

"Thus carbon and sulfur are both solid elements, one black and the other yellow, as generally found. If these two elements are mixed together under ordinary conditions no change occurs. The result is simply a mixture of carbon and sulfur. But, if this mixture is heated in a retort which excludes the air, the carbon and sulfur unite into a chemical compound called carbon disulfid. This compound is neither black, yellow, nor solid; but it is a colorless, limpid liquid; and yet it contains absolutely nothing except carbon and sulfur."

"That seems strange," remarked Mr. Thornton. "Yes, but similar changes are going on about us all the time," replied Percy. "We put ten pounds of solid black coal in the stove and an hour later we find nothing there, except a few ounces of ashes which represent the impurities in the coal."

"Well, the coal is burned up and destroyed, is it not?"

"The carbon is burned and changed, but not destroyed. In this case, the heat has caused the carbon to unite with the element oxygen which exists in the air in the form of a gas, and a chemical compound is formed which we call carbon dioxid. This compound is a colorless gas. This element oxygen enters the vent of the stove and the compound carbon dioxid pa.s.ses off through the chimney. If there is any smoke, it is due to small particles of unburned carbon or other colored substances.

"As a rule more or less sulfur is contained in coal, wood, and other organic matter, and this also is burned to sulfur dioxid and carried into the air, from which it is brought back to the soil in rain in ample amounts to supply all of the sulfur required by plants.

"Everywhere over the earth the atmosphere contains some carbon dioxid and this compound furnishes all agricultural plants their necessary supply of both carbon and oxygen. In other words, these are the two elements that plants secure from the air. The gas, carbon dioxid, pa.s.ses into the plant through the breathing pores on the under side of the leaves. These are microscopic openings but very numerous. A square inch of a corn leaf may have a hundred thousand breathing pores."

"Now, as we go on, I am especially anxious to get at this question of supply and demand," said Mr. Thornton. "I think I understand about iron and sulfur, and also that these two elements, carbon and oxygen, are both contained in the air in the compound called carbon dioxid, and that this must supply our crops with those two elements of plant food. I'd like to know about the supply. How much is there in the air and how much do the crops require?"

"As you know," said Percy, "the atmospheric pressure is about fifteen pounds to the square inch."

"Yes, I've heard that, I know."

"Well, that means, of course, that there are fifteen pounds of air resting on every square inch of the earth's surface; in other words, that a column of air one inch square and as high as the air goes, perhaps fifty miles or more, weighs fifteen pounds."

"Yes, that is very clear."

"There is only one pound of carbon in ten thousand pounds of ordinary country air. Now, there are one hundred and sixty square rods in an acre, and since there are twelve inches in a foot and sixteen and one-half feet in a rod, it is easy to compute that there are nearly a hundred million pounds of air on an acre, and that the carbon in this amounts to only five tons. A three-ton crop of corn or hay contains one and one-fourth tons of the element carbon; so that the total amount of the carbon in the air over an acre of land is sufficient for only four such crops; while a single crop of corn yielding a hundred bushels to the acre, such as we often raise in Illinois on old feed-lots or other pieces of well treated land would require half of the total supply of carbon contained in the air over an acre. However, the largest crop of corn ever grown, of which there is an established authentic record, was not raised in Illinois, but in the state of South Carolina, in the county of Marlborough, in the year 1898, by Z. J. Drake; and, according to the authentic report of the official committee that measured the land and saw the crop harvested and weighed, and awarded Drake a prize of five hundred dollars given by the Orange Judd Publis.h.i.+ng Company,--according to this very creditable evidence, that acre of land yielded 239 bushels of thoroughly aid-dried corn; and such a crop, Mr. Thornton, would require as much carbon as the total amount contained in the air over an acre of land."

"Well, that is astonis.h.i.+ng! Then there must be some other source of supply besides the air."

"There is no other direct source from which plants secure carbon; but of course the air is in constant motion. Only one-fourth of the earth's surface is land, and perhaps only one-fourth of this land is cropped, and the average crop is about one-fourth of three tons; so that the total present supply of carbon in the air would be sufficient for about two hundred and fifty years. But as a matter of fact the supply is permanently maintained by the carbon cycle. Thus the carbon of coal that is burned in the stove returns to the air in carbon dioxid; and all combustion of coal and wood, gra.s.s and weeds, and all other vegetable matter returns carbon to the atmosphere. All decay of organic matter, as in the fermentation of manure in the pile and the rotting of vegetable matter in the soil, is a form of slow combustion and carbon dioxid is the chief produce of such decay. Sometimes an appreciable amount of heat is developed, as in the steaming pile of stable refuse lying in the barnyard, while the heat evolved in the soil is too quickly disseminated to be apparent.

"In addition to all this, every animal exhales carbon dioxid. The body heat and the animal force or energy are supplied by the combustion of organic food within the body, and here, too, carbon dioxid is the chief product of combustion.

"Thus, as a general average, the amount of carbon removed from the atmosphere by growing plants is no greater than the amount returned to the air by these various forms of combustion or decay. In like manner the supply of combined oxygen is maintained, both carbon and oxygen being furnished to the plant m the carbon dioxid.

"As a matter of fact, the air consists very largely of oxygen and nitrogen, both in the free state, but in this form these elements cannot be utilized in the growth of agricultural plants. The only apparent exception to this is in case of legume crops, such as clover, alfalfa, peas, beans, and vetch, which have power to utilize the free nitrogen by means of their symbiotic relations.h.i.+p with certain nitrogen-fixing bacteria which live, or may live, in tubercles on their roots.

"Carbon and oxygen const.i.tute about ninety per cent. of the dry matter of ordinary farm crops, and with the addition of hydrogen very important plant const.i.tuents are produced; such as starch, sugar, fiber, or cellulose, which const.i.tute the carbohydrate group.

As the name indicates, this group contains carbon, hydrogen, and oxygen, the last two being present in the same proportion as in water.

"Water is composed of the two elements, hydrogen and oxygen, both of which are gases in the free state. Water is taken into the plant through the roots and decomposed in the leaves in contact with the carbon dioxid under the influence of sunlight and the life principle. The oxygen from the water and part of that from the carbon dioxid is given off into the air through the breathing pores, while the carbon, hydrogen, and part of the oxygen, unite to form the carbohydrates. These three elements const.i.tute about ninety-five per cent. of our farm crops, and yet every one of the other seven plant food elements is just as essential to the growth and full development of the plant as are these three."

"Then so long as we have air above and moisture below, our crops will not lack for carbon, oxygen, and hydrogen. Is that the summing up of the matter?"

"Yes, Sir," Percy replied.

"And those three elements make up ninety-five per cent. of our farm crops. Is that correct?"

"Yes, Sir, as an average."

"Well, now it seems to me, if nature thus provides ninety-five per cent. of all we need, we ought to find some way of furnis.h.i.+ng the other five per cent. It makes me think of the young wife who told her husband she could live on bread and water, with his love, and he told her that if she would furnish the bread he'd skirmish around and get the water. But, say, did that South Carolina man use any fertilizer for that immense crop of corn?"

"Some fertilizer, yes. He applied manure and fertilizer from February till June. In all he applied 1000 bushels (about 30 tons) of farm manure, 600 bushels of whole cotton seed, 900 pounds of cotton seed meal, 900 pounds of kainit, 1100 pounds of guano, 200 pounds of bone meal, 200 pounds of acid phosphate, and 400 pounds of sodium nitrate."

"I would also like to know the facts about this nitrogen business,"

said Mr. Thornton. "I've understood that one could get some of it from the air, and I would much rather get it that way than to buy it from the fertilizer agent at twenty cents a pound. Cowpeas don't seem to help much, and we don't have the cotton seed, and we never have sufficient manure to cover much land."

"It is a remarkable fact," said Percy, "that of the ten essential elements of plant food, nitrogen is the most abundant, measured by crop requirements, and at the same time the most expensive. The air above an acre of land contains enough carbon for a hundred bushels of corn per acre for two years, and enough nitrogen for five hundred thousand years; and yet the nitrogen in commercial fertilizers costs from fifteen to twenty cents a pound. At commercial prices for nitrogen, every man who owns an acre of land is a millionaire.

"You mean he has millions in the air," amended Mr. Thornton.

"Yes, that is the better way to put it," Percy admitted, "but the fact is he can not only get this nitrogen for nothing by means of legume crops, but he is paid for getting it, because those crops are profitable to raise for their own value. Clover, alfalfa, cowpeas, and soy beans are all profitable crops, and they all have power to use the free nitrogen of the air.

"There are a few important facts to be kept in mind regarding nitrogen:

"A fifty-bushel crop of corn takes 75 pounds of nitrogen from the soil. Of this amount about 50 pounds are in the grain, 24 pounds are in the stalks, and 1 pound in the cobs. A fifty-bushel crop of oats takes 48 pounds of nitrogen from the soil, 33 pounds in the grain, and 15 in the straw. A twenty-five bushel crop of wheat also takes 48 pounds of nitrogen from the soil, 36 pounds in the grain and 12 in the straw.

"These amounts will vary to some extent with the quality of the crops, just as the weight of a bushel of wheat varies from perhaps 56 to 64 pounds, although as an average wheat weighs 60 pounds to the bushel."

"You surely remember figures well," remarked Mr. Thornton as he made some notations.

"It is easy to remember what we think about much and often," said Percy; "as easy to remember that a ton of cowpea hay contains 43 pounds of nitrogen as that Blairville is 53 miles from Richmond."

"I have added those figures together," continued Mr. Thornton, "and I find that the three crops, corn, oats, and wheat, would require 171 pounds of nitrogen. Now suppose we raise a crop of cowpeas the fourth year, how much nitrogen would be added to the soil in the roots and stubble?"

"Not any."

"Do you mean to say that the roots and stubble of the cowpeas would add no nitrogen to the soil? Surely that does not agree with the common talk."

"It is even worse than that," said Percy. "The cowpea roots and stubble would contain less nitrogen than the cowpea crop takes from a soil capable of yielding thirty bushels of corn or oats. Only about one-tenth of the nitrogen contained in the cowpea plant is left in the roots and stubble when the crop is harvested. Suppose the yield is two tons per acre of cowpea hay! Such a crop would contain about 86 pounds of nitrogen, and about 10 pounds of nitrogen per acre would be left in the roots and stubble."