The organic matter is constantly undergoing more or less rapid decay unless the soil be perfectly dry or frozen solid. Stirring and cultivating the soil hasten this decay.
As the organic matter decays it adds available plant food to the soil, particularly nitrogen.
As it decays, it produces carbonic acid and other acids which are able to dissolve mineral plant food not soluble in pure water and thus render it available to plants.
Plants, although they require the same elements of plant food, take them in different amounts and different proportions.
Plants differ in the extent and depth of root growth and therefore take food from different parts of the soil. Some are surface feeders while others feed on the deeper soil.
Plants differ in their power to take plant food from the soil; some are weak feeders, and can use only the most available food; others are strong feeders, and can use tougher plant food.
Plants vary in the amount of heat they require to carry on their growth and development.
THE ONE CROP SYSTEM
We are now ready for the question. What effect has the continuous cultivation, year after year, of the same kind of crop on the soil conditions necessary to the best growth and development of that crop or any other crop? Suppose we take cotton for example.
How does cotton growing affect soil humus?
During the cultivation of cotton, the organic matter or humus of the soil decays in greater quant.i.ties than are added by the stalks and leaves of the crop. Therefore, cotton is a humus wasting crop, and the continuous cultivation of this crop tends to exhaust the supply of organic matter in the soil.
How does cotton growing affect soil texture?
Cotton growing wastes soil humus and therefore injures soil texture by making the lighter soils more loose and open, and the heavier soils more dense and compact.
How does cotton growing affect soil water?
By wasting humus cotton growing injures soil texture and so weakens the water holding and water pumping power of light soils and weakens the water absorbing power of heavy soils. Therefore the continuous cultivation of cotton weakens the power of the soil over water, that most important factor in crop growth.
How does cotton growing affect soil ventilation?
Continuous cotton culture, by wasting humus, injures texture and therefore injures soil ventilation, causing too much ventilation in the lighter soils and too little in heavier soils.
How does cotton culture affect plant food in the soil?
Continuous cotton growing wastes plant food:
Because it wastes organic matter which contains valuable plant food, particularly nitrogen.
Because by wasting organic matter it increases the leaching of the lighter soils and the surface washing of the heavier soils.
Because its roots occupy largely the upper soil and do not make use of much food from the lower soil.
Because it grows only during the warm part of the year and there is no crop on the land to check loss of plant food from leaching and surface wash during the winter.
Because it is a weak feeder of phosphoric acid, and can use only that which is in the most available form. In applying fertilizer to cotton it is necessary for best results to apply at least twice as much phosphoric acid as the crop can use, because it can use only that which is in the most available form and the remainder is left in the soil unused.
Continuous cotton culture then has an injurious effect on all the important soil conditions necessary to its best growth and development, and the result is a diminishing yield or an increasing cost in maintaining fertility by the use of fertilizer.
How does continuous cotton culture affect the economics of the farm?
The injury to the soil conditions necessary to root growth diminishes the yield and therefore increases the cost of production.
The poor soil conditions tend not only to diminish yield but also to diminish the quality of the crop, which tends to lower the price received for the cotton.
Keeping the land constantly in cotton tends to increase the insect enemies and the diseases of the crop.
The continuous growing of cotton does not permit the constant employment of one set of laborers throughout the year.
The continuous growing of cotton generally means that most of the farm goes into cotton. A small patch of corn is planted for the stock, which are apt to suffer from a lack of variety in food.
The same is true with reference to home supplies. Very few vegetables are grown for the table and there is little milk, b.u.t.ter or eggs for home use or exchange for groceries or drygoods at the store.
Thus we see that the continuous growing of cotton on the soil, year after year, has a bad effect on conditions necessary to its best growth and development and also on the economics of the farm.
These facts are true to a greater or less degree in the case of nearly all of the farm crops. The grain crops are often considered as humus makers because of the stubble turned under, but Professor Snyder, of Minnesota, found that five years" continuous culture of wheat resulted in an annual loss of 171 pounds of nitrogen per acre, of which only 24.5 was taken by the crop, the remaining 146.5 pounds were lost through a waste of organic matter.
THE ROTATION OF CROPS
Now, suppose that instead of growing cotton on the same soil year after year, we select four crops--cotton, corn, oats and cowpea--and grow them in regular order, a rotation practiced in some parts of the South.
We will divide the farm into three fields and number them 1, 2 and 3, and will plant these crops as indicated by the following diagrams:
[Ill.u.s.tration: Plan of farm.]
Plan for planting.
FIELD 1. FIELD 2. FIELD 3.
+----------------+----------------+----------------+ | | OATS, | CORN, | 1st year | | harvested in | followed by | or 1905. | COTTON | spring, | oats, | | | followed by | planted in | | | COWPEAS. | fall. | +----------------+----------------+----------------+ | CORN, | | OATS, | 2d year | followed by | | harvested in | or 1906. | oats, | COTTON. | spring, | | planted in | | followed by | | fall. | | COWPEAS. | +----------------+----------------+----------------+ | OATS, | CORN, | | 3d year | harvested in | followed by | | or 1907. | spring, | oats, | COTTON. | | followed by | planted in | | | COWPEAS. | fall. | | +----------------+----------------+----------------+
Each of these crops occupies one-third of the farm each year, and yet the crop on each field changes each year so that no one kind of crop is grown on any field oftener than once in three years. The cotton is grown for market, the corn partly to sell, partly to feed, the oats to feed and the cowpeas to plow under. All cotton and corn refuse is plowed under.
What effect will such a system have on the conditions necessary for plant growth? Suppose we follow the crops on Field 1. Cotton, corn, and oats are humus wasting crops but the pea crop which is grown the third year is plowed under, and largely, if not entirely, remedies the loss by furnishing a new supply of organic matter, and the ill effects which we noticed would follow the loss of organic matter due to the continuous growing of cotton are avoided, soil texture is preserved, soil ventilation is not injured, and the power of the soil over water is preserved.
What is the effect on plant food in the soil?
Before answering this question let us see what amounts of plant foods these crops take out of the soil.
We will a.s.sume that the soil is a good loam at the start and will produce:
One bale of five hundred pounds of lint cotton per acre, sixty bushels sh.e.l.led corn per acre, thirty bushels oats per acre, or two tons cowpea hay per acre.
Such a yield of crop would take from the soil the following amounts of plant food per acre:
----------------------+-----------+------------+------------ | | Phosphoric | | Nitrogen, | Acid, | Potash, | pounds. | pounds. | pounds.