*Possible agricultural area estimated at 25o million acres. twill ultimately be greater.
$Possible agricultural area estimated at 30o million acres. §Possible agricultural area estimated at 700 million acres.
With our rapidly increasing world population two major soil problems present themselves, first, the possibility of increasing the area of lands growing crops other than forests, and second, methods of preventing soil depletion and consequent crop decrease. The utilization of virgin and well watered grass lands and the reduction of our forest areas have met the first problem in times past. Unfortunately soils have often been deforested and put under cul tivation when they should have been left with their forest cover.
The reclamation of swamps and the better drainage of crop bearing lands have increased the area and effectiveness of the cul tivable soils. (See DRAINAGE; RECLAMATION OF LAND.) The utilization of arid and semi-arid lands is particularly prom ising in certain countries. Such soils, developed under low rainfall, are potentially fertile. Thousands of acres of arable and very highly productive lands will ultimately be added to that now avail able for crop production through irrigation (q.v.).
Another important means of utilizing semi-arid soils is by dry land farming. (See AGRICULTURE.) By proper methods of culti vation, moisture is conserved in the soil and carried from one season to the next. Farming operations thus become possible in areas where the annual rainfall is as low as 20 inches.
The second major soil problem relates to the maintenance of soil fertility. Land can not be cropped without certain losses taking place. These losses are four in number: first, erosion or the removal of the surface soil, the most fertile portion, by water or wind; second, the loss of constituents by the drainage of water through the soil, called leaching; third, the removal of nutrients by plants; and fourth, the loss of organic matter as a result of biochemical oxidation and the liberation of carbon dioxide.
Water erosion may be controlled by small dams, by terraces, by sod, and by strip-cropping. In spite of such possibilities thousands of tons of fertile soil are swept annually into rivers. In many cases this is the major cause of soil depletion and is closely correlated with deforestation and flood devastations.
The losses through drainage, crop removal and organic oxidation are susceptible of but little control. Farming operations, even the most careful, encourage rather than check such depletions.
The major features of soil fertility maintenance may be very simply set forth if it is taken for granted that the soil drainage is good, that tillage is adequate, that erosion is under control, and that a suitable crop rotation, if possible, is utilized. Under such conditions the problem is to add certain constituents in such amounts and proportions as at least partially to off-set the inevi table losses. At the same time the physiological balance of the plant nutrients should be maintained as nearly as possible.
While plants require fourteen nutrients for their normal growth and probably several more besides, ordinarily only four are likely to become especially critical, namely nitrogen, potas sium, calcium and phosphorus. Sulphur also may be a limiting factor as well as copper, manganese, boron, and zinc.
The nitrogen losses may be met by applying farm manure, turn ing under crop-residues and by utilizing, either as a crop in the rotation or as a green manure, certain legumes. Commercial nitro gen may be resorted to, although it is generally used to stimulate the crop directly rather than to reduce the drain on the soil nitrogen. Obviously the methods utilized in maintaining the soil nitrogen automatically add organic matter as well. As this mate rial decays and humus is formed a definite ratio is established be tween the organic matter and the nitrogen. The nitrogen is always in the minimum and apparently controls the amount of humus that finally is present in the soil. The advantage of legumes as green manures over non-legumes is quite apparent, since they obtain nitrogen from the air and are especially rich in this constituent. The soil organic matter besides carrying nitro gen is a source of energy for various biochemical transformations and encourages a suitable physical condition of the soil.
The presence of this decaying organic matter tends to make available to plants certain soil constituents, especially potassium. Therefore, it is often unnecessary to add a potassium fertilizer to a soil rich in this element if the organic matter is properly main tained and is actively decomposing.
Many soils are low in active calcium and as a consequence be come acid. This deficiency also develops other conditions that may be detrimental to some plants but by no means to all. Liming, therefore, must be resorted to for certain crops and is thus an important feature in the maintenance of soil fertility the world over. (See FERTILIZERS.) The lime not only reduces the soil acidity but also adjusts other physiological soil conditions.
Phosphorus exists in the soil in small amounts and is often very slowly available to plants. In fact it is so slightly soluble as to be lost but sparingly, if at all, in the drainage water. The only way of dealing with this element is by direct addition of phosphorus fertilizers (q.v.). As the losses of soil constituents increase, the necessity becomes greater for commercial fertilizers.