Loss of Lime. Nature made the value of land as a producer of food utterly dependent upon the activity of lime, and at the same time gave it some power to shirk its work. In a normal soil is a percentage of lime that came from the disintegration of rock of the region or was transported by action of water on a huge scale. Possibly rarely would it be in insufficient amount to keep a soil in a condition friendly to plant life, and to feed the plant, if it stayed where nature placed it and kept in form available for the needs it was intended to meet. There is land that always was notably deficient in this material, and there is land that was known in the early history of the world's agriculture to be "sour," but the troubles of our present day in the case of the farming country in the humid region of the United States is less due to any natural absolute shortage than to combination that destroys value and to escape by action of water.

Prevalence of Acidity. The results of experiment station and farm tests are conclusive that the soils of the greater part of all the humid region of the United States show lime deficiency. Formerly, acidity was associated in our minds with wet, low-lying land, but within the last twenty years we have learned that it prevails in light seashore sands along the Atlantic shore, in clays, loams and shales stretching to the Appalachian system of mountains, on top of mountain ranges and across foothills to our central states, and through them in stretches to the semi-arid lands of the west. While not all this land has fallen into the lime-deficient class, and the great part of some states remains alkaline, the tendency toward acidity is continuous.

Crop production in great portions of the Mississippi valley is restricted by lack of lime in the soil, and some states to the eastward have one-half to nine-tenths of their acreage too low in lime for the best results. Calcareous soils have been losing their distinctive feature, and the immense areas of land naturally low in lime have remained hampered in ability to make full returns for labor, fertilizer and seed. It is this situation that brings the right use of lime on land to the front as a matter of fundamental importance to the farmer.

Causes of Soil Acidity. If any discussion of the causes of soil acidity would delay a decision to apply lime where needed, the time given to such discussion would be worse than wasted. It is much more important to be able to detect the presence of harmful acids and to neutralize them than it is to know why the soil should be in such plight that it could not supply the required lime and had become dependent upon its owner for assistance. On the other hand, some of us find it difficult to accept a fact without seeing a reason for it, and we may do well to consider several causes that may be at work to put a soil out of the alkaline class.

Leaching. One cause that appears obvious and easy of acceptance is leaching. In the case of one Pennsylvania farm, lying in a limestone valley, the lime had been washed out by action of water so freely that caverns formed under the surface, and a test showed a marked deficiency in the top soil. This land ceased to grow clover, and plantain and sorrel abounded. This case, which is not an isolated one, showed an unusually rapid loss, but we always expect to find the water from wells and springs in a limestone country strongly impregnated with lime. Drainage waters contain it. The draft by action of water is continuous, and in some types could easily account for sufficient loss to change the nature of the soil. We may place undue emphasis upon this factor, as other causes are at work, but leaching is a leading source of loss.

Chemical Compounds. A serious cause of lime exhaustion that is being studied by soil chemists is the presence of compounds in the soil that combine with the lime and rob it of ability to serve the soil when new acids form. The practical farmer accepts the statements of the chemists on this point, and probably would not have his interests served by any exact knowledge of the nature of these agents.

Decaying Vegetation. A cause of acid conditions that is widely known and accepted, and that may therefore stand out in our thinking with undue prominence, is connected with the decay of green vegetable matter in the soil. Many of us have seen fields rendered temporarily unproductive by the plowing down of a mass of immature plants in midsummer. All organic matter, indeed, in its decay makes a draft upon the lime content of the soil in which it may be buried.

Removal in Crops. Lime is taken out of land by plants, and the loss is a considerable item, but our interest is in the form of lime that can correct soil acidity, and we know that compounds of lime that are worthless for this purpose may be the chief source of the lime in our crops. A determination of the lime in the ash of a crop does not give data of much practical value.