It is not intended to convey the idea that lime carbonate is the only essential factor in developing and maintaining the best physical condition of soils. It is, however, one of the essential factors, especially in the heavy clay types; and all limestone soils which result from the decay of lime rock in place are, with few exceptions, of the clay type.

Development of available plant food. From the standpoint of the chemical and life processes in the soil which are concerned in the elaboration of plant food, lime carbonate plays an extremely important role, and the part it takes in producing from humus the nitrogen food of crops takes first rank. In the decomposition of humus by bacteria in the soil nitric acid is formed, just as vinegar is the final product in the fermentation of sugar by yeast and the mother of vinegar. The presence of any considerable amount of free nitric or other acid in the soil moisture checks and finally arrests completely the further formation of nitric acid from the humus. But when there is

present an abundance of lime carbonate, associated at the same time with other conditions favorable for nitrification, the lime of the carbonate is appropriated by the nitric acid formed, setting free the carbonic acid and leaving, in place of the lime carbonate, lime nitrate, which is the chief compound from which agricultural crops derive their supply of nitrogen. In this neutralization of the nitric acid by the lime the soil is prevented from becoming sour, and nitrification goes forward continuously and at a rapid rate. At the same time that the nitric acid is passing from the organism which excretes it as a waste product to the lime of the carbonate to become neutralized, this free nitric acid and the carbonic acid too, in its turn, when it is set free from the lime by the nitric acid, act together in increasing the dissolving power of the soil moisture for the other essential plant food elements in the soil. Thus it is seen that the carbonate of lime not only takes a necessary part in putting the nitrogen of humus in food form to supply crops, but it also aids

in bringing other plant food elements into available form as well; and it is a common experience that soils comparatively poor in potash and phosphoric acid, if only the humus and lime content are kept high, may yet produce large yields, because the same action which develops from the humus the lime nitrate, sets free in the soil moisture more potash and more phosphoric acid. Lime sulphate, or land plaster, exerts a similar effect in favoring nitrification, but is never naturally so abundant in soils. Other bases, if free to unite with the nitric acid, may favor nitrification, but none of the naturally available bases is as serviceable as lime.

We have thus stated two of the main reasons, one physical and the other biochemical, why "a lime country is a rich country"; why an abundance of lime carbonate in a soil is generally associated with one highly productive. To set out all of the ways in which lime carbonate aids in maintaining soil productivity would require the discussion of at least a dozen other lines of activity or influence.

CHAPTER VII

PHYSICAL FEATURES OF SOILS WHICH INFLUENCE THEIR ABILITY TO FEED CROPS

SOILS

OILS are mechanical mixtures of air, water, organic matter and a great variety of minerals. Through the play of physical, chemical and vital forces this mixture is in constant interaction. The product of this interaction is a very complex solution, which is the soil moisture.

Plants place their roots so as to withdraw this solution from the soil and, except the carbon derived from the air through the leaves, this solution constitutes their entire food. If this solution is continuously maintained in the soil; if it is sufficiently abundant; if it has the right concentration and composition, then, with the right temperature and abundant sunshine, only accident or disease can prevent large yields.

DEPTH OF SOIL

When all other conditions are similar, the best soils are those which are congenial to the roots of crops throughout a depth of four feet. There is a wide difference in the depth of root penetration into the soil, but these differences are determined, in our opinion, more by soil conditions than by the habit of the plant. There are few crops which will not draw upon more than four feet in depth of soil if the soil is in good condition. It is important to know and to control the conditions throughout a depth of at least four feet of soil.

PROPORTIONAL PARTS

When the surface foot of soil is in prime physical condition, about seven inches out of the 12 are occupied by air and soil moisture, and only five inches by the mineral grains and organic matter.

In southern Wisconsin we have found the ratio between the solids of the surface four