percentage of fertilizing ingredients in the crop is not of so much importance as the total amount removed by it from each acre of ground. The next table gives the amounts of nitrogen, phosphoric acid and potash removed from an acre by a few of the common crops. (Adapted from Van Slyke.)

An interesting point brought out by the table is the great difference in the total amount of plant food removed from an acre by the various crops. It is readily seen that certain crops must exhaust the fertility of the soil more rapidly than others, and common experience shows that the plants which remove large quantities of the essential plant foods are those that most quickly render the land infertile.

Amount of Plant Food in the Soil.-The bearing of the above facts upon the question of the maintenance of fertility can not be fully shown, unless the amount of plant food existing in the soil is determined. Large numbers of analyses of soils have been made and, as might be expected, these analyses show great variations in the composition of the soils. The following table gives the amount of nitrogen, phosphoric acid and potash in the first foot of typical sandy loam, clay loam and clay soils.

AMOUNT OF PLANT FOOD PER ACRE IN THE SURFACE FOOT

The large amount of plant food present in the soil is surprising, in view of the fact that it is so hard to maintain a satisfactory yield of crops. Comparing the last two tables it is seen that the analysis of the clay loam soil shows the presence of sufficient nitrogen for 77 crops of wheat yielding 30 bushels to the acre; enough phosphoric acid for 246; and potash to supply 1,724 such crops. The second and third foot contain nearly as much phosphoric acid and potash as the surface foot, so that so far as these two substances are

concerned the supply seems almost inexhaustible. Although the chemical analyses of many of the soils upon which wheat has been grown show fully as large amounts of plant food as the clay loam under discussion, experience has demonstrated that long before the smallest number of crops mentioned above (i. e.

The physical condition of the soil is as important as its chemical composition. The lumpy soil contains as much plant food as the friable soil, but the plant roots cannot penetrate the hard lumps to obtain it.

77) have been produced the yield will have so decreased as to be unprofitable.

Chemical Analysis does not Show Available Plant Food. The reason for the apparent inconsistency between the analyses of soils and actual results in raising crops is found in the fact that the chemical analyses give the total amount of nitrogen, phosphoric acid and potash in the soil, but do not indicate what part of these foods is available to the plant. The greater proportion of these substances is locked up in insoluble compounds, in which form the plant is incapable of using them. Smaller quantities have been changed by the forces of nature into a condition in which they are

available to plants. While the amounts of these materials removed by the crop seem insignificant when compared with the total plant food in the soil, they may be very large in comparison with the available part. The unavailable, or "potential," plant food is gradually being made available, but not with sufficient rapidity to replace that removed from the field at harvest. It will thus be seen that the present fertility of the soil depends not upon the potential plant food it contains,

but upon that which is immediately available to the plant, and that the yield will be limited by the element of this available plant food present in least quantity. Continuous cropping of the soil with the removal of everything from the field results in the exhaustion of the plant food which has been rendered available during the past ages. It will be interesting to study the origin of the plant food, and the manner in which it became available to the plants.

CHAPTER V

ORIGIN OF THE SOIL

The Primary Soils.-All soils are derived primarily from the igneous, or original rocks, of which the granites and trap are good examples. Geology teaches that the earth was once a molten mass, and that upon

cooling it solidified into rocks, of which those mentioned are types. These rocks must have contained all of the mineral or ash elements of plant food, as no other source of them is conceivable. This plant food, however, was present in insoluble compounds, and in