Of course, an excess of lime in the soil may be hurtful, though less so in arid than in humid regions.  Some authors state that from 8 to 20 per cent of calcium carbonate makes a soil unfitted for plant growth.  There are, however, a great many agricultural soils covering large areas and yielding very abundant crops which contain very much larger quantities of calcium carbonate.  For instance, in the Sanpete Valley of Utah, one of the most fertile sections of the Great Basin, agricultural soils often contain as high as 40 per cent of calcium carbonate, without injury to their crop-producing power.

In the table are two columns headed “Soluble Silica” and “Alumina,” in both of which it is evident that a very much larger per cent is found in the arid than in the humid soils.  These soil constituents indicate the condition of the soil with reference to the availability of its fertility for plant use.  The higher the percentage of soluble silica and alumina, the more thoroughly decomposed, in all probability, is the soil as a whole and the more readily can plants secure their nutriment from the soil.  It will be observed from the table, as previously stated, that more humus is found in humid than in arid soils, though the difference is not so large as might be expected.  It should be recalled, however, that the nitrogen content of humus formed under rainless conditions is many times larger than that of humus formed in rainy countries, and that the smaller per cent of humus in dry-farming countries is thereby offset.

All in all, the composition of arid soils is very much more favorable to plant growth than that of humid soils.  As will be shown in Chapter IX, the greater fertility of arid soils is one of the chief reasons for dry-farming success.  Depth of the soil alone does not suffice.  There must be a large amount of high fertility available for plants in order that the small amount of water can be fully utilized in plant growth.

Summary of characteristics.—­Arid soils differ from humid soils in that they contain:  less clay; more sand, but of fertile nature because it is derived from rocks that in humid countries would produce clay; less humus, but that of a kind which contains about 3-1/2 times more nitrogen than the humus of humid soils; more lime, which helps in a variety of ways to improve the agricultural value of soils; more of all the essential plant-foods, because the leaching by downward drainage is very small in countries of limited rainfall.

Further, arid soils show no real difference between soil and subsoil; they are deeper and more permeable; they are more uniform in structure; they have hardpans instead of clay subsoil, which, however, disappear under the influence of cultivation; their subsoils to a depth of ten feet or more are as fertile as the topsoil, and the availability of the fertility is greater.  The failure to recognize these characteristic differences between arid and humid soils has been the chief cause for many crop failures in the more or less rainless regions of the world.