Soils, their formation, properties, composition, and relations to climate and plant growth in the humid and arid regions

by Eugene W. Hilgard

In the case of native annuals, either the duration of their vegetation is extremely short, ending with or shortly after the cessation of rains; or else their tap roots descend so low, and the nutritive rootlets are developed at such depth, as to be beyond reach of the summer’s heat and drought. For while it is true that rootlets immersed in air-dry soil may absorb plant-food, this absorption is very slow and can only be auxiliary to the main root system which, instead of terminating in the surface soil as in the humid region, will be found to begin to branch off at depths of 15 and 18 inches, and may then in sandy lands descend to from 4 to 7 feet even in the case of annual fibrous-rooted plants like wheat and barley.[64] In the case of maize the roots of a late-planted crop may sometimes be found descending along the walls of the sun-cracks in heavy clay land poorly cultivated; and it frequently matures a crop without the aid of a single shower after planting. See figures 33, 34.

Fig. 31.—Hop Root from Sacramento
Bench-land.

The annexed plate (No. 32) shows the main roots of two native perennial weeds of California, the goosefoot (Chenopodium californicum) and the figwort (Scrophularia californica), common on the lower slopes of the coast ranges. The soil was a heavy clay loam or “black adobe” resulting from the weathering of the clay shale bedrock, fragments of which are so abundantly intermixed with the substrata that excavation of the roots became very difficult. Yet the main root of the goosefoot went down below the depth of eleven feet.

The main root of the figwort, also, was followed below the depth of ten feet without reaching the extreme end. This proves clearly that the great penetration of the goosefoot was not, as might be supposed, due to its bulbous root. Yet such thickening of the root just below the crown is a rather common feature in arid-region plants, and can here be noted even in the figwort, within whose botanical relationship bulbous roots are almost unknown.

Any one accustomed to the cornfields of the Middle West, where in the after-cultivation of maize it is necessary to restrict very carefully the depth of tillage to avoid bringing up a mat of white, fibrous roots, will be at once impressed with the remarkable adaptability of maize to different climatic conditions, as exhibited in such cases and shown in figures 33, 34. In southern California, in the deep mesa or bench soils, corn stalks so tall that a man standing on horseback can barely reach the tassel, and with two or three large ears, are quite commonly grown under similar rainfall-conditions.

Importance of proper Substrata in the Arid Region.—The paramount need of deep penetration of roots in the arid region renders the substrata below the range of what is usually understood by subsoil in the humid climates, of exceptional importance. A good farmer anywhere will examine the subsoil to the depth of two feet before investing in land; but more than this is necessary in the arid region, where the surface soil is often almost thrown out of action during the greater part of the growing season, while the needful moisture and nourishment must be wholly drawn from the subsoil and substrata; an examination of which should therefore precede every purchase of land, or planting of crops.

Fig. 32.—Deep-Rooting of
Native California Goosefoot
and Figwort.