4 Development and Activities of Roots of Crop Plants. 



and their excretion of carbonic acid (Truog, 1918). A knowledge of root 

 position and extent is of prime importance in studies of aeration and the 

 closely allied phenomenon of soil toxicity. Passageways left in the soil, 

 after the roots have decayed, afford greatly increased possibilities for aeration. 



The important problem of the relation of various methods of tillage to root 

 development is one of which we are almost entirely ignorant. The value of 

 various depths of plowing, listing, or subsoiling, and the preparation of the 

 seed-bed, as well as the time, depth, and manner of subsequent cultivation, 

 is judged entirely by the increase or decrease in growth and yield of the 

 above-ground parts with scarcely a thought as to the more direct cause, the 

 effect of these practices upon root activities and development. It should be 

 clear that a knowledge of root development under each of the diverse methods 

 of tillage, systems of mulching, etc., will not only give a logical answer for the 

 results obtained by these practices, but will form a scientific basis for the appli- 

 cation of other methods or combinations which may result in greater yields. 



Crop rotations on different types of soil and under different climatic con- 

 ditions should be worked out with constant reference to root relations. It 

 may be found practicable, especially in semiarid regions, to grow short-rooted 

 and densely-rooted crops alternately with those of longer and more spreading 

 root systems. In humid regions, under intensive agricultural conditions, 

 two such crops might be grown in the same field at the same time. In fact, 

 this is a common practice in India, where mixed cultures usually outyield 

 pure ones. The selection, breeding, and adaptation of crops for arid and 

 semiarid regions should logically center about their efficiency as absorbers 

 and conservers of water. Plasticity of root systems as to depth, lateral 

 spread, degree of branching, etc., goes far toward determining the ability of 

 a crop to make sufficient growth and yield to warrant its cultivation in dry 

 lands. Moreover, under these conditions root competition is an important 

 factor in determining the rate of seeding. 



It seems not improbable that some of our best yielding crops may be able 

 to outstrip others largely because of their greater efficiency in securing a larger 

 and more constant supply of water and nutrients. Why certain artificial 

 mixtures of grasses and other herbs may thrive in pastures and meadows, 

 while others do less well, must depend to a large degree upon competition of 

 root systems. This is the case in native grassland, where it is usual for 200 

 to 300 individuals or groups of individual plants to grow in a single square 

 meter, due to lessened competition resulting from absorption at different 

 soil-levels and from maximum above-ground activities at different times of 

 the growing-season. 



Just as a knowledge of root systems of native plants makes far more exact 

 their value as indicators of lands with the possibility of crop production, 

 likewise this knowledge of underground parts is of great value in solving 

 problems of soil erosion, whether by wind or by water. This holding of the 

 soil for cultivation applies equally to plants of pasture and meadow lands 

 and to a certain extent to annual crops. Likewise, keeping the soil free from 

 noxious weeds, and especially perennials, is again a problem concerned pri- 

 marily with underground plant parts. 



The relation of the disease resistance of crop plants to root development 

 is an important one and warrants thorough study. Recent investigations 



