424 SOILS: PROPERTIES AND MANAGEMENT 



with the mycclia of fungi. It is thought that the mycor- 

 rhiza aid the higher plants to obtain nutriment that they 

 must strive for in competition with the fungi. 



Mycotrophic plants are also able to grow with a very 

 small transpiration of moisture, as is well known to be 

 the case with many conifers; and this restricted tran- 

 spiration would doubtless result in lack of nutriment were 

 it not for the assistance of the mycorrhiza. 



346. Plant roots. The roots of plants assist in pro- 

 moting productiveness of the soil both by contributing 

 organic matter and by leaving, on their decay, openings 

 which render the soil more permeable to water and which 

 also facilitate drainage and aeration. The dense mass of 

 rootlets, with their minute hairs that are left in the soil 

 after every harvest, furnish a well-distributed supply of 

 organic manure, which is not confined to the furrow slice, 

 as is artificially incorporated manure. The drainage and 

 aeration of the lower soil, due to the openings left by the 

 decomposed roots, are of the greatest importance in heavy 

 soil, and the beneficial effects of clover and other deep- 

 rooted plants are due in no small measure to this function. 



MICROORGANISMS OF THE SOIL 



Of the microorganisms commonly exist- 

 ing in soils, the greater part belong to 

 plant rather than to animal life. Of the 

 latter, the only organisms of well-known 

 economical importance are the nematodes 

 (Fig. 58), whose injurious effect on plant 

 growth is accomplished through the for- 

 mation of galls on the roots, in which the 

 FIG. 58. Nema- young are hatched and live to sexual 



todes entering ' 



a plant root. maturity. 



