160 PLANT GROWTH 



take the place of a mulch. It increases the temperature of 

 the soil, prevents the growth of weeds, and retards to a high 

 degree the evaporation of soil moisture. 



Hoeing or cultivating soil is a common practice, but many 

 experiments have failed to give evidence that it is valuable. 

 It appears to have no merit in a direct way, but when done 

 properly increases the absorption of rainfall, increases the 

 aeration of the soil, destroys weeds, and probably increases 

 the bacterial action in the soil. All this can be accomplished 

 with a good mulch of grass, leaves, or other materials applied 

 over the loose soil, in which case there is no need to hoe. If 

 cultivating or hoeing is deep enough to destroy many roots 

 the objection is obvious. 



The water-holding power of the soil can be improved, 

 even though it depends largely on the size of the soil particles. 

 In a small flower bed it may sometimes be desirable to add 

 sand to a heavy clay soil or to add clay to a sandy soil. It is 

 usually easier, however, to improve a heavy or a light sandy 

 soil by adding humus. The humus has a high water-holding 

 capacity and it will increase the bacteria and animal popula- 

 tion, both of which will aid in increasing the water-holding 

 capacity. There are probably few cases where the water- 

 holding capacity is too great and in such cases good aeration 

 should overcome any difficulty. 



Most plants require a well-drained soil; one in which the 

 roots will not be immersed in water. Areas which are not 

 drained naturally should have underground drainage in the 

 form of tile about three feet below the surface. Drainage 

 often increases the available soil water for plants by increas- 

 ing the root depth, and makes the soil more porous. Roses, 

 for example, which failed before drainage, may grow perfectly 

 after drainage. 



Humus includes all stages of decaying organisms (plant 

 and animal) in the soil. In most soils the humus is largely 

 of plant origin because the lignified cell walls decompose very 



