22O THE FIRST BOOK OF FARMING 



Plant roots need for their growth and develop- 

 ment (see Chapter II) : 



A mellow yet firm soil. 



A moist soil. 



A ventilated soil. 



A warm soil. 



A soil supplied with plant food. 



Decaying organic matter or humus is one of the 

 most important ingredients of our soils. Because: 



It greatly influences soil texture and therefore the 

 conditions necessary for root growth. 



Its presence or absence greatly influences the at- 

 titude of soils toward water, the most important 

 factor in plant growth. Its presence helps light, 

 sandy soils to hold more water and to better pump 

 water from below, while it helps close, heavy soils 

 to better take in 'the water which falls on their sur- 

 face. Its absence causes an opposite state of affairs. 



The presence of organic matter checks excessive 

 ventilation in too open, sandy soil by filling the pores, 

 and improves poor ventilation in heavy clay soils 

 by making them more open. 



Humus, on account of its color, influences the 

 heat absorbing powers of soils. 



The organic matter is constantly undergoing more 

 or less rapid decay unless the soil be perfectly dry 

 or frozen solid. Stirring and cultivating the soil 

 hasten this decay. 



As the organic matter decays it adds available 

 plant food to the soil, particularly nitrogen. 



As it decays, it produces carbonic acid and other 



