

222 THE LIVING ORGANISMS OF THE SOIL [chap. 



helps to explain both its sterile nature when brought 

 to the surface, and the difficulty and length of time 

 required to develop a state of fertility, especially when 

 dealing with a clay soil in which percolation and aera- 

 tion have been deficient. 



The effect of a low temperature in checking the 

 formation of nitrates is well seen in the way the growing 

 corn turns yellow through nitrogen starvation whenever 

 a cold and drying north-east wind chills the ground 

 in spring : the bright green colour returns as soon as 

 warmer and moister soil conditions restore the activity 

 of the bacteria in the surface layer. King found in 

 the top foot of soil when oats were turning yellow only 

 0-26 parts of nitric nitrogen per million of dry soil, 

 whereas in soil where the oats were green on the same 

 date there was 2-55 parts of nitric nitrogen per million, 

 itself a small amount. The greater warmth of a light 

 soil also causes it to form nitrates quickly in the spring 

 and so assists in producing an early growth. 



But in obtaining early crops, even when the land is 

 rich, a dressing of ready-formed nitrate is often of the 

 greatest assistance, for the development of very early 

 crops may easily outstrip the rate at which the nitrates 

 they require can be formed in the still unwarmed soil. 

 Nitrates are much more freely formed in the summer 

 than in the winter, and as they are not retained by 

 the soil, they may easily be washed away when the 

 crop has been removed, unless weeds or a catch crop 

 sown to that end are present to take up the nitrates 

 and store them as organic compounds of nitrogen for 

 the future enrichment of the land. 



The need for aeration in connection with the nitrify- 

 ing process has already been alluded to when discussing 

 drainage : all processes of working and cultivating the 

 soil assist nitrification, both by the thorough aeration 



