194 THE LIVING ORGANISMS OF THE SOIL [chap. 



active production of nitrates from the organic com- 

 pounds of nitrogen present in the soil — and this is 

 necessary if the crop is to be kept supplied with the 

 nitrogen required for its growth — the following condi- 

 tions are requisite : — The presence of the nitrifying 

 organisms in sufficient quantities, a certain degree 

 of temperature, darkness, sufficient moisture for the 

 development of the bacteria, free aeration of the soil 

 to supply the oxygen necessary, and a base to neutralise 

 the acids as they are produced. 



The scanty number of nitrifying bacteria in any 

 subsoil below the cultivated layer 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 aeration 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 nitrifying bacteria in the surface layer. King 

 found in the top foot of soil when oats were turning 

 yellow only 0-026 parts of nitric nitrogen per million 

 of dry soil, whereas in soil where the oats were green 

 on the same date there was 0-255 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 



