THE MAIN TYPES OF SOIL 63 



character of the weather in autumn and 

 winter has, however, much influence on the 

 supply of nitrates ; if the rainfall is heavy 

 they are largely washed out of the land, 

 whereas if the season is dry they are more 

 likely to be utilized by winter crops. So 

 intimate is the relationship between the 

 rainfall of the last quarter of the year, and 

 the supply of nitrates, that by a study of the 

 rainfall-figures it is possible to make a fairly 

 accurate forecast of the yield of the succeed- 

 ing wheat crop. If, on the other hand, 

 winter wheat should not have been well 

 supplied with natural nitrates, it will be 

 found to respond readily to spring dressings 

 of nitrate of soda. Other crops, like turnips, 

 which are not sowed until summer, make 

 their growth at a time of the year when the 

 conditions of nitrification are at their best. 

 As a consequence such crops are well pro- 

 vided with nitrogen, which has been con- 

 verted into an available form during their 

 period of growth, and they are therefore not 

 so dependent on supplies of artificial nitrogen 

 as a crop that makes its growth at a season 

 of the year when temperatures are lower. 

 While there is no doubt that turnips respond 

 in many cases to artificial dressings of nitrate 

 of soda or sulphate of ammonia, the quantity 



