90 NITROGENOUS MANURES [chap. 



a single ploughing, and this light preliminary prepara- 

 tion of the soil is not followed up by any further 

 cultivation except rolling, and possibly a single hoeing. 

 The winter rainfall not only washes away much of the 

 nitrate that may have been present in the soil at the 

 end of the summer, but it also tends to set the soil down 

 into a close mass, in which aeration may become defec- 

 tive. The main growth of the crop takes place also 

 during the early spring months when low temperatures 

 prevail ; and all these causes work together to reduce 

 the rate of decay and nitrification, and so keep the soil 

 poorly supplied with nitrates. In consequence, wheat is 

 specially responsive to an early supply of some active 

 compound of nitrogen, such as nitrate of soda or sul- 

 phate of ammonia. But apart from this difficulty in 

 obtaining nitrogen, wheat possesses a very extensive root 

 system and also has a comparatively prolonged period 

 of growth, by which means it is able to satisfy its 

 requirements for potash and phosphoric acid, even on 

 comparatively poor land. 



Barley, on the other hand, is a comparatively shallow- 

 rooted crop, occupying therefore a much more restricted 

 layer of soil, and possessing but a short period of growth ; 

 it has not the same opportunity as wheat to search for 

 phosphates, and thus becomes specially dependent upon 

 an artificial supply. Barley, further, makes its chief 

 growth at rather a later date in the spring than 

 wheat does; the land receives a spring cultivation 

 before the barley is sown and the tilth is not destroyed 

 by the winter rains. Thus the nitrification of the 

 natural reserves of the soil can count for much more 

 in the nutrition of barley, and in consequence external 

 supplies of nitrogen are rarely required. 



Swede turnips afford another example of a crop 

 comparatively indifferent to nitrogenous manuring, 



