THE MAIN TYPES OF SOIL 51 



thus add materially to the supply of humus. 

 The crop which in this country is most usually 

 cultivated for this purpose is white mustard, 

 which within six weeks or two months of 

 being sowed may have grown to a height of 

 a foot or more, producing excellent results 

 when incorporated with the soil. Other 

 crops that are used for a like purpose are rape, 

 and certain leguminous plants, such as lupins, 

 serradella, and clover. Other things being 

 equal, it is better to use a leguminous plant 

 for this purpose, because, as is well known, 

 such plants not only add organic matter to 

 the soil, but they also increase the stock of 

 nitrogen through the power that they are 

 known to possess of fixing this material from 

 the supply present in limitless quantity in the 

 atmosphere. But the seeds of leguminous 

 plants are more costly than those of mustard, 

 and, moreover, their cultivation demands a 

 longer period of growth, and is more de- 

 pendent upon suitable conditions of soil and 

 weather. 



Although theoretically one should select a 

 leguminous crop for the supply of humus, 

 experiments on green manuring conducted 

 by the Royal Agricultural Society of England 

 on sandy soil at Woburn, in Bedfordshire, 

 show that better crops are often got after 



