GENEKAL MANUEES -ANIMAL AND VEGETABLE 301 



VEGETABLE MANURES. 



Unmixed vegetable matter is not very largely used 

 as manure on farms. It can generally be employed 

 more economically either as fodder or as litter for 

 animals. It contains comparatively small proportions of 

 fertilising ingredients ; these only become available as 

 the organic matter decays, and decomposition, as a rule, 

 takes place somewhat slowly. Vegetable manures are 

 generally very bulky, and produce considerable mechani- 

 cal effects. 



Green Manures. Crops which are neither cut nor 

 eaten off, but simply ploughed into the land in the fresh 

 condition are called green manures. They add little 

 or nothing in the shape of plant foods, but they increase 

 the quantity of humus and are chiefly valuable for the 

 effects of the latter upon the physical properties of the 

 soil. It is to be noted that though all, or nearly all, 

 the fertilising ingredients in green manures are derived 

 from the soil, a certain proportion may have come from 

 the lower depths, and helps to enrich the surface soil. 

 Green manures may also be useful as a means of prevent- 

 ing loss of nitrogen ; the crops used for green manuring 

 are generally raised as " catch crops " in the autumn, and 

 are thus able to take up the nitrates produced during the 

 late summer and which would otherwise be liable to be 

 washed out of the soil by the winter rains. When taken 

 up by the crops, the nitrogen is converted into insoluble 

 organic compounds which must undergo nitrification be- 

 fore it again becomes available. As the time available 

 for the growth of an autumn catch crop is short, quick- 

 growing plants, e.g., mustard, rape, etc., are considered 

 most suitable for the purpose. If a leguminous crop, 

 such as clover, vetches, lupines, etc., be used, a con- 

 siderable quantity of nitrogen may be actually added to the 



