MANURES 21 



far the most important plant food element thus made 

 into an available form for plants is nitrogen. 



Besides being a great storehouse of plant food, humus 

 improves the tilth of soils by making heavy soils more 

 friable and light soils less open, and also increases the water- 

 holding capacity. 



Stable manure contains a large amount of humus- 

 forming materials, which undoubtedly add very much to 

 its value, and it is probably on this account that it often 

 gives better returns than commercial fertilizers contain- 

 ing the same quantities of what are termed the essential 

 elements. 



The action of manures in the soil can be, and general- 

 ly is, both direct and indirect. Manures act directly when 

 they contain available plant food or when by their decay 

 they yield it to the plant; they act indirectly when they 

 cause chemical action that decomposes soil particles and 

 sets free mineral plant food from the soil itself. Almost 

 all manures act in this indirect way to some extent. A 

 moderate application of stable manure, by its decomposi- 

 tion (which is chemical action) in the soil, has been known 

 to increase the temperature of the soil by three degrees. 

 Lime in itself is a plant food and is largely used by some 

 crops. Most soils, however, contain it in great abundance, 

 yet if quicklime be added to a soil already rich in common 

 limestone it generally serves to increase growth. This 

 is not due to the plant taking up more lime, but 

 rather to the fact that the quicklime starts chemical action 

 in the soil by which some of the locked-up stores of plant 

 food, especially potassium, are made available. The same 

 may be said of unleached wood ashes, which, however, 

 contain potash in addition to lime. Common salt also 

 acts to some extent as a fertilizer, but it is of no value as a 

 plant food. 



