MANUAL FOR SUGAR GROWERS. 21 



more freely than a compact one, is self-evident and 

 calls for no further comment here. 



Porosity of soils, then, must be maintained by til- 

 lage and the introduction of vegetable matter as a 

 means both of retaining the requisite quantity of 

 moisture and of enabling the excess to drain away. 



Plant-food in soils. — All plants appear to require 

 essentially the same kinds of plant-food, but in va- 

 rying proportions ; the adjustment of the kind and 

 quantity by artificial means constitutes the art and 

 science of manuring. 



Plant-food derived from soil may be classed as 

 mineral and nitrogenous. The mineral matters found 

 in plants are potash, lime, magnesia, oxide of iron, 

 silica, phosphoric acid, sulphuric acid, chlorine (as 

 chlorides), with others of apparently less importance, 

 such as soda, alumina, manganese, etc. 



Now, from what has been already said it is evident 

 that, to be of value, these must exist in the soil in a 

 form capable of being absorbed by the plant-roots ; 

 that is, they must be capable of being dissolved. It 

 might therefore be assumed that the quantity of 

 mineral matter available as plant-food in a soil might 

 be easily estimated by finding how much was capable 

 of being extracted by water. If the experiment be 

 made, it will be found that little or no potash or 

 phosphate, for instance, can be removed by treatment 

 with water, even from soils bearing luxuriant crops ; 

 it follows, therefore, that the root must possess some 

 solvent power not possessed by water, by which 

 means it is enabled to dissolve and assimilate that 

 portion of the plant-food insoluble in water. From 



