583 



The nitrifying organisms in the soil cannot develop in the pres- 

 ence of a free acid and it often happens that a soil becomes sour being 

 overcharged with nitric acid. In cases of this kind the application of 

 lime will counteract the acidity and restore the soil to a healthy 

 state. Most soils, however, contain a sufficiency of lime for this pur- 

 pose and it need only be applied iu rare instances. 



Sulphur and phosphorus are both taken from the soil by plants. 

 Both substances are found in the form of calcium salts. Sulphur is 

 found largely in the form of gypsum. Phosphorus is found in most 

 soils in combination with iron. Plants deficient in phosphorus assume 

 a red appearance. 



Potassium is essential to all plant life in the formation of carbo- 

 hydrates such as starch, sugar, and cellulose. If potassium is absent 

 from the soil the growth of plants stops and the leaves refuse to con- 

 tinue their functions, that is the formation of starch within the chloro- 

 phyll grains (green colouring matter). 



Magnesium enters a plant in the form of sulphates and phos- 

 phates as also does calcium to which it is closely allied. Magnesium 

 is said to work with nitrogen in the formation of chlorophyll and pro- 

 toplasm. 



Iron is widely distributed in soils and is also necessary for the 

 formation of chlorophyll of plants. 



All combinations of lime that are necessary for plant nutrition 

 have their origin in calcium which is chiefly of use in strengthening 

 the fabric of the plant. Besides this it is valuable in fixing oxalic acid 

 which although produced by most plants is poisonous to them. 



The subsidiary nutritive substances such as sodium and silicium 

 also play their important parts in the growth of plants. Sodium is 

 taken up by most plants in the form of common salt which in itself 

 has considerable manurial value when applied to soils for certain 

 crops. 



Silicium is always present in soil as silica and like lime is used to 

 strengthen the cell walls. 



Other substances of this class are occasionally present in plants 

 but are not sufficiently important to require mention in detail here. 



The question of how to replace those nutritive substances which 

 are abstracted from the soil during the growth of plants is an import- 

 ant one to the agriculturist and embraces the all important subject of 

 manuring. Green manuring has already been touched on, other 

 manures are generally divided into three kinds, vis. (a) Stable or Farm- 

 yard manure, (b) inorganic manures, (c) organic manures. 



Stable manure contains all the food substances of plants and 

 when available is the best means of replacing those substances which 

 have been exhausted from the soil during the growth of the crop. In 

 this country it is not obtainable in sufficient quantities, unfortunately, 

 to supply anything like the demand, but when possible it should be 

 used in preference to all others. 



Bones for field crops or fruit trees are extensively used as manure. 

 The fertilizing qualities are lasting when the bones are used in a 

 crushed state and more active when in the form of powder. Bones 

 contain large quantities of phosphoric acid which is a valuable ferti- 



