Building up the Fertility of the Soil. 759 



very widely used. These soluble pliosphntes when applied to soil 

 become insoluble in a very finely divided form which plants generally 

 can make use of, and which, in the presence of lime, is quite safe. 



A very superficial study of fertilizer price lists with their pub- 

 lished analyses will show in the case of mixed fertilizers for specified 

 crops that they uniformly contain a very hig-h proportion of 

 phosphoric oxide. There are several'reasons for this. They are: — 



(a) Phosphoric oxide is not liable to loss from the soil even 

 when freely applied; nitrogen is. 



(h) Phosphoric oxide is never dang'erous owing' to its numerous 

 insoluble forms. Potash salts are dangerous when freely 

 used, because they are all very soluble in water. 



(c) Phosphoric oxide is comparatively very widely deficient 



and there is more need, therefore, to afford a supply. 



(d) Phosphoric oxide is returned to the soil less freely in the 

 various residues than are nitrogen and potash. 



(e) Phosphoric oxide occurs more plentifully in produce sold 



oft' the farm than does potash. 



(/) Phosphoric oxide is not replenished from the air as nitrogen 



is. 



(g) The smallest part of phosphoric oxide that can be used by 

 the plant is about five times as heavy as the smallest part 

 of nitrogen that can be so used. 



(h) Phosphoric oxide is built up into the seed to a greater 

 extent than potash. Potash is used in the plant over and 

 over again to bring about the changes in which it is active. 

 Less of it is therefore required. 



Special fertilizers containing a low proportion of phosphoric 

 oxide are rarely needed, and when they are used, the reasons why 

 should be well understood, as loss and harm may easily follow. 



The order of importance of the five requirements in building up 

 the fertility of the soil is : — 



1. Humus, because (a) it is so commonly deficient in amount; 



(6) it serves so many useful purposes; (c) it is the longest 

 step towards the desired goal. 



2. Phosphoric oxide, because it is nearly always required and 



is sold in most of the produce sent off the farm anti can 

 only be replaced by purchased fertilizer or foodstuff. 



3. Lime. 



4. Nitrogen. 



5. Potash. 



When considering the building up of fertility, the beneficial 

 effects of a change of crop ought not to be forgotten. In this connec- 

 tion, nearly any change adapted to the farmer's requirements can be 

 made to yield the main advantages of a well-designed rotation of 

 crops. 



