28 FOOD FOR PLANTS 



tilizers and manures as just so much Nitrate, phos- 

 phoric acid and potash, as we can then at once com- 

 pare the usefulness of all fertilizer materials. No 

 doubt, other substances are necessary for the proper 

 development of crops, but soils so generally supply 

 these in ample quantities that they may safely be 

 neglected in a consideration of soil needs and plant 

 foods. The food of plants may therefore be under- 

 stood to mean simply Nitrate, Phosphoric Acid and 

 Potash. 



Farmyard manure acts in promoting plant growth 

 almost wholly because it contains these three sub- 

 stances; green manuring is valuable for the same 

 reason and largely for that only. Various refuse 

 substances, such as bone, wood ashes, etc., contain 

 one or more of these plant food elements, and are 

 valuable to the farmer and planter on that account. 



The Quality of Manures and Fertilizers. 



While plant food is always plant food, like all other 

 things it possesses the limitation of quality. Quality 

 in plant food means the readiness with which plants 

 can make use of it. In a large sense, this is dependent 

 upon the solubility of the material containing the 

 plant food — not merely solubility in water, but solu- 

 bility in soil waters as well. Fertilizer substances 

 freely soluble in water are generally of the highest 

 quality, yet there are differences even in this. For 

 example. Nitrate of Soda is freely soluble in soil 

 liquids and water, and is the highest grade of plant 

 food Nitrogen; sulphate of ammonia is also soluble 



