able slowly. — Roberts has compiled the analyses of 49 represen- 

 tative soils, made by American chemists, and the following is the 

 result : ' ' The tables reveal the fact that even the poorer soils 

 have an abundance of plant-food for several crops : while the 

 richer soils in some cases have sufficient for two hundred to three 

 hundred crops of wheat or maize. Theaverageof 34 analysesgives 

 to each acre of land, eight inches deep, 3,217 pounds of nitrogen, 

 3,936 pounds of phosphoric acid, and 17,597 pounds of potash, 

 and this does not include that which is contained in the stones, 

 gravel and sand of the soil which will not pass through meshes 

 of one fiftieth of an inch, which, by weathering and tillage, 

 slowly give up their valuable constituents. — Roberts' ''Fertility 

 of the Land,'' p. 16. 



Fortunately, this great store of plant-food is locked up, else it 

 would have leached from the soil or have been used up long ago. 

 By careful husbandr>'. a little of it is made usable year by 

 year ; and the better the management of the land the more of 

 this food is available to the plant. When the farmer has done 

 his best to get out of the land all that it will give him, then he 

 may add fertilizers for bigger results. 



Plant-food is available when it is in such condition that the 

 plant can use it. It must be both soluble and in such chemical 

 form that the plant likes it. Plant-food which is not soluble in 

 rain water, may still be soluble in soil water (which contains 

 acids derived from the humus) ; and the acid excretions from the 

 roots may render it soluble. But solubilit}^ is not necessarily 

 availability, for, as we have said, the materials must be in such 

 combination that the plant will take them. Thus, nitrate of 

 soda (Na NO3) is available because it is both soluble and in the 

 form in which the plant wants it. But nitrite of soda (Na NOJ 

 is not available although it is soluble, — the plant does not like 

 nitrites. 



1 1 . Nitrogen must probably be in the form of nitrates before it can 

 be usedby most plants. — Nitrogen is abundant. It is approximately 

 four- fifths of the atmosphere, and it is an important content of 

 every plant and animal. Yet, it is the element w^hich is most 

 difficult to secure and to keep, and the most expensive to buy. 

 This is because the greater part of it is not in a form to be avail- 



