HOW BOILS L08E MINERAL MATTER 111 



what the plants did not use would soon be washed out 

 by rains and the soil left with none at all. Fortunately, 

 most of the phosphorus in the Boil is combined with 

 iron and aluminum, in which form it is nearly insolu- 

 ble in water and unavailable for plants. By some pro- 

 cesses in the soil, probably the action of the acid- in 

 humus, the insoluble phosphates are gradually being 

 converted into soluble compounds which become food 

 for growing plants. This change is so gradual that 

 just about enough soluble phosphates are made each 

 year to supply the demand of the yearly growth of 

 nature's crops. Nature allows her crops to go on 

 glowing year after year, but little in the way of plant 

 food being removed from the soil; so each year only 

 enough plant food is required to supply the season's 

 growth, and the supply is always abundant. 



On cultivated lands a crop is usually gathered each 

 year, and with it go from the soil the soluble phos- 

 phate* it hai taken up in growing. The n.\i year the 

 soil must supply more plant food for the season's crop, 

 and so this goes on each year, The formation of solu- 

 ble phosphates in manv soils is so slow that there is 

 not sufficient to supply this constant demand, and inch 

 soil? arc said to be deficient in phosphoric acid. Verjf 

 often they ha\e a large store of insoluble phosphates, 

 but arc unable to manufacture the soluble compounds 

 fast enough to supply the demand of the crops. The 

 great store of phosphates contained in most soils is 

 shown by the following figures: Au am- cOTen 

 square feet; then if the soil be twelve inches deep, we 

 have for an acre of ground 13^60 cubic feel of surface 



