Plant Food in the Soil 81 



Then again, a soil may be abundantly supplied with all 

 the elements of plant food in a perfectly available form, 

 but if there is no water to dissolve them for the use of 

 plants the soil will be a barren one, or produce only such 

 plants as are able to exist in desert conditions. The re- 

 sults of irrigating the desert lands of the West show this 

 very plainly. Still again, the soil may possess an abund- 

 ance of plant food, but the temperature of the soil may be 

 kept too low by the presence of water near the surface, 

 the water, too, shutting out the full supply of air which is 

 essential to plant roots. So the soil will be unproductive 

 except in such plants as prefer these saturated conditions, 

 and the growing of farm crops on such land imperatively 

 demands the drainage of the surplus water. Then, too, 

 as we have said, the physical and mechanical nature of 

 the soil may prevent the prosperity of plant Hfe on it, and 

 means for ameliorating these conditions are usually within 

 the reach of the farmer, as we shall endeavor to show 

 later. 



While all the elements named are essential to plant life, 

 some of them exist in all soils so abundantly that there is 

 never any need for using them as additional plant food. 

 Iron, for instance, is the most abundant of all the ele- 

 ments. Our clays are colored with the oxidized iron, 

 and we could hardly make up a mixture of fertilizing 

 materials without at the same time getting some iron in it. 

 The green color of plant leaves caused by the chlorophyll 

 grains, is due to iron, and so long as trees and plants in 

 general make green leaves it is evidence of an abundance 

 of iron. Sulphur is essential, but it is always in combi- 

 nation with other elements in the soil and the fertilizers 



