228 THE LURE OP THE LAND 



feldspar, and in this condition be wholly inaccessible 

 for food purposes. These compounds must be broken 

 down and the potash liberated before its maximum of 

 utility is obtainable. Potash is especially useful in 

 growing root crops such as potatoes, and in many soils, 

 such as the blue grass region of Kentucky, adds greatly 

 to the yield of Indian corn. 



IMPORTANCE OF HUMUS. 



Another thing which a fertile soil requires is a lot 

 of decaying organic matter, known as humus. In the 

 decay of organic matter not only is nitric acid formed 

 from the protein which is present in the material, but 

 also the salts of potash and phosphoric acid which are 

 present in the organic materials are set free in a form 

 suitable for plant nutrition. In addition to this, the 

 presence of the decaying organic matter known as humus 

 profoundly modifies the physical condition of the soil, 

 rendering it more porous, more retentive of moisture, 

 and more easily cultivated. 



To supplement any deficiency in plant foods, manures 

 and fertilizers are to be freely used. The common un- 

 derstanding of the difference between these bodies is 

 that a manure is something produced on the farm, 

 principally by the farm animals, while a fertilizer is a 

 manufactured article sent from the centers of manu- 

 facture for the use of farmers. High-grade fertilizers 

 containing 4 or 5 per cent, of potash, 2 to 4 per cent, 

 of nitrogen-bearing materials, and 8 to 12 per cent, of 

 available phosphoric acid, are applied to soils in large 

 quantities in order to furnish materials for vigorous 

 plant growth. The fertility of the soil, therefore, is 

 simply its feeding ability. The difference between a 

 poor soil in this respect and a rich one is that which is 



