SOIL. 91 



261. How many crops will the phosphoric acid sup- 

 ply? 



262. How many crops will the potash supply? 



263. A crop of oats yielding 60 bushels per acre, re- 

 quires of nitrogen 73.3 pounds, of phosphoric acid 25.7 

 pounds, of potash 61.5 pounds. How many crops of such 

 oats will the nitrogen content of this soil supply ? Phos- 

 phoric acid? Potash? 



Important truth. Nature has not stored these ele- 

 ments in the soil in such a form as to be wholly plant 

 food. Only a small percentage of any one is directly 

 available at any time. Available plant food is readily lost 

 by leaching and drainage. Hence, if all the elements had 

 been in an available form originally the soil would have 

 lost its fertility ages ago. Great quantities of plant food 

 are locked up in the soil in .such forms that they can be 

 made available only by good tillage, crop rotation, winter- 

 growing crops, humus, stable manure, and legumes. These 

 together form the key which unlocks the door to Nature's 

 richest storehouse. 



Moisture of the soil. Water is important to plant 

 growth. It dissolves plant food and carries it to all parts 

 of the plant body. Crops often fail or are greatly in- 

 jured by an insufficient supply of moisture in the soil. 

 How to keep enough water in the soil during the grow- 

 ing season for all the needs of the growing plant is a very 

 important problem. 



Saving moisture by plowing. 



264. Samples of soil from a field partially plowed 

 were taken to a depth of 12 inches, two weeks after plow- 

 ing was done. The sample taken from the plowed land 

 showed 13.87 per cent of water in the soil, while the sam- 



