Factors Limiting Closeness of Planting 183 



its occupants, but disaster as well. Nor do the roots 

 of the plants which occupy the field constitute the only 

 demand for free oxygen in the soil, for the various 

 fermenting germs which transform humus into avail- 

 able nitrates must have free oxygen, or the all- 

 important nitric acid cannot be made, and the farm- 

 yard manures applied to the soil must lie there unal- 

 tered and of no avail. 



Soil temperature reduced by too close planting. Then, 

 again, too heavy verdure above the soil so completely 

 absorbs the heat from the surrounding air and dissi- 

 pates it again into space, that the soil temperature can- 

 not rise high enough to produce the maximum rate 

 of solution and production of plant -food, nor the 

 maximum root pressure so essential to sending the dis- 

 solved and prepared food into the foliage above, where 

 assimilation takes place ; while the humus and ma- 

 nure-fermenting germs themselves must work the slower 

 the lower the soil temperature is after it falls below 

 98 F. It is true that available nitrates may be applied 

 to the soil direct, and other of the ash ingredients in 

 soluble form may be added, or the soil may receive 

 thorough and repeated tillage before the crop is put 

 upon it, and thus a supply in advance be generated, 

 which leaves more of the oxygen and of the soil warmth 

 for the service of the roots; but neither of these con- 

 ditions can be attained except at added cost. 



The sunshine itself is limited. Even when we come 

 to the item of sunshine itself, it is easy to so increase 

 the number of plants that not enough sunshine can be 

 absorbed to produce normal growth, and a diminished 



