32 FIELD CROPS 



extend their roots to ol)tain water, nor is tliere sufficieni air in 

 the soil for the healthy growth of roots. Consequentl>^ 

 plants in wet locations often suffer first when drj- weather 

 comes, for their root systems are so small and so shallow- 

 that they are unable to obtain enough w^ater. For the same 

 reason, plants are more likely to be damaged by a drought 

 which follows a w^et spring than by one succeeding a moder- 

 ately dry one. Enough w^ater, but not too much, is essential 

 for the best growth of plants. Good drainage helps by tak- 

 ing off the surplus water and allowing air to penetrate the 

 soil, thus inducing deeper rooting. 



31. Roots Take in Plant Food. When the chemist ana- 

 lyzes a plant, he finds many things besides the carbon which 

 is taken from the air and the w^ater wdth which it is com- 

 bined to make starch. He finds compounds of nitrogen, 

 phosphorus, potassium, calcium, and other substances. Now 

 these elements, wdth the exception of nitrogen, are not to 

 be found in the air in appreciable quantities, and the nitro- 

 gen of the air is not in a form in which most plants can 

 use it. Phosphorus and potassium and the other things are 

 to be found in the soil, for soil is simply decayed or disinte- 

 grated rock, and these elements are a part of all rocks. The 

 surface soil which is penetrated by the roots also contains 

 decaying organic matter, and it is from this that plants obtain 

 their supply of nitrogen. Nitrogen, phosphorus, potassium, 

 and the other elements can not be taken in by the roots of 

 plants and pass through their tissues in a solid state, hence 

 they must be in a soluble form so that they can be carried by 

 the water w^hich is drawn in by the roots. Most of the com- 

 pounds of these elements are not soluble in pure water, but 

 the water in the soil contains some carbon dioxide given off 

 by the roots, and this carbon dioxide is an efficient aid in 

 dissolving the material in the soil particles. These com- 

 pounds of potassium, nitrogen, phosphorus, etc., are known 

 as plant food. 



