68 THE FIRST BOOK OF FARMING 



especially in those parts of the country where the 

 land does not freeze. The phosphorus, potassium 

 and calcium also exist in most soils in considerable 

 quantities, but often are not available; thorough 

 tillage and the addition of organic matter will help 

 to make them available, and new supplies may be 

 added in the form of fertilizers. Calcium is found 

 in nearly all soils in sufficient quantities for most 

 crops, but sometimes there is not enough of it for 

 such crops as clover, cowpea, alfalfa, etc. It is 

 also used to improve soil texture. The entire sub- 

 ject of commercial fertilizers is based almost en- 

 tirely on the fact of the lack of these four elements 

 in the soil in sufficient available quantities to grow 

 profitable crops. The plant gets its phosphorus 

 from phosphoric acid, its potassium from potash, 

 and its calcium from lime. 



There is a class of plants which have the power 

 of taking free nitrogen from the air. These are 

 the leguminous plants; such as clover, beans, cow- 

 peas, alfalfa, soy bean, etc. They do it through the 

 acid of microscopic organisms called bacteria which 

 live in nodules or tubercles on the roots of these 

 plants (Figs. 34-35). Collect roots of these plants 

 and find the nodules on them. The bacteria take 

 nitrogen from the air which penetrates the soil and 

 give it over to the plants. Here is another reason 

 for good soil ventilation. 



This last fact brings us to another very important 

 property of soils. Soils have existing in them many 

 very small plants called bacteria. They are so very 



