32 Practical Farming 



itself it does not feed plants it is important as clay in 

 increasing the ability of soils to retain moisture and plant 

 food. Clay is essential to bind together the particles of 

 silica and to make clayey loams, which are more retentive 

 than pure sands. 



The metal potassium is one of the most important of 

 all the elements that contribute to the growth of plants in 

 the soil. The first material that we can discover that is 

 formed within the tissues of a plant is starch, and from 

 this starch all the long series of substances known as 

 carbohydrates are made, and the woody structure of 

 plants is built up of these by the living matter in the 

 plant cells. But without potassium in a soluble form no 

 plant can make starch, and hence can make no growth. 

 Not that there is any potassium in starch, but it has been 

 well proved that in its absence no starch can be formed. 

 Plants could not use potassium in a metallic form, and it 

 is found in combination with oxygen, making potash. 

 The granitic rocks contain the silicate of potassium in 

 their feldspar and the clays formed from the decomposi- 

 tion of these rocks are rich in potassium, which becomes 

 slowly available to plants as potash through the action of 

 the carbonic acid in the soil waters. It has been found, 

 too, that the carbonate of lime acts on this insoluble potash 

 and sets it free for plants, and lime sulphate also has a 

 similar effect. In New Jersey and other sections of the 

 Atlantic coast there are beds of greensand marls which 

 contain a mineral known as glauconite. This has a very 

 considerable percentage of potash, which becomes slowly 

 available when appHed to the soil. Kaolin, the white 

 clayey material from which chinaware is made, also con- 



