A FERTILE SOIL 



149 



phosphates. These are often not available to plants, 

 but can to a certain extent be made available 

 through tillage and by adding humus to the soil. 



Potash in the soil. The plant finds potassium in 

 potash which exists in the soil. Potash like phos- 

 phoric acid often exists in forms which the plant 

 cannot use but may be made available to a certain 

 extent by tillage, the addition of humus, and the ad- 

 dition of lime to the soil. 



Lime in the soil. Most soils contain the element 

 calcium or lime, the compound in which it is found, 

 in sufficient quantities for plant food. But lime is 

 also of importance to the farmer and plant grower 

 because it is helpful in causing chemical changes in 

 the soil which tend to prepare the nitrogen, phos- 

 phoric acid and potash for plant use. It is also 

 helpful in changing soil texture. 



The chemical changes which make the plant foods 

 available are dependent on moisture, heat, and air 

 with its oxygen, and are therefore dependent largely 

 on texture, and therefore on tillage. 



When good tillage and the addition of organic 

 matter and lime do not render available sufficient 

 plant food, then the supply of available food may 

 be increased by the application of manure and fer- 

 tilizers. 



It will be seen that all these classes of properties 

 are necessary to furnish all the conditions for root 

 growth. 



The proper chemical conditions require the pres- 

 ence of both physical and biological properties and 



