86 ELEMENTS OF AGKICTJLTURE 



of red or yellow. The amount of iron in most culti- 

 vated soils seldom falls below 1 per cent. 



88. Other Elements in Soils. — Sodium, magnesium, 

 chlorine, aluminum, and a number of other elements 

 besides those already mentioned are found in soils; 

 but they are of little importance as plant foods, so need 

 only be mentioned. 



89. Analyses of Soils. — To analyze a soil means to 

 determine the various elements and compounds it con- 

 tains. The following figures show the comparative 

 composition of samples of rich and poor soils. These 

 figures are from actual analyses, the rich loam being a 

 sample of soil from a rich bluegrass region, and the 

 poor sand a soil from a very poor region of country: 



Rich Loam. Poor Sand. 



Limestone Soil. Sandstone Soil. 



Organic matter 5 per cent. 2 per cent. 



Sand .' 75 per cent. 90 per cent. 



Clay 20 per cent. 8 per cent. 



With the sand and clay: ' 



Potash 0.5 per cent. 0.1 per cent. 



Phosphoric acid 0.2 per cent. 0.05 per cent. 



Lime 0.5 per cent. 0.05 per cent. 



Iron 2. per cent. 0.8 per cent. 



Questions 



1. How do sand and clay aid the growth of plants? 



2. Name four mineral elements necessary to plant growth. 



3. From what does the phosphorus in soil come? 4. With 

 what other compounds is it usually combined? 5. Give 

 some idea of the amount of phosphoric acid found in soils. 



6. From what rocks does the soil's supply of potash come? 



7. With what substance is the potash of soils usually com- 

 bined? 8. What rocks form soils rich in lime? 9. Why do 



