SOILS 103 



Oxygen exists in the air as a free element, is taken in by 

 respiration through the leaves of the plant, and functions in 

 a similar way as it does in the animal body. 



Carbon, oxygen and hydrogen constitute the largest part 

 of the agricultural plant, but plant growth is not possible 

 without seven other elements supplied by the soil. Iron is 

 one of the essential elements of plant-foods, but the amount 

 required is small and the amount contained in the soil is 

 large. Sulphur is found in plants in small amounts and is 

 essential to plant growth. The supply of sulphur in normal 

 soils is not large, but with the decay of organic matter a 

 great deal of sulphur passes into the air and is brought back 

 to the soil, dissolved in rain. Under normal conditions the 

 sulphur supply is ample to meet the needs of the farm crops. 



There are five other essential elements of plant-food that 

 require special consideration in connection with permanent 

 soil fertility. They are nitrogen, phosphorus, calcium, mag- 

 nesium, and potassium. In studying these five elements we 

 must note the following points: the soil's supply, the crop 

 requirements, the" loss by leaching, the methods of liberation, 

 and the means of renewal. The neglect of one or more of 

 these points will reduce the fertility of cultivated soils and 

 bring about conditions that are well known in the impov- 

 erished older farm lands of the United States. Intelligent 

 attention to these factors will restore and make productive 

 such lands. 



Fertility in normal soils. Of the important mineral ele- 

 ments potassium is the most abundant in common soils. Doc- 

 tor Cyril G. Hopkins of Illinois states that in an average of 



