PART I. AGRONOMY 



CHAPTER I 

 THE SOIL AND THE PLANT 



What Plants Get from the Soil. About 5 per cent of 

 the weight of a plant is made up of elements that come 

 from the soil, and about 95 per cent is composed of ele- 

 ments that come from air and water. The most impor- 

 tant elements taken from the soil are nitrogen, phosphorus, 

 potassium, and calcium. The elements that a plant takes 

 from air and water are carbon, oxygen, and hydrogen. 

 When a plant is burned these last three elements and nitro- 

 gen disappear into the air. In the ash that remains are 

 all the elements, except nitrogen, that came from the soil. 

 At the outset it is well to keep in mind that although four 

 fifths of the air is nitrogen, plants cannot get this element 

 directly from the air, but obtain it from nitrogen-con- 

 taining substances in the soil. 



What Soil Is. The soil consists of mineral particles; 

 decaying plants and animals; living organisms, such as 

 bacteria and worms ; soil water ; and soil air. The min- 

 eral particles have been formed by the breaking up and 

 decay of solid rock. Rain, air, frost, and streams are 

 some of the agencies that have changed solid rock into 

 soil. Even earthworms and many other small forms of 

 animal life are active agencies in making soil. The de- 

 caying plants and animals form the organic matter of the 



