THE MAKE-UP OF THE SOIL 11 



chlorine. The humus of the soil contains a great deal of 

 carbon, but the plant gets its carbon from the air through the 

 pores on the leaves. Besides the above, there are some other 

 substances found in the plant which come from the soil. 



The substances mentioned above are called elements of 

 plant food. They are dissolved in the water in the soil and 

 enter the plant through the roots and are carried slowly up to 

 the leaves. In the leaves they are acted upon by the sunshine 

 and are united in various ways with each other and the car- 

 bon of the air to make the different compounds like starch, 

 sugar, fat and protein which are found in the plant. Hydro- 

 gen and oxygen united form water, but hydrogen, oxygen, and 

 carbon combined make starch, sugar, and cellulose, the last of 

 which forms the woody part of the plant. Rankness of growth 

 indicates an abundance of nitrogen; a pale color of the leaves 

 shows need of iron; lime, phosphorus, and magnesium appear 

 in the seeds quite largely; strong stems and a good heading 

 out and earing out of grain plants show a sufficiency of pot- 

 ash. When the different compounds have been formed in 

 the leaves they have to be moved to other parts of the plant, 

 for instance, in the potato plant starch is moved down to the 

 potato. Potash, magnesia, lime, and chlorine are important 

 in these movements. Nitrogen, sulphur, and phosphorus are 

 necessary for life processes to go on in the plant. The part 

 which sodium and silicon play in the plant has not, as yet, 

 been fully determined. In some cases they appear to be 

 necessary, while in other cases they are not. 



It has been shown by analysis that most soils contain 

 enough of the different plant foods to furnish crops for hun- 

 dreds of years. Professor King, of Wisconsin, has demon- 

 strated that there is enough potash in the surface soil one 



