36 FIELD CROPS 



to a greater or less extent in all soils, but sandy soils contain 

 less of it than clay and loam soils, while the supply in peat 

 and muck is comparatively small. A very large part of the 

 potassium in the soil is in a form which is not available for 

 the use of plants and, as it becomes available very slowly, 

 it occasionally is not present in sufficient quantity for plant 

 growth. It is much less likely to be lacking than phosphorus 

 or nitrogen, however. The supply of potassium in com- 

 mercial fertilizers is obtained from mines, the most important 

 of which are in Germany. Wood ashes are also used to 

 supply potassium to the soil. 



35. Other Elements. Other elements which are neces- 

 sary to plant growth, but which are usually present in all 

 soils in sufficient quantity so that no attention need be paid 

 to them, are calcium, iron, magnesium, and sulphur. Silicon, 

 chlorin, and sodium are also usually present in plants, but 

 they do not appear to be necessary for growth. Calcium 

 contained in lime corrects the soil acidity, which is harmful 

 to most plants; it is also essential to leaf growth. Lime is 

 also necessary for the development of the bacteria which 

 change the nitrogen of the air and that in decaying vegetable 

 matter into forms which can be used by plants. Iron is an 

 essential part of the green coloring matter of plants (chloro- 

 phyll), without which carbon dioxid can not be broken up 

 and starch manufactured. Lime is the only one of these 

 elements which is at all likely to become depleted; it may 

 sometimes be necessary to supply it on wet or acid soils. 



36. Sources of Plant Food. The rocks from which the 

 soil is made are the principal source of all the mineral ele- 

 ments of plant food. Decaying vegetable matter is also an 

 important source of all the elements of plant food, for all are 

 taken up by plants and naturally they are returned to the 

 soil when these plants decay. The two most important 



