ELEMENTS OF PLANT FOOD 35 



bones are very rich in this element. The ordinary fcrms 

 of phosphate fertihzers are raw bone meal, raw rock phos- 

 phate, and acid phosphate. 



34. Potassium. Potassium, or potash, the latter the 

 form of potassium to which reference is usually made, is the 

 third great element of plant food. Like phosphorus, it is 

 present 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. Sea weed now promises to be 

 a source of this element. Wood ashes are also used to 

 supply potassium to the soil. 



35. Other Elements. Other elements which are neces- 

 saiy 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, 

 chlorine, 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 dioxide cannot be broken up 

 and starch manufactured. Lime is the only one of these 

 elements which is at all likely to become depleted. Liming 

 to correct soil acidity is very commonly practiced in the 

 eastern and southern United States. 



