Plants as Affected by Lnsufficient Food. 145 
Ammonium sulfate is mostly changed to nitrates in the 
soil before it is used by plants, and hence is less prompt 
in its action than sodium nitrate. It is more tenaciously 
held by the soil than sodium nitrate, and is therefore less 
likely to be lost by washing. 
263. Phosphorus is used by plants in the form of 
soluble phosphoric acid, which exists in the soil in combi- 
nation with lime, iron and alumina, as phosphates of these 
substances. It may be purchased in the form of mineral 
phosphate of lime, and ground bone. The former is insolu- 
ble in water unless it has been treated with strong acid, 
when it is known as acid phosphate or superphosphate. Phos- 
phoric acid is not readily washed out from the soil, even in 
its soluble form. 
264. Potassium is used by plants in the form of pot- 
ash, i. e., potassium combined with oxygen. Potash ex- 
ists in the soil, mainly, in combination with chlorin (chlorid 
or muriate of potash), with sulfuric acid (sulfate of potash), 
or with nitric acid (nitrate of potash). All these forms of 
potash are freely soluble in water, and are therefore imme- 
diately available as plant food. Nitrate of potash (saltpeter) 
is a most valuable fertilizing material, since it contains both 
potash and nitrogen, but unfortunately its price is too high 
to render its use for this purpose economical. The muriate 
either pure or in crude form (Kainit), and sulfate may, on 
the other hand, be purchased at reasonable prices. The lat- 
ter is considered preferable for tobacco and potatoes as it is 
thought to produce a better quality of product. The muri- 
ate acts more promptly than the sulfate, however. 
265. Wood Ashes are a Valuable Fertilizer, espe- 
cially when unleached, as they contain both potash and 
