i8 



PRINCIPLES OF AGRICULTURAL CHEMISTRY 



is taken up in water, in carbon dioxide, and as free 

 oxygen. Nitrogen enters the plant in nitrates, ammonia, as free 

 nitrogen, and to some slight extent in organic bodies. Hydrogen is 

 taken up as water and ammonia. Since these elements are pres- 

 ent in the soil in the oxidized condition, and are taken up by 

 plants in that form, and not as elements, plant food is usually 

 referred to and estimated in the form of the oxides. We speak 

 of phosphoric acid (P^Og), potash (K 2 O), soda (Na 2 O), lime 

 (CaO), magnesia (MgO), oxide of iron (Fe 2 O 3 ) and silica 

 (SiO 2 ). These terms are used almost exclusively in agriculture 

 and especially in the analysis of soils and fertilizers. We speak 

 of nitrogen (N) and chlorine (Cl), for the former may or may 

 not be present in the oxidized condition, and the latter is injur- 

 ious when oxidized. 



Quantity of Plant Food Required. The method of determining 

 the exact quantity of plant food required is tedious and difficult 

 and has been applied only to two or three plants. 



To determine the minimum quantity of phosphoric acid re- 

 quired by oats, WViff 1 grew eight sets of six oat plants each in 

 water cultures. One solution contained an excess of all the 

 essential forms of plant food, except phosphoric acid. The other 

 solutions received increasing amounts of phosphoric acid. All 

 conditions were the same, excepting the varying amounts of phos- 

 phoric acid. When the oats were ripe, they were harvested and 

 subjected to analysis. The following are some of the results: 



1 Jahresber. Agr. Chem., 1873, p. 293. 



