SOILS AND ALKALI. 
5 
mon agricultural products, and do their best upon soils 
containing from 5 to 8 per cent, of organic matter. This 
organic matter is sometimes termed humus. The plants 
decay and add organic matter or humus to the soil, and 
the roots of plants have the power to decompose the rocks 
themselves, and there is a constant accession of mineral 
matter or soil. From the putrefaction or decay of 
organic matter, the elements form new combinations; the 
carbon combines with the oxygen to form carbonic acid ; 
the nitrogen combines with the hydrogen to form 
ammonia; this ammonia undergoes a further decomposi¬ 
tion called nitrification, resulting, like the original putre¬ 
faction, from the action of oxidizing microbes, and changes 
the ammonia into nitric acid. Plants are capable of re¬ 
ceiving food, either in the form of gas through the instru¬ 
mentality of their leaves, or in solution by their roots. 
Of the total weight of the plants, about 5 per cent, is of 
soil or mineral origin; the remaining 95 per cent, is 
wholly of atmospheric origin; most of which becomes 
added to the soil mass on the death and decomposition of 
the plants. The following elements are found in 
plants: Carbon, hydrogen, nitrogen, oxygen, sulphur, 
phosphorus, chlorine, silicon, potassium, sodium, calcium, 
magnesium, iron, manganese, and in rare cases a few 
others that may be called accidental. Of these, the car¬ 
bon, hydrogen, nitrogen, oxygen, sulphur and phosphorus 
are grouped together to form the various organic com¬ 
pounds furnished by plants. The remaining elements are 
generally arranged in the following forms: Chlorides 
and silicates of potassium and sodium, calcium sulphate, 
phosphates of iron, calcium, magnesium and ammonium 
(and possibly manganese), salts of potassium, sodium and 
calcium, with vegetable acids. When plants are 
burned, the mineral elements remain as the ash. The 
amount of the different elements may vary. For in- 
