PLANT-FOOD MATERIALS iW SOILS, 



101 



Table 22. 



Yields of Crops and Composition of Soil and 

 Water Extract of Soil 



It may be observed that the water extract of the soil from 

 the plats treated with any fertilizer ingredient was much 

 richer in that constituent than were the plats not so treated, 

 while the total quantities found in the soil were not propor- 

 tionately increased. 



118. Other forms of available plant-food materials in soil. 

 — The natural weathering of soil that goes on continually 

 makes soluble a part of the originally insoluble mineral mat- 

 ter and this is absorbed just as are the fertilizer salts. When 

 land is cropped each year, this soluble matter is used by 

 plants about as quickly as it is formed, but when land is 

 bare fallowed the dissolved matter is largely absorbed, and 

 thus a bare fallow increases the quantity of available nutri- 

 ents for the following crop. 



Another, and very important supply of available plant 

 nutrients, is that combined with the organic matter in soils. 

 When organic matter is incorporated with soil, decomposi- 

 tion begins, acids are formed and these unite with mineral 

 matter previously in a difficultly soluble condition. The 

 result is a compound, partly organic and partly inorganic. 

 These compounds decay still further until all the organic 

 matter passes off as we have already seen (§ 50), and the 



