394 Sources of the Constituents of Minnesota Soils. 



and others must be worked over in divers ways. All these 

 changes direct and indirect, immediate and remote are necessary 

 to convert chemical elements and compounds into protoplasmic 

 food, that is, to assimilate them. 



The essential food elements of plants. — There are twelve 

 chemical elements which may be considered the essential foods of 

 plants : Of these six occur in large amounts, viz : Carbon, oxygen, 

 hydrogen, sulphur, phosphorus and nitrogen ; and six others in 

 small amounts, viz : Calcium, magnesium, iron, silicon, potassium 

 and sodium. Let us now see what extent these food elements 

 exist in Minnesota rocks and, proportionately, in the soils derived 

 from them. 



Quite as essential as the mineral matters or the average 

 moisture in the soil are occasional showers of rain. Dry weather 

 causes the soil and sub-soil to hold these constituents as solids, 

 and in this condition plants cannot take them up ; but let a shower 

 fall and dissolve them and they are rapidly carried into the plant 

 tissues and there assimilated. A hint at this condition is seen 

 upon prairies in the almost white coating of magnesic and calcic 

 salts that cover the dark loam in the dried out sloughs and swamps 

 of nearly every portion of Minnesota. A shower of rain will 

 cause this white coating to disappear, again to appear as soon as 

 the rain has been absorbed by the earth or evaporated once more 

 into the air. 



Here is the reason for the phenomena often seen in dry 

 -weather of the lower and mature leaves of a plant shriveling and 

 disappearing; the salts so essential to the growth of the' new 

 shoots and leaves of the plant are taken from the mature parts of 

 the plant itself instead of from the soil, where they lie in the con- 

 dition of salts on which it is impossible for the plant to feed. 



The chemical composition of soils. — When soils are spoken 

 of in their chemical relations to plants and plant food the chemical 

 composition of soils should be understood that the capacity of soils 

 to feed plants may be seen. When the sources of the constituents 

 of soils are discussed the amount and character of these constitu- 

 ents should be known. So little work has thus far been done on 

 the chemistry of Minnesota soils that the following table is made 

 up of analyses drawn from the geological survey reports of Wis- 

 'consin. They are taken from Salisbury's table, vol. i, p. 307, as 

 fairly representative of similar soils of Minnesota. 



