148 SOIL 



between particles. Water may be raised by capillarity to distances 

 varying from about a foot to as much as 5 feet in different soils. 



95. Soil Minerals.— Ten chemical elements are necessary as food 

 materials. These are carbon, hydrogen, oxygen, nitrogen, sulphur, 

 phosphorus, potassium, calcium, magnesium and iron. The first 

 three of these are obtained from carbon dioxide and water. The 

 other seven are absorbed from the soil in the form of soluble salts. 



The rocks from which sand and clay are formed usually consist 

 largely of silica and alumina and a few other oxides that are entirely 

 unavailable as food materials. Mixed with these oxides, however, 

 are small quantities of compounds containing the plant nutrients. 

 Many rocks lack certain nutrients entirely, but since most soils 

 are of rather complex origin there are usually some of all the essential 

 elements present. However, they are present in varying amounts 

 and they may be lost by being leached out by percolating water or 

 by the removal of crop plants that have grown on the soil in question. 

 It is often necessary, therefore, to replace certain of the essential 

 elements in order to maintain the fertility of the soil. 



Iron is used by plants only in very minute quantities and is usually 

 present in the soil in rather large quantities. It is very seldom neces- 

 sary, therefore, to add iron to agricultural soil. Sulphur is used in 

 rather large amounts and is ordinarily present in the soil in relatively 

 small amounts. Sulphur, however, is washed into the soil by rain. 

 Experiments have shown that in Illinois an average of 40 pounds of 

 sulphur per acre is added to the soil annually by this means and this 

 appears to be enough to maintain the supply indefinitely. It is 

 seldom necessary to add magnesium to a soil. Potassium is abun- 

 dant in most soils but in the case of peaty soils it is often necessary 

 to add potassium, usually in the form of the sulphate. Calcium, 

 when needed, may be profitably added, usually, in the form of pul- 

 verized limestone, and phosphorus in the form of powdered rock 

 phosphate. Limestone not only adds calcium to the soil but corrects 

 the acidity of soils that have become "sour." The most practical 

 means of keeping up the nitrogen supply in the soil is to grow legume 

 crops periodically so that the nitrogen-fixing bacteria may build up 

 nitrates from atmospheric nitrogen. Of course, when sufficient 

 manure is available for addition to a soil often nothing else is needed 

 since manure ordinarily contains all of the necessary mineral 

 nutrients. 



Although there is, as we have indicated, an abundance of inor- 

 ganic salts in most soils, the actual concentration of salts in solution 



