158 ROOTS 



and thus aid in decomposing it. Earthworms are important in 

 bringing soils from below to the surface. Their activities tend 

 to keep the lower and upper layers of soil mixed. Experiments 

 show that earthworms play an important role in maintaining 

 soil fertility. Some worms, as the Nematodes, often injure 

 plants by destroying their roots. In addition to worms, there 

 are numerous insects which no doubt have some influence on soil 

 fertility. It is obvious that soils are exceedingly complex and 

 that there are many things to consider in maintaining soil fertility. 

 Soil Solution. The soil water and the various mineral mat- 

 ters and organic substances dissolved in it constitute the soil 

 solution. The dissolved organic substances are of use to the soil 

 micro-organisms, but it is mainly water and mineral matter that 

 higher plants need to obtain from the soil solution. The most 

 important of the m neral elements for crops are nitrogen, phos- 

 phorus, potassium, sulphur, calcium, iron and magnesium. These 

 occur in compounds known as mineral salts, which, although very 

 essential to plant growth, are present in very small quantities, 

 usually constituting less than one percent of the best of soil solu- 

 tions. Of these, nitrogen, phosphorus, and potassium are in 

 most demand by crops, and the ones 'most likely to be lacking. 

 Consequently, in maintaining soil fertility, the chief problem is 

 to conserve and restore these elements. The value of artificial 

 fertilizers and manures depends chiefly upon the amount of these 

 elements contained. In most soils, iron, sulphur, and magnesium 

 are present in sufficient quantities. Calcium must always be 

 present to neutralize the acids, for both roots and soil Bacteria 

 are very sensitive to acids. Calcium is added to the soil in the 

 form of lime or limestone. The recent soil surveys conducted in 

 the various states have revealed the fact that even the richest of 

 soils, as those of the prairie states, commonly contain too much 

 acid and are often much improved by the addition of lime or 

 limestone. The amount of lime and limestone added to soils to 

 reduce the acidity' is rapidly increasing. On the other hand, 

 when soils contain too much of an alkali, such as sodium carbo- 

 nate, plants will not do well until the condition is changed by 

 the addition of gypsum or some other substance capable of break- 

 ing up the alkali. In the Western States there are many tracts 

 of land that, in addition to being too dry, contain too much 

 alkali for ordinary plants to grow. 



