SOIL SOLUTION 



157 



many kinds of soil Bacteria, breaking up compounds in different 

 ways to secure energy and food, not all results of their activi- 

 ties in the soil can be expected to be desirable ones. 



The Protozoa, which are small one-celled animals of which 

 the Amoeba is one type, are abundant in rich soils, where 

 they are thought to exert a harmful in- 

 fluence on the soil fertility by feeding 

 on the Bacteria. The evidence for this 

 accusation is that soils are more fertile 

 after being subjected to temperatures 

 or poisons which kill the Protozoa but 

 leave the Bacteria unharmed. 



Soil Solution. — The soil water and 

 the various mineral matters and or- 

 ganic substances dissolved in it consti- 

 tute the soil solution. The dissolved 

 organic substances are of use to the soil 

 micro-organisms, but it is mainly water 

 and mineral matters that higher plants 

 need to obtain from the soil solution. 

 The most important of the mineral ele- 

 ments for crops are nitrogen, -phosphorus, 

 potassium, sulphur, calcium, iron, and 

 rnagnesium. 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 per 

 cent of the best of soil solutions. Of these, nitrogen, phos- 

 phorus, and potassium are in most demand by crops, and the 

 ones most hkely 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 de- 

 pends 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. On the other hand, when soils contain too 

 much of an alkali, such as sodium carbonate, plants will not 

 do well until the condition is changed hy the addition of gypsum 

 or some other substance capable of breaking up the alkali. 



Fig. 138. — Nodules on 

 the roots of a Pea. Modi- 

 fied from Palladin. 



