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 Amceba 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- Fig. 138. — Nodules on 
ments for crops are nitrogen, phosphorus, the roots of a Pea. Modi- 
potassium, sulphur, calcium, iron, and Horns Falladis, 
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 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 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 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 by the addition of gypsum 
or some other substance capable of breaking up the alkali. 
