FERTILIZERS 1 3 



directly from the air, and how much is contributed 

 by plant life, is not known. But it is generally be- 

 lieved that most of these elements as found in soils 

 are derived from plants, either through the agency 

 of beneficial bacteria or from the decay of vegetable 

 tissue. 



16. The character of vegetation changes with the 

 soil. Pure clay soils support very little plant life, as 

 there is no drainage, no ventilation and no yielding 

 to root penetration. The right degree of porosity, 

 due to the presence of sand and humus, allows the 

 roots to enlarge rapidly which in turn nourishes a 

 large plant. Hence the same species of plants will 

 vary in size and appearance according as the nature 

 of the soil encourages or restricts their growth. A 

 well developed root means a well developed plant 

 and vice versa. 



If potassium or sodium or chlorine are in excess, 

 the soil is alkaline and tolerated by certain classes of 

 plants like the salt bushes, some of the mallows, 

 tussock grass, etc. 



Peat lands, bogs and meadows have a characteris- 

 tic vegetation. Peat may contain as high as 80% 

 humus or decayed organic matter. Here, nitrogen 

 is naturally in excess and moisture plenty, so that 

 the growth is vigorous and succulent. Tall grasses, 

 and willows thrive here. 



The well drained "mesa" of the arid west, sup- 

 ports another variety of vegetation and a dense 

 forest cover or leaf mould, still another, according to 

 their moisture holding powers. The various com- 

 binations of sand, clay, humus and plant food are 

 almost infinite. 



