88 BACTERIA AND SOIL FERTILITY 



is not directly utilizable by higher plants or animals as are oxy- 

 gen and carbon dioxid. When in the soil it tends to become 



O 



soluble, and hence, leached from it by excessive irrigation water 

 or rain. Most soils are deficient in nitrogen. 



Phosfhorus — The next element used by the plant in large 

 quantities from the soil is phosphorus. It is required by every 

 cell. It is necessary for the production of chlorophyll. In its 

 absence neither starch nor sugar is formed in the leaf. To the 

 farmer of the arid districts it is especially important for two 

 reasons: (i) In the early stages of growth it promotes root for- 

 mation to a remarkable extent. This is especially noted in clay 

 soils where there is a tendency for scant root formation. (2) 

 In the later stages of growth phosphorus hastens ripening. In 

 northern districts where frost is a menace this may at times be 

 an important factor. At maturity the phosphorus has accumu- 

 lated to a large extent in the seeds. Many soils are deficient in 

 phosphorus. 



Potassium— TYiis element is demanded by all plants but by 

 some in larger quantities than by others. Potatoes and sugar- 

 beets require it in large quantities, as it seems to play a vital part 

 in the formation of starch and sugar in the plant. Potassium 

 starvation is even more easily detected than phosphorus starvation. 

 Potassium-starved plants have a dull, poor color and tend to die 

 early at the tops. Beets are low in sugar. Grains are under 

 weight. Potassium-starved plants are the first to succumb to dis- 

 ease. During the ripening of the plant the potassium of the leaves 

 and stalks becomes soluble, and hence may be washed back into 

 the soil. Few soils are lacking in potassium, although many soils 

 show an increased yield on applying potassium salts. This may 

 be due to the soil having little available potassium, or the added 

 potassium may liberate other essential needed elements. 



Calcium — This element gives tone and vigor to the plant. It 

 occurs more in the leaves and stems than in the seed. All soils 

 have sufiScient calcium for plant needs. Some soils may be acid, 

 under which condition they are benefited by the addition of lime- 



