PHYSICAL, CHEMICAL, BIOLOGICAL 59 



absence of particular elements. The first essential to profitable crops is 

 the production of a healthy and vigorous plant. Added to this is a high 

 degree of fruitfulness. A deficiency in phosphorus may not prevent a 

 satisfactory development of the plant, but may seriously curtail the pro- 

 duction of seed. This is often illustrated in the case of wheat which 

 makes a rank growth of straw and a comparatively small yield of wheat. 

 The absence of available nitrogen is often indicated by the yellow color 

 of the foliage. 



The form in which the elements are combined may influence the 

 quality of the product. This is illustrated in tobacco when the applica- 

 tion of muriate of potash causes a poor burning quality of the leaf that is 

 to be used for cigars. Better results with a cigar tobacco are secured 

 when the potash is applied in the form of sulphate or carbonate. Further- 

 more, the essential plant food constituents dominate in the development 

 of certain parts of the plant or in the performance of certain vegetative 

 functions. For example, potash is believed to be largely instrumental 

 in the development of starch, and fertilizers for starch-producing plants, 

 such as potatoes, generally contain a high percentage of potash. It is 

 believed also that the color of fruits is controlled to a certain extent by 

 the presence or absence of certain essential elements, such as potash or iron. 



Elements Essential to Plants. — The essential elements of plant food 

 may be grouped as follows: First, those obtained from air and water, 

 consisting of oxygen, hydrogen and carbon; second, those constituents 

 that are frequently deficient in soils and are supplied through the use of 

 commercial fertilizers, namely, nitrogen, phosphorus and potassium; the 

 third group is not likely to be deficient as elements of plant food. These 

 consist of calcium, magnesium, sulphur and iron. In this group calcium 

 and magnesium in the carbonate form may become so deficient that soils 

 become sour, in which case the practice of applying lime is advisable. 

 The five other elements commonly present and fitting into a fourth group 

 are silicon, aluminum, sodium, chlorine and manganese. 



Soil Bacteria. — Bacteria are microscopic plants. They are composed 

 chiefly of protoplasm, and differ from higher plants in that they contain 

 no chlorophyll. Bacteria are generally single-celled, and they are so 

 small that it would require about one and one-half millions brought to- 

 gether in a mass in order to be visible to the naked eye. These small 

 plants are omnipresent. Soils are teeming with millions upon millions of 

 them. They are present in the air and in the water of the lakes and 

 rivers, and occur on all vegetation and are present in the foods we eat. 

 These minute organisms were unknown until the high power microscope 

 was invented a comparatively short time ago. They play a very important 

 part in all life processes. More than a thousand species of bacteria have 

 already been identified and described, and new species are being discovered 

 every day. 



Bacteria Make Plant Food Available. — The bacteria of the soil are 



