140 PLANTS AND MAN 



necessity be secured from the soil. Analysis of plant foods and 

 related products shows that eleven elements are present in addi- 

 tion to the carbon, hydrogen and oxygen found in the ba'sic 

 glucose. These are nitrogen, potassium, phosphorus, mag- 

 nesium, calcium, sulphur, iron, manganese, boron, zinc and 

 copper. Nutrient solutions — recently publicized as water cul- 

 ture of plants — have long been known to botanists; these contain 

 all of the essential elements for plant growth, dissolved in distilled 

 water. They usually contain definite weights of calcium nitrate, 

 potassium nitrate, potassium phosphate, magnesium sulphate 

 and iron phosphate. 



With this more recent appreciation of what soil fertility 

 means, it has become possible to restore to soils the amounts and 

 kinds of chemicals withdrawn from them, in correlation with the 

 kinds of food substances elaborated by the crop in question. Such 

 fertilizers need include only those mineral salts which are with- 

 drawn in great quantities by plants — chiefly nitrogen, sulphur, 

 potassium and phosphorus. Nitrogen is returned to the soil, as 

 we have seen, by bacteria; but it can also be put into the soil in 

 the form of sodium nitrate, calcium cyanimide, calcium nitrate 

 and various ammonium compounds. Phosphorus is often 

 restored to the soil in the form of ground bone (calcium phos- 

 phate) but this is being replaced by rock phosphates. Potassium 

 is replenished in the soil by addition of wood ash, seaweeds or 

 potassium salts. Some of these substances are important in that 

 they regulate vital plant activities, in addition to being necessary 

 for the formation of proteins or other plant products. Phosphorus, 

 because of its stimulation of root growth, is used for root crops 

 such as carrots or turnips; it also speeds up the movement of 

 carbohydrates and stimulates the formation of flowers and fruits. 

 Potassium often determines the general health of the plant, per- 

 haps acting as a catalyst in the synthesis of carbohydrates and 

 proteins. Other necessary substances, such as the magnesium 

 used in chlorophyll production, are withdrawn in such small 

 quantities that they rarely need replacing. 



From this it can be seen that green plants produce a great 

 variety of foods and allied substances, most of which are de- 

 signed specifically for use by the plants themselves. Man has 



