CHAPTER XXXV ^ 



r 

 PLANT LIFE. OSMOSIS. FERTILIZERS 



FOR successful growth, plants require carbon, hydrogen, oxy- 

 gen, phosphorus, potassium, nitrogen, sulphur, calcium, iron, and 

 magnesium. The carbon is chiefly supplied by the carbon dioxide 

 in the air, as we saw in the preceding chapter. Water supplies 

 hydrogen and oxygen, entering through the leaves, and also 

 through the roots and stems. Oxygen is also supplied directly 

 by the air. The oxygen produces oxidation of substances in the 

 plants, and gives heat. Phosphorus, nitrogen, and sulphur are 

 absorbed from the soil by the roots as soluble phosphates, nitrates, 

 and sulphates, and nitrogen sometimes as ammonia. Potassium 

 commonly enters as carbonate or bicarbonate. Calcium and 

 magnesium are absorbed as phosphate, nitrate, sulphate, or 

 bicarbonate; and iron as ferric hydroxide (Fe(OH) 3 ). Man- 

 ganese, chlorine and silicon are also present in many plants. 

 In some species, sodium salts can take the place of potassium 

 salts. 



The last named elements are used in such small amounts, 

 relative to the available supply in the soil, that they rarely need 

 attention. Nitrogen, phosphorus, potassium and calcium, how- 

 ever, the presence of which in quantity is essential to the life and 

 development of plants, often need to be added to soils which 

 are deficient in these elements. Suitable compounds of nitro- 

 gen, phosphorus, potassium and calcium are therefore used hi 

 enormous quantities as fertilizers. 



Before discussing individual fertilizers, we may profitably take 

 up the general question of how the plant derives its food from the 

 soil. 



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