O2 FARM DEVELOPMENT 



Substances used by plants. Plants take from the soil 

 and require for their growth and development the follow- 

 ing elements : Potassium, phosphorus, nitrogen, iron, 

 calcium, magnesium, sulphur and chlorine. These 

 elements, together with carbon, hydrogen and oxygen, 

 which may be considered as derived in some form from 

 the air, are absolutely necessary for the growth of 

 all higher plants. In the absence of any one of these 

 elements none of the higher plants can reach maturity. 

 Other elements, such as silicon, sodium and aluminum, 

 are also invariably present in plants, but they are not 

 necessary-, as shown by accurate experiments. Besides 

 these, such elements as titanium, copper, manganese and 

 others have also been found, but their presence seems to 

 be accidental ; that is, they have been taken up because 

 they happened to be present in the soil where the plants 

 grew, thus making it impossible for the plants to reject 

 them when they occur in solution in the soil water. It 

 must be understood, of course, that plants do not 

 take up these elements as such, but find them in the soil 

 in combination with other substances. For example, 

 calcium is not taken up in its elementary form, but 

 occurs in the soil, combined with nitrogen and oxygen, 

 as calcium nitrate, and as such may be taken up. It 

 also occurs in many other compounds. 



The food material which the plant takes from the soil 

 forms only a small per cent of its weight, as shown by 

 the percentage of ash found on burning. The amount 

 varies, according to the plant, between, approximately, 

 i per cent and 10 per cent. The bulk of the plant is 

 made up of the elements derived from the air and water 

 carbon, hydrogen and oxygen. The cell walls, 

 starches, sugars, organic acids, etc., are composed almost 

 exclusively of these three elements; while certain other 

 compounds like proteids, as the gluten of wheat, contain 

 in addition to these, nitrogen and small amounts of sul- 



