PLANT .NUTRIENTS 3 



oxygen, water, nitrogen, phosphorus, sulfur, potassium, calcium, 

 magnesium, and iron. The two gases first mentioned are derived 

 directly from the air, through the respiratory organs of the plant. 

 Water is taken into the plant chiefly from the soil, through its 

 fibrous roots. All the other elements in the list are taken from the 

 soil, nitrogen being derived from decaying organic matter (the 

 original source of the nitrogen is, however, the atmosphere, from 

 which the initial supply of nitrogen is obtained by direct assimila- 

 tion by certain bacteria and perhaps other low forms of plant life), 

 and the remaining ones from the mineral compounds of the soil. 



Carbon dioxide and oxygen, being derived from the air, are 

 always available to the leaves and stems of growing plants hi 

 unlimited supply; but the supply available to a seed when ger- 

 minating in the soil, or to the roots of a growing farm crop, may 

 sometimes become inadequate, especially in soils of a very com- 

 pact texture, or " water-logged " soils. In such cases, the defi- 

 ciency of these gaseous food elements may become a limiting 

 factor in plant growth. 



Water is often a limiting factor in plant growth. Experiments 

 which have been repeated many times and under widely varying 

 conditions show that when water is supplied to a plant in varying 

 amounts, by increasing the percentage of water in the soil in which 

 the plant is growing by regular increments up to the saturation 

 point, the growth of the plant, or yield of the crop, increases up 

 to a certain point and then falls off because the excess of water 

 reduces the supply of air which is available to the plant roots. 

 Hence, abundance of water is, in general, a most essential factor 

 in plant growth. 



Under normal conditions of SLIT and moisture supply, however, 

 the plant food elements which may be considered to be the lim- 

 iting factors in the nutrition and growth of plants are the chemi- 

 cal elements mentioned in the list above. 



AVAILABLE AND UNAVAILABLE FORMS 



The plant food materials which are taken from the soil by a 

 growing plant must enter it by osmosis through the semi-permeable 

 membranes which constitute the epidermis of the root-hairs, and 

 circulate through the plant either carried in solution in the sap or 

 by osmosis from cell to cell. Hence, they must be in water-soluble 



