ABSORPTION OF NUTRIENTS BY PLANTS 17 



compounds of calcium, iron, or aluminum. Most of these com- 

 pounds are only slightly soluble. Crops which are grown on 

 soils which contain large total amounts of phosphorus frequently 

 develop much better following the application of phosphatic 

 fertiUzers. Similarly, the potassium, calcium, and sulfur exist 

 in combinations from which they are not liberated rapidly into 

 the soil solution. 



Some soils contain such an abundance of basic salts that plani 

 growth is markedly depressed. The influence of this condition 

 on plant development may be due to at least two effects: (1) 

 In the so-called alkali soils of arid regions the concentration of 

 soluble salts may be so great that the plants fail to grow. (2) 

 In other soils the reaction may be so basic that iron will not go 

 into solution in amounts sufficient to satisfy the requirements of 

 the plants ; this brings about a yellowing or mottHng of the green 

 surfaces of the plants, stunts the growth and may lead to 

 death. 



Absorption of Nutrients by Plants. — Carbon is taken from 

 the carbon dioxide which is present in limited concentrations 

 (0.03 per cent) in the atmosphere. Only green plants, which 

 contain chlorophyll and which can utihze, by photosynthesis, 

 energy of the sun, and a few bacteria, are able to use this source 

 of carbon. The carbon dioxide is reduced in the leaves and com- 

 bined with water to give formaldehyde, which is then built up to 

 form carbohydrates. Soil activities are particularly important 

 in this process, even if indirectly, by replenishing the carbon 

 dioxide of the atmosphere by a constant stream of the gas origi- 

 nating from the activities of the microbes in the soil. Oxygen 

 comes from the carbon dioxide obtained from the air, from the 

 water drawn from the soil, and from numerous other substances. 

 Hydrogen is derived principally from water. 



Nitrogen in various combinations is absorbed by the plant 

 roots from the soil. Although from 75 to 80 per cent of the 

 gases of the atmosphere consists of elemental nitrogen, none of it 

 can be used by the plants, since they can assimilate only com- 

 bined nitrogen in the form of ammonia (NH4+) or nitrate 

 (NOs"). Plants store in their tissues considerable quantities of 

 nitrogen, which becomes available to subsequent plant growth 

 only after these tissues undergo decomposition through the 

 agency of the soil microbes. Very few plants (legumes) are 



