SOIL AERATION 139 



plants that grow in water possess well-developed air spaces but this 

 characteristic is much more conspicuous in plants growing in stand- 

 ing Avater than in those growing in running water. 



The presence of algse and other water j^lants often influences very 

 greatly the amounts of oxygen and carbon dioxide in water. In 

 lakes and ponds where algse are abundant the water sometimes be- 

 comes supersaturated with oxygen as a result of photosynthesis. 

 The amount frequently reaches a saturation of 300 per cent or more. 

 The same process prevents the accumulation of large amounts of 

 carbon dioxide. In this way the algae become very important in 

 furnishing a supply of oxygen to the submerged roots of higher 

 plants. This phenomenon is of especial economic importance in rice 

 fields where the algse have been shown to be valuable for the aeration 

 of the roots of rice plants. 



88. Soil Aeration.— Roots of different plants vary considerably 

 in their oxygen requirements. The roots of swamp plants, for 

 example, require much less oxygen than the roots of drier land 

 plants. All roots, however, must carry on respiration in order to 

 live and so must have some oxygen. The aeration of the soil, there- 

 fore, is very important to the plants growing in it. 



The carbon dioxide that is present in the soil is derived almost 

 entirely from respiration and decay processes that take place within 

 the soil. Therefore, as would be expected, it regularly increases 

 in amount with the depth of the soil, since at the greater depths it is 

 more difficult for it to escape into the atmosphere. The oxygen of the 

 soil, on the other hand, is obtained from the atmosphere and de- 

 creases with an increase in depth of soil. The amounts of carbon 

 dioxide and oxygen in the soil vary greatly at different times of year, 

 with varying conditions of temperature and rainfall, with different 

 types of soil, and with varying kinds and numbers of plants growing 

 in the soil. As would be expected the greatest amount of carbon 

 dioxide is found in the soil in summer and the smallest amount in 

 winter. Ordinarily sand contains the most oxygen and the least 

 carbon dioxide while peat soil is just the opposite in these respects 

 and clay holds an intermediate position. An increased amount of 

 plant growth augments the carbon dioxide and decreases the oxygen 

 supply and similar effects are produced by an increased amount of 

 dead organic matter in the soil. 



Since oxygen dissolves very slowly in water a soil that is saturated 

 with water is almost invariably poorly aerated. Plants which 

 ordinarily do not grow in water may be actually drowned if the soil 



