312 GENERAL SCIENCE 



a time, as is done by the corn planter, the wheels of which 

 press the soil around the grains of corn. 



Some lands are too dry for cultivation, and so they are 

 irrigated by digging small canals through which the 

 water flows and seeps out into the soil, from which the 

 plants take it and the food that is dissolved in it. 



216. Soil Air. About half the volume of ordinary 

 soils when they are dry is air. A cubic foot of dry soil 

 contains about half a cubic foot of air. Take a known 

 quantity of dry soil, say one peck, and see how much 

 water can be poured into the vessel containing it. As the 

 water goes in the air comes out of the soil, and hence 

 the quantity of water poured in will be the measure of 

 the air that was in the dry soil. From this it can be 

 seen that, as the water in soils increases, the amount of 

 air decreases, and that when soils are saturated there is 

 very little air in them. Soil air is just as necessary for 

 the growth of farm crops as it is for the life of animals. 

 When water excludes all the air from the soil, the crops 

 will suffer and drown just as surely as a person drowns 

 in water, but not so quickly. This is very noticeable 

 when river valleys are flooded late in the spring. Corn 

 in fields that were under water for a few days has a 

 yellowish color, while much of the corn dies if the soil 

 is covered with water for a week or more. Plants turn 

 yellow when too much water is in the soil, because they 

 cannot then get a sufficient amount of the element nitro- 

 gen which is necessary for the making of green coloring 

 matter in the leaf. Plants are not healthy without this 

 green coloring matter. Air is also necessary for the? 

 growth of bacteria in the soil. The bacteria decompose 

 the humus and leave it in soluble form for the plants to 

 absorb with the soil water. 



