Soil contains two types of pores, macro- pores and micro pores. Macro pores generally allow the ready exchange of air and movement of water. In micro pores, both air and water movement is restricted.
Some chemical fertilizers harden the soil and reduce aeration. Nitrate of soda is a typical offender. In yearly applications of this fertilizer, plants use up much of the nitrate but little of the soda. This keeps piling up in the soil, and combines with carbon to form carbonate of soda (washing soda). Where large amounts of nitrate of soda are used, the soil can become so hard that it can be cultivated only after a rain.
Being heavy, it requires aeration. In its natural state it will contain no earthworms (ants and tunneling gophers aerate it for the native vegetation).
Poor aeration results in a decrease in the activity of soil microorganisms. When oxidation of organic matter slows, aerobic organisms are unable to function properly. Higher plants are adversely affected in several ways. Root and top growth is slowed, absorption of nutrients and water is decreased and certain inorganic compounds toxic to plant growth may form.
The subsoiler made by many tractor manufacturers is pulled through the soil at depths of ten to 20 inches, shattering hard pans and improving aeration. The soil must not be wet when subsoiling is going on or the hard pan will not be broken.
A legume crop is a good natural subsoiler on soil with a high enough oxygen content to support it. Alfalfa roots have been known to go down 15 feet or more. The decaying roots of this crop after harvest provide air passage ways to very low depths.
Tilling is an excellent means of improving soil aeration and reducing the carbon dioxide content of the soil.
When one plows or tills corn and then cultivates three times while the crop grows, much air is mixed with the soil, weeds are killed and crop yields increase.
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