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.

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).

The earthworm will burrow down six feet and more, leaving its passageways as means for the entry of air. Applications of organic matter automatically multiply the earthworm population. In well-run organic farms and gardens there should be millions of earthworms per acre.

Role of Soil in Air is needed in the soil for the decomposition of organic matter, and is necessary for the proper functioning of soil microorganisms and oxidation of mineral matter. In poorly aerated soil, few minerals are available for plant sustenance. The presence of sufficient oxygen helps to balance the supply of carbon dioxide, and in the process of plant respiration, oxygen is absorbed by plant roots. Better soil aeration enables plants to develop a bigger root system and results in higher yields. In the process of plant growth. the leaves absorb carbon dioxide from the atmosphere and give off oxygen. The reverse takes place in the roots, which take in oxygen and give off carbon dioxide. In the decay of organic matter, carbon dioxide is given off.

The soil will become easier to till. Moisture will seep down and be held instead of running off. In this converted adobe, earthworms may be planted and will flourish. Since humus is a neutralizer, the alkalinity will be brought down to a point suitable for many garden plants.

Even the vital iron, which has been there all the time (the red in adobe is iron oxide), and the locked-up phosphates may be freed to some extent.

A waterlogged condition is so detrimental to plant growth that even a short period may seriously endanger some plants. This may occur either in poorly drained soils composed of fine particles or in well-drained soil if enough water is rapidly applied. Prevention of this type of poor aeration necessitates removing water from the land, either by controlled runoff or drainage; the latter can be particularly effective.