Nitrogen in the soil.—­Plant roots use nitrogen in the form of nitric acid and salts of nitrogen called nitrates.  But the nitrogen of the soil is very largely found in the humus with the roots cannot use.  A chemical change must take place in it and the nitrogen be built into nitric acid and nitrates.  This, we have learned, is done through the aid of the nitrifying germs.

Phosphoric acid in the soil.—­Phosphorus does not exist pure in the soil.  The plant finds it as a phosphoric acid united with the other substances forming phosphates.  These are often not available to plants, but can to a certain extent be made available through tillage and by adding humus to the soil.

Potash in the soil.—­The plant finds potassium in potash which exists in the soil.  Potash like phosphoric acid often exists in forms which the plant cannot use but may be made available to a certain extent by tillage, the addition of humus, and the addition of lime to the soil.

Lime in the soil.—­Most soils contain the element calcium or lime, the compound in which it is found, in sufficient quantities for plant food.  But lime is also of importance to the farmer and plant grower because it is helpful in causing chemical changes in the soil which tend to prepare the nitrogen, phosphoric acid and potash for plant use.  It is also helpful in changing soil texture.

The chemical changes which make the plant foods available are dependent on moisture, heat, and air with its oxygen, and are therefore dependent largely on texture, and therefore on tillage.

When good tillage and the addition of organic matter and lime do not render available sufficient plant food, then the supply of available food may be increased by the application of manure and fertilizers.

It will be seen that all these classes of properties are necessary to furnish all the conditions for root growth.

The proper chemical conditions require the presence of both physical and biological properties and the biological work in the soil requires both chemical and physical conditions.

From the farmer’s standpoint the physical properties seem to be most important, for the others are dependent on the proper texture, moisture, heat and ventilation which are controlled largely by tillage.

Therefore the first effort of the farmer to improve the fertility of his soil should be to improve his methods of working the soil.

Every one of these properties of the fertile soil, and consequently every one of the conditions necessary for the growth and development of plant roots, is influenced in some way by every operation performed on the soil, whether it be plowing, harrowing, cultivating, applying manure, growing crops, harvesting, or anything else, and the thoughtful farmer will frequently ask himself the question:  “How is this going to effect the fertility of my soil or the conditions necessary for profitable crop production?”