It must be well supplied with moisture.

It must be well supplied with air.

It must have a certain amount of heat.

It must be supplied with available plant food.

In order to furnish these needs or conditions the soil must possess certain characteristics or properties.

These properties may be grouped under three heads: 

Physical properties; the moisture, heat and air conditions needed by the roots.

Biological properties; the work of very minute living organisms in the soil.

Chemical properties; plant food in the soil.

PHYSICAL PROPERTIES OF A FERTILE SOIL

Three very important physical properties of a fertile soil are its

  Power to take water falling on the surface. 
  Power to absorb water from below. 
  Power to hold water.

The fertile soil must possess all three of these powers.  The relative degrees to which these three powers or properties are possessed determine more than anything else the kind of crops or the class of crops that will grow best on a given soil.

These powers depend, as we learned in Chapter IV, on the texture of the soil or the relative amounts of sand, silt, clay and humus contained in the soil.

The power of admitting a free circulation of air through its pores is also an important property of a fertile soil, for air is necessary to the life and growth of the roots.  This property is dependent also on texture.

Two other important properties of a fertile soil are power to absorb and power to hold heat.  These depend upon the power of the soil to take in warm rain and warm air, and also upon density and color.  The denser or more compact soil and the darker soil having greater power to absorb heat.

The compactness of the soil which gives it greater powers to absorb heat weakens its powers to hold it, because the compactness allows more rapid conduction of heat to the surface, where it is lost by radiation.

The more moisture a soil holds, the weaker is its heat-holding power, because the heat is used in warming and evaporating water from the surface of the soil.

These important properties or conditions of moisture, heat and air, are, as we have seen, dependent on soil texture and color, which in turn are dependent upon the relative amounts of sand, clay and humus in the soil.  We are able to control soil texture and therefore these physical properties to a certain degree by means of tillage and the addition of organic matter or humus (see Chapter IV).

BIOLOGICAL PROPERTIES OF A FERTILE SOIL

Biology is the story or science of life; and the biological properties of the soil have to do with living organisms in the soil.

The soil of every fertile field is full of very small or microscopic plants called bacteria or germs.  They are said to be microscopic because they are so small that they cannot be seen without the aid of a powerful magnifying glass or microscope.  They are so small that it would take about 10,000 average-sized soil bacteria or soil germs placed side by side to measure one inch.