Of the dry soils, then, the humus averaged warmest, because, on account of its dark color, it absorbed heat more readily than the others.  The dry clay was warmer than the sand on account of its color and compact texture.  Of the wet soils the sand was the warmest, because, on account of its holding less moisture, less heat was required to raise its temperature and there was less cooling by evaporation, while the other soils, although they absorbed more heat than the sand, lost more on account of greater evaporation, due to their holding more moisture.  Why are sandy soils called warm soils and clay soils said to be cold?

How may we check losses of heat from the soil?

If we make a mulch on the surface of the soil evaporation will be checked and therefore loss of heat by evaporation will be checked also.  The mulch will also check the conduction of heat from the lower soil to the surface and therefore check loss of heat by radiation from the surface.

VALUE OF ORGANIC MATTER

Figure 33 illustrates a simple way to show the value of organic matter in the soil.  The boxes are about twelve inches square and ten inches deep.  They were filled with a clay subsoil taken from the second foot below the surface of the field.  To the second box was added sufficient commercial fertilizer to supply the plants with all necessary plant food.  To the third box was added some peat or decayed leaves, in amount about ten per cent. of the clay subsoil.  The corn was then planted and the boxes were all given the same care.  The better growth of the corn in the third box was due to the fact that the organic matter not only furnished food for the corn but during its decay prepared mineral plant food that was locked up in the clay, and also brought about better conditions of air and moisture by improving the texture of the soil.  The plants in the second box had sufficient plant food, but did not make better growth because poor texture prevented proper conditions of air and moisture.  “And that’s another witness” for organic matter.  Decaying organic matter or humus is really the life of the soil and it is greatly needed in most of the farm soils of the eastern part of the country.  It closes the pores of sandy soils and opens the clay, thus helping the sand to soak up and hold more moisture and lessening excessive ventilation, and at the same time helping the roots to take a firmer hold.  It helps the clay to absorb rain, helps it to pump water faster, helps it to hold water longer in dry weather, increases ventilation, favors root penetration and increases heat absorption.  We can increase the amount of organic matter in the soil by plowing in stable manure, leaves and other organic refuse of the farm, or we can plow under crops of clover, grass, grain or other crops grown for that purpose.

CHAPTER VIII

PLANT FOOD IN THE SOIL