s.

Plant roots need for their growth and development (see Chapter II):

A mellow yet firm soil.

A moist soil.

A ventilated soil.

A warm soil.

A soil supplied with plant food.

Decaying organic matter or humus is one of the most important
ingredients of our soils. Because:

It greatly influences soil texture and therefore the conditions
necessary for root growth.

Its presence or absence greatly influences the attitude of soils
toward water, the most important factor in plant growth. Its presence
helps light, sandy soils to hold more water and to better pump water
from below, while it helps close, heavy soils to better take in the
water which falls on their surface. Its absence causes an opposite
state of affairs.

The presence of organic matter checks excessive ventilation in too
open, sandy soil by filling the pores, and improves poor ventilation
in heavy clay soils by making them more open.

Humus, on account of its color, influences the heat absorbing powers
of soils.

The organic matter is constantly undergoing more or less rapid decay
unless the soil be perfectly dry or frozen solid. Stirring and
cultivating the soil hasten this decay.

As the organic matter decays it adds available plant food to the soil,
particularly nitrogen.

As it decays, it produces carbonic acid and other acids which are
able to dissolve mineral plant food not soluble in pure water and thus
render it available to plants.

Plants, although they require the same elements of plant food, take
them in different amounts and different proportions.

Plants differ in the extent and depth of root growth and therefore
take food from different parts of the soil. Some are surface feeders
while others feed on the deeper soil.

Plants differ in their power to take plant food from the soil; some
are weak feeders, and can use only the most available food; others are
strong feeders, and can use tougher plant food.

Plants vary in the amount of heat they require to carry on their
growth and development.

THE ONE CROP SYSTEM

We are now ready for the question. What effect has the continuous
cultivation, year after year, of the same kind of crop on the soil
conditions necessary to the best growth and development of that crop
or any other crop? Suppose we take cotton for example.

How does cotton growing affect soil humus?

During the cultivation of cotton, the organic matter or humus of the
soil decays in greater quantities