soils | properties
| classes |
caliche | depth |
components | pH
SOILS AND FERTILIZERS: SOILS
Ch. 2, pp. 15 - 17
The effective depth of a soil for plant growth is the vertical distance
into the soil from the surface to a layer that essentially stops
the downward growth of plant roots. The barrier layer may be rock,
sand, gravel, heavy clay, or a cemented layer (e.g. caliche).
Terms that are used to express effective depth of soil are:
Very Shallow surface is less
than 10 inches from a layer that retards root development.
Shallow Soil surface is 10 to 20 inches from a
layer that retards root development.
Moderately deep Soil surface is 20 to 36 inches
from a layer that retards root development.
Deep Soil surface is 36 to 60 inches from a layer
that retards root development.
Very deep Soil surface is 60 inches or more from a
layer that retards root development.
Soils that are deep, well-drained, and have desirable
texture and structure are suitable for the production of most
garden or landscape plants. Deep soils can hold more plant
nutrients and water than can shallow soils with similar textures.
Depth of soil and its capacity for nutrients and water frequently
determine the yield from a crop, particularly annual crops that
are grown with little or no irrigation. Plants growing on shallow
soils also have less mechanical support than those growing in deep
soils. Trees growing in shallow soils are more easily blown over
by wind than are those growing in deep soils.
Soils that have a lost part or all of their surface are usually harder to
till and have lower productivity than those that have desirable
thickness of surface soil. To compensate for surface soil loss,
better fertilization, and other management practices should be
used. Increasing the organic matter content of an eroded soil
often improves its tillage characteristics, as well as its water
and nutrient capacity. Erosion can be the result of running water
or wind, or can be the result of land leveling during home
construction. Whatever the cause, generous use of soil amendments,
organic materials and necessary fertilizers can help speed the
conversion of poor quality subsoil into high quality top soil.
Organic matter in soil consists of the remains of plants and animals. When
temperature and moisture conditions are favorable in the soil,
earthworms, insects, bacteria, fungi, and other types of plants
and animals use the organic matter as food, breaking it down into
humus (the portion of organic matter that remains after most
decomposition has taken place) and soluble nutrients. Through this
process, materials are made available for use by growing plants.
In addition, organic material has a very high cation exchange
capacity, so nutrients are retained in plant-available form. The
digested and decomposing organic material also helps develop good
In sandy soil, organic material occupies some of the
space between the sand grains, thus binding these together and
increasing water-holding capacity. In a finely textured or clay
soil, organic material on and around soil particles creates
aggregates of the fine soil particles, allowing water to move more
rapidly around these larger particles. This grouping of the soil
particles into aggregates or peds makes soil mellow and easier to
Organic matter content depends primarily on the kinds
of plants that have been growing in a soil, the long-term
management practices, temperature, and drainage. Soils that have
native grass cover for long periods usually have a relatively high
organic matter content in the surface area. Those that have desert
or native forest cover usually have relatively low organic matter
content. In either case, if the plants are grown on a soil that is
poorly drained, the organic matter content is usually higher than
where the same plants are grown on a well-drained soil. This is
due to differences in available oxygen which is needed by the
organisms that attack and decompose the organic material. The
activity of soil microorganisms is temperature dependent. Soils in
a cooler climate such as in Northern Arizona have more organic
matter than those in the southern Arizona deserts where the
climate is much hotter.
Water and Air
Water in the soil ultimately comes from precipitation (rain, snow, hail,
or sleet), entering the soil through cracks, holes, and openings
between the soil particles. As the water enters, it pushes the air
out. Oxygen is taken up by roots for respiration. If air is
unavailable for too long, the roots will die.
Plants use some water, some is lost by evaporation, and
some moves so deep into the soil that plant roots cannot reach it.
If it rains very hard or for a long time, some of it is lost
through surface run-off.
When organic matter decomposes in the soil, it gives
off carbon dioxide. This carbon dioxide replaces some of the
oxygen in the soil pores. As a result, soil air contains less
oxygen and more carbon dioxide than the air above the soil
surface. Carbon dioxide is dissolved by water in the soil to form
a weak acid (carbonic acid). This solution reacts with the
minerals in the soil to form compounds that can be taken up and
used as foods by the plants.
Plants need 18 elements for normal growth. Carbon, hydrogen, and oxygen
(come from air and water). Nitrogen is a major plant constituent.
Although the atmosphere is 78% nitrogen, it is not directly
available for plant use. However, certain bacteria that live in
nodules on the roots of legumes are able to fix nitrogen from the
air into a form available to plants. Beans, peas, and Mesquite and
Acacia trees, and alfalfa, are examples of legume plants.
The other 14 essential elements are iron, calcium,
phosphorus, potassium, copper, sulphur, magnesium, manganese,
zinc, boron, chlorine, cobalt, nickel and molybdenum. These
elements come from the soil.
With the exception of nitrogen, phosphorus, and iron there is usually a
large enough quantity of each of these elements in Arizona soils
for cultivation of crops. Irrigation and rain water also can
contain ample amounts of some essential plant nutrients.