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Pesticides
and Human Health Environmental
Issues Soil &
Plant Nutrition Nutrient
Content
Genetic Engineering Community
Food Security
Soil and Plant Nutrition
The relationship between soil quality and plant quality
 The
importance of soil, as a medium for plant growth, is sometimes overlooked.
Most evidently seen to serve the purpose of physical support; soil
also provides plants with essential nutrients, protection from various
toxins, and can function as a moderator of temperature fluctuations.
In addition, soil aides in water availability due to its water holding
capacity, and plant respiration due to its ability to provide ventilation.
Soil Basics
Physical properties: Soil functionality and manageability
is dictated by its physical properties. Primary characteristics
include texture, structure and color. Texture (or soil particle
size distribution) usually is considered to encompass soil fragments
ranging 2mm (sand) to 0.002mm (clay) in size. The middle ground
is termed silt and is typically the ideal texture for agricultural
conditions. Structure refers to how the soil particles are grouped
or aggregated (glossary link) together, defining pore space between
particles. Whereas texture remains fairly constant over time, structure
is susceptible to alteration depending on any number of activities
that may affect the soil at any given time such as drainage, grazing,
or tillage. Color can be a good indicator of the amount of soil
organic matter (SOM), a crucial component in a given soil; light
color indicates low SOM levels and darker color indicates higher
SOM levels.
Chemical properties: Soil particles are highly reactive
with electrically charged surfaces which serve many purposes such
as aggregate stability and more tightly bound binding nutrients
among others. Cation exchange capacity (CEC) (glossary
link), base saturation (glossary link),
and pH (glossary link) buffering are some
of the major illustrations of these important chemical properties
of soil particles. The exchangeability of positive charges (cations)
is an important property that is helpful in classifying soils in
more defined terms for the assessment of their fertility and environmental
behavior. Increasing base saturation increases the availability
of essential nutrients in the soil. Base saturation is determined
by the ratio of CEC present in the form of basic cations such as
calcium, magnesium potassium and sodium; this is inversely proportional
to the acidity of the soil. The pH buffering mechanism of soils
is an attempt to correct for limit extremes of over acidity or alkalinity
(glossary link) that may be occurring so neither extreme prevails.
SOM: Composed of a variety of carbon containing organic
substances, soil organic matter (glossary link)
plays a critical role in the CEC, soil water-holding capacity, and
global carbon cycle. Although SOM is only a small percent of soil
mass, it is largely responsible for aggregate formation and stability
as well as a nutrient supply for soil microorganisms. The quality
of SOM is crucial to the growth and enhancement of plants and thus
illustrates the quality of the soil.
Photo: Topsoil
produced from composting.
Nutrients: Essential Elements
Macronutrients: There are 17 macronutrients needed for
normal plant growth. Those available from the air and water include
carbon, hydrogen and oxygen. Minerals obtained from the soil that
are used in large amounts (hence MACROnutrients) include calcium,
magnesium, nitrogen, phosphorus, potassium and sulfur.
Read more about Nutrient
Topics & Deficiencies from Iowa State University Extension.
Micro or trace nutrients: Other soil minerals that are
used in smaller amounts (hence trace or MICROnutrients) include
boron, cobalt, copper, chlorine, iron, manganese, molybdenum, and
zinc.
Processes or methods of fixation, release, and removal are major
influences on the availability of nutrients to plants. Some of these
nutrients can be taken in through the leaves, but most are taken
up by the roots. The pH of the soil may affect the absorption of
nutrients or the level of nutrient toxicity present. Properly balanced
nutrients are more likely to produce successful agricultural crops,
maintain fertile soil conditions, and promote healthy biological
activity.
Soil Testing
What is it? Generally used for agricultural purposes, the testing
of soils is meant to evaluate the chemical status of soils and measure
the degree of nutrients accessible for plant uptake. Soil organic
matter is often what is being tested for, and since the levels present
are susceptible to change, regular testing is recommended.
Why is it important? Soil tests usually assess SOM, able to test
for base saturation, CEC, and soil pH. With such information, recommendations
for soil enhancement can be made more available.
Where do I go for a soil test? Visit A&L
Labs in Modesto, CA and Portland, OR or the North Carolina Department
of Agriculture & Consumer Services Agronomic
Division.
How do I interpret
my soil test?
A good report includes samples from various sites on a given plot
of land as it is common for soil compositions to change within short
distances. In addition, textural classification of the soil being
sampled should be known.
Practices
Tillage: Used as a means of manipulating the soil
in order to modify conditions, tillage is often used in crop production.
Tillage practices can have beneficial effects such as assimilation
of organic matter into the soil. Over long periods of time, adverse
effects of excessive tillage may occur. Eventually aggregates break
down too small, which leads to decreasing pore size and hindrance
of water infiltration. An alternative approach is a variety of conservation
tillage practices by means of minimum tillage, mulch tillage, no-tillage,
ridge till, or strip till practices which may be employed to reduce
unfavorable consequences.
For reduced tillage practices, visit: www.reducedtillage.ca
Photo examples of conservation tillage:
Fertilizers: Application of fertilizers is often used to supply
soils with salts that it is otherwise lacking. The most common nutrients
used in fertilizers are nitrogen, phosphorus, and potassium. These
are the main macronutrients required for plant growth that would
normally exist in fertile soils under optimum conditions. Fertilizers
must be carefully selected based on individual needs and instructions
of applications must be carefully read (some compounds are not compatible
with others and should not be mixed).
For information on efficient fertilizer use, visit: www.back-to-basics.net/efu/efu.html
Potential Problems
Leaching (glossary link)
of nutrients into the environment can occur from weathering of minerals
and natural organic compounds or from synthetic chemicals that have
been applied to the soil. Leaching results in loss of nutrients
necessary for plant growth. Also leaching can cause the potential
consequence of eutrophication in natural lakes, streams and estuaries,
in which the accumulation of chemicals in natural bodies of water
nutrients leads to anaerobic (glossary link)
conditions in which life is unable to exist.
Erosion is the term used to imply the loss of topsoil and degradation
of soil quality making the soil less productive. It is of major
concern due to the fact that it is a continually increasing process
and it is an extremely hard condition to counteract. Human activity
has led to land degradation which allows erosion to occur via water
flow and wind intensity. The problem will persist unless precautionary
steps are taken to prevent future erosion and repair that which
has already occurred. For more on soil erosion, visit: www.soilerosion.net
Photo: Severe
soil erosion in a wheat field near Washington State University.
Environmental Consequences
With increasing population size there is an increasing need for
land use. Activities such as grazing, timber harvesting, and cultivation
lead to rapid soil degradation and erosion. Over time, the consequence
is less productive soils that are unable to provide necessary benefits
for human survival. The greater degree of degradation, the harder
it is to perform soil restoration and the greater the spread of
desert like conditions of unworkable soil. For these reasons, it
is important to practice good soil management and conservation.
Related website links
Institute
of Plant Nutrition and Soil Science
Food
& Fertilizer Technology Center
TFREC Soil
and Plant Nutrition
Kids
World - Plant Nutrition
The
Soils Online page
Glossary:
Aggregate - numerous soil particles held in a single mass
or cluster.
Alkalinity - having the nature of a class of substrates
that liberate hydroxide ions in water, pH > 7.
Anaerobic - without air, oxygen-free environment.
Base saturation - total CEC saturated with exchangeable
nonacid cations.
Cation exchange capacity - total number of cations a soil
can absorb, expressed in centimeters of charge per kilogram (cmolc/kg)
of soil.
Leaching - to be removed from the soil by means of liquid
percolation.
pH - a logarithm of the reciprocal hydrogen-ion concentration
in moles per liter of a solution, a measurement of the degree of
acidity/alkalinity.
SOM - (soil organic matter) portion of the soil which contains
plant and animal residues at various stages of decomposition, established
by the amount of organic material within a soil sample passed through
a 2mm sieve.
Sources:
Dixon, J.B., and D.J. Schulze. 2002 Soil Mineralogy with Environmental
Applications. (Madison, Wis.: Soil Sci. Soc. Amer.)
Hudson, N. 1995. Soil Conservation, 3rd ed. (Ames, Iowa: Iowa State
University Press).
Hughes, H.A. 1980. Conservation Farming. (Moline, III.: John Deere
and Company).
McCarthy, D.F. 1993. Essentials of Soil Mechanics and Foundations,
4th ed. (Englewood Cliffs, NJ: Prentice Hall).
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