All nutrients are needed at moderate
amounts. Excessive concentrations will cause toxicity while insufficient
concentrations will stunt growth. One of the more frequent causes of mediocre
growth is too much fertility or the addition of the incorrect product.
Waiting to analyze the soil until discoloration, burned
leaves, wilting and diseases occur decreases the probability of successful
corrections. Green plants does not necessarily mean that growth conditions are
ideal. Green plants may have hidden hunger. Optimum balance of the
thirteen essential mineral nutrients, irrigation and environmental factors
allow for the proper growth and vigor of the plants. Moderate mineral
deficiencies will not cause major changes in the appearance of the tissues, but
they can substantially lower the vitality. The worst cases are multiple
simultaneously. With critical deficiencies or nutrient toxicities,
discoloration follows with a rapid decline of the plant.
Soil testing as a management tool is greatly under utilized.
The most recent data for lawns and gardens are from 1987. The frequency of soil
testing is the highest in the Southern States where it is one test per about
200 people per year. The per capita rate of soil testing is very low in the
Southwestern States. California has a rate of one test per 2,091 people per
year while Arizona has a rate of one test per 958 people per year. Ideally,
every site should be tested every few years. The frequency of needed soil
testing depends upon the amount of irrigation, the quality of irrigation water,
the use of nutrients and amendments and the initial soil properties. Testing
could help to prevent and solve many problems.
WHAT IS SOIL?
Soil is formed from the parent minerals contained in rocks.
Through the influence of climate (rain, wind, heating, freezing etc.) and
organisms the rocks weather. Simple plants like lichens and microorganisms use
the minerals released in the weathering process and continue with the formation
of soil. As organisms grow and die, organic matter accumulates which interacts
with the mineral particles. Eventually, a horizon or profile of developed soil
is generated which is called a topsoil.
Rain leaches the soluble minerals into the deeper soil
profiles. The topsoil profile is called an "A horizon" while the soil
profile which receives the minerals that are moved into the soil by water is
called a "B horizon" or subsoil. Below the B horizon is the "C
horizon" or the unweathered rock.
The properties of the topsoil
depend mainly upon native vegetation and upon the amount of rain. These are a
function of climate. Weathering of rocks releases salts which had previously
been encapsulated, usually as part of the structure of the rock. In the desert
and semiarid zones of the Southwest, the salt content of the soil is very high
due to little leaching of the soils. The salts also impart an alkaline
condition to the soil. Native plant species are desert shrubs which are
tolerant of the local conditions. Due to poor plant growth, the accumulation of
organic matter is generally low and the soil has poor physical properties. The
soils are light colored.
The Great Plains has more rainfall than do the Western
deserts; the salinity or salt level is decreased, but there is not an excessive
amount of mineral leaching. The topsoil supports the growth of grasses. Organic
matter accumulates and the soils have good tilth, are fertile and have a dark
brown or black appearance due to the accumulation of organic matter.
As the amount of rain increases such as in the eastern
regions of the country, the nutrient content of the topsoil is lower. The
minerals have been leached into the deeper soil profiles. The soil is a
gray-brown. The native species are broadleaf deciduous trees in high rainfall
locations and the trees are needle-leaf trees in higher rainfall areas. Organic
matter decomposes forming organic acids making acidic conditions. The acidity dissolves the nutrients which
are leached into the groundwater by the rains.
Tropical soils near the surface are red because of the
extremely low level of organic matter which unmask the presence of iron oxides
which are very prevalent in soils. The fertility is extremely low and mostly
what is available comes from the recycling of the nutrients from decaying
vegetation and parasitic growth from plants growing on host plants. Weathering
is rapid in the hot, humid conditions which releases some nutrients from the
rocks.
Variations of the above conditions exist. Former marine
sediments can be exposed in the normally alkaline west containing deposits of
sulfur or iron sulfide which were formed from sulfate ions in the ocean. When exposed to the air, sulfuric acid is
produced by oxidation leading to acidic soils. Also earth slides, erosion or
grading can expose alkaline deposits from the "B" horizon in areas
which are normally acidic.
Soils with the best physical properties exist with the
highest level of soil organic matter. This occurs in the areas with moderate
rain. The extremes of too little water and of too much rain decrease soil
organic matter with a reduction in the tilth of the soil.
MODIFICATION OF SOIL PROPERTIES
The
application of soil amendments and fertilizers can increase directly or
indirectly the level of soil organic matter, increase the fertility of the soil
and change the salt level and pH of the soil. Acidifying fertilizers such as
ammonium sulfate and soil sulfur at high rates can make soil too acidic in
normally alkaline conditions. Alkaline forming fertilizers such as calcium
nitrate and potassium nitrate or the addition of excessive amounts of limestone
can also cause growth inhibition.
It is surprising for most people to learn that
plants have growth optimum conditions for nutrients; too much can be as bad as
too little. When too much fertilizer is applied which is common for many sites,
the rate of plant growth is decreased. Part of the inhibition is an induced
deficiency of another nutrient caused by competition. Additionally, excessive
nutrients increases the salt level of the soil which interferes with the
moisture absorption by most plants.
In some cases, the soils are very resistant to change. This
can be the situation for the arid and semiarid climates of the Southwest where
limestone is present -- it is extremely difficult to acidify the soil. Plant
growth problems exist in plants species (acid-loving plants) adapted to acidic
soils when they are grown under alkaline conditions. Iron deficiency recognized
by yellow leaves with narrow green veins is caused by the limestone and
bicarbonates. In extreme cases the yellow leaves are very small; totally white
leaves are easily burned from saline conditions. Special iron products are
available to correct these problems. The iron deficiency condition is so common
in some locals that many people believe that some species are normally yellow
when they are not.
GUIDANCE FROM SOIL TESTING
Soil Acidity If the soil is too acidic, aluminum is dissolved
causing a specific ion toxicity. The plant growth is stunted and the leaf
coloration is sometimes deep green.
If the soil
is too alkaline, some plant nutrients are unavailable causing a mineral
deficiency. The source of these two problems can be the use of too much plant
fertilizers of the incorrect type. A soil acidity (pH test) is required to know
the soil acidity status.
Managing alteration in soil acidity with choice of
nitrogen fertilizers – Fertilizer products are not interchangeable. Each
product has a particular advantage and benefit over other materials. Use of the
incorrect product will exacerbate problems while the correct on will enhance
growth. For instance with nitrogen products, ammonium sulfate (21-0-0) will
acidify the soil; ammonium nitrate (34-0-0) will be pH neutral if not over
applied; calcium nitrate (15.5-0-0) will slightly increase the soil pH; urea
(46-0-0) needs to be hydrolyzed before it is available. Nitrate nitrogen will
supply soil oxygen. Ammonium nitrogen consumes oxygen when it is nitrified to
nitrate.
Slow-release nitrogen materials
also have certain benefits. Ureaformaldehyde (38-0-0) release nitrogen
according to temperature and biological activities. IBDU releases in the
presence of moisture and acidity, not according to plant growth. If coated
products are broken, they become rapid-release.
Presence of limestone If limestone (calcium
carbonate or chalk) is present, acid-loving plants become iron deficient unless
corrective measures are taken.
Lime Requirement In areas of high rainfall, there are inadequate levels
of potassium, calcium and/or magnesium due to the acidic soil. Tests for the
required level of limestone or dolomite needed to raise the soil pH to a safe
level are essential.
Excess Salts in the Soil The term used by laboratories is salinity. If salts
have excessively accumulated in the soil, many plants are unable to use the
moisture in the soil and may have toxicity from sodium and/or chloride. A
salinity test is required to determine if this is a problem. The salinity can
be controlled by leaching unless soils have drainage problems. A soil high in
salinity is called "saline."
Excessive Sodium Excessive sodium or a "sodic" soil most
often has an elevated pH level. Soils high in pH values are suspect. Sodium can
cause toxicity, but the more likely problem is soil compaction and poor
drainage caused by the reaction of sodium on the clay.
Gypsum Requirement Excessive sodium can be
corrected with the addition of gypsum. Another cause of high pH values is the
presence of bicarbonates. Gypsum is also used to precipitate the excessive
bicarbonates and lower the very high soil pH values. A laboratory test shows
how much gypsum is needed.
Fertility Most plants require at least 16 nutrients. Three
nutrients are supplied by the water and by the air (oxygen, hydrogen and
carbon). Thirteen are mineral nutrients. If any one is too low, the plants will
not grow. In some cases, too much fertilizers have been applied causing an
adverse reaction. Too much phosphorus, for instance, inhibits the plant uptake
of iron, manganese, zinc and copper causing induced deficiencies. The best
method to determine if a problem is caused by a true deficiency or is an
induced deficiency is soil testing. Soil analysis is used to assess the
nutrient levels of the soil. Plant tissue testing is also used to ascertain
which nutrients have reduced availability in the soil.
Toxicity Soils may contain toxic metals. They either exist in
the soil naturally or have been introduced as contaminants in amendments. Mined
minerals and waste products are the frequent contaminant sources. These
elements prevent plant growth. If a vegetable garden is to be grown and if the
presence of heavy metals is suspected, the soil should be tested as a
precaution for human poisoning. Lead can be present in urban soil at levels
which do not injure plants but can accumulate in produce at levels which may
harm humans. Excess levels of selenium and molybdenum are problems with wild
life or cattle. Other common toxic elements are aluminum, cadmium, chromium,
nickel, arsenic, silver, and vanadium.
Soil Compaction Excessive compaction impedes root growth, impairs
water penetration and reduces soil aeration. Reduced aeration hinders the
absorption of nutrients. In addition, slow water penetration exasperates the
problem. Soil compaction can be measured and corrected with soil conditioners.
Their need can be detected with soil testing.
