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Soil
pH
The
soil pH is a measure of the soil's acidity. This is measured
in the lab by taking a measured quantity of soil, adding
a measured amount of distilled water, shaking the sample
for 10 minutes, then using a pH meter and electrode to measure
the pH of the soil/water solution. The pH is an index, with
7.0 being neutral. This means that a pH of 7.0 is neither
acidic nor alkaline. Numbers above 7.0 are considered to
be alkaline, and numbers below are considered to be acidic.
Most lawn and garden plants prefer a soil pH in the range
of 6.0 to 7.0, or just slightly acidic. Some plants,
such as blueberries and azaleas, actually prefer the soil
to be even more acidic. To raise the soil pH, lime can be
applied. To lower the soil pH, sulfur can be applied.
Buffer pH
The
buffer pH is a test run to determine the soil's ability
to resist a pH change. If your soil's pH is already above
7.0, this test will not be run. However, if your soil's
pH is less than 7.0, we need to determine how much lime
it will take to increase the pH. The test is done in the
laboratory by adding a measured quantity of a buffer
to the same solution that was used to measure the soil pH.
After shaking the sample again, the pH is measured a second
time. The difference between the two readings gives us an
indication of how the soil will react to a lime application.
For example, let's imagine two different soils that both
have a soil pH of 5.0:
_________Sample #________Soil pH_______Buffer pH
__________1_______________5.0_____________6.0
__________2_______________5.0_____________7.0
Relatively
speaking, sample #1 is more difficult to change than sample
#2, so more lime would have to be applied to reach the same
final soil pH.
Phosphorus
The
phosphorus test result is reported in terms of pounds per
acre. A theoretical target level of 60 pounds per acre is
what we would like to see for most garden crops. The bar
graph of your test results reflects whether your test result
is above or below this target level.
Phosphorus
is one of the major nutrients essential for plant growth.
Phosphorus is relatively immobile in the soil, meaning it
is not easily leached
through the soil profile. If an application of phosphorus
is needed, it must be mixed into the soil for proper distribution.
The primary role of phosphorus in plants is the storage
and transfer of energy. Phosphorus is a structural component
of many membranes and other cell structures and helps increase
root growth. Plants need an adequate supply of phosphorus
at early growth stages when reproductive parts are being
formed. Phosphorus is essential for seed formation; relatively
large quantities are found in seeds and fruits.
The
most common form of phosphorus in fertilizers is phosphate.
In a fertilizer formulation, the second number listed in
a complete fertilizer is the phosphorus content. For example,
a fertilizer called a 24-6-4 would contain 6% phosphorus,
or 6 pounds of phosphorus per one hundred pounds of fertilizer.
Potassium
The
potassium test result is reported in terms of pounds per
acre. A theoretical target level of approximately 350 pounds
per acre is what we would like to see for most garden crops.
The bar graph of your test results reflects whether your
test result is above or below this target level.
Potassium
is one of the major nutrients essential for plant growth.
Potassium in plants is involved in enzyme activation, starch
formation, nitrogen assimilation, translocation of sugars,
and other metabolic processes. Potassium is relatively mobile
in the soil, meaning it is readily leached
through the soil profile and can be lost from the root zone.
Although potassium does move through the soil, if large
quantities need to be applied it is best to work it into
the soil. When a deficiency occurs, the plant will translocate
potassium to newer growth. As a result, potassium deficiency
will usually appear on older leaves first as browning along
the leaf edges. Because potassium regulates the opening
and closing of leaf pores, plants deficient in potassium
are susceptible to water stress. With insufficient potassium,
leaf pores close too slowly, and too much water may be lost
through transpiration.
With adequate potassium, the pores close more rapidly and
less water is lost under dry conditions.
The
third number listed in a complete fertilizer is the potassium
content. For example, a fertilizer called a 24-6-4 would
contain 4% potassium, or 4 pounds of potassium per one hundred
pounds of fertilizer. The most common fertilizer source
of potassium is potassium chloride (KCl), often called "muriate
of potash" or "potash".
Calcium
The
calcium test result is reported in terms of pounds per acre.
It is difficult to choose one number as a target for calcium,
since the optimum level varies with soil type and the plant
to be grown. The bar graph of your test results takes these
factors into account and reflects whether your test result
is above or below a target level.
Calcium
is considered a secondary nutrient; however, it is essential
to all plants. Plants require calcium in fairly large quantities
because it is part of cell walls and other structural tissue.
It is also the primary element used to correct soil acidity.
Calcium does not move to the top of plants as quickly or
easily as other nutrients. Therefore, calcium deficiency
symptoms are likely to appear in young tissues first. A
lack of calcium may prevent the development of terminal
buds and roots. Fruits and vegetables often show calcium
deficiency when pH, water supply, and other essential nutrients
are inadequate. Inadequate soil moisture can lead to calcium
stress. In fruits and vegetables, deficiencies often appear
as disorders in the fruit or vegetable storage organs. Blossom-end
rot in tomatoes, carrot cavity, black heart in celery, tipburn
in lettuce and cabbage, and bitter pit in apples are examples.
The
most common sources of calcium include lime, gypsum, marl,
and calcium chloride.
Magnesium
The
magnesium test result is reported in terms of pounds per
acre. It is difficult to choose one number as a target for
magnesium, since the optimum level varies with soil type
and the plant to be grown. The bar graph of your test results
takes these factors into account and reflects whether your
test result is above or below a target level.
Magnesium
is considered a secondary nutrient; however, it is essential
to all plants. Plants use magnesium in the formation of
chlorophyll, the pigment that causes plants to have a green
color. Without chlorophyll, photosynthesis
cannot occur. Plants use about 15 to 20 percent of the
magnesium they take up for chlorophyll formation. The rest
occurs in organic compounds or as free ions in the cells.
Magnesium is active in enzyme systems and aids in the
translocation of phosphorus
in plants. Magnesium is fairly mobile in plants, and is
translocated from older to younger plant parts. Therefore,
deficiencies may appear first on older leaves. Plants deficient
in magnesium may develop chlorosis
of the leaf. In more advanced stages, leaf tissue may become
uniformly yellow or develop a pink or purple color.
Common
sources of magnesium include dolomitic limestone and Epsom
salts.
Organic
Matter
The
organic matter test result is reported in terms of a percentage.
That is, for a given weight of soil, a certain percent is
determined to be organic matter. There is no "correct" level
for organic matter in the soil. This is why there is no
bar graph on our soil printout for organic matter. Two to
four percent organic matter is typical. On coarse textured
soils (sand, loamy sand) where organic matter is low, leaching
may be a problem. High organic matter may lead to high acidity
and a lack of nutrient availability.
The
organic matter in soil is useful for functions as varied
as improving water-holding capacity to providing a growth
medium for useful organisms such as soil bacteria and earthworms.
As organic material breaks down, it tends to produce acidic
by-products, so it can be detrimental to soil pH if there
is too much in the soil.
The
organic matter percentage in the soil may be lowered by
adding sand, or increased by adding manure, compost, peat
moss, or higher organic matter topsoil.
Cation
Exchange Capacity
The
cation exchange capacity (or C.E.C.) is reported in terms
of milliequivalents per 100 grams of soil. Typical C.E.C.
numbers will range from around 5.0 to 50 or more, although
numbers from 0.1 to 100 are theoretically possible. This
is actually a calculated number which is based on the potassium,
calcium, magnesium, and (where applicable) the buffer pH
tests.
The
C.E.C. is an indication of how many places nutrients can
be attached to the soil. The higher the number, the more
sites there are within the soil to hold nutrients. In fairly
unscientific terms, the C.E.C. is a measure of the soil's
ability to both hold on to and give up nutrients. It is
important for the soil to be able to hold on to nutrients,
or the first rain would wash away all of your fertilizer.
It is equally important for the soil to be able to give
up nutrients, or the plant roots would not be able to get
the nutrients away from the soil! Numbers from 10 to 25
are probably a good range for the typical gardener. The
C.E.C. can also be used as somewhat of a rough indicator
of soil texture. Soils with very low C.E.C. numbers tend
to be very sandy, while very high numbers are often associated
with high clay content or highly organic soils.
The
C.E.C. cannot be easily changed except by changing the actual
composition of the soil itself. This means the addition
of sand or organic materials to the existing topsoil.
Percent
Base Saturation
The
percent base saturation is tied somewhat to the cation
exchange capacity.
While the C.E.C. is an indication of the soil's ability
to hold nutrients, the percent base saturation shows how
those sites are being used. That is, of all the potential
places that could have a nutrient attached to them, what
percentage are being used by potassium, calcium, magnesium,
or hydrogen? Only acidic soils will have a hydrogen percentage,
so it is usually best to see this number low or non-existent.
This may not be true if you are growing acid-loving plants,
however. There are no hard and a fast rule for what is optimum
in terms of base saturation. As a general guideline, however,
we would expect to see the potassium at about 2-5%, the
magnesium at 10-25%, and the calcium at 70-80%. It is possible
to have very good results from soil that does not exhibit
these ratios . However, this is what we expect when all
other nutrients are at their optimum levels.
Glossary
of terms
Buffer
- a solution that is a known pH and is resistant to change
in pH
Chlorosis
- a condition where the area between the veins of the leaves
turns yellow while the veins remain green; often caused
by lack of iron or magnesium
Leaching
- the process by which water (rain, etc.) carries nutrients
downward through the soil
Photosynthesis
- the process by which plants utilize the energy they receive
from sunlight to create their own energy for growth
Stomate
(plural - stomata) - the pores in leaf surfaces that allows
them to release water and exchange gases
Translocation
- the process by which nutrients are moved within a plant
via its vascular system
Transpiration
- the process by which water is allowed to exit the leaf
surface via the stomata
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