nutrients, CeC, aCidity, and alkalinity
The potential available nutrients in a soil, whether natural or added in
manures or fertilizer, are only in part utilized by plants . . .
—t.l. lyon and e.o. FiPPin, 1909
OTHER NUTRIENTS
quantities for optimum yields of crops. It’s generally
Although farmers understandably focus on nitrogen and
available to plants as a cation, and the soil’s c ation
phosphorus—because of the large quantities used and
exchange capacity (CEC) is the main storehouse for this
the potential for environmental problems—additional
element for a given year’s crop. Potassium availability
nutrient and soil chemical issues remain important.
to plants is sometimes decreased when a soil is limed to
Overuse of other fertilizers and amendments seldom
increase its pH by one or two units. The extra calcium,
causes problems for the environment, but it may waste
as well as the “pull” on potassium exerted by the new
money and reduce yields. There are also animal health
cation exchange sites (see the next section, “Cation
considerations. For example, excess potassium in feeds
Exchange Capacity Management”), contributes to lower
for dry cows (cows that are between lactations) results
potassium availability. Problems with low potassium
in metabolic problems, and low magnesium availability
levels are usually dealt with easily by applying muriate
to dairy or beef cows in early lactation can cause grass
of potash (potassium chloride), potassium sulfate, or
tetany. As with most other issues we have discussed,
sul-po-mag or K-mag (potassium and magnesium sul-
focusing on the management practices that build up and
fate). Manures also usually contain large quantities
maintain soil organic matter will help eliminate many
of potassium.
problems or at least make them easier to manage.
Magnesium deficiency is easily corrected if the soil
Potassium (K) is one of the N-P-K “big three”
is acidic by using a high-magnesium (dolomitic) lime-
primary nutrients needed in large amounts, but in
stone to raise the soil pH (see “Soil Acidity,” p. 230). If K
humid regions it is frequently not present in sufficient
is also low and the soil does not need liming, sul-po-mag
Photo by Dennis Nolan
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for the full development of garlic’s pungent flavor.)
The risk for sulfur deficiency varies with the soil type,
Much of the sulfur in soils occurs as organic matter,
the crops grown on the soil, the manure history, and
so building up and maintaining organic matter should
the level of organic matter in the soil. A deficiency is
result in sufficient sulfur nutrition for plants. Although
more likely to occur on acidic, sandy soils; soils with
reports of crop response to added sulfur in the Northeast
low organic matter levels and high nitrogen inputs; and
are rare, it is thought that deficiencies of this element
soils that are cold and dry in the spring, which condi-
may become more common now that there is less sulfur
tion decreases sulfur mineralization from soil organic
air pollution, originating mainly in the Midwest. Some
matter. Manure is a significant supplier of sulfur, and
fertilizers used for other purposes, such as sul-po-mag
manured fields are not likely to be S deficient; how-
and ammonium sulfate, contain sulfur. Calcium sulfate
ever, sulfur content in manure can vary.
(gypsum) also can be applied to remedy low soil sulfur.
—S. PLACE ET AL. (2007)
The amounts used on sulfur-deficient soils are typically
20 to 25 pounds of sulfur per acre.
is one of the best choices for correcting an Mg deficiency.
Zinc deficiencies occur with certain crops on soils
For a soil that has sufficient K and is at a satisfactory
low in organic matter and in sandy soils or soils with a
pH, a straight Mg source such as magnesium sulfate
pH at or above neutral. Zinc problems are sometimes
(Epsom salts) would be a good choice.
noted on silage corn when manure hasn’t been applied
Calcium deficiencies are generally associated with
for a while. Zinc also can be deficient following topsoil
low pH soils and soils with low CECs. The best remedy
removal from parts of fields as land is leveled for fur-
is usually to lime and build up the soil’s organic mat-
row irrigation. Cool and wet conditions may cause zinc
ter. However, some important crops, such as peanuts,
to be deficient early in the season. Sometimes crops
potatoes, and apples, commonly need added calcium.
outgrow the problem as the soil warms up and organic
Calcium additions also may be needed to help alleviate
sources become more available to plants. Applying
soil structure and nutrition problems of sodic soils (see
about 10 pounds of zinc sulfate (which contains about
“Remediation of Sodic (Alkali) and Saline Soils,” p. 233).
3 pounds of zinc) to soils is one method used to correct
In general, if the soil does not have too much sodium, is
zinc deficiencies. If the deficiency is due to high pH, or
properly limed, and has a reasonable amount of organic
if an orchard crop is zinc deficient, a foliar application is
matter, there will be no advantage to adding a calcium
commonly used. If a soil test before planting an orchard
source, such as gypsum. However, soils with very low
reveals low zinc levels, zinc sulfate should be applied.
aggregate stability may sometimes benefit from the extra
Boron deficiencies show up in alfalfa when it grows
salt concentration and calcium associated with surface
on eroded knolls where the topsoil and organic matter
gypsum applications. This is not a calcium nutrition
have been lost. Root crops seem to need higher soil boron
effect but a stabilizing effect of the dissolving gypsum
levels than many other crops. Cole crops, apples, celery,
salt. Higher soil organic matter and surface residues
and spinach are also sensitive to low boron levels. The
should do as well as gypsum to alleviate this problem.
most common fertilizer used to correct a boron deficiency
Sulfur deficiencies are common on soils with low
is sodium tetraborate (about 15% boron). Borax (about
organic matter. Some soil testing labs around the coun-
11% boron), a compound containing sodium borate, also
try offer a sulfur soil test. (Those of you who grow garlic
can be used to correct boron deficiencies. On sandy soils
should know that a good supply of sulfur is important
low in organic matter, boron may be needed on a routine
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basis. Apply no more than 3 pounds of actual B (about 27
cATION ExcHANgE cAPAcITY MANAgEMENT
pounds of borax) per acre at any one time—it can be toxic
The CEC in soils is due to well-humified (“very dead”)
to some plants at higher rates.
organic matter and clay minerals. The total CEC in a soil
Manganese deficiency, usually associated with
is the sum of the CEC due to organic matter and due to
soybeans and cereals grown on high-pH soils and veg-
clays. In fine-textured soils with medium- to high-CEC
etables grown on muck soils, is corrected with the use
clays, much of the CEC may be due to clays. On the other
of manganese sulfate (about 27% manganese). About 10
hand, in sandy loams with little clay, or in some of the
pounds of water-soluble manganese per acre should sat-
soils of the southeastern U.S. that contain clays with low
isfy plant needs for a number of years. Up to 25 pounds
CEC, organic matter may account for an overwhelming
per acre of manganese is recommended if the fertilizer
fraction of the total CEC.
is broadcast on a very deficient soil. Natural, as well as
There are two practical ways to increase the abil-
synthetic, chelates (at about 5% to 10% manganese) usu-
ity of soils to hold nutrient cations such as potassium,
ally are applied as a foliar spray.
calcium, magnesium, and ammonium:
Iron deficiency occurs in blueberries when they
• Add organic matter by using the methods discussed
are grown on moderate- to high-pH soils, especially a
in earlier chapters.
pH of over 6.5. Iron deficiency also sometimes occurs
• If the soil is too acidic, use lime (see “pH Manage-
on soybeans, wheat, sorghum, and peanuts growing on
ment,” p. 231) to raise its pH to the high end of the
soil with a pH greater than 7.5. Iron (ferrous) sulfate
range needed for the crops you grow.
or chelated iron is used to correct iron deficiency. Both
One of the benefits of liming acid soils is increasing
manganese and iron deficiencies are frequently cor-
soil CEC. Here’s why: As the pH increases, so does the
rected by using foliar application of inorganic salts.
CEC of organic matter as well as some clay minerals. As
Copper is another nutrient that is sometimes deficient
hydrogen (H+) on humus is neutralized by liming, the
in high-pH soils. It is also sometimes deficient in organic
site where it was attached now has a negative charge and
soils (soils with 10–20% or more organic matter). Some
can hold Ca++, Mg++, K+, etc.
crops—for example, tomatoes, lettuce, beets, onions, and
Many soil testing labs will run CEC if asked.
spinach—have a relatively high copper need. A number of
However, there are a number of possible ways to do the
copper sources, such as copper sulfate and copper chelates,
test. Some labs determine what the CEC would be if the
can be used to correct a copper deficiency.
soil’s pH was 7 or higher. They do this by adding the
estIMAtInG orGAnIc MAtter’s contrIbutIon to A soIL’s cec
The CEC of a soil is usual y expressed in terms of the number of milliequivalents (me) of negative charge per 100 grams of soil.
(The actual number of charges represented by one me is about 6 followed by 20 zeros.) A useful rule of thumb for estimating the CEC due to organic matter is as follows: For every pH unit above pH 4.5, there is 1 me of CEC in 100 grams of soil for every percent of organic matter. (Don’t forget that there will also be CEC due to clays.) SOM = soil organic matter.
Example 1: pH = 5.0 and 3% SOM → (5.0 – 4.5) x 3 = 1.5 me/100g
Example 2: pH = 6.0 and 3% SOM → (6.0 – 4.5) x 3 = 4.5 me/100g
Example 3: pH = 7.0 and 3% SOM → (7.0 – 4.5) x 3 = 7.5 me/100g
Example 4: pH = 7.0 and 4% SOM → (7.0 – 4.5) x 4 = 10.0 me/100g
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pH
soIL AcIdItY
Background
4
5
6
7
8
9
10
• pH 7 is neutral.
acidic
neutral
basic
• Soil with pH levels above 7 are alkaline; those of less
than 7 are acidic.
• The lower the pH, the more acidic is the soil.
Figure 20.1. Soil pH and acid-base status.
Note: Soils at pH 7.5 to 8 frequently contain fine particles of lime
Note: Soils at pH 7.5–8 frequently contain fine particles of lime
• Soils in humid regions tend to be acidic; those in
(calcium carbonate).Soils above pH 8.5 to 9 usually have excess
(c sodium (sodic, also called alkali, soils).
alcium carbonate). Soils above pH 8.5–9 usual y have excess sodium
semiarid and arid regions tend to be around neutral
(sodic, also cal ed alkali, soils).
or alkaline.
SOIl AcIDITY
• Acidification is a natural process.
• Most commercial nitrogen fertilizers are acid form-
Background
ing, but many manures are not.
Many soils, especially in humid regions, were acidic
• Crops have different pH needs—probably related
before they were ever farmed. Leaching of bases from
to nutrient availability or susceptibility to aluminum
soils and the acids produced during organic matter
toxicity at low pH.
decomposition combined to make these soils natu-
• Organic acids on humus and aluminum on the CEC
rally acidic. As soils were brought into production and
account for most of the acid in soils.
organic matter was decomposed (mineralized), more
Management
acids were formed. In addition, all the commonly used
• Use limestone to raise the soil pH (if magnesium is
N fertilizers are acidic—needing from 4 to 7 pounds of
also low, use a high-magnesium—or dolomitic—
agricultural limestone to neutralize the acid formed
lime).
from each pound of N applied to soils.
• Mix lime thoroughly into the plow layer.
Plants have evolved under specific environments,
• Spread lime well in advance of sensitive crops if at
which in turn influence their needs as agricultural crops.
all possible.
For example, alfalfa originated in a semiarid region
• If the lime requirement is high—some labs say
where soil pH was high; alfalfa requires a pH in the
greater than 2 tons; others say greater than 4 tons—
range of 6.5 to 6.8 or higher (see figure 20.1 for common
consider splitting the application over two years.
soil pH levels). On the other hand, blueberries, which
• Reducing soil pH (making soil more acid) for acid-
evolved under acidic conditions, require a low pH to
loving crops is done best with elemental sulfur (S).
provide needed iron (iron is more soluble at low pH).
Other crops, such as peanuts, watermelons, and sweet
acidity that would be neutralized if the soil was limed to
potatoes, do best in moderately acid soils in the range of
the current soil CEC. This is the CEC the soil would have
pH 5 to 6. Most other agricultural plants do best in the
at the higher pH but is not the soil’s current CEC. For
range of pH 6 to 7.5.
this reason, some labs total the major cations actually
Several problems may cause poor growth of acid-
held on the CEC (Ca++ + K+ + Mg++) and call it effective
sensitive plants in low pH soils. The following are three
CEC. It is more useful to know the effective CEC—the
common ones:
actual current CEC of the soil—than CEC determined at
• aluminum and manganese are more soluble and can
a higher pH.
be toxic to plants;
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chAPter 20 other Fertility issues: nutrients, CeC, aCidity, and alkalinity
• calcium, magnesium, potassium, phosphorus, or
• by enhancing bacterial activity, including the rhizo-
molybdenum (especially needed for nitrogen fixation
bia that fix nitrogen in legumes; and
by legumes) may be deficient; and
• by making aluminum and manganese less soluble.
• decomposition of soil organic matter is slowed and
Almost all the acid in acidic soils is held in reserve
causes decreased mineralization of nitrogen.
on the solids, with an extremely small amount active in
The problems caused by soil acidity are usually less
the soil water. If all that we needed to neutralize was the
severe, and the optimum pH is lower, if the soil is well
acid in the soil water, a few handfuls of lime per acre
supplied with organic matter. Organic matter helps
would be enough to do the job, even in a very acid soil.
to make aluminum less toxic, and, of course, humus
However, tons of lime per acre are needed to raise the
increases the soil’s CEC. Soil pH will not change as
pH. The explanation for this is that almost all of the acid
rapidly in soils that are high in organic matter. Soil
that must be neutralized in soils is reserve acidity associ-
acidification is a natural process that is accelerated by
ated with either organic matter or aluminum.
acids produced in soil by most nitrogen fertilizers. Soil
organic matter slows down acidification and buffers
pH Management
the soil’s pH because it holds the acid hydrogen tightly.
Increasing the pH of acidic soils is usually accomplished
Therefore, more acid is needed to decrease the pH by a
by adding ground or crushed limestone. Three pieces of
given amount when a lot of organic matter is present.
information are used to determine the amount of lime
Of course, the reverse is also true—more lime is needed
that’s needed:
to raise the pH of high-organic-matter soils by a given
1. What is the soil pH? Knowing this and the needs of
amount (see “Soil Acidity” box, p. 230).
the crops you are growing will tell you whether lime
Limestone application helps create a more hospi-
is needed and what target pH you are shooting for. If
table soil for acid-sensitive plants in many ways, such as
the soil pH is much lower than the pH needs of the
the following:
crop, you need to use lime. But the pH value doesn’t
• by neutralizing acids;
tell you how much lime is needed.
• by adding calcium in large quantities (because lime-
2. What is the lime requirement needed to change the
stone is calcium carbonate, CaCO3);
pH to the desired level? (The lime requirement is
• by adding magnesium in large quantities if dolomitic
the amount of lime needed to neutralize the hydro-
limestone is used (containing carbonates of both
gen, as well as the reactive aluminum, associated
calcium and magnesium);
with organic matter.) A number of different tests
• by making molybdenum and phosphorus more
used by soil testing laboratories estimate soil lime
available;
requirements. Most give the results in terms of tons
• by helping to maintain added phosphorus in an
per acre of agricultural grade limestone to reach the
available form;
desired pH.
Soil testing labs usual y use the information you provide about your cropping intentions and integrate the three issues (see the discussion under “pH Management,” above, of the three pieces of information needed) when recommending limestone application rates. Laws govern the quality of limestone sold in each state. Soil testing labs give recommendations based on the use of ground limestone that meets the minimum state standard.
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A
10
Organic Matter (%)
A) silty clay loams
5.0
B) loams and silt loams
3.0
8
C) sandy loams
2.0
D) sands
1.0
B
reach pH 6.8
6
C
4
tons of limestone to
D
2
0
4.5
5.0
5.5
6.0
6.5
7.0
soil pH before liming
Figure 20.2. Examples of approximate lime needed to reach pH 6.8. Modified from Peech (1961).
3. Is the limestone you use very different from the one
clovers. As pointed out above, most of the commonly
assumed in the soil test report? The fineness and the
grown crops do well in the range of pH 6.0 to 7.5.
amount of carbonate present govern the effective-
There are other liming materials in addition to
ness of limestone—how much it will raise the soil’s
limestone. One commonly used in some parts of the U.S.
pH. If the lime you will be using has an effective
is wood ash. Ash from a modern airtight wood-burning
calcium carbonate equivalent that’s very different
stove may have a fairly high calcium carbonate content
from the one used as the base in the report, the
(80% or higher). However, ash that is mainly black—
amount applied may need to be adjusted upward (if
indicating incompletely burned wood—may have as little
the lime is very coarse or has a high level of impuri-
as 40% effective calcium carbonate equivalent. Lime
ties) or downward (if the lime is very fine, is high in
sludge from wastewater treatment plants and fly ash
magnesium, and contains few impurities).
sources may be available in some locations. Normally,
Soils with more clay and more organic matter need
minor sources like these are not locally available in suf-
more lime to change their pH (see figure 20.2). Although
ficient quantities to put much of a dent in the lime needs
organic matter buffers the soil against pH decreases, it
of a region. Because they might carry unwanted con-
also buffers against pH increases when you are trying to
taminants to the farm, be sure that any new by-product
raise the pH with limestone. Most states recommend a
liming sources are field tested and thoroughly evaluated
soil pH of around 6.8 only for the most sensitive crops,
for metals before you use them.
such as alfalfa, and of about 6.2 to 6.5 for many of the
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chAPter 20 other Fertility issues: nutrients, CeC, aCidity, and alkalinity
“Overliming” injury. Sometimes problems are cre-
concentrate when the soil dries. Another way is to grow
ated when soils are limed, especially when a very acidic
crops or varieties of crops that are more tolerant of soil
soil has been quickly raised to high pH levels. Decreased
salinity. Saline-tolerant plants include barley, Bermuda
crop growth because of “overliming” injury is usually
grass, oak, rosemary, and willow. However, the only way
associated with a lowered availability of phosphorus,
to get rid of the salt is to add sufficient water to wash it
potassium, or boron, although zinc, copper, and man-
below the root zone. If the subsoil does not drain well,
ganese deficiencies can be produced by liming acidic