| Important terms:
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CEC: Cation Exchange Capacity - The ability
of a soil to hold cations such as potassium, magnesium and calcium, on the
surfaces of colloids in the soil. Is usually expressed in meq/100g soil. The
higher the number, the more cations a soil can hold on exchange sites. Cations
held on exchange sites are readily available to the soil solution and,
therefore, the plant. Sandy soils usually have low CEC’s of less than 5.
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| Nitrogen
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| Soil Persistence:
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The primary forms of inorganic N in soil
are nitrate ions (NO3-) and ammonium ions (NH4+).
Nitrates do not persist in soil. They are easily leached through soils and
they are also converted to nitrogen gases by denitrification under anaerobic
conditions (wet or compacted soils). While NH4 may be held by clay
soils to some degree, they are readily nitrified (converted to NO3)
in warm, well-drained soils, especially when pH and fertility are maintained
at favorable levels. Once in the NO3 form, they may be more easily
leached from the rooting zone of most crops.
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| Deficiency Symptoms:
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Yellowing of older (lower) leaves and
stunting of the plant are characteristic of nitrogen deficiency. Leaves will
begin to die at the tips and will eventually die completely.
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| Phosphorus
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| Soil Persistence:
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| Phosphorus is not very mobile in soils
(except coarse sands and muck soils), but it can be lost by
water runoff and erosion. Phosphorus is readily fixed in the soil to
unavailable forms. Soils with low levels of P may fix more of the applied
fertilizer than soils that are high in P, thus affecting application methods.
Since P is relatively immobile in the soil, root growth is even more important
to uptake by the plant. Factors limiting root growth (such as cold soil
temperatures, compaction, etc.) may
increase the chance of having a P
deficiency.
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Phosphorus availability in soil increases
as pH increases from 5.0 to 7.0.
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| Deficiency Symptoms:
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Plants may be stunted and have fewer leaves
with less flowers. Phosphorus deficient cotton may be stunted and a darker
green. Most obvious P deficiency symptom is purpling of leaves. This often
occurs in spring when temperatures remain cool.
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| Other:
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At pH’s above 6.5, may have interaction
between P and Ca resulting in a Ca induced P deficiency. Sandy Coastal Plains
soils can have P induced Zn deficiency at high pH’s and high P levels. Can
also have P induced Fe deficiency at high pH’s (iron chlorosis in turf).
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| Potassium
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| Soil Persistence:
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More mobile in soil than P, but less than
nitrate. The potassium cation (K+) is held on exchange sites in
soil, so leaching is more of a problem on very sandy soils.
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| Deficiency Symptoms:
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Leaf margins may turn brown in K deficient
plants. Increased lodging may occur in grain crops. Deficient plants are less
drought tolerant. In pastures with legumes and grasses, legumes can be
expected to suffer more than grasses from low K levels. Potassium deficiency
may also allow greater incidence of some diseases.
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| Other:
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In sandy soils, may see Ca induced K
deficiency if soil is limed heavily to raise pH from a low pH to a pH of about
6.5.
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| Magnesium
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| Soil Persistence:
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| Similar to K for most Alabama soils.
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| Deficiency Symptoms:
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Magnesium deficiency may cause interveinal
chlorosis in older leaves (N deficiency yellows in middle of leaves, Mg on the
edges). May exhibit purple color similar to P. Deficiencies occur most readily
on sandy, low CEC soils.
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Deficiencies may be more likely when using
high rates of NH+ and K+ on low CEC soils.
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| Calcium
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| Soil Persistence:
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| Similar to Mg.
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| Deficiency Symptoms:
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A key symptom of Ca deficiency is death of
the growing point (bud), although this is rare. Calcium deficiencies are also
involved in excessive "pops" in peanuts and blossom end rot of
tomatoes and watermelons. Deficiencies occur on sandy soils most often. Poor
root growth (nematode damage, cold wet soils, compacted layer in soil, low pH,
herbicide damage, etc.) also contributes to Ca deficiency.
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| Sulfur
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| Soil Persistence:
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Sulfur is converted in the soil to the
sulfate anion (SO42-) which is easily leached from the
rooting zone (though not as easily as NO3-). Sulfur
accumulates in clay subsoil, therefore deficiencies are more common in the
early part of the season.
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| Deficiency Symptoms:
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Sulfur deficiency may include yellowing of
younger leaves. There is greater need for S fertilizer today because of
increased use of P fertilizers that do not contain S and because there is less
S air pollution.
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| Boron
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| Soil Persistence:
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Boron is easily leached from soil.
Recommendations for B are very crop specific. What is needed by one crop may
be toxic to another.
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| Deficiency Symptoms:
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| A common B deficiency is hollow heart in
peanuts. Peanuts, cotton and vegetables respond well to B applications.
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| Interactions with other nutrients:
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High Ca or K and low B may result in B
deficiency. Low K and high B can lead to B toxicity.
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| Zinc and
Manganese |
| Soil Persistence:
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Zinc and Mn are more available to plants at
lower pH’s. Care should be taken when planting crops that are sensitive to
high Zn levels behind crops that respond to Zn fertilization (e.g. planting
peanuts behind corn).
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Buds of young plants turning white or light
yellow demonstrate Zn deficiency in corn.
Yellow bands on either side of the leaf midrib can also occur.
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Mn deficiency is more common in high water
table soils with pH>6. Soybeans with Mn deficiency exhibit interveinal
chlorosis.
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