As
important as the primary nutrients but taken up in smaller quantities.
Not
as much attention is given to adding these nutrients to the soil
because they are added incidentally.
-
When
the pH is raised by liming Ca is added to the soil. Mg is
added when dolomitic limestone is used.
-
Soil
sulfur is replenished by sulfur dioxide from the air brought
down by precipitation. Regulations on air pollution have
reduced this in recent years. Added incidentally with other
fertilizers ordinary superphosphate.
Calcium
A.
Form used by plants
Absorbed
by plants as the Ca2+ from exchange sites or soil
solution
B.
Soil Calcium
- Contained
in rocks and minerals from which the soil is formed - dolomite,
calcite, apatite, calcium feldspars.
- In
humid regions even soils formed from limestone are acid in
the surface layer because Ca2+ may be removed by
excessive leaching. H2O + CO2 ===>
H2CO3 Carbonic acid
C.
Calcium added to Soil
- Limestone
used to improve acid soils add Ca
- Gypsum
is used to supply Ca without changing the pH.
- Phosphorus
fertilizers
D.
Behavior of calcium in the soil
- Availability
is dependent on the percent of the cation exchange complex
occupied by Ca2+ rather than the absolute amount.
- Type
of Clay
a. Kaolitic clays (1-1) can supply enough Ca to plants at saturation
values of 40-50?
b. Montmorillonitic clays require Ca situation of 70% to supply
plants
E.
Calcium Nutrition
1.
General
a.
Forms compounds that a part of the cell walls -- to strengthen
plant structure.
b.
Stimulate root growth and leaf development.
c.
Calcium is very immoble in the plant. Ca in the leaves is
not translocated to the fruit. The Ca must be supplied by
the soil when the fruit is forming.
d.
Ca deficiencies can occur in the fruit when soil moisture
is low. ex. blossom
end rot of tomatoes.
2.
Specific Crops
a.
Peanuts
Ca is required for good peg development. Since Ca is immoble
in the plant, gypsum is broadcast to the soil surface to insure
adequate uptake.
b.
Soybeans (legumes)
Ca is required for the rhizobia to form nodules on the roots.
Magnesium
Form
taken up by plants Mg2+
Soil
Magnesium
Dolomite,
olivine, biotite mica the minerals containing magnesium are
easily weathered. The soil is depleted of weatherable Mg minerals
relatively sooner than most Ca, Mg and K minerals.
Exchangeable
Mg2+ is the largest source of plant available Mg.
Twelve
to 18% of exchangables bases are usually Mg2+ ions compared
to 75-85% for Ca.
The
Mg2+ hydrates to form a larger ion than Ca therefore,
it is adsorbed less strongly by the cation exchange complex.
Mg
is excessive if it occupies more than 40-60% of CEC deficient
if less than 3-8%
Preferential
adsorption of these double charged ions results in a lower percentage
in soil solution than other cations like K and Na.
High
K fertilization can induce Mg deficiency
Magnesium
in plants
Vital
to photosynthesis -- every chlorophyll molecule contains a Mg
ion
Mg
deficiency causes grass tetany in ruminants
potatoes
and tobacco are particularly sensitive to Mg deficiency
Sand
drown of tobacco
Epsom
salts MgSO4 is a fertilizer source other than dolomitic
limestone
Sulfur
1.
Form utilized by the plant : absorbed by plants as SO42-
anion small amount through leaves as SO2
2.
Soil Sulfur
a.
found in soil solution as the SO42-.
Negative charge so there is little adsorption on clays. Moves
with the soil water and is easily leached.
b.
organic matter is the source of soil sulfur. N-S ratio 10-1
3.
Sources of sulfur added to the soil.
a.
atmosphere - (SO2) sulfur dioxidebrought down by
precipitation.
b.
Fertilizer S
1.
ordinary superphosphate 9-12% S
2.
gypsum - soluble - neutral in reaction 17% S copper sulfate
4.
elemental Sulfur - when added to soil it is oxidized to H2SO4
by microorganisms very acid. 300 lbs./acre would require 1000
lbs. of CaCO3 to neutralize.
5.
Sulfate movement and retention
a.
reacts with the same anion exchange sites as H2PO4-,
HPO42- is held more strongly
so there is little room for SO42-
on these sites in surface soils.
b.
leaches readily - accumulation in B horizon
6.
Sulfur
Nutrition - part of three amino acids Cystine, Cysteine,
Methionine
a.
Deficiencies occur on sandy soils low in organic matter in
areas with high rainfall.
b.
Deficiency
symptoms plants show a pale green color
c.
S is part of protein. Plant need a N:S ratio of 15:1
d.
application rates
15-20
lbs/acre S where leaching is not a problem; 40-60% lbs /
acre an deep sandy soils
e. Vidalia
Onions-low sulfur soils are needed to grow sweet onions.
Mattamuskeet
sweets