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Relative Availability of Three Mg Sources
to Corn and Soybean
F. M. Rhoads
University of Florida
North Florida Research and Education Center
Route 3 Box 4370
Quincy, Florida 32351
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Relative Availability of Three Mg Sources to Corn and Soybean!/
F. M. Rhoads"/
ABSTRACT
Dolomite and magnesium sulfate are widely used sources of
fertilizer Mg consequently there is little information available
on plant uptake of Mg from magnesia (MgO). The main objective of
this experiment was to determine relative Mg uptake from
Dolomite, MgSO4, and MgO by corn and soybean. Corn and soybean
were grown in greenhouse pots and supplied with 0, 50, and 100 mg
kg-I of Mg from the above sources. Dry-matter yield, Mg concen-
tration, and Mg content of shoots were determined after five and
seven weeks of growth of corn and soybeans respectively. Dry-
matter yield of soybean was not influenced by any of the Mg
sources. Corn dry-matter yield was increased by MgO only. All
Mg sources increased tissue concentration of Mg in both corn and
soybean. The 50 mg kg-1 Mg rate was as effective as the 100 mg
kg-1 rate in all cases for total Mg uptake. Magnesia was as
effective in supplying Mg to corn and soybean as dolomite and
MgSO4.
Additional Index Words: Dolomite, magnesium sulfate, magnesia,
MgO, MgSO4, Mg uptake, Dry-matter yield.
Cost and ability to obtain a particular material are
important factors when determining the source of fertilizer
!/North Florida Research and Education Center Research Report No.
87-8.
2/Professor of Soil Science, North ?lorida Resrch nd Zd:: in
Center, Quincy, University of Florida.
magnesium (Mg) to use in crop production. However, availability
of Mg to the crop after it is applied to the soil may alter the
cost:return ratio for a particular crop.
Dolomite is generally used as a source of liming material
which also supplies calcium (Ca) and Mg (11%). When lime is not
needed, dolomite may be considered as a source of Mg based on its
Mg content. Dolomite will have very little influence on soil pH
when applied in amounts required for Mg needs of most crops.
Magnesium sulfate can be purchased in several forms, among
them are epsom salts (10% Mg) and dehydrated MgSO4 (20% Mg)
(Berg. 1986). Both compounds are highly soluble in water. Cost
per lb of Mg is usually higher for MgSO4 than for other forms of
Mg fertilizers.
Magnesia (MgO) is about 56% Mg and not as soluble as MgSO4,
but it is usually less expensive per unit of Mg. Some specialty
fertilizers contain MgO but it is seldom used as a Mg source in
general crop production.
Objectives of this experiment were to determine relative Mg
uptake from Dolomite, MgSO4, and MgO by corn and soybean and
determine the effect of these Mg sources on dry matter
production.
METHODS
Corn and soybean were seeded in 0.2 m diameter greenhouse
pots containing 3.2 kg of soil on 1 May 1986. Dolomite, MgSO4,
and MgO were added in amounts to supply approximately 50 and 100
mg of Mg kg-1 of soil. Seven treatments consisting of two levels
of Mg from each source and a control with no Mg added were
replicated five times for both corn and soybean. In addition to
the Mg source, each pot of soil was mixed with 0.5 g of triple
super phosphate (46% P20 ), 1.0 g of K2SO and 3 g of calcitic
lime. Each pot of soil containing corn was supplied with 1.0 g
of ammonium nitrate after the plants emerged.
Corn shoots were harvested 5 June 1986 and soybean shoots
were harvested 17 June 1986. Plant tissue was dried to constant
weight at 700C and weighed to determine dry matter production.
The dry tissue was ground and ashed for Mg determination on an
atomic absorption spectrophotometer.
Soil samples were taken from each pot after harvest,
extracted with Mehlich I extractant (double acid), and analyzed
for Mg on an atomic absorption spectrophotometer.
The experimental design was a randomized complete block.
Analysis of variance procedures and single degree of freedom
comparisons were used to identify treatment differences.
RESULTS AND DISCUSSION
Magnesium source or rate did not influence dry-matter yield
of soybean (Table 1), but MgO increased corn dry-matter by
approximately 29%. Dolomite and MgSO4 did not increase
dry-matter yield of corn in comparison to the zero Mg treatment
(Table 1).
All Mg sources increased tissue concentration of Mg above the
zero Mg treatment in both corn and soybean (Table 2). There were
no significant differences between Mg sources regarding their
influence on Mg concentration in soybean tissue (Table 2).
Dolomite was not as effective as MgO and MgSO, for increasing Mg
annncradcn in -as e ec .-e as he i mg kg f aSO
and MgO was nct as effective as the 100 mg ka-i race for
Table 1. Effect of magnesium (Mg) source on dry-matter (DM)
yield of corn and soybean in greenhouse pots.
Mg
Source rate Corn Soybean
mg kg1 ----------g pot-1
Check 0 11.8 a' 9.2 a
MgSO4 50 11.3 a 8.7 a
MgSO4 100 12.1 a 7.7 a
Dolomite 50 12.2 a 9.2 a
Dolomite 100 11.3 a 9.9 a
MgO 50 15.1 b 10.7 a
MgO 100 15.2 b 8.6 a
Means within columns followed by the
icantly different (P>0.05).
same letter are not signif-
Table 2. Effect of magnesium (Mg) source on Mg concentration of
corn and soybean tissue grown in greenhouse pots.
Mg
Source rate Corn Soybean
mg kg ---------g kg1----------
Check 0 2.3 a' 2.5 a
MgSO4 50 2.9 c 3.2 bI
MgSO4 100 3.4 3.5
Dolomite 50 2.5 bi 3.2 bI
Dolomite 100 2.8 3.2
MgO 50 2.8 c 3.2 bl
MgO 100 3.1 3.5
Means within columns followed by the same letter are not signif-
icantly different (P>0.05). Letters indicate differences be-
tween sources and vertical lines indicate no difference between
rates within sources.
increasing tissue Mg concentration in corn. However, 50 mg kg-"
of Mg from all sources was as effective for increasing Mg con-
centration in soybean as 100 mg kg-.
Total Mg uptake from all three sources was higher in both
corn and soybean than from the zero Mg treatment (Table 3).
However, the 50 mg kg-1 Mg rate was as effective as the 100 mg
kg-1 rate in all cases for total Mg uptake. Uptake of Mg by
soybean was about equal among sources (Table 3). Dolomite was
least effective for increasing Mg uptake by corn and MgO was most
effective. The higher Mg uptake by corn from MgO was due mainly
to increased plant size (Tables 1,2, and 3).
Soil-test Mg was increased by each source in proportion to
the amount applied (Table 4). All sources were equally effective
in increasing soil-test Mg. Regression analyses showed a highly
significant linear relationship between soil-test Mg and rates of
fertilizer Mg (Fig. 1 and 2). The quadratic component was not
significant (P > 0.10) A smaller proportion of the variation in
soil-test Mg was accounted for by fertilizer Mg in corn soil
samples than in soybean soil samples (Fig. 1 and 2). An increase
of 0.265 mg kg"1 soil-test Mg per unit of fertilizer Mg was pre-
dicted in corn soil samples (Fig. 1), while an increase of 0.309
mg kg-1 soil-test Mg per unit of fertilizer Mg was predicted in
soybean soil samples (Fig. 2). However, the slope of the regres-
sion line (soil-test Mg vs fertilizer Mg) for corn was not sig-
nificantly different from that of soybean (Steel and Torrie,
1960).
Table 3. Effect of magnesium (Mg) source on Mg content of corn
and soybean tissue grown in greenhouse pots.
Mg
Source rate Corn Soybean
mg kg- --------mg pot--------
Check 0 26.6 a' 23.1 a
MgSO4 50 32.6 cj 28.0 b
MgSO4 100 41.5 26.7
Dolomite 50 30.6 bI 29.0 b
Dolomite 100 31.6 31.0
MgO 50 41.8 dl 34.1 b
MgO 100 48.5 30.6
Means within columns followed by the same letter are not signif-
icantly different (P>0.05). Letters indicate differences be-
tween sources and vertical lines indicate no difference between
rates within sources.
Table 4. Effect of magnesium (Mg) source on soil test Mg after
harvest of corn and soybean in greenhouse pots.
Mg
Source rate Corn Soybean
--------------------mg kg------------------
Check 0 32 a' 38 a
MgSO4 50 41 b 52 b
MgSO4 100 55 69
Dolomite 50 48 b 49 b
Dolomite 100 56 67
MgO 50 48 b 49 b
MgO 100 65 67
Means within columns followed by the same letter are not signif-
icantly different (P>0.05). Letters indicate differences be-
tveen sources and vertical lines indicate no difference between
rates within sources.
S60-
-
50-
E-40
-,-4
o 30-
Cn
? 60-
4J 48-
tn
a)
,-I
o 36-
cn
R2= 0.511**
Y = 32.2 + 0.265X
I I I I I
0 20 40 60 80 100
Fertilizer Mg (mg kg- )
Fig. 1. Soil-test Mg as a function of
fertilizer Mg in soil samples taken
from pots where corn was grown.
R2= 0.782**
Y = 36.1 + 0.309X
Si 1 I I I
0 20 40 60 80 100
Fertilizer Mg (mg kg-1)
Fig. 2. Soil-test Mg as a function of
fertilizer Mg in soil samples taken
from pots where soybean was grown.
4 *
CONCLUSIONS
Dolomite is the best source of Mg for corn and soybean if the
soil pH is low (Usherwood and Miller, 1967). Magnesia (MgO) was
as effective in supplying Mg to corn and soybean as dolomite and
MgSO Therefore, MgO would be the best source of Mg for corn
and soybean when cost is favorable and lime is not needed. The
growth response of corn to MgO should be investigated further.
REFERENCES
1 Berg, G. L. 1986. Farm Chemicals Handbook. Meister
Publishing Co., Willoughby, OH.
2 Steel, R. G. D. and J. H. Torrie. 1960. Principles and
Procedures of Statistics. McGraw-Hill, New York. p.
173-174.
3 Usherwood, N. R. and J. R. Miller. 1967. Effect of soil pH
on the availability of magnesium to corn (Zea mays L.) from
magnesium sulfate and high magnesium liming materials. Soil
Sci. Soc. Amer. Proc. 31:390-393.
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