Agriculture Reference
In-Depth Information
6.2.4 n
ItroGen
versus
m
aGnesIum
Magnesium is a macronutrient essential for all plant growth and development. The adequacy of its
level in the soil is important for producing maximum economic yields. Like calcium (Ca
2+
), magne-
sium (Mg
2+
) deficiency in crop production is more common in highly weathered acid soils (Fageria
and Souza, 1991). A deficiency of Mg
2+
may also occur in coarse-textured soils of humid regions
with low cation exchange capacities. The functions of Mg
2+
in plants are many and the most impor-
tant are the enzyme activator and also part of the chlorophyll molecule. Magnesium is a mineral
constituent of plant chlorophyll, so it is actively involved in photosynthesis. Magnesium also aids in
phosphate metabolism, plant respiration, and the activation of several enzyme systems involved in
energy metabolism (Fageria and Gheyi, 1999). Magnesium aids in the formation of sugars, oils, and
fats. It also activates the formation of polypeptide chains from amino acids (Tisdale et al., 1985).
Magnesium is also an essential element for microbial growth and was implicated in microbial ecol-
ogy in the early studies of soil microbiology, since magnesium carbonate applied to certain soils
increases the reproduction of soil bacteria (Jones and Huber, 2007).
Looking into the importance of Mg, the knowledge of factors affecting its uptake is important in
the management of this element for higher or sustainable crop yields. Nitrogen has a positive inter-
action in the uptake of magnesium (Fageria and Baligar, 2005). Data in Table 6.11 show that Mg
uptake in dry bean plant shoots was significantly and quadratically increased with increasing N lev-
els in the range of 0-200 kg ha
−1
. The variation in the uptake of Mg was 79% with the addition of N.
6.2.5 n
ItroGen
versus
s
ulfur
Sulfur is the fourth major plant nutrient along with N, P, and K. Deficiency of sulfur has been
reported in many soils and crops in many parts of the world (Scherer, 2001). Among different
regions, Asia represents the region with the highest S fertilizer requirement (Aulakh and Malhi,
2005). Sulfur plays many important roles in the growth and development of plants. Fageria and
Gheyi (1999) have summarized the important functions of sulfur in plants: (i) Sulfur is an important
component of two amino acids, cysteine, and methionine, which are essential for protein formation.
Since animals cannot reduce sulfate, plants play a vital role in supplying essential S-containing
amino acids to them (Fageria and Gheyi, 1999). (ii) It plays an important role in enzyme activation.
(iii) It promotes nodule formation in legumes. (iv) It is necessary in chlorophyll formation, although
it is not a constituent of chlorophyll. (v) The maturity of seeds and fruits is delayed in the absence of
adequate sulfur. (vi) Sulfur is required by plants in the formation of nitrogenase. (vii) It increases the
crude protein content of forages. (viii) It improves the quality of the cereal crop for milling and bak-
ing. (ix) It increases the oil content of oilseed crops. (x) It increases winter hardiness in plants. (xi) It
TABLE 6.11
Influence of N on the Uptake of Mg in the Shoot
of 60-Days-Old Dry Bean Plants
N Rate (kg ha
−1
)
Mg Uptake (kg ha
−1
)
0
1.0
40
2.1
80
3.2
120
3.3
160
5.1
200
6.5
R
2
0.79**
**Significant at the 1% probability level.
