Agriculture Reference
In-Depth Information
40
30
20
Y = 175.1192 - 43.1805X + 2.9082X
2
R
2
= 0.8983**
10
0
4
6
Soil pH in H
2
O
7
5
FIGURE 6.13
Influence of soil pH on the uptake of N in the shoot of dry bean plants. **—Significant at the
1% probability level. (Adapted from Fageria, N. K. and V. C. Baligar. 1999.
J. Plant Nutr
. 22:23 -32.)
anaerobic N fixers (
Azotobactor
and
Clostridium
) and also to the symbiotic
Rhizobium
. The most
important soil factor affecting the availability of Mo to the plant is the soil pH. As an acid former,
Mo appears to increase its solubility with the increase in soil pH. Soil solution molybdenum levels
increase 10-fold for each unit increase in soil pH. Where Mo has been added to the soil, the amount
adsorbed will decrease as the pH increases (Barber, 1995). Generally, liming of an acid soil is suf-
ficient to correct Mo deficiency. However, if parent materials are too low in Mo, this statement is not
valid. Soils on which deficiencies were found were all acidic, while excess Mo occurred on alkaline
soils (Okajima et al., 1975).
The plant requirement for Mo is lower than for any other mineral element. The critical defi-
ciency levels range from 0.1 to 1 mg kg
−1
plant dry weight. The molybdenum requirement is much
greater for legumes, because it is used in the nodule for nitrogen fixation, and levels needed for
plant growth are much lower than levels needed to supply the root nodule with sufficient molybde-
num for nitrogen fixation. An antagonistic relationship between Mo and Cu and sulfate was found,
whereas phosphate promotes the absorption of Mo. Molybdenum is an essential component of two
major enzymes, nitrogenase and nitrate reductase, both of which depend on an Mo valency change
for their function (Shuman, 1994). One could generalize by saying that this element is closely
associated with N metabolism in most crop plants (Blevins, 1994). The function of Mo in plant
metabolism is to reduce nitrate N to nitrite N; thus, this element functions in a way that is differ-
ent from other elements. If a plant absorbs N in the nitrate form, Mo is indispensable for normal
growth. On the other hand, if N has been supplied in the ammonium form, the presence of Mo will
not be necessary (Ishizuka, 1978). Molybdenum is immobile in the plant and hence Mo deficiency
first appears in the younger leaves. The molybdenum deficiency symptoms are very similar to the
symptoms of iron deficiency.
