Agriculture Reference
In-Depth Information
at different N levels. In Figure 6.14, plant growth increased with increasing N levels when the N
source was ammonium sulfate. However, in Figure 6.15, plant growth was reduced when the N rate
was 400 mg kg
−1
supplied with urea. In this case, nickel deficiency may be suspected for reduction
in rice growth at a higher urea level because, in addition to reduced growth, tips of rice leaves were
showing necrotic or toxic symptoms of urea as described by Fageria (2009).
6.4 NITROGEN VERSUS SOIL SALINITY AND ALKALINITY
Salt-affected soils limit crop yields around the world. It is especially prevalent in irrigated agricul-
ture and in marginal lands associated with poor drainage or high water tables. Estimates for the
extent of salinity damage vary from 25% to 50% of the world's irrigated land (Adams and Hughes,
1990). The knowledge of how nutrient availability is affected in plants growing on salt-affected soils
is important to adopt appropriate management practices to satisfy plants' nutritional requirements
and improve yields to meet food demands of increasing world populations. In the salt-affected
environment, plants are required to absorb essential nutrients from a dilute source in the presence
of highly concentrated nonessential nutrients (Fageria et al., 2011b). Further, among nutrients, N
is one of the most widely limiting elements for crop production, when plants are subjected to salt
stress. The uptake of N by rice was inhibited under high NaCl and Na
2
SO
4
concentration of the root
medium, and the excess amount of absorbed Na depressed NH
+
absorption (Palfi, 1965; Mahajan
and Sonar, 1980). Reduction in root permeability and the consequent decrease in water and nutrient
uptake under high salt concentrations (Frota and Tucker, 1978) have been associated with impaired
N absorption by plant under salt stress conditions.
Grattan and Grieve (1999) reviewed the literature on the interaction between salinity and N
accumulation in crop plants and concluded that salinity can reduce N accumulation. These authors
further reported that this is not surprising since an increase in Cl
−
uptake and accumulation is often
accompanied by a decrease in shoot NO
−
of many crops (Khan and Srivastava, 1998; Kaya and
Higgs, 2003). This may be associated with the competition between NO
−
and Cl
−
ions. Kafkafi et al.
(1992) reported that the nitrate influx rate or interaction between NO
−
and Cl
−
might be related to
the salt tolerance of the cultivar under investigation. These authors found that salt tolerance cultivars
had higher NO
−
influx rates than the more sensitive cultivars. Forms of N also influence the interac-
tion of salinity with N. The NH
+
feed plants were more sensitive to salinity than NO
−
feed plants in
nutrient solutions (Grattan and Grieve, 1999).
Soil salinity may disrupt symbiotic N
2
fixation systems in several ways. Salts can limit nodule
formation by reducing the population of
Rhizobium
in the soil or by impairing their ability to infect
root hairs. The direct effects of salinity on the host plant can limit N fixation, independently of the
effects of salinity on the
Rhizobium
bacteria and the nodulation process (Keck et al., 1984; Fageria,
1992). Stunted growth of the host plant may reduce the supply of photosynthate to the root nodules.
Since photosynthate supply is a major limiting factor in N
2
fixation, this indirect effect can be quite
important (Fageria, 1992).
Sodic solonetzic soils are also deficient in calcium for optimum plant growth and can be reclaimed
by applying gypsum to replace Na
+
with Ca
2+
on the cation exchange capacity (Aulakh and Malhi,
2005). In a black solonetzic soil in Alberta, Canada, an application of gypsum increased the con-
centration of extractable Ca and reduced the sodium adsorption ratio (Malhi et al., 1992). In this
experiment, an N × Ca interaction not only improved the yield but also enhanced the concentration
of Ca, K, and Zn in the flag leaf of barley while decreasing the Na concentration.
6.5 CONCLUSIONS
The knowledge of interactions among essential plant nutrients is important in formulating a balanced
supply of fertilizers to crop plants. Nutrient interactions may affect the uptake and utilization of essen-
tial plant nutrients depending on the type of interaction. The interaction may be positive (synergistic),