Agriculture Reference
In-Depth Information
6.3.7 n
ItroGen
versus
C
hlorIne
In 1954, the essentiality of chlorine for plant growth was confirmed (Broyer et al., 1954). For more
than 20 years, it was generally assumed that field-grown crops would not benefit from the applica-
tion of Cl-containing fertilizers because it is ubiquitous in the environment (Fixen, 1993). Pacific
Northwest studies in the United States (Taylor et al., 1981; Christensen et al., 1981) in the early
1980s provided the first evidence that field-grown wheat could benefit from chlorine fertilization.
Engel et al. (1998) reported that wheat grain yield increased at an average of 417 kg ha
−1
(9.7%) in 86
cases where significant responses to Cl
−
were measured. The largest grain yield responses (>800 kg
ha
−1
) occurred at sites with the lowest plant Cl
−
concentration (<0.50 g kg
−1
). Kernel size was the
most important yield component affected by applied Cl
−
. Approximately 73% of the yield response
to applied Cl
−
could be accounted for by the larger kernel size. Biological functions of Cl
−
in plants
are presumed to require a concentration of no more than 0.10 g kg
−1
(Fixen, 1993). The beneficial
effects of chlorine are more likely due to its osmoregulatory role in the plant (Flowers, 1988). The
importance of this function on plant growth and grain yield should be highly dependent on the
growing environment (e.g., water and temperature).
Among micronutrients, chlorine is absorbed in maximum quantity by crop plants. Chlorine takes
part in the capture and storage of light energy through its involvement in photophosphorylation
reactions in photosynthesis. It is not present in the plant as a true metabolite but as a mobile anion.
It is involved with K in the regulation of osmotic pressure, acting as an anion in counterbalance
to cations. General plant symptoms are chlorosis in younger leaves and overall wilting. Chlorine
is ubiquitous in the environment and appears to be involved in several plant processes, including
photosynthesis, sugar translocation, and maintaining or increasing water potential (Voss, 1993).
Nitrogen and chlorine interact via several mechanisms including both soil and plant processes.
The rates of some steps in the mineralization of soil organic matter are affected by Cl
−
(Fixen,
1993). This can influence the form of N absorbed by crop plants. At the root surface, the NO
−
and
Cl
−
ions are known to compete with each other in the uptake process (Fixen, 1993). Increasing the
supply of either one tends to reduce the tissue concentration of the other. Nitrate and Cl
−1
compete
with each other for uptake in many species (Glass and Siddiqi, 1985; Christensen and Brett, 1985;
Goos et al., 1987). However, under some conditions, positive interactions between NO
−
and Cl
−
have
been reported (Murarka et al., 1973). At soil Cl
−
levels greater than 19 mg kg
−1
soil, spring wheat
Cl
−
concentrations in South Dakota increased with increasing soil NO
−
levels (Fixen et al., 1987).
Below 19 mg Cl
−
kg
−1
soil, they decreased with increasing soil NO
−
.
6.3.8 n
ItroGen
versus
n
ICkel
The essentiality of nickel (Ni) for higher plants is the most recent addition to the list of micronutri-
ents. Nickel was suspected of being an essential plant nutrient in the early twentieth century, when
it was discovered to be a constituent of plant ash (Wood and Reilly, 2007). Responses of crops in
the field of foliar Ni sprays were noted to increase yields of wheat, potatoes, and broad beans as
early as 1946 (Roach and Barclay, 1946), and responses in other crops were noted in subsequent
years (Dixon et al., 1975; Welch, 1981). Its essentiality for higher plants was established in the 1980s
(Welch, 1981; Eskew et al., 1983) using soybean as a test plant. Soybean plants were grown in a
highly purified nutrient solution (without Ni) and urea accumulated in the toxic level in the tips of
leaflets, which become necrotic. When Ni was supplied to soybean plants in the concentration of
1 µg L
−1
, no excess urea was accumulated in the leaf tips and necrosis was also absent (Epstein and
Bloom, 2005).
Subsequent research by Brown et al. (1987a,b) established the essentiality of Ni for other crop
species such as barley. These authors reported that Ni is essential for plants supplied with urea.
The Ni-deficient plants accumulate toxic levels of urea in leaf tips, because of reduced urease
