Agriculture Reference
In-Depth Information
By modern chemical methods, it is possible to analyze for the nutrient element in a plant sample
with great accuracy. In plant analysis, concentration is the nutrient content per unit of dry plant
material. Plant analysis results (concentration) are expressed in percentage (%), g kg
−1
, or mg kg
−1
.
3.4.1 n
ItroGen
C
onCentratIon
In
p
lants
This approach is used to determine the
critical nutrient level
or optimum concentration of a nutri-
ent, including N in the plant tissue of a crop. Critical nutrient level is defined as the concentration
of a nutrient in a plant tissue at 90% or 95% relative yield. Ulrich and Hills (1973) reported that the
critical nutrient level in a plant tissue can be calculated by using a 10% reduction in yield. They also
defined critical nutrient concentration as that concentration at which the growth rate of the plant
begins to decline significantly. It involves adding increasing amounts of a deficient element to oth-
ers present in adequate amounts, and relating the growth or yield to the nutrient concentration. The
critical concentration is estimated best through the use of solution culture and, to a lesser extent, by
the soil culture or field experiment technique (Ulrich, 1950). The reason for this is that in solution
culture it is easy to create a large variation in nutrient concentration and get a good crop response
curve.
Ulrich and Hills (1973) further stressed that plant response to nutrient addition is largely inde-
pendent of their source, and consequently it makes very little difference to the plant whether its
roots are in a culture solution or in a soil. The symptoms and nutrient concentration of the affected
leaf blades are, for all practical purposes, identical in both growth media. It follows, therefore, that
the critical values have a nearly universal application once they are properly calibrated (Ulrich and
Hills, 1973). Varietal differences have not been observed in critical concentrations of sugar beet
(Ulrich and Hills, 1973). Similarly, the authors did not observe significant differences in critical
levels of N among upland and lowland rice cultivars having similar yield levels. However, Barker
and Bryson (2007) reported that N concentrations in plants vary within species and with varieties
within species.
In the author's opinion, yield level is more important in defining adequate or critical N concen-
tration rather than variety. Nutrient concentration at a higher yield level will be relatively lower
compared with that at a lower yield level due to dilution effects, especially in the vegetative growth
stage. During the grain filling stage, nutrient concentration is more or less constant in the crop spe-
cies. A hypothetical relationship between nutrient concentration in plant tissue and relative yield is
presented in Figure 3.26. According to Ulrich and Hills (1973), the most useful calibration curve
is one in which the transition zone is sharp, that is, there is a narrow range in nutrient concentra-
tion between plants that are deficient and those that are well supplied with the nutrient in ques-
tion. Barker and Bryson (2007) reported that herbaceous crops from fertilized fields commonly
have concentrations of N that exceed 3% (30 g kg
−1
) of the dry mass of mature leaves. Leaves of
grasses (Gramineae, Poaceae) (1.5-3.5% N) are typically lower in total N concentration than those
of legumes (Leguminosae, Fabaceae) (>3%). Leaves of trees and woody ornamentals may have
<1.5% N in mature leaves. The choice of tissue for plant analysis is important in N disorder diag-
nosis (Table 3.3).
However, Novoa and Loomis (1981) reported that the N concentration 1.4-1.6% (14-16 g kg
−1
) in
mature biomass reflects the final balance, which is achieved between the grain and straw fractions.
These authors reported that the demand for N is determined by the growth rate and the N compo-
sition of the new tissues. In the field, both the growth rate and tissue composition will vary with
the nitrogen and water supply, plant competition, and other environmental factors. The maximum
demand for N will be achieved under nonlimiting conditions for photosynthesis when the growth
rate approaches its genetic potential (Novoa and Loomis, 1981).
In the literature, several different terminologies have been used in classifying nutrient concen-
trations in plant tissue. On the basis of Figure 3.26, nutrient concentrations can be classified as
deficient, marginal, excess, and toxic. When nutrients are in the deficiency range, the plant growth
