This method involves a comparison of the obtainednutrient value to the critical value on a nutrient to nutrient basis. It allowsinterpretation of plant nutrient status using the following ratings:deficiency, sufficiency, luxury consumption and excess (Bates, 1971).
Criticalvalues (CVA) and sufficiency ranges (SRA) are the most widely use univariateapproach for the diagnosis of nutritional status of a given crop (Camacho et al., 2012; Serra et al., 2012; Beaufils et al.,1973; Walmorth and Sumner, 1987). Deviation form Optimum Percentage (DOP) wasalso proposed by (Montañésa et al.
, 1993) as a univariate approach fordiagnosing nutritional status of a given crop.2.5.1.1 Critical ValuesCritical values have beendefined as the concentration at which there is a 5–10% yield reduction. The useof critical values for practical interpretation has limited value. It is bestsuited to diagnose severe deficiencies and has little application inidentifying hidden hunger.
Symptoms are generally evident when nutrientconcentrations decrease below the critical value. Critical values play animportant role in establishing lower limits of sufficiency ranges. The nutrientis said to be deficient if it is lower than the critical value and sufficientwhen greater than the critical value. If the analyzed value is equal to the critical value itis said to be optimum (Rao et al., 1990). Thus, a nutrient concentrationfar below or above reference values is associated with decreasing crop growth,yield, and quality (Mourão-Filho, 2005).
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2.5.1.2 Sufficiency RangesSufficiency rangeconsists of optimum ranges of nutrient concentration to establish thenutritional status of a given crop (Serra etal., 2013). Sufficiency range interpretation offers significant advantagesover the use of critical values.
First, hidden hunger in the transitional zonecan be identified since the beginning of the sufficiency range is clearly abovethe critical value. Sufficiency ranges also have upper limits, which providesome indication of the concentration at which the element may be in excess. 2.5.
1.3 Deviation form Optimum Percentage (DOP)DOP (Montañésa et al., 1993) index is definedas the percentage deviation of the concentration of an element with respect tothe optimum content taken as the reference value. The DOP index is calculatedas: 1Where C is the concentration of a givennutrient; Cref is the optimal nutrient concentration. A DOP index can bepositive, zero, or negative, the negative DOP index values mean that areundersupplied and positive DOP index values mean that are oversupplied. The sumof the absolute values of the DOP indexes (?DOP) gives an indication of thesufficiency, deficiency or excesses of the nutrient in question. If the sampleis near to an adequate nutritional status, the ?DOP will be near zero (Montañeset al., 1993).
DOP is not widely used as the useful references are thedeficiency (Xu et al., 2015).2.5.1.4Limitation of univariate approachGenerally, univariatemethods try to evaluate isolated deficiency or excess values without measuringthe overall nutritional imbalance. Similarly, although the critical value andsufficiency range approaches have been used to make accurate diagnoses, some ofthe disadvantages are that the values vary with the concentration of othernutrients, plant age, and varieties.
And as such, there is often difficulty inestablishing consistent critical values and relate them to high yields.Similarly, this approach is erroneous in that critical nutrient concentrationare not independent but can vary in magnitude as the background concentrationof other nutrients increases or decreases in crop tissue (Walworth and Sumner.,1986, 1991 and 1993). The method does not diagnose which nutrient is “mostlimiting” when two or more nutrients are simultaneously deficient (Bailey et al., 1997). This approach assessesonly the sufficiency status of a single nutrient (e.
g N) on the basis of itsabundance relative to one other nutrient (e.g P) and makes no allowance forpotential imbalances with other nutrients (Bangroo et al., 2010).
These comparisons based on the standard values are abasic methodology which considers each nutrient independently and this waydifficult the application of the concept of nutritional balance since it canonly identify a nutrient at each time (Jones, 1981; Beverly, 1991). Thus, theuse of such methods does not allow the rank of nutrient limitations (Maeda etal., 2004; Meyer, 1981). On the other hand, they consider differentnutritional mechanisms of plants not the interactions issue between nutrients(Schaller et al., 2002). Furthermore, thestandardization of the sampling period required by the above-referred methodsis based on the principle that higher nutrient requirements are met at theflowering stage which, in case of annual crops, prevents its results from beingused on the benefit of the crop from which the sample was taken (Srivastava andSingh, 2008; Nachtigall, 2004; Harger et al.
, 2003; Meyer, 1981). Theevolution of tissue maturation, and therefore the instability of nutritionalconcentrations, is another difficulty in the interpretation and theestablishment of these standards as well as the quantification of its impact onproduction (Sumner, 1979; Nachtigall, 2004; Srivastava and Singh, 2008). Finally as these methodsdo not consider environmental factors or other nutritional conditions, it isassumed that the reference values listed are not unique or universallyapplicable, which justifies the existence of a large number of critical valuesreferred in the literature for the same crop (Srivastava and Singh, 2008). Theseapproaches have been used for several crops including potatoes (O’Sullivan et al., 1997), maize and sorghum (Jones et al., 1990; Westfall et al.
, 1990).
