Agriculture Reference
In-Depth Information
The current N fertilizer recommendations for spring wheat are mainly based on the determina-
tion of the soil NO
3
-N content (0-60 cm) at sowing (Calvo et al., 2013). To use it, different N avail-
ability thresholds (soil + fertilizer) have been suggested, which vary according to the area, farming
systems, and crop yield objective. A recent study conducted with different wheat genotypes has
shown that critical levels of N availability for applications at sowing and at tillering were 174 and
152 kg ha
−1
, respectively (Barbieri et al., 2012). These kinds of simplified models do not explicitly
take into account the N supply through mineralization, however, which represents one of the main
N sources for crops (Campbell et al., 1993, 2008), especially in soils with high organic matter con-
tent (Calvo et al., 2013). Under medium- to high-yielding conditions, mineralization of N from soil
organic pools during the growing season meets 30% of wheat (Gonzalez Montaner et al., 1997) and
60% of corn (Steinbach et al., 2004) demands for N. It is worth mentioning that soil potential N
mineralization is affected by cropping history, soil management practices, and weather conditions
(Calvo et al., 2013).
Soil analyses are direct measurements of the soil N status. Several approaches have been adopted
to directly assess soil N (Stanford, 1982). Gerik et al. (1998) reported that in the western states of
the United States, where arid conditions prevail, soil
NO
3
−
has been successfully used to determine
the existing N levels and to adjust N application rates. Other locations have adopted a preseason soil
NO
3
−
test for adjusting fertilizer N rates based on the existing
NO
3
−
levels. As with fertilizer tests,
soil analysis should be confined to the general location and soil type where testing was performed
(Gerik et al., 1998).
3.4 PLANT TISSUE TEST
Plant tissue analyses were developed to overcome variations inherent in fertilizer tests and soil
analyses. They play an increasingly important role in modern crop production. A plant tissue test or
plant analysis is one of the most important diagnostic techniques in determining nutrient deficiency,
sufficiency, or toxic levels in plants. The concept of using a plant tissue test or plant analysis as a
guide to identify the nutritional status of the plant is relatively old. However, there is renewed inter-
est in recent years in using this technique due to the increase in the number of spectrographs or
the analytical technique. In addition, large quantities of data have been accumulated in the last few
decades to facilitate the interpretation of tissue analysis results or reference standards are available.
The third reason is that farmers and those who assist them in crop growing techniques are becom-
ing increasingly sophisticated in the technology they demand and are capable of testing. The fourth
reason is that fertilization has contributed significantly in increasing crop yields in the last few
decades of the twentieth century of important food crops, which permitted the economical use of
tissue tests as a diagnostic technique. Smith et al. (1990) reported that to date tissue tests have found
application mainly as a diagnostic tool to indicate the N deficiency/sufficiency for high-value crops,
including cotton and corn.
A plant tissue test or plant analysis is defined as the determination of the nutrient concentra-
tion in plants or plant parts with analytical procedures (Soil Science Society of America, 2008).
According to Ulrich and Hills (1973), plant analysis, in its simplest terms, is a study of the relation-
ship of the nutrient content of the plant to its growth. Reference concentrations of mineral nutrients
in specific plant parts are determined and used as a guide to indicate how well plants are supplied
with essential plant nutrients at a certain time of sampling. Such reference concentrations provide
a tool to assist the agronomist in evaluating nutrient disorders and in improving fertilization in the
present or succeeding crops (Ulrich and Hills, 1973). Ulrich and Hills (1973) further reported that
a basic concept is that the concentration of a nutrient within the plant at any particular moment is
an integrated value of all the factors that have influenced the nutrient concentration up to the time
of sampling. Rauschkolb et al. (1984) stated that an attractive feature of a tissue test is that the plant
root system tends to integrate the spatial variability of soil N supplying power over a relatively large
field volume.
