Essential Nutrients for Plant Growth
Obtaining representative plant sample (s) is a key step toward successful plant analysis. Step by step, explain how you will obtain representative plant sample from farmer’s field and process the sample for laboratory analysis.
Plant Sampling is a process by which a predetermined number plant
parts are taken from a larger population of plants for laboratory nutrient
analysis (Paul, Ramesh, and Pandey,
2017).
The main objective of drawing a sample is to make inferences about the larger
population from the smaller sample. It is critical for plant
analysis and statistical analysis of the results
as plant nutrient composition varies with age, the portion of the plant
sampled, and many other factors. Therefore, proper selection of the plant part
to sample, proper stage of plant growth and time of sampling, number of plants
and plant parts selected are very crucial for obtaining the interpretable
nutrient analysis results. Based on these considerations, standards, against
which the sample is evaluated, have been selected to represent the plant part
and time of sampling that best define the relationship between nutrient
composition and plant growth. The following are the steps that are usually
followed when collecting plant sample
1) The
farm to be sampled is divided in to plots with homogeneous characteristics in
terms of plant age, variety, spacing, soil and manipulations, the area should not
be bigger than 10 ha.
2) From
each plot, the indicated plant part is collected or the whole plant is uprooted
using the random or zigzag direction. Wash the roots and the basal part of the
shoot with distilled water
3) To
be accurate, do not take samples from dead, diseased, insect damaged or
mechanically injured plants. Also avoid plants from unusual areas in the field,
including border areas and places where plants are under water stress or where
nutrient availability is atypical. Do not collect samples during the hottest
part of the day, particularly in summer. Sampling must never be conducted after
fertilization or spraying.
4) From
each plot, at least 20 leaves or whole plants are collected and mixed to make a
composite sample before sending to the laboratory.
5) Place
the samples in the clean unused paper bags (don’t use plastic bags and metal
containers to avoid decay of samples and contamination, respectively).
6) Label
the plant sample bag or use tags. The label should include date and location of
sampling, name of farmer and purpose of sampling on the bag. Samples should be
immediately sent to the laboratory. Fresh plant tissue is perishable and
therefore sample material must be kept cool and in a drying atmosphere till it
reaches to the laboratory (If the collected plant tissue begins to decay then
this will lead to a significant reduction in dry weight and some elements like
N and S will also be lost via volatilization.).
Sample
processing
Maintain the integrity of the collected
samples, care should be taken to ensure that the sample is not altered chemically
or contaminated by extraneous materials. The following are processes done on
plant samples after collection
Sample
decontamination
When only macro nutrients are to be
determined in samples washing may be plain, to eliminate gross contaminations like
dust. Just shaking the sample under tap water and rinsing with distilled water
will be enough but the procedure must be fast to avoid the loss of soluble
elements. To avoid loss of soluble inorganic constituents the washing stages
must not take more than 30 seconds (Prado & Caione, 2012). Contaminations
by pesticides and foliar fertilizers (especially when applied with surfactants
in the spraying mixture) are difficult to remove by washing. Collection of
samples in these cases must be carefully overseen (Prado & Caione, 2012).
Drying
Dry the samples in a draft-oven at 70˚ C
- 80oC (± 5˚ C) until a constant dry weight is obtained (about 72 hours) (Very
high temperatures may cause thermal decomposition of the sample while lower
temperatures are inefficient). This is so because the elemental concentration
of any sample is presented on dry weight basis of the sample. Therefore, any
condition that affects the dry weight of collected samples will affect its
elemental composition as well. Record the oven-dry weights when drying is
completed. Do not expose the samples to the atmosphere for long time before
weighing to avoid moisture absorption. If the samples are broken, it is
advisable to weigh each sample in the bag in which it was dried. Then remove
the sample and obtain the weight of the bag. For precise analysis, kill fresh
tissue by placing it in boiling alcohol for 3 minutes. Remove the roots with
scissors when sample is dried. The dried tissue is then stored in a moisture
free atmosphere prior to further processing.
Grinding
(particle size reduction)
Cut the samples into small pieces and
then grind them in a mill that is free of grease and thoroughly cleaned between
each sample grinding. The finer the ground powder, more homogeneous the sample
will be. Care should be taken when selecting the mill as in most mills,
particles of the contact surfaces will be added to the sample, Cu and Zn are
added from brass fittings and even Fe can be added when fittings, cuttings and
crushing surfaces are made up of steel or other containing material.
Store the ground samples in glass
bottles with tight stoppers in a cool, dark place. Be sure that the samples are
properly labeled before storing: dates of sampling are essential.
Before
weighing samples for chemical analyses, redry the container of ground tissue at
70˚ C (± 5 ˚ C) for 24 hours.
2. The
use of critical concentration (C.C)
is a commonly used method of interpreting plant analysis results. Step by
steps, explain how critical values of nutrient concentrations of a given crop
can be developed for interpretation of plant analysis results where such values
are not available (Not established).
Critical
level or concentration is a term used to describe nutrient concentration in
soil and plant analysis that is associated with 90% of maximum yield and
growth, which is also a reasonable division of the zones of adequacy and
deficiency in the figure below (Kalala, Amuri, & Semoka, 2016). The critical
level for toxicity may be similarly defined in the division of the plateau and
descent (toxicity) in the same figure.
The
Critical levels of plant nutrients in soils and plants are established by using
graphical method of Cate and Nelson (Kalala et al., 2016). This method
consists of constructing graphs of the relative yield (RY) on the Y axis and
nutrient concentration on the X axis where the positive and negative quadrants
of fertilizer response and non-response, respectively are demarcated.
The
graph is created in form of considerable scatter when soil test values are
plotted against actual yields. To eliminate some of the scatter, most soil test
correlation work uses relative or percentage yields.
Relative yield is
defined as being 100 times the yield of a treatment which provides adequate but
not excessive amounts of all nutrients other than the one being correlated,
divided by the yield of a treatment which is the same except that it includes
the nutrient under study (Gate & Nelson, 1971).
This
technique of establishing the critical concentration of nutrient consists of
the following steps:
I.
The plant samples and soil samples are
analyzed in the laboratory to obtain their nutrient concentrations
II.
The analysis data are ordered in an
array based upon rankings of the X values i.e., soil or plant test values. The
(X, Y) pairs are maintained in this order throughout the analyses.
III.
The yield data of the crop grown in the
same soil under different concentrations of nutrients as supplied by the
different fertilizer rates is collected and recorded.
IV.
The percentage relative yield is
calculated on basis of the yield data
The
percentage relative yield (s) = [GY or DMY of Nutrient control treatment(s)
x 100
A treatment giving maximum yield
Where;
GY = Grain yield
DMY = dry matter yield
V.
The graph is constructed using the
relative yield (RY) on the Y axis and nutrient concentration on the X axis and
the positive and negative quadrants of fertilizer response and non-response
respectively are demarcated.
VI.
Dividing line between two categories
(high probability of response and low probability of response) is determined
approximately by a graphical technique in which vertical and horizontal lines
are superimposed on a scatter diagram so as to maximize the number of points in
the positive quadrants. The point where the vertical line intersects the X axis
is used to divide the data into two classes. This dividing line has been termed
the "critical level."
Example;
Hypothetical data for phosphorous concentration in plant shoot and the relative
yield in a graph
|
Percentage relative yield Kg |
P concentration in shoot PPM |
|
26 |
0 |
|
28 |
20 |
|
38 |
40 |
|
46 |
60 |
|
50 |
80 |
|
68 |
100 |
VII.
Starting with the X value that will
place two or more points to the left of a vertical dividing line, one then
calculates the corrected sums of squares of the deviations from the means of
the two "populations" that result from moving to each successive X value.
The sum of the two corrected sums of squares at each X level is then
determined, and this pooled sum of squares is subtracted from the total
corrected sum of squares of deviations from the overall mean of all Y
observations.
VIII.
By this simple iterative process, one
obtains a series of R2 values for divisions made at various levels of X. One
picks the critical level of X as that where R2 is maximum. In other words,
using this procedure one finds the value of X which best divides the data into
two populations or classes, from the point of view of prediction. The method is
general in the sense that an extension of the two-mean separation procedure may
be used to divide the data into more than two populations.
3. Give
an account of all factors affecting plant analysis results and how to minimize
the effects associated with each factor while interpreting plant analysis
results
The
following are the factors that determine the quality of the sample
I.
Sampling
technique; obtaining the sample which is not representative
of the whole population will lead to wrong interpretation of the results and
inference of the whole population. Some sampling techniques are completely
biased, especially when a plant sampler does not use the sampling regulations. Method
of mixing and reduction (grinding, homogenization) as well as the sample size
will also affect the representativeness of the plant sample after analysis. To
minimize the effect of this factor, appropriate sampling technique and sample
size should be selected.
II.
Contamination;
sample
handling and types of storage determine the extent of contamination of the
sample. Some materials add extraneous nutrients into the sample leading to the
misleading results of the analysis. For example, metal based storage will
probably add Fe into the sample and the analysis results will have more Fe than
the sample have. To avoid this, appropriate sampling materials and storage
should be selected so as to reduce the extent of contamination of the sample.
When fungicides or foliar fertilisers containing zinc and/ or manganese are
used, the levels of these two elements will be high even after the leaves are
washed. When copper fungicides are used during the season, they will raise the
levels of copper in leaves.
III.
Weight
variation sample; most often, the analytical results are
expressed as concentration per unit weight of the sample analyzed. However, the
weight of the sample may vary greatly depending on the efficiency of drying and
the amount of moisture content left in the sample. This will lead to wrong
inference about the whole population although the analytical procedure were
correct
IV.
Analytical
variation; the method of analysis used will also
determine the accuracy of the analytical results. Some technique will show high
nutrient concentration while there is actually low nutrient concentration in
the sample. Proper calibration of the instruments is essential for avoiding
these errors
V.
The
part of plant sampled and the time of sampling;
the concentration of nutrients in the plant is
not fixed, but constantly changes. It may vary from month to month and even
from day to day. The concentration of nutrients even differs between various
parts of the same plant. In order to learn the rate in which a nutrient is
absorbed, it is necessary to take samples from several plant parts at different
growth stages. Samples should be taken from plants which are at the same
physiological stage and from the same parts of the plants. It is recommended to
avoid sampling plant tissues which are physiologically very young, since their
nutrient content undergoes rapid changes. Very old plant tissues are not
representative as well. Younger tissues will contain more N,P and K, while in
older tissues it is expected to find higher concentrations of Calcium,
magnesium, manganese and boron. Plant analysis results of adjacent plants may
vary considerably, even if the plants were fertilized at the same fertilizer
rates. Under conditions of nutrient deficiencies, the variance between the
plants is considerably greater. Interpretation of the results The nutrient
content of the plant is expressed on a dry weight basis. Therefore, any
condition that affects the dry weight of the collected sample will affect its
nutrient composition.