The most important
aspect of soil, plant and water analysis is knowing how to take
representative sample. Sample integrity must be maintained. Analytical
results are only as good as the samples collected and the method of
preparation. Because a small amount of sample is used for any particular
test, it is essential that subsamples be carefully selected and
thoroughly mixed, and the quantity prepared should be at least 10 times
greater than the final sample analyzed. The Analytical Service Laboratory
has prepared the following sampling guide.
1.0 PLANT SAMPLE PREPARATION
for Chemical Analysis
surface contamination, leaves are cleansed with a damp linen cloth or by
gentle brushing with a stiff-bristled brush followed by brief rinsing
with distilled water.
greenhouse experiments) contaminated with soil may be washed under
running tap water. Washing must be done quickly to minimize loss of
soluble constituents, and should be followed by rinsing with distilled
water and drying with cloth or tissue paper.
Sand or soil
adhering to roots can be washed away under running tap water, then roots
must be rinsed with distilled water and dried with a cloth or tissue
activity can alter the composition of plant tissue material. To keep
metabolic activity to a minimum, keep the samples cold or frozen.
other plant material are cut into small pieces before drying.
by dust should be avoided, especially when Fe, Mn, Cu, and Zn are to be
put in an oven and dried overnight at 80 ± 2°c. Longer drying times are
required for materials high in silica.
possible loss of B from the samples, dry samples at 60°C prior to B
plants be sampled?
take all plants in each pot and make a composite sample; if that is not
possible, take two predetermined plants per pot.
plants per 3 x 5 m2 plots depends on variability. For rice, 4
tillers/hills are recommended as the minimum if entire plant is used. If
sample is a single lead, use at least 10 plants. Make a composite sample.
Quarter if too bulky.
using random pairs of numbers with the first number corresponding to the
width and the second to the length of the plots for spaced plants. Pairs
of such numbers, the first between 1 to 10 and the second between 1 to 20
are chosen from a table of random numbers. This set of random pairs can
be used for each plot in a block. A new set is necessary for each block.
Do not sample
plants attacked by insects or diseases or are injured mechanically or are
under physical stresses.
Do not include
dead leaves in the sample except in straw samples at harvest.
Do not sample
a plant adjacent to a missing hill.
deficiencies or toxicities: sample 2 or 3 plants showing symptoms and 2
or 3 healthy plants at same growth stage from adjacent area; avoid dead
or nearly dead plants.
surveys, divide the field into a suitable number of blocks of equal area
and take a sample from the center of each block or depending on the
1.3 What plant parts should be sampled?
deficiencies and toxicities: use whole plant if small, the most recently
matured leaf blade if large.
nutrient uptake: entire above ground plant at different growth stages.
uptake: entire above ground plant including panicle at maturity.
recommendations: most recently matured leaf.
hidden hunger or toxicity: tillering phase.
recommendations: before reproductive phase.
uptake: at maturity
studies: at successive developmental stages.
1.4 Plant sample collection and decontamination
culture plants: cut at the base with scissors, wipe the tops with dry
muslin cloth, tie a label and place in a labeled muslin cloth bag. If
iron and manganese are to be determined, wipe the plant with muslin cloth
dipped in 0.2% Teepol, rinse rapidly twice with RO water, and dry with
muslin cloth. If root analysis is needed, dry the roots with muslin
cloth, label, and put in labeled cheese cloth bag.
culture plants: cut 3 cm above soil level and treat as in above.
culture plants: cut 2 cm above the water mark on the plant and treat as
plants: cut 5 cm above ground level, brush off soil at the base of the
plant and treat as in above.
cut 5 cm above the water mark and treat as in above.
1.5 Plant washing and preparation
Fresh sample is
pre-washed at the greenhouse with tap H20, then rinse with RO 4x.
Excess water is
blotted off using a paper towel or clean cheesecloth
sample in cloth bags or perforated paper bags. Do not tie samples
Place in an 80
C oven and allow to dry for 12 to 32 hours. Drying time depends on the
type of samples and the amount of samples in the oven.
1.6 Plant Grinding
homogeneous powders, samples are finely ground, using an Cyclone Udy Mill
with stainless steel screen to pass through a 20-mesh sieve.
are first ground through a standard Beater Cross grinder and are then
reduced by quartering to a manageable size.
These are then
ground by the Cyclone Udy Mill or Intermediate Wiley Mill.
are used for the determination of N, P, K, Ca, Mg, Na, and other
the mill is thoroughly cleansed with a stiff-bristled brush or compressed
air in order to avoid cross-contamination
determination of Fe, Mn, Cu, and Zn, the samples are ground in the
Stainless Steel Beater Cross grinder or an agate or porcelain mortar to
avoid metallic contamination. The sample mesh can be important, but for
routine analyses samples ground to pass a 20-mesh sieve are satisfactory.
the whole sample must be mixed thoroughly.
are transferred to tightly capped glass jars or sealed polyethylene
bags, labeled clearly, and stored for further analysis.
oven dried overnight at 60°C for B and 80°C for other determinations
before being weighed for analysis. If a sample is dried at 60°C for B, it
can be used for other determinations after drying at 30°c. For analysis,
the material is subsampled by
2.0 SOIL SAMPLE
consideration is the degree of urgency associated with certain analyses.
For example, it may be desirable to measure pH, nitrate (NO3-N),
ammonium (NH4-N), etc., before handling the samples any
are made on air-dried samples. In some cases, however, NH4-N,
NO3-N, pH, electrical conductivity, and some other properties
are determined on moist samples (field condition) immediately after
arrival at the laboratory. Drying some soils, particularly organic
horizons, can cause irreversible changes in some properties (Bartlett and
James 1980; Davey and Conyers 1988; Leggett and Argyle 1983; Peverill et
al. 1935; Schalscha et al. 1965; Searle and Sparling 1983); If analyses
cannot be done immediately after collection, then moist samples are
stored at 2°C or frozen at -20°C, depending upon the length of time
before analysis can be done. Stored samples must be tightly closed. In
some instances it might be necessary to air-dry part of the sample and to
maintain the other part in the field-moist state. Problems associated
with obtaining a representative sample of moist soils can be reduced by
blending moist samples prior to subsampling.
Soil samples should
be air-dried soon after collection to prevent microbial changes. Soils
are air-dried at 20-25°C and with relative humidity of 20-60% (Oackson
1958); the term "air-dried" refers to soil conditioned to ambient
temperature and humidity. Large lumps of moist soil are broken by hand
and spread on paper in a room free of fumes, dust, etc. If large clods
are not broken, they will take an unduly long time to dry and will also
be harder to grind. When dry, the soil is rolled gently with a wooden
roller. Coarse concretions, stones and pieces of macro-organic matter
(roots, leaves, and other vegetative material) are picked out.
Grinding is essential
to homogenize the soil and reduce sub sampling error as well as to
increase the specific surface. After air-drying, the soil is ground to
pass a 2-mm sieve using a modified Rukuhia soil grinder (Day and Dixon
1965). The grinder consists of three cylinders into which the samples and
metal pestles are placed. The cylinders are rotated horizontally by
electrically driven rollers. As the cylinders rotate, the sample is
ground by the pestle and falls through the mesh of the cylinder walls
into a tray below. Remaining gravel (weathered and non-weathered rock
fragments) and organic residue (e.g., fibrous material from roots) are
removed. These materials are weighed and their percentage in the total
sample is determined. Approximately 500 g of homogenized subsample fine
earth (less than 2 mm soil) is obtained by the quartering method (Jackson
1958) or by using a riffle sampler, in which a soil sample is
automatically halved by a series of chutes. The process is repeated as
many times as necessary. It is stored in a cardboard or glass container.
determinations are carried out on the fine earth fraction (less than 2
mm). If less than 1 g of sample is required for a particular analysis,
then the 2-mm fraction might not be sufficiently representative. A
smaller-sized sample is obtained by grinding a 2-mm sub sample with
pestle and mortar or a Udy Mill. For organic carbon, for example, soil is
ground to a 35-mesh size.
The composition of
the grinding and sieving apparatus is important, particularly if trace
elements are to be determined. For heavy metals (such as Cu and Zn), the
soil is ground in an agate or porcelain mortar with a pestle (preferred
over a Rukuhia soil grinder), then passed through a nylon 2-mm sieve (or
smaller if required). Iron, copper, and brass sieves are avoided.
Treatment with a metallic grinder can also result in serious
contamination for some analyses (e.g., iron can interfere with organic
All grinding is
performed using clean, dry equipment. The grinder must be thoroughly
cleaned between samples to avoid carryover. When grinding with a mortar
and pestle, the complete subsample must be ground to pass the sieve and
none is discarded.
Soils may undergo
significant changes during storage (air-dried or frozen), particularly
with respect to extractable nutrient concentrations (Maynard et al. 1983;
Peverill et al. 1935; Searle and Sparling 1983). As a result, many
extractable analyses are carried out on moist samples and, therefore, the
soils are stored frozen until the analyses can be performed.
The long-term effect of frozen storage of moist samples has not been
sufficiently evaluated. This must be considered when interpreting data on
soils analyzed immediately after collection and those analyzed after
storage for any length of time. Segregation of particles by size can
involuntarily occur during grinding, sieving, and storage; therefore, the
ground sample must be mixed well before a sample is weighed for analysis.