Sampling Guide
Understanding Result

Sampling Guide

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.


1.1  Plant Sampling for Chemical Analysis

a)      To remove 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.

b)      Shoots (from 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.

c)      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 paper.

d)      Metabolic activity can alter the composition of plant tissue material. To keep metabolic activity to a minimum, keep the samples cold or frozen.

e)      Leaves and other plant material are cut into small pieces before drying.

f)        Contamination by dust should be avoided, especially when Fe, Mn, Cu, and Zn are to be determined.

g)       Samples are put in an oven and dried overnight at 80 ± 2°c. Longer drying times are required for materials high in silica.

h)      To avoid possible loss of B from the samples, dry samples at 60°C prior to B determination.


1.2   How should plants be sampled?

a)       Greenhouse experiments: take all plants in each pot and make a composite sample; if that is not possible, take two predetermined plants per pot.



b)       Field experiment

·         Sample all replications

·         Exclude 2 border rows

·         Number of 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.

·         Locate plants 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.

c)       Production fields

·         Nutritional 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.

·         Nutrient 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 experimental design.

1.3  What plant parts should be sampled?

a)      Nutrient deficiencies and toxicities: use whole plant if small, the most recently matured leaf blade if large.

b)      Pattern of nutrient uptake: entire above ground plant at different growth stages.

c)      Total nutrient uptake: entire above ground plant including panicle at maturity.

d)     Fertilizer recommendations: most recently matured leaf.

e)      Detecting hidden hunger or toxicity: tillering phase.

f)       Fertilizer recommendations: before reproductive phase.

g)      Total nutrient uptake: at maturity

h)      Physiological studies: at successive developmental stages.

1.4  Plant sample collection and decontamination

a)      Solution 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.

b)      Dry soil culture plants: cut 3 cm above soil level and treat as in above.

c)      Wet soil culture plants: cut 2 cm above the water mark on the plant and treat as in above.

d)     Dry land plants: cut 5 cm above ground level, brush off soil at the base of the plant and treat as in above.

e)      Wetland rice: cut 5 cm above the water mark and treat as in above.

1.5  Plant washing and preparation

a)      Fresh sample is pre-washed at the greenhouse with tap H20, then rinse with RO 4x.

b)      Excess water is blotted off using a paper towel or clean cheesecloth

c)      Place the sample in cloth bags or perforated paper bags. Do not tie samples together.

d)     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

a)      To obtain homogeneous powders, samples are finely ground, using an Cyclone Udy Mill with stainless steel screen to pass through a 20-mesh sieve.

b)      Large samples are first ground through a standard Beater Cross grinder and are then reduced by quartering to a manageable size.

c)      These are then ground by the Cyclone Udy Mill or Intermediate Wiley Mill.

d)      These samples are used for the determination of N, P, K, Ca, Mg, Na, and other elements.

e)      Between samples the mill is thoroughly cleansed with a stiff-bristled brush or compressed air in order to avoid cross-contamination

f)       For the 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.

g)      After grinding, the whole sample must be mixed thoroughly.

h)      Ground samples are transferred to tightly capped glass jars or sealed polyeth­ylene bags, labeled clearly, and stored for further analysis.

i)        Samples are 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 quartering.



The first 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 further.

2.1  Drying

Most determinations 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.

2.2  Grinding

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.

Nearly all 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 carbon determination).

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.

2.3  Storage

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.

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