Sampling techniques vary with the crop. In some cases, such as cereal crops, leaf samples are taken. In a few cases, for example suspected boron deficiency in sugar beet, the roots should be sampled.

or

In any sampling of plant material great care should be taken to avoid soil contamination. This is true when sampling for major element analysis but it is critically important when trace element analysis is concerned because the contaminating soil may contain very much more of some trace elements than does the crop and a false result will be obtained.

It is essential to take a representative sample of the crop. One or two whole plants taken from an area may be completely unrepresentative. Samples should be taken from 50-100 plants in a given area. The advisory services, if called in may wish to take samples themselves, or may advise on which part of the plant to sample.

50-100

If a deficiency or toxicity condition is suspected in only a part of a field, 'good' and 'poor' areas should be sampled for comparison in the laboratory. Leaves or other parts of the plant, of the same age or growth stage, should be taken at the same time.

It is extremely important to use containers for transporting the samples which will not contaminate them. Tins, other metal containers and even some paper bags can cause serious contamination of the sample. The best container is a clean polythene bag. After taking the sample, labels should be attached, the bag sealed and transferred to the laboratory as quickly as possible.

The basic principle behind this technique is that the nutrient concentration of plants is related to the amount of nutrient element available in soil. General range of nutrient content in fully developed leaves of vegetable crops is given in table. If the nutrient level in the tissues falls below the critical concentration, the soil may be deficient in that element for optimum plant growth.

Fully developed leaves

Different plant parts of the same plant contain different concentrations of the same nutrient. Nutrient concentration again varies with the stage of the crop. So, leaf samples for analyses should be selected on the basis of physiological age, i.e., developmental stage. Stages of leaf sampling for the vegetable crops have been presented in table. It is important that the sample must be free from diseases, insect damage and physical or chemical injury. Leaf near the fruit should not be sampled as the nutrients, it might have contained, are often translocated to the fruits.

Table . Sampling procedures for collecting leaf and plant tissue for a plant analysis (Plank, 1979)

Plant part to sample

Number of plants to sample

Seedling stage (less than 30 cm)

All the above ground portion

20-30

Prior to tasseling

Tasseling

The entire leaf blade fully developed below the whorl

15-25

From tasseling and shooting to silking

The entire leaf blade at the ear node (or immediately above or below it)

* Sampling after silking occurs is not recommended

Prior to or during initial flowering

Initial flowering

The leaflets on 2 or 3 fully developed leaves at the top of the plant

* Sampling after pods begin to set not recommended.

Prior to heading

Heading

The 4 uppermost leaf blades

Hay Pasture, or Forage Grasses

Prior to seed head emergence or at the optimum stage for best quality forage

40-50

Prior to or at 1/10 bloom stage

Mature leaf blades taken about 1/3 of the way down the plant

Prior to bloom

Mature leaf blades taken about 1/3 of the way down from top of the plant

Prior to or at first bloom or when first squares appear

Youngest fully mature leaf blades on main stem.

30-40

Before bloom

Uppermost fully developed leaf blade

8-12

Sorghum

Prior to or at bloom stage

Leaflets of mature leaves from both the main stem and either cotyledon lateral branch

Peanuts (Groundnuts)

Early growth (35-40 days after planting)

Tomato Fourth to sixth leaf from growing tip

Early bloom

20-25

Chilli and Sweet pepper Young mature leaves

Early fruit set

Cauliflower Young mature outside leaves

Button stage of curd

10-20

Cabbage First mature leaf from central whort

Broccoli Young mature leaves

First bud formation stage

Brussels sprouts Young mature leaves

Mid growth

Watermelon, Muskmelon, Cucumber, Pumpkin, etc. Mature leaves near the base portion of plant on main stem.

Early growth prior to fruit set

Beans (French bean, Cowpea, Lima bean, Hyacinth bean, etc.) Two to three fully developed leaves at top of the plant

Pea Leaves form third node down from top of the plant

30-60

Root crops (Carrot, Radish, Beet, Turnip) Young from third node down from top of the plant

Bulb crops (Onion, Garlic etc.) Young mature leaves from centre

Prior to bulbing

Celery Petiole of youngest fully elongated leaf

Midgrowth (30-35 cm tall)

Midgrowth

30-35

Lettuce (leaf type) Midrib of wrapper leaf

Lettuce (leaf type) Youngest mature leaf

Leafy greens (Palak, Spinach, etc.) Youngest mature leaf

Sweet potato Fourth to sixth leaf from the growing tip

Prior to root enlargement

Asparagus Top 10 cm of the new fern branch

Midgrowth of the ferm

Sweet com Entire leaf at the ear node

Mid season

Leaf blade near base of current year's growth or from spurs

Current year's growth

Youngest fully expanded mature leaves, without petioles

50-75

6 to 8 weeks after bloom

Leaflets from terminal shoots, (taking the pairs from the middle of the leaf)

30-45

Middle leaflet pairs from mature shoots

End of bloom period

Petioles from leaves adjacent to fruit clusters

60-100

Youngest mature leaves on laterals or "primo" canes (without leaf petioles)

20-40

Fully developed leaves (without petioles)

30-100

During normal growing season

Leaf blades. Clip by hand to avoid contamination with soil or other material

½ liter of material

During flower production

Upper leaves on the flowering stem, without petioles

Prior to or at flowering

Upper leaves on flowering stem

1. Unpinched plants

Pinched plants

4th or 5th leaf pairs from base of plant

5th or 6th leaf pairs from top of primary laterals

Most recently mature, fully expanded leaf blades

15-20

1. Mailing Kit. Many laboratories have plant analysis mailing kits. Instructions for sampling and submitting samples should be followed specifically.
2. What to sample. The sampling procedures for collecting leaf and plant tissue for analysis is shown in Table 3.4. When no specific sampling instructions are given for a particular crop, the general rule of thumb is to sample the upper, recently matured leaves. The recommended stage of growth to sample is just prior to the beginning of the reproductive stage for many plants. Roots from total plant sample should be removed. It is well to take a soil sample from the same area at the same time.
3. Comparison samples. Where a deficiency is suspected, take samples from normal plants in an adjacent area as well as from the affected plants. Take a soil sample from each area also.
4. Washing to remove contaminants. Dusty plants should be avoided but if dust is present, brushing or wiping with a clean damp cloth may be sufficient. If not, rinse briefly in running water while the material is still fresh.
5. What not to sample.

1. Diseased or dead plant material, damaged by insects, or mechanically injured.
2. Plants stressed severely by cold, heat, moisture deficiency or excess.
3. If roots are damaged by nematodes, insects or diseases.

1. The questionnaire. This is the means of communication between the sample and the laboratory. Completion of the questionnaire is important if the interpreter is to evaluate properly the analysis and make a recommendation.
2. Packaging the plant tissue. Partially air dry and put in a clean paper bag or envelope. Do not put in polyethylene bags or tightly sealed containers, since this permits molding.

Selection of the tissue to be tested in various plants and interpretation of the analyses under the varying environmental conditions of any given plant and conditions of any given plant are much more complex. Since the plant is a dynamic system growing in a dynamic soil system and subject to other ecological influences, many factors will influence the level of the soluble or unassimilated nutrients in a given plant at any given time. Some of the most pertinent of these which should be considered in the interpretation of the tests are: first general performance and vigor of the plant; Second, level of other nutrients in the plants; third, occurrence of insect damage, disease or other known disturbances; forth, climatic condition at the time of testing; and fifth, time of day at which tests are made. In other words, the plant diagnostician must be well versed with the physiology of the plant if he is to make most effective use of plant – tissue tests.