Sampling of plant material
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.
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.
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.
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)
Stage of growth |
Plant part to sample |
Number of plants to sample |
Field crops |
|
|
Maize* |
|
|
|
Seedling stage (less than 30 cm) |
All the above ground portion |
20-30 |
|
Prior to tasseling |
The entire leaf blade fully developed below the whorl |
15-25 |
|
or |
|
|
|
From tasseling and shooting to silking |
The entire leaf blade at the ear node (or immediately above or below it) |
15-25 |
* Sampling after silking occurs is not recommended |
Soybeans or Other Beans* |
|
Seedling stage (less than 30 cm) |
All the above ground portion |
20-30 |
|
or |
|
|
|
Prior to or during initial flowering |
The leaflets on 2 or 3 fully developed leaves at the top of the plant |
20-30 |
* Sampling after pods begin to set not recommended. |
Small Grains (including rice)* |
|
Seedling stage (less than 30 cm) |
All the above ground portion |
50-100 |
|
or |
|
|
|
Prior to heading |
The 4 uppermost leaf blades |
|
* Sampling after pods begin to set not recommended. |
Hay Pasture, or Forage Grasses |
Prior to seed head emergence or at the optimum stage for best quality forage |
The 4 uppermost leaf blades |
40-50 |
Alfalfa |
|
|
Prior to or at 1/10 bloom stage |
Mature leaf blades taken about 1/3 of the way down the plant |
40-50 |
Clover and Other Legumes |
|
|
Prior to bloom |
Mature leaf blades taken about 1/3 of the way down from top of the plant |
40-50 |
Cotton |
|
|
Prior to or at first bloom or when first squares appear |
Youngest fully mature leaf blades on main stem. |
30-40 |
Tobacco |
|
|
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 |
15-25 |
Peanuts (Groundnuts) |
Prior to or at bloom stage |
Leaflets of mature leaves from both the main stem and either cotyledon lateral branch |
40-50 |
Potato
Fourth to sixth leaf from growing tip |
Early growth (35-40 days after planting) |
20-30 |
Tomato
Fourth to sixth leaf from growing tip |
Early bloom |
20-25 |
Chilli and Sweet pepper
Young mature leaves |
Early fruit set |
20-25 |
Cauliflower
Young mature outside leaves |
Button stage of curd |
10-20 |
Cabbage
First mature leaf from central whort |
Prior to heading |
10-20 |
Broccoli
Young mature leaves |
First bud formation stage |
10-20 |
Brussels sprouts
Young mature leaves |
Mid growth |
10-20 |
Watermelon, Muskmelon,
Cucumber, Pumpkin, etc.
Mature leaves near the base portion of plant on main stem. |
Early growth prior to fruit set |
20-30 |
Beans (French bean, Cowpea, Lima bean, Hyacinth bean, etc.)
Two to three fully developed leaves at top of the plant |
Initial flowering |
20-30 |
Pea
Leaves form third node down from top of the plant |
Initial flowering |
30-60 |
Root crops (Carrot, Radish, Beet, Turnip)
Young from third node down from top of the plant |
Initial flowering |
20-30 |
Bulb crops (Onion, Garlic etc.)
Young mature leaves from centre |
Prior to bulbing |
20-30 |
Celery
Petiole of youngest fully elongated leaf |
Midgrowth (30-35 cm tall) |
10-20 |
Lettuce (leaf type)
Midrib of wrapper leaf |
Heading |
30-40 |
Lettuce (leaf type)
Youngest mature leaf |
Midgrowth |
30-40 |
Leafy greens (Palak, Spinach, etc.)
Youngest mature leaf |
Midgrowth |
30-40 |
Sweet potato
Fourth to sixth leaf from the growing tip |
Prior to root enlargement |
20-30 |
Asparagus
Top 10 cm of the new fern branch |
Midgrowth of the ferm |
20-30 |
Sweet com
Entire leaf at the ear node |
Tasseling |
20-30 |
FRUITS AND NUTS |
|
|
Apple, Apricot, Almond, Prune, Peach, Pear, Cherry |
|
Mid season |
Leaf blade near base of current year's growth or from spurs |
50-100 |
Strawberry |
|
|
Mid season |
Youngest fully expanded mature leaves, without petioles |
50-75 |
Pecan |
|
|
6 to 8 weeks after bloom |
Leaflets from terminal shoots, (taking the pairs from the middle of the leaf) |
30-45 |
Walnut |
|
|
6 to 8 weeks after bloom |
Middle leaflet pairs from mature shoots |
30-35 |
Grapes |
|
|
End of bloom period |
Petioles from leaves adjacent to fruit clusters |
60-100 |
Raspberry |
|
|
Mid season |
Youngest mature leaves on laterals or "primo" canes (without leaf petioles) |
20-40 |
Ornamentals and Flowers |
|
|
Ornamental trees |
|
|
Current year's growth |
Fully developed leaves (without petioles) |
30-100 |
Ornamental Shrubs |
|
|
Current year's growth |
Fully developed leaves |
30-100 |
Turf |
|
|
During normal growing season |
Leaf blades. Clip by hand to avoid contamination with soil or other material |
½ liter of material |
Roses |
|
|
During flower production |
Upper leaves on the flowering stem, without petioles |
20-30 |
Chrysanthemums |
|
|
Prior to or at flowering |
Upper leaves on flowering stem |
20-30 |
Carnations |
|
|
1. Unpinched plants |
4th or 5th leaf pairs from base of plant |
20-30 |
Pinched plants |
5th or 6th leaf pairs from top of primary laterals |
20-30 |
Poinsettias |
|
|
Prior to or at flowering |
Most recently mature, fully expanded leaf blades |
15-20 |
Taking plant samples for analyses of nutrient content
- Mailing Kit. Many laboratories have plant analysis mailing kits. Instructions for sampling and submitting samples should be followed specifically.
- 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.
- 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.
- 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.
- What not to sample.
- Diseased or dead plant material, damaged by insects, or mechanically injured.
- Plants stressed severely by cold, heat, moisture deficiency or excess.
- If roots are damaged by nematodes, insects or diseases.
- 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.
- 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.
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