The validity and usefulness of any chemical plant analysis is dependent on how representative the plant tissue sample is. A poor sample can result in incorrect diagnosis, leading to corrective treatments which are costly yet ineffective.
Obtaining a representative plant sample is essential for plant analysis; nevertheless, the researcher or scientist should be aware that this is a complex task requiring expert knowledge. Suggestions for gathering representative samples should be followed closely. They include:
Location: It is recommended that plant samples be taken from mature leaves located just below the growing tip of the main branches and stems. Their nutrient composition is stable compared to that of younger leaves which can undergo rapid change.
Maturity: The composition of a plant’s nutrient elements can vary considerably, depending on plant maturity. Sampling should occur when the relationship between plant nutrient concentrations and yield, and the plant’s physical appearance is at its best.
The best time: for sampling is just before the plant begins its reproductive growth. When plants begin setting and developing fruits or seeds, the nutrient concentration of the vegetative portions change considerably. Particularly, in the case of grain crops, sampling after pollination is not recommended.
Physical and Chemical Conditions: Plants subjected to stress over a long period of time, can develop nutrient concentrations which are often misleading. One way of avoiding this is to take samples when the plant has been initially affected. Plant tissue taken from normal plants within the same field, and at the same stage of development, can also aid in the interpretation.
Sampling is not recommended if the plant is covered with soil or dust, damaged by insects, mechanically injured or diseased. Dead plant tissue should not be included in a sample either, in addition, sampling is not recommended if plants are under stress caused by moisture or temperature changes.
Read Also: Principles of Plant Analysis Data Interpretation
Sample Preparation Methods
Once plant samples have been obtained, they must undergo four preparatory steps before any analysis can be carried out on them:
1. Cleaning of Material
It is not uncommon to find plant samples which have been recently collected, covered with a thin film of dust. It is important that this be removed, since its presence can affect the samples’ concentrations of Fe and Mg.
Dust is difficult to remove by mechanical wiping or brushing only; therefore, washing the tissues in a 0.1 – 0.3% solution to detergent followed by a rinse in pure water, is recommended.
This washing should be done quickly in order to prevent any leaching of nutrients such as Na, K or Ca.
2. Drying
Following washing, plant tissue samples should be dried as rapidly as possible in order to minimize any chemical or biological changes.
Drying at 650C is considered adequate to stop enzymatic action. The sample is placed in a hot air oven and dried until the weight is constant. This may require from 24 to 48 hours. After drying, the sample is ready to be ground.
3. Grinding
This is beneficial for two reasons, it allows the sample to be manipulated with greater ease, and even more important, it ensures greater ease, and even more important, it ensures greater uniformity in terms of composition.
When selecting a mill, it is important to choose one which minimizes the possibility of contamination. This is especially critical when analysis is being performed to determine the presence of any micronutrients.
Equipment having grinding surfaces of either steel or stainless steels are recommended. Grinding equipment available includes the hammer mill, wiley mill, jar mill with flint, porcelain with mullite balls, agate or glass mortar and pestle.
4. Store
Under humid tropical conditions, samples which have been poorly dried are frequently in danger of becoming moldy. Any samples which have to be stored for a prolonged period should be kept in a refrigerator, preferably at temperatures of 50C or lower.
Read Also: Problems Associated with Plant Analysis
