Sampling
Sampling plays a critical role in plant analysis. When analyzing the nutrient status of plants, it is essential to select the plant part for chemical analysis that reflects the status of the particular element of interest. Four samplings during a growing season are usually sufficient to characterize seasonal nutritional patterns. One sampling should be early in the growing season, two in mid-season and the last one just prior to harvest. Four samples should be collected from each field or management unit. Each sample should contain material from at least 20 plants to ensure adequate, representative material for analytical testing. Separate samples should be taken from areas that appear different from the rest of the field.
A young mature leaf is generally selected for analysis. The sample can be subdivided into blade and petiole. The status of Cl, NO3-N, NH4-N, extractable K and P, in the form of PO4-P (2% acetic acid) are generally determined through analysis of the petiole. Blades are used when evaluating the status of K, Ca, Mg, Na, Fe, Mn, Zn, Cu, B, Mo, SO4-S and total-N in plants. For diagnostic purposes, only leaves that have recently developed symptoms should be collected for chemical analysis.
Drying Recommendations
After collection, plant material should be washed to remove any residual soil or dust.
For phenols: Dry samples at 50 - 52°C for 24 hours using a forced air oven. Avoid drying higher than 55°C as it could affect phenolic quantification. Best practice is to immediately freeze samples with liquid nitrogen and then freeze-dry the samples without thawing. Store the freeze-dried material in a dark, dry location (e.g., a desiccator).
Fresh samples or those suspected to be moist: Place samples into paper bags (with adequate room for air movement within the bag) and dry in a forced air oven at 55-60°C. In general, adequate drying time is approximately 12 hours or until the material snaps or breaks easily. All samples, except freeze drying material, should be turned every 24 hours.
Drying Exceptions:
- Range samples take 24 hours or more to dry
- Power plant ash samples take three days to dry
- Solid pieces of wood take 24 hours to dry
- Any material larger than gallon size will take one or more days to dry
- Fruits and vegetables should be freeze dried and pureed (weigh the samples before and after the freeze-drying process)
Note: These time frames are based on use of a large, forced-air oven. Times need to be adjusted accordingly for other types/sizes of ovens.
Grinding Recommendations
Most plant and feed samples should be ground to pass through a 40-mesh screen. Large wood samples must be splintered into smaller pieces before grinding. Average grinding time is one minute per sample if less than 10 grams or three to four minutes for samples over 10 grams.
Grinding Exceptions:
- Grape blades – use a 20-mesh screen due to tricomb separation from leaf
- Sugar beet petioles – use a 20-mesh screen due to high sugar content
- Walnut leaf – use a 20-mesh screen due to fibers in veins
- Corn stalks – use a 10-mesh screen due to high sugar content
- Freeze dried samples – do not need to be ground as they are easily crushed manually