One of the most important aspects of analytical work is the proper collection and preparation of representative samples. Below are some guidelines on sampling techniques for soils, plants and water. However, these guidelines are very general and information concerning your specific project should be obtained prior to sampling for submission to the Grotek Analytical Lab.

Soils

Soil sampling is a particularly difficult task when attempting to get a representative sample. Normally a 500-gram sample is submitted to the laboratory for analysis. This 500-gram sample may represent 10 or more acres. If the area covered by the sample is not uniform, the chemical analysis may not accurately reflect the nutrient status of specific sites. Factors that need to be considered when sampling soil include the depth and time of sampling. Proper sampling depth is affected by the crop being grown, past cropping, depth of plowing and also the nutrient of interest. Subsoil samples are important for most crops. Standard sampling times should be used due to the difficulty in comparing samples taken at different times. The fertility level of a field will vary over the course of the year and interpreting results for samples taken at different times of the year will be very difficult. Sampling between crops will give more consistent results.

When sampling soils, the area should be subdivided into as homogeneous sections as possible. Between 10 and 20 subsamples should be composited from each area. Subsamples should be small enough that the composite sample will be of a size that can be completely processed for analysis. The depth of the sampling is determined by the crop, the elements of interest and existing knowledge about the soil profile. Samples for cultivated crops are taken from the plow layer. Pasture and sod crop soil samples should be collected from the top four inches. Samples for nitrate, soluble salts and available micronutrients should be taken at the root depth.

Once samples have been collected, they must be processed promptly to prevent any changes that might affect the analysis. Soils should be air dried and crushed in a soil pulverizer to pass through a 2-mm sieve. Freezing the samples will prevent changes in the nitrate levels.

Plants

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, NO 3-N, NH 4-N, extractable K and P, in the form of PO 4-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, SO 4-S and total-N in plants. For diagnostic purposes, only leaves that have recently developed symptoms should be collected for chemical analysis.

After collection, plant material should be washed to remove any residual soil or dust, placed in paper bags with adequate room for air movement within the bag and dried at 55-60°C for approximately 24 hours or until the material breaks easily. Samples should be ground to pass through a 40-mesh screen.

Water

Samples must be representative. Samples should be collected in clean, plastic bottles that have been rinsed three times prior to use. Well-water samples should be collected after pumping for at least 30 minutes. Sampling from distribution systems should be done after the lines have been flushed sufficiently to ensure that the sample is representative of the supply. If NO 3-N is an element of interest and the analysis cannot be completed within three hours of collection, the samples should be frozen or kept below 40°F. Samples collected for alkalinity, conductivity, phosphate and sulfate should also be refrigerated until analysis can be completed. Due to the problems of absorption or precipitation, if micronutrients are of interest, the sample, or a subsample, should be acidified to pH <2 as soon as possible after collection.

References

Reisenauer, H. M. (ed.) 1978. Soil and Plant-Tissue Testing in California, Division of Agricultural Sciences, University of California, Bulletin 1879.