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Laboratory soil sample preparation includes the soil sampling, soil drying, soil grinding and soil digestion. Its the essential procedure before soil analysis.
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 sub-samples should be composited from each area. Sub-samples 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. Break up large chunks of soil and spread out to air dry where the sample will not be contaminated, particularly by fertilizer dust. The sample may also be placed in a forced air oven set between 35°C and 55°C.
Soil should be crushed in a soil pulverizer to pass through a 2mm mesh screen. Large clumps of hard soil should be pounded into smaller clumps first. Soil pulverizing time is normally one to four minutes per sample.
Analysis of Carbon, Nitrogen (including Total Kjeldahl Nitrogen), Calcium Carbonate, and total elements (Arsenic, Cadmium, Copper, Chromium, Iron, Lead, Manganese, Molybdenum, Nickel, Phosphorus, Selenium, Zinc) requires that the sample be ground to a powder fine enough to pass through a 60 mesh screen. Grinding with mortar and pestle or mechanical grinder may be necessary to powder the sample, especially with soils containing sand.
Soils with a high percentage of sand or organic material may require grinding to pass through a 60 mesh screen to achieve sample homogeneity.
Before the soil sample be sent for analysis, it needs acid digestion for testing like atomic absorption, atomic fluorescence and ICP=AEC.