Soil Sampling
FGV-00044 View this publication in PDF form to print or download.
Why test your soil?
Unless a specific problem is suspected, the most common reason to sample soil is to determine levels of essential nutrients. Soil samples submitted to a laboratory will be analyzed for macronutrient (nitrogen, phosphorus and potassium) content as well as pH levels. Knowledge of current soil nutrient levels, combined with a relative understanding of crop nutrient needs, aids in developing useful fertilizer recommendations to reach or maintain desired nutrient levels.
Some independent laboratories include nutrient recommendations with soil sample results; however, these recommendations are not necessarily appropriate for soil conditions in Alaska. Various laboratories use different soil test methodologies that are often only suited or calibrated for soils and crops in their area. Location appropriate recommendations are available from your district Cooperative Extension Service personnel or from the National Resource Conservation Service.
Why sample correctly?
Only a few tablespoons of the soil sample are used in the lab for the actual analysis, so it is important that the small amount truly represents the area of soil being tested. Soil improvement recommendations are based on the test results, and the accuracy of test results depends on the quality of the sample. If recommendations are made based on an unrepresentative sample, the crop could suffer in yield and/or appearance and money could be wasted. In addition, improper fertilization could also have adverse environmental impacts. Proper sampling greatly reduces these risks.
Soil sampling guidelines:
The soil sample that goes to the lab should represent the entire area sampled. The sample should not be collected from the best part of the lawn or garden, nor should it come from the worst part. It should represent the "average" part of the area being tested. For example, if there is a small sandy corner of the garden, don’t sample there, but if half of the garden is sandy, include soil from both halves.
Areas that are managed separately should be sampled separately. Lawns, gardens and flower beds tend to have different management practices; therefore, each area should be sampled separately. Many gardeners in Alaska use raised beds, and often each bed has a separate crop or separate soil properties. When using raised beds with one type of crop in each, it is important to sample each bed separately, because the soil will have different nutrient characteristics with different crop history. If all raised beds are treated exactly the same and generally contain the same crops or else a mixture of crops, it is sufficient to sample each bed and combine the soil into one sample for the lab. The general rule for sampling is that each area should be analyzed separately if it is going to be treated differently. The reason is that each type of crop will have different nutritional needs and will leave the soil with different characteristics.
How do you take a soil sample?
There are commercially available soil probes that allow many soil samples to be collected in a short time, but these probes are expensive and generally not worth the cost for the home gardener’s occasional use. A clean shovel (or spade), along with a clean pail and a trowel or old kitchen knife, is quite adequate for collecting soil samples.
If using a shovel or spade...
- Insert the shovel straight into the ground, about 6 inches deep for flower and vegetable