Soil Sampling and Analysis - March 30, 2016
Jeff Schalau, Agent, Agriculture & Natural Resources
University of Arizona Cooperative Extension, Yavapai County
Laboratory soil tests can help you increase soil productivity by assaying the essential plant nutrients available in your soil. Soil test results identify nutrient deficiencies and provide guidance as to the appropriate amounts of fertilizers and amendments. Soil testing is not absolutely necessary for small home vegetable gardens. However, it is not expensive and can be of great value where plant nutrition problems could be limiting production. I recommend testing your garden soil every few years to monitor nutrient availability and how fertilizer additions may have changed this over time.
Once the area is stratified, sampling can begin. Each soil type being tested should have 15-20 subsamples collected from that area. These are combined to create a composite sample for analysis. Avoid small areas with odd conditions (manure piles, other animal waste, compost piles, etc.). Use clean sampling tools made of steel (not brass, bronze, or galvanized). Brass, bronze, and galvanized coatings may contaminate soil samples with iron, copper, manganese, or zinc which becomes problematic if you are testing for micronutrients.
Each subsample should be collected systematically and at the same soil depth (usually 3 to 4 inches). This is the depth at which most plant roots will be extracting nutrients from the soil. Other depths can be chosen depending on the crop and management objectives. Only a small amount of soil is needed from each subsample location (2 or 3 tablespoons). Use a shovel, trowel, and ruler or a coring tool specially designed for soil sampling. Mix all the subsamples well to ensure uniformity of the composite sample and allow them to air-dry. Use a clean container to store the soil sample. Label it clearly and keep notes of where and when the sample was collected. Most labs request one half-pound of soil for analysis (check with your lab to be sure).
The expense of testing increases when additional nutrients are assayed. For baseline soil testing, it is good to test for pH, nitrate-nitrogen, phosphorus, potassium, calcium, magnesium, sodium, sulfur, and electrical conductivity. You may also choose additional micronutrient testing (zinc, iron, and manganese). Organic matter and soil texture may also be of interest to you. Nitrogen results are not extremely useful unless you have sampled at more than one depth. Nitrogen is dynamic and highly mobile within a soil profile and almost always limiting to plant growth.
Upon receiving your soil test results, you are then faced with their interpretation. Most labs have relative nutrient values interpreted for you. For example, along with a quantitative result for each nutrient tested (usually listed in pounds per acre), there would be qualitative interpretations such as low, medium, or high. If a rating is low, then you may expect a growth response when adding that nutrient in a fertilizer or amendment.
Soil pH is often the reason plant experience micronutrient deficiencies. Iron deficiency is common on limestone-derived soils because the alkalinity (high pH) caused by the excess calcium carbonate causes the iron to become less available in the soil. A soil test will identify these conditions and to what degree you may expect to improve the situation. Phosphorus is not highly mobile in soil. If a soil test indicated phosphorus deficiency, then amending the soil with a phosphorus fertilizer at the root zone is the best alternative.
If you have a small garden or farm on a sizeable area, soil testing can be a very useful tool. Recently, I have been recommending the Texas A & M Soils Lab for folks interested in soil testing. It is relatively inexpensive ($10/sample for basic testing) and they provide a quick turnaround time. There are also many independent soil testing labs in Arizona. See the online edition for links to these and other resources.
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Texas A & M Agrilife Extension Service Soil, water and Forage Testing Laboratory
Laboratories Conducting Soil, Plant, Feed, or Water Testing
University of Arizona Cooperative Extension
Interpretting a Soil Test Report
Ohio State University
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Last Updated: March 24, 2016
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