   Soil Sampling and Testing - December 11, 2002 Jeff Schalau, County Director, Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County Laboratory soil tests will help you develop and maintain more productive soil and increase production by providing information on the available nutrient content of your soil. Soil testing helps you select the correct kind and amount of fertilizer and amendments. Soil testing is best used when objectives are specific or problem soils have caused losses. The expected improvements in production/plant health should also outweigh the expense. Great care should be taken in collecting and packaging the sample. The best time to sample a soil is before planting the crop (or landscape). If multiple soil types are present, then the areas should be stratified (defined) and these areas sampled separately. Examples for the Verde Valley are river bottom terraces vs. uplands or limestone vs. sandstone vs. gypsum derived soils. For some areas, this information may be available on a soil survey map. 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 that area. 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 6 to 9 inches). This is 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. Use a clean container to mix and store the soil sample. Label it clearly and keep notes of where 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 with added nutrients. 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 symptoms show a micronutrient deficiency. 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 (6 to 9 inches deep) is the best alternative. Soil testing is not for everyone. In fact, only serious gardeners and farmers should give it a try. However, if you really want to get to know your soil and improve it, then soil testing is a great idea. For labs that conduct soil testing, get the University of Arizona Cooperative Extension publication titled Laboratories Conducting Soil, Plant, Feed, and Water Testing. It is available on the web at ag.arizona.edu/pubs/ or by calling your local Cooperative Extension office. The University of Arizona Cooperative Extension has publications and information on gardening and pest control. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 or E-mail us at mgardener@verdeonline.com and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site: http://ag.arizona.edu/yavapai/anr/hort/byg/. |