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Don’t Fertilize Without A Soil Test: A Protocol on soil sampling
Written by Mark A. Webber
Board Certified Master Arborist #OH-0824B
For any tree or plant to be successful it must have an ample supply of nutrients from the soil that can be transported from the roots to various parts of the tree, including the leaves, to support the plants basic functions like photosynthesis. For generations, the thought of many green industry professionals has been that all plants, including trees, need to be fertilized on a regular and on-going basis. New industry regulations and standards suggests that fertilizer applications should be carefully considered and done only if there is a laboratory test that indicates that the target plant is lacking in one or more of the required nutrient’s to support a healthy plant.
The ANSI A300 (Part 2) - 2011 Soil Management a. Modification, b. Fertilization, and c. Drainage states that soil modification shall be used to meet an objective (section 10.2.1 “Reason”). More importantly, the ANSI standard under section 15.2 states that: “soil and/or foliar nutrient analysis should be used to determine the need, formulation, and rate of fertilizer.”
Without soil testing and/or foliar analysis it is impossible to determine the need, formulation, or rate of fertilizer to use when fertilizing a plant. . To get an accurate and useful soil test result: the soil sample must follow a proper soil collection (sampling) protocol.
These are the critical factors of soil sampling protocols:
• Collection devices: The best way to collect soil sample cores is with the use of a soil probe. Most soil probes feature a window slot in the cylinder of the probe for easy sample recovery. Some probes are used without a liner and others can be purchased to be used with a liner. Probes can be purchased at varied lengths and diameters. Un-plated, rust-resistant nickel-plated or stainless steel models are available.
(Photo source Mwebber 2015)
• Root Zone Depth: Collecting soil cores at the right depth is critical and pulling the samples from the soil zone where a plants root system is located will provide the most accurate sample results. Most trees in the urban landscape have roots at a depth of 4-24” depending on the site and species. It’s important to adjust the depth of collections by doing sample cores and looking for plant/tree roots that exist in that zone of collection to know the best sample zone to pull samples from.
• Taking a representative sample: Most soil scientists tell us that a minimum of 10-12 cores (sub-samples) or about one cup of soil should be collected for every 8,000 square feet of area sampled. The general rule is to pull one sub-sample at a distance of every 10-15 feet from the last one collected.
• Avoid Contamination: When collections are made be sure to remove debris like grass/thatch or mulch from the soil sample. Then place all the sub-sample cores collected into a clean non-metallic bucket. Thoroughly mix the sub-samples in the same collection bucket before bagging the final sample to send to the lab for testing. Especially if you are testing for microorganisms, the sample should be kept out of direct sunlight, due to the potential perishability of the microorganisms.
• Sample Identification/paperwork: Before the final mix of multiple sub-sample cores are bagged for shipment, mark the sample bag with a water-proof permanent marker. Make sure if the lab requires special codes/names that they have been placed on the bag before filling it. Before the final paperwork is filled out and placed with the final sample be sure to record any sample numbers or codes on the corresponding paperwork like the work order for the collection. This is done so that you have generated a second set of numbers from the final sample bag
• Which test do you want? Soil testing labs can vary on what tests they run, so make sure the lab you utilize can test for what your needs are.
Additionally I have seen soil test results that often offer recommendations under low to high fertility practices. It’s important as well for you to have a good relationship with your testing lab as they may be able to help explain issues that the test results may not show or that cannot be tested for.