Soil Testing for Beginning Gardeners
While there are exceptions, most vegetables flourish in a pH range of 5.5 to 6.5. Outside this range, plants cannot access many of the soil’s nutrients, even if they are present. It is recommended that gardeners test their soil at least every two to three years and make changes as needed to both pH and nutrient levels. An excess of nutrients may have accumulated from over-fertilization by zealous gardeners or nutrients may have been depleted from continued gardening or leaching. The WVU Soil Testing Lab offers free basic soil testing to West Virginia residents; organic matter levels can be requested at an additional cost.
Benefits of Soil Testing
Soil testing determines the extractable soil phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg) as well as the soil acidity reflected as pH. The test does not evaluate the soil’s physical properties, such as poor drainage or compaction. For many crops, the lab will also make nitrogen (N) recommendations based upon the crop specified and the intended yield. Nitrogen, phosphorus and potassium can be supplied individually or in a combination fertilizer. 10-10-10 for instance has 10% of each nitrogen, phosphorus and potassium (N-P2O5-K2O). A consultation with a local WVU Extension agent can help gardeners determine the best option for their garden or lawn depending upon the soil test and local sources available.
Adequate levels of each necessary nutrient allow for maximum plant growth. Levels below a critical value for each plant species will limit plant growth. When crops, such as vegetables, are removed from the garden, the plant tissue taken away contains some nutritive material, which decreases the soil nutrient content over time. This material must be replaced by fertilization of those lost nutrients to the critical level or future plant growth may suffer. Adding additional fertilizer above the necessary amount is not only expensive but can cause environmental problems. Excess nitrogen can cause toxic nitrate levels in the water supply and excess phosphorus can cause algae blooms in surface water, which is detrimental to aquatic life.
The three most commonly supplied nutrients are nitrogen, phosphorus and potassium. Nitrogen is utilized by plants for photosynthesis and green growth. Excessive amounts may lead to too many leaves, vining or spindly growth, while deficits may lead to yellowing of leaves and inadequate fruit production. Phosphorus is necessary for cell division, root development and reproduction; excess amounts can cause imbalances with other nutrients. Potassium is essential for water regulation and enzyme activation in plants. Deficiency is often displayed as browning or scorching of leaf tips or yellowing between the leaf veins; root, seed and fruit development also are often reduced. Secondary nutrients are necessary for many essential functions including cell wall development (calcium) and photosynthesis and protein synthesis (magnesium). A soil test is the only accurate way to determine whether your soil is lacking in these nutrients and how much needs to be supplied.
How To Take A Soil Sample
A soil test uses an “average” sample to determine the nutrients available in the soil. Your results can be presented on an acre basis or per 100 square feet; for most home gardeners, the 100-square-feet option is generally most appropriate. To receive accurate results, it is essential that the sample is representative of the area sampled.
To ensure that the sample represents the soil that the plants are growing in, follow these suggestions:
- Send off an adequate sample, typically 1 to 2 cups of soil.
- Sample areas that represent a uniform area of land – anything that has a different slope, soil texture or has been treated differently with different fertilization, lime or tillage should be sampled separately.
- Do not sample shortly after applying lime or fertilizer or use containers or tools contaminated by lime or fertilizer.
- Do not pull samples from areas that are atypical, such as where compost, manure or lime was piled or spilled.
- Do not pull samples from areas that are contaminated with other material, such as leaves, roots or rocks.
- Samples should not be taken when the soil is wet, frozen or excessively dry – it is ok to lightly air dry the sample on a clean paper out of direct sun, if needed, but do not dry in an oven, hot sun or using an aluminum or iron pan.
- Sample to the depth of tillage.
WVU Extension generally recommends a soil sampler probe to auger samples, but if the garden or raised bed has loose tilled soil, a small trowel or spade can be used. All tools and buckets should be either stainless steel or plastic as iron can interfere with test results. Before taking a sample, move aside any surface debris to expose mineral soil. Take several small samples to the depth of tillage, usually 10 to 20 randomly selected cores. Put the samples into a bucket or similar container. Once all samples for the area are taken, gently crush and mix the samples and pull a 1- to 2-cup sample out to send to the lab. A slotted spoon can be used to remove rocks, roots and other organic matter. Gardeners may send off multiple samples if they have areas they intend to sample and treat differently or have treated differently in the past, such as a potato patch.
Gardeners should fill out the digital form and submit it with their sample, using a separate sheet for each sample submitted. The form to submit with a soil sample and more information about sampling and interpreting results can be found on the WVU Soil Testing Laboratory website at https://soiltesting.wvu.edu/.
Rayburn, E. and Basden, T. (2014). Soil Sampling and Testing. WVU Extension. https://extension.wvu.edu/files/d/a00cb30b-5077-4802-aa19-7a81a3ff0a03/soil-sampling-and-testing.pdf
Jodi Richmond – WVU Extension Agent – Mercer County, Ed Rayburn, WVU Extension Specialist – Forages and Agronomy, and Tom Basden – WVU Extension Specialist – Nutrient Management