Soil Testing

Soil Testing

  • By Janet Anastasi
  • /
  • April 2015 - Vol 1. No. 4

A soil test is one of the best investments a gardener can make. The purpose of a soil test is to provide the gardener with information necessary to make a wise investment in fertilizer and soil amendment choices. Performing a soil test is one of the first things any gardener should do regardless of what is to be planted. A soil test report will provide a wealth of information:   the pH level, available phosphorus (P),  potassium (K), calcium (C), magnesium (M), zinc (z), manganese (Mn), copper (C) and iron (Fe) components of your soil. These elements are essential for healthy productive plants, trees and lawns.

One of the most important soil factors that affect a plant’s growth and health is the pH level of the soil. The soil pH test measures how acidic or basic the soil is. The pH scale is a logarithmic scale and ranges from 0-14;  a pH level of 7 is neutral. A pH less than 7 is acid and a pH greater than 7 is basic. Since the pH scale is logarithmic, a soil with a pH of 5.7 is ten (10) times more acid than a soil with a pH of 6.7,  and a soil testing 4.7 is 100 times (10X10) more acidic than a soil testing 6.7. The same principle holds true for soils testing above 7.0 —  each is whole number 10 times more alkaline or basic than the next whole number. This explanation may be confusing for those of us who are “math challenged,”  but the main thing to remember is that when the pH needle moves one whole point,  it’s not just one point it’s 10 times that because the pH scale is logarithmic.

The degree of acidity or alkalinity of the soil is directly related to the availability and uptake of soil nutrients to plants. At pH extremes, some nutrients become partially or completely locked up in the soil and become unavailable to plants. In short, the pH factor is the keeper of the nutrient key. Adding amendments or fertilizer to soils with extreme pH levels will have little or no effect on plant growth.  Correcting the pH level opens the  nutrient door and makes amendments and fertilizers effective.

For lawns, The Soil Lab at Virginia Tech will recommend adding soil amendments (primarily lime) to soils testing less than a pH of 6.2.

The Effect of Soil pH on Nutrient Availability

Source: National Soil Survey Manual (NRCS)

Source: National Soil Survey Manual (NRCS)

As we can see, the thicker the bar the more available the nutrient. A pH above 6.2 will insure that all nutrients are available for a beautiful lawn. In our area, in very rare cases, an application of sulfur is recommended to lower the pH  to the 6.2-6.8 range; but a high pH in our area usually means that improper quantities of amendments (lime and or wood ashes) have been added to the soil.

Soil test kits are available at your local extension office and also at garden centers throughout Charlottesville and Albemarle County.  The Albemarle County/Charlottesville office of Virginia Cooperative Extension is located at 460 Stagecoach Road, just off of Fifth Street Extended in Charlottesville.

Some general rules about soil sampling:

  1. If the soil is too wet to work, it is too wet for a sample. Slightly wet samples can be dried on newspaper prior to sending the sample in for analysis.
  2. Rocks, twigs, grass or any other debris must be removed from the sample.
  3. Samples from mulched beds should be taken after the mulch is removed from the area.
  4. Do not take samples from areas where the soil conditions are different from the rest of the landscape such as pet areas, brush piles, wet spots, and landscape borders.

When and how should soil samples be collected? Ideally soil samples should be taken a few months before any new landscaping is planned. This allows any nutrients that are added to start working in the soil. Generally, a  soil test should be taken once every 3 to 4 years. A soil test should also be considered if there is abnormal growth or a change in plant color. However, samples should not be taken within six to eight weeks of fertilizing or liming.

Where and how many unique soil samples should be taken? Generally in the home landscape, a different soil sample should be taken from each planting area;  thus, separate samples should be taken from the lawn, the perennial flower beds, and the vegetable gardens. Additional samples may be necessary if, for example, one part of the lawn area is fescue and the other Bermuda.  The key is that all the soil in a sample box should come from a uniform area because what your lawn needs is usually quite different from what your shrub border or flower bed needs.

Tools for taking the soil sample include a shovel, trowel, spade, or soil sampler, and a clean plastic bucket. The tools should not be brass, bronze or galvanized because they can contaminate the sample with copper or zinc.

Sample depths depend on the type of plant and range from 2-4 inches for lawns, 6-8 inches for vegetable and flower gardens and 6 inches for trees and shrubs. The sample is taken by inserting the shovel into the soil at the appropriate depth, and removing a soil plug 2 inches wide and one inch thick. The sample is placed in the plastic bucket and another sample is taken from a different location but in the same uniform area as discussed above. Thus, you will collect several samples from different parts of your lawn, but they will all go together into one sample box to be sent for analysis.  A zig- zag pattern across the sampling area provides an accurate cross- section of the soil in that area. At least 10 samples should be taken of each area, but even more samples may produce more accurate results.

Once all the samples are collected they should be thoroughly mixed together, removing the debris, and if necessary, dried out thoroughly and packed tightly in the soil test box. It is very important to give each box a unique identification number with the same number on the box as well as on the accompanying information form. Using the correct plant code list from the soil sample information sheet will insure the most accurate recommendation for your planting area.

The soil test report comes from Virginia Tech in a few weeks and is the valuable end result of all this work. The report contains the following information: name of the crop, lab results, fertilizer recommendations, recommended lime amounts to add, and soil organic matter.  For a detailed explanation of the contents of a soil test report, look at https://pubs.ext.vt.edu/452/452-701/452-701.html).

Because every soil test is different, it is very important to review the report for each unique area sampled. For example, the explanation of soil test reports for lawns are found at VCE Pub. No. 452-717 (cool season grasses) and at VCE Pub. No. 452-718, (warm season grasses).  For vegetables and flower gardens, explanatory keys for soil test reports can be found at VCE Pub. No. 452-719.

With your soil test report in hand, you will be able to add necessary amendments in the right amounts to create a fertile environment for your lawn, your flower bed and your vegetable garden.  Doing a soil test is one of the best ways and most certainly least expensive ways of insuring that your landscape will flourish.