Soil Sampling Your Lawn and Garden

— Written By Eli Snyder and last updated by
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One of the best garden chores for the fall is to take and submit a soil sample to the North Carolina Department of Agriculture (NCDA). Soil tests report the pH, soil nutrient levels, and also make plant or crop-specific fertilizer recommendations. Soil tests can alert you to specific nutrient concerns before you see plant problems. The report also helps select the best fertilizer for your soil and for your specific crop. When plants grow in soil with correct pH and appropriate nutrition, they achieve maximum yields and better resist pests.

Soil pH is critical because it affects nutrient availability. Soil pH is reported on scale of 0-14.0, with 7.0 being neutral. Values below 7.0 are acidic, and soil with pH above 7.0 is basic or alkaline. For many plants, an optimum pH lies between 6.0 and 7.0 but some plants tolerate or even thrive with a lower pH. Caldwell County soils are naturally acidic. Over time, our soils becomes more acidic with rainfall and as plant residues breakdown. Managing soil pH for optimum plant growth is essential for gardeners.

The primary products used to raise pH are calcitic or dolomitic limestone, commonly called “lime”. This mined substance takes time to alter soil pH. Applying lime in the fall gives time for it to neutralize acid in the soil for the following season. Applying lime in fall can also relieve us of that duty in spring when there are many other garden chores beckoning. Having the soil test results in the fall can get us ahead in planning garden beds and crop fields.

North Carolina Department of Agriculture soil tests are $4 per sample when submitted between December and March. There is typically a one to two-week turn-around period for obtaining results, but this varies throughout the year based on volume of submitted samples.

How to take a good soil sample

You can obtain a soil sampling kit and instructions on how to take a soil sample at the Caldwell Extension Center (120 Hospital Ave NW Suite 1, Lenoir). Identify the number of areas that will be sampled separately; for example, if you have a lawn and a permanent flower bed that you would like to sample, treat these as two separate samples. Use a soil probe or trowel to collect 10-12 soil cores or “slices” from random locations throughout each sample area.

Woman taking soil sample.

To take a soil sample, combine 10-12 cores or slices per sample area in a clean bucket, mix thoroughly, and submit a subsample of this soil along with your soil test form. Test kits are available at 120 Hospital Ave NW, Lenoir, NC.

Avoid spots that lie wet or other areas that are not representative of the rest of the sample area. Sampling depth should be 4” in undisturbed beds or lawns, and 6” in tilled areas. Thoroughly mix the soil cores in a clean bucket, and use this soil to fill the sample box to the appropriate line. Specific instructions on sample packaging are described in the soil test kit instructions. Fill out the form according to instructions and ship to the NCDA Soil Testing service.

How often should I take a soil sample?

It is best to sample soil in a particular bed or field at least once every three years. Since it takes time for lime to react with the soil and change the pH, the soil test will recommend a total amount of lime that needs to be applied over the three year period. When submitting a sample, indicate the amount of lime, if any, applied to a field in the previous 12 months. Do not apply the total recommended amount of lime each year without taking a new soil sample- this may result in over-application of lime and a high pH, which is difficult to correct. Lime can be incorporated through tilling it in or it can be left on the soil surface. Surface applications of lime will take longer to alter the pH, and only 60 lb/1000 sq ft should be applied at a time. If the recommendations call for more lime, split the applications and apply every 6-9 months to achieve the recommended total amount.

The soil test will also provide information on plant nutrients present in the soil such as phosphorus (P) and potassium (K), calcium (Ca), and micronutrients. However, the soil test does not provide a nutrient index for nitrogen (N) because nitrogen is very mobile in the soil and levels can vary based on recent weather conditions or other factors. Fertilizer recommendations are made based on the estimated needs for the indicated plant or crop and the soil nutrient index values. If your soil nutrient index shows P and K levels that are below optimum, the recommendation will be to apply fertilizer to correct the nutrient levels in the soil and supply enough for the plants to be grown. Nitrogen recommendations are made based on crop needs. Generally, it is best to apply fertilizers close to the time when the plant will need the fertilizer the most; i.e. when it is putting on the most growth. Soil test results will recommend the best time to apply fertilizers for different crops.

Soil tests provide us with basic but valuable information, and help us provide the foundation for healthy gardens, lawns, and crops. Stop by the Caldwell Cooperative Extension Center to pick up our soil sampling kit today, and contact us (828-757-1290) with any questions about soil sampling, lime, or fertilizer recommendations.