Soil Test for Optimum Plant Growth

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If you ever called the Caldwell Extension Center with a yard or garden issue there is a good chance we recommended taking a “soil test”. A soil test is a chemical test that measures soil nutrient levels. The information learned from a soil test helps gardeners know how much lime and fertilizer to add for optimum plant growth.

In North Carolina, the NC Department of Agriculture operates the soil testing lab. This lab exists to support farmers and gardeners in the State. The lab makes lime and fertilizer recommendations based on research conducted over the past century in cooperation with NC State University.

Last year, 353 soil samples were analyzed from Caldwell County by the NC Department of Agriculture soil lab. This is up a little from the previous two years. In 2015 there were 333 soil samples and in 2016 there were 303 samples analyzed.

We recommend taking soil samples every three years to maintain correct nutrient levels in lawns and gardens. Over the last three years, 2015, 2016, and 2017 there have been 989 soil samples analyzed representing 989 locations (gardens, yards, and fields) in Caldwell County. These 989 locations represent a significant portion of the cultivated areas in Caldwell County.

With all this data in hand, I thought it would be interesting to see what it indicates about Caldwell County soils.

Soil pH is important for nutrient uptake by the roots from the soil. Not surprisingly, pH has a major effect on plant growth. Nutrients in the soil and added fertilizer are both equally affected by pH.

Low soil pH causes some nutrients to become chemically unavailable to plant roots.The ability of legumes to fix nitrogen is reduced with a low soil pH. A high soil pH is just as detrimental to plant growth. High soil pH also affects nutrient availability. Typically trace elements such as iron, manganese, copper, and zinc are most affected by a high pH.

The ideal soil pH for nutrient availability is 6.5. For practical purposes, a soil pH of 6.0 to 6.5 is ideal for most gardens and crop fields in Caldwell County. I say most because there are always exceptions. The typical acid-loving plants around here are blueberries, azaleas, and rhododendrons. A pH of 5.0 to 5.5 is a good range for these plants.

I used a spreadsheet to calculate the average soil pH from the 2017 Caldwell County samples. The average came out to be 5.9. This is actually much than I expected. However, the pH range of all the samples went from 4.6 on the low end to 8.5 on the high end. It turns out that 74% of the samples had pH values that were outside the ideal range of 6.0 to 6.5. Over half the samples had a low soil pH, but surprisingly 20% had a soil pH too high for optimal plant growth.

Phosphorus and potassium, commonly referred to as potash, are important macronutrients for plants. Just over half, 51%, of the samples were low in phosphorus. Potash is typically not as lacking in our soil. Only 20% had low potash.

When looking at all the soil samples, only 13% of all the samples had a pH in the optimum range with adequate phosphorus and potash.

This quick study of the 2017 soil sample data shows we often have goldilocks issues. Sometimes our pH too low and sometimes it is too high. And occasionally our soil pH is just right.

I can say with confidence that testing is a good idea. There are no blanket statements that can be made about Caldwell County soils.

 Virginia Lopez demonstrates how to take a proper soil sample with a soil probe.

Virginia Lopez demonstrates how to take a proper soil sample with a soil probe.

It will pay dividends to know the pH and nutrients in your soil. And take the time to soil test your soil.

If you stop by the Caldwell County Cooperative Extension Center at 120 Hospital Ave, Lenoir, we have the tools and materials to conduct a soil test. There is a $4 charge for soil samples now, but after April 1st there is no fee.