Photo: Tom Ford, Penn State
Over the last year (2017), we have been working with 27 farmers across Pennsylvania to get a better picture of the soils in their high tunnels. Farmers collected soil samples and sent them to Penn State’s Agricultural Analytical Services Laboratory (hereafter “the Lab”) for analysis. We are focusing on soil pH in this article.
Most vegetables grow best with a soil pH between about 6.0 and 7.0. Sweet potatoes and some white potato cultivars are exceptions and grow best with a soil pH around 5.2. pH uses a logarithmic scale which means that a 1 unit change is a 10-fold difference. For example, a pH if 8.0 is ten times higher than 7.0 and 100 times higher than 6.0. Out of the 27 high tunnels, 13 (48 percent) had a soil pH within the optimal range of 6.0 to 7.0, one (4 percent) was below the range, and 13 (48 percent) were above the range.
Photo: Tom Ford, Penn State
The Lab also tracks soil pH for all samples received for commercial vegetables. This would be a combination of vegetables grown in the field and high tunnels. Out of 1,359 samples in 2016-17, roughly half had a pH within the optimal range, similar to what we saw in the high tunnels. For the other half, about 40 percent were below the optimum range, and 60 percent were above the optimum range.
When soil pH was out of the optimal range in the high tunnels, it was almost always higher than 7.0 compared to the combination of field and high tunnel samples where about 40 percent of the time the soil pH was below 6.0 and 60 percent of the time above 7.0. The average soil pH in the high tunnels was 6.9. The average soil pH for commercial vegetable samples at the Lab was a little lower at 6.6.
(Each point on the graph represents the soil pH from an individual high tunnel. The optimal range for most vegetable crops falls between the two red lines.)
Five of the farms we have been working with are certified organic; the remaining 22 are conventional, with several farmers indicating that they use organic growing methods. Certified organic or conventional farming status did not seem to be linked with soil pH with an average of 7.0 from certified organic tunnels and 6.9 from conventional ones.
We wondered if there was a link between using compost in high tunnels and high soil pH. Fourteen of the farmers used compost in their high tunnels. Compost was used in eight tunnels with soil pH above 7.0, and five did not. From another perspective, compost was used in six tunnels with soil pH within the optimal range, and seven did not. A link between using compost and having a pH above 7.0 does not appear to exist. Other issues relating to compost, the amount or type used, for example, may play a role. We were not able to determine that.
Soil pH affects nutrient availability to plants. When pH is not in the optimal range, plants have a hard time getting the nutrients they need. That is why when the pH value is not within the optimum range; it is commonly the first thing we recommend addressing. If the soil pH in your high tunnel is out of the optimal range, it is likely high, and you may need to add sulfur to bring it down.
Below is a table from the Lab showing the amount of sulfur to add based on the soil pH you want. A good target is 6.5. For example, if your current pH is 7.5 and you want to change it to 6.5, you would add 1.25 pounds of sulfur per 100 square feet on a loam soil.
Table 1. Amount Of Sulfur Needed To Lower Soil pH To Optimum Level
|From Current Soil pH||To Optimum Soil pH||Sulfur lb/100sq ft|
Apply sulfur at the above rates for a loam soil. On heavier soil (silt loams) use one-third more than the amount shown. On lighter soils (sandy loams) use one-half of the amounts shown. If aluminum or ferrous sulfate is used to lower pH, multiply the above amounts by 2.5. Follow the same suggestions as above for soil types. If four or more pounds are needed, divide the amount in half and make two applications, six months apart.