High Tunnel Soil Test Report: Soil Nutrient Levels

Phosphorus (P), potassium (K), magnesium (Mg), and calcium (Ca) levels in a survey of 27 high tunnels across Pennsylvania.
High Tunnel Soil Test Report: Soil Nutrient Levels - News

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High tunnel tomatoes. Photo: Tom Ford, Penn State

Over the last year (2017), we have been working with 27 Pennsylvania farmers to better understand soils in high tunnels. Farmers sent soil samples from their high tunnels to Penn State’s Agricultural Analytical Services Laboratory (hereafter “the Lab”) for analysis. In this article, we are taking a look at phosphate, potash, magnesium, and calcium levels.

Fertility baselines for high tunnel crops have not yet been determined for Pennsylvania. In the interim, we are using field data for commercial vegetable crops as the baseline for all nutrient management decisions coupled with regular plant analysis (tissue testing.)

Soil test values reported for commercial vegetables from the Lab fall into the categories “deficient,” “optimum,” or “exceeds crop needs.” Crops normally produce best when nutrient levels fall in the “optimum” category. When nutrients are in the “deficient” category, adding those nutrients should increase yield. When nutrient levels are in the “exceeds crop needs” category, more than enough of a nutrient is present in the soil. Having nutrients in the “exceeds crop needs” category can be as bad as being in the “deficient” category. High soil nutrient levels might not only represent an economic loss, but may also result in crop, animal, or environmental problems.

Example from a soil test report showing the categories Deficient, Optimum, and Exceeds Crop Needs.

Phosphate values higher than about 321 lb/acre fall in the “exceeds crop needs” category. The range for the high tunnels surveyed was 289 lb/acre to 5038 lb/acre, and 26 of the 27 (96 percent) fell into the “exceeds crop needs” category. The average phosphate value was 1580 lb/acre. The Lab tracks the average nutrient value for commercial vegetable soil samples. These samples include soil from fields and high tunnels. The average phosphate level for the 1359 commercial vegetable samples analyzed in 2016-17 was about 751 lb/acre. This value also falls in the “exceeds crop needs” category and is about 52% lower than the average of the 27 high tunnels.

Potash values higher than about 336 lb/acre fall in the “exceeds crop needs” category. The range for the high tunnels was 228 lb/acre to 8215 lb/acre, and 23 of the 27 (85 percent) fell into the “exceeds crop needs” category. The average potash value in the high tunnels was 1282 lb/acre. The average phosphate value for commercial vegetable samples sent to the Lab in 2016-17 was about 550 lb/acre. This value also falls in the “exceeds crop needs” category and is about 57% lower than the average of the 27 high tunnels.

Magnesium (as magnesium oxide) values higher than about 398 lb/acre fall in the “exceeds crop needs” category. The range for the high tunnels was 515 lb/acre to 4977 lb/acre, and all (100 percent) of the samples were in the “exceeds crop needs” category. The average magnesium value in the high tunnels was 1619 lb/acre. The average magnesium value for commercial vegetable samples sent to the Lab in 2016-17 was about 717 lb/acre. This value also falls in the “exceeds crop needs” category and is about 56% lower than the average of the 27 high tunnels.

Calcium (as calcium oxide) levels are a bit more complicated because recommendations are based on the calcium level, pH goal, and exchangeable acidity. The range for the high tunnels was 3472 lb/acre to 21,040 lb/acre, and 23 of the 27 (85 percent) of the samples were in the “exceeds crop needs” category. The average calcium value in the high tunnels was 9290 lb/acre. The average value for commercial vegetable samples sent to the Lab in 2016-17 was about 5516 lb/acre. This value also falls in the “exceeds crop needs” category and is about 40% lower than the average of the 27 high tunnels.

Nutrient levels in these high tunnels largely exceeded crop needs. Averages for these nutrients in all commercial vegetable samples also exceeded crop needs, but not to the same extent as in the high tunnels. Phosphate, potash, and magnesium levels from high tunnels were over twice the values seen in all commercial vegetable samples.

As with soil pH , a link between using compost and nutrient levels exceeding crop needs did not exist. (See In high tunnels, nutrients can accumulate to “exceeds crop needs” levels when using compost, inorganic fertilizers, or other organic nutrient sources.

If nutrient levels in your high tunnels exceed crop needs, avoid adding more of these nutrients to the tunnels to start to correct the situation. Use other nitrogen sources, including granular and liquid forms, to meet crop needs. Look for sources with grades containing very low or no phosphorus or potash, for example, dried blood, feather meal, or urea.

Nitrogen needs still need to be met, when phosphate, potash, magnesium, and calcium levels are in the “exceeds crop needs” category.

For nitrogen, another option is using cover crops – typically legumes. For example, planting a red clover can provide 100 to 110 lbs of nitrogen per acre. Tables exist that can help to determine how much nitrogen can be added by various legume cover crops. You can find one on page B16 of the Mid-Atlantic Commercial Vegetable Production Recommendations  available to purchase as a hard copy. It is also available as a free pdf.

When using compost, be aware that most composts do not contain nutrients in balanced amounts as needed by plants. They can have an excess of phosphorus and potassium relative to plant demand for nitrogen. These nutrients can accumulate to levels exceeding crop needs with repeated applications or even a single large application.

This is not to say that you should not use compost, just that it is better used along with other nutrient sources. We have a publication for free download called Using Organic Nutrient Sources , which helps interpret soil test reports for using organic nutrient sources. It includes a formula (on page 11) for calculating how much compost to apply.

Over time, with the production of your crops the phosphate, potash, magnesium, and calcium levels should decrease. You can speed up the process by growing cover crops. The key with this method is to remove the cover crops from the tunnel versus turning them into the soil.

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