Want to worry less about micronutrients? Get your pH right for high tunnel tomatoes this off season

Published: August 1, 2011

I’ve been surprised by how many growers are concerned with micronutrient deficiency in high tunnel tomato production. The concern seems to center on boron and manganese, sometimes zinc. Why the concern? Is it justified? What to do about it? These will be addressed below. There is a lot of foliage and vines growth, which growers are advised to remove from the area for disease prevention. Due to the high yield and plant residue removal a lot of macronutrients (N, P, K, Sulfur, Magnesium, and Calcium) and micronutrients are being removed from the soil.

Why the concern?

• The foremost reason is because the production system of high tunnel tomatoes is very demanding of soil fertility. Growers are often picking 20 or more pounds per plant when training the plants up strings.
• Early season tomatoes are quite valuable; every pound counts!
• A soil test or a tissue analysis may indicate a micronutrient is lacking.

Is it justified?
Of course if a soil test or tissue test indicates a micronutrient is lacking, one should remedy the situation. But what is not as appreciated, is why this might be developing in the first place.

Micronutrients are generally in sufficient supply in most Missouri soils
Most producers are growing on soils that have ample silt and clay as primary soil components (Missouri is not known for sandy soils except along the major rivers and in the boot heel). Soils rich in clay and silt in Missouri generally have good levels of micronutrients.

Micronutrient availability is sensitive to high soil pH
As the soil pH approaches and goes over 7, most micronutrients become less available (the exception is Molybdenum). A soil test may recommend raising the level of a micronutrient because some of what is in the soil is not available at a higher pH. A tissue analysis indicating a micronutrient is lacking may mean that some of it is in an unavailable form for plant uptake. See the pH and nutrient chart on this page.

Missouri soils in high tunnel production are likely to increase in pH
High tunnel tomatoes are primarily irrigated; in Missouri well water frequently has a somewhat high pH and a bit of alkalinity, generally due to calcium carbonate. (Both are easily tested for and we have been providing these tests at no cost when we’ve been at the produce auctions this year and last) The more years a crop is grown in a high tunnel, the more water that has been applied, and the more risk the pH of the soil is to have increased. While surface water sources (e.g. ponds) are lower in alkalinity levels and/or pH, they are not commonly used for high tunnel irrigation.

What to do about it?
Growers should review their soil pH and the pH and alkalinity level of their irrigation water. If the pH is over 7, adding sulfur should be considered. If the pH is 6.7 to 7.0 AND the irrigation water has a pH over 7.0 or has an alkalinity level above 300 ppm, adding sulfur should be considered. Normal soil test recommendations are for field production and sulfur as a pH reducing amendment will not be recommended unless the pH reaches around 7.5 or 8.0, depending on the crop and soil test lab***. Most soil test labs do not have recommendations tailored for intensive high tunnel tomato production.

Finely ground elemental sulfur should be applied in the fall and incorporated. As with lime, it can take up to 6 months to have full effect. An ideal time to apply is just before or after seeding an off season cover crop.

How much to apply?
The best way to determine this is contact the soil test lab you used for your high tunnel sample. Give them the target pH you’d like for your soil, and they can calculate a specific rate. According to the Midwest Vegetable Production Guide for Commercial Growers the optimum range for tomato is 6.0 to 6.8, with 6.4 ideal. If the pH in your soil is likely to increase, shooting a little low (e.g. 6.2) might be a good idea. If you don’t have the time or a phone to call the soil test lab, below are two standard rates for finely ground elemental sulfur.

• To lower a clay loam soil pH by 1.0 (e.g. from 7.2 to 6.2)- apply 20 pounds per 1,000 sq feet
• To lower a silt loam soil pH by 1.0- apply 12 pounds per 1,000 sq feet

* When you want to increase calcium, but not raise soil pH, use gypsum
** We strongly discourage growers from using 'home kits' for determining their soil pH or nutrient levels; a quality soil sample should be submitted to a reputable soil test lab.
***Most soil test labs use a water based pH test. Some labs use a salt based pH test; the MU soil test lab uses the salt based test. To convert a salt pH test results to a water based pH test, add 0.5. E.g. an MU soil pH test of 7.0 is really 7.5 for a comparable water based test. The recommendations given in this article, and in publications like the Commercial Vegetable Production Guide for Commercial Growers, assume a water based soil pH test.

Dos and Don’ts reminders

Do not add lime unless is prescribed. It raises the pH.*
Do not add wood ashes, as they act similar to lime and increase the pH.*
Do take a soil test annually and follow the recommendations. Get a complete analysis, including the micronutrients. **
Do add micronutrients when recommended, but be sure to broadcast evenly over the entire soil area. Adding micronutrients to the soil in a band or any concentrated method can lead to micronutrient toxicity. Micronutrient toxicity is difficult to correct.
Do understand the pH testing method your soil test lab uses.***

pH and nutrient chart