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Does your soil have a high pH? Fall is the best time to amend it.

Authors: Natalie Hoidal & Annie Klodd
The ideal pH range for growing fruits and vegetables is 6.0 - 7.0 (with the exception of a few specific plants such as blueberries, which thrive in acidic conditions). Outside of this range, nutrients become less available to your plants, even if they are abundant in the soil. This article will cover some strategies for improving the pH of your soil, including longer-term management practices to keep it in the ideal range.

Soils that are too acidic (below 6) can be amended with agricultural lime. A basic soil test will provide instructions for the amount of lime needed to adjust your soil. Soils that are too basic (above 7) also need to be amended, but a soil test will not provide specific guidance. A high pH in soil is caused by a few things:
  • Some soils simply have a naturally high pH (or a naturally low pH)
  • Soils that have received excess compost, especially composted manure, tend to have a higher pH due to the build-up of base cations
  • High tunnels sometimes increase in pH over time. Without rain water to wash nutrients through the soil, they can build up overtime, increasing the alkalinity of your soil. This tends to drive the pH up as well.

Acidifying your soil

Above a pH of 7.0, iron, boron, zinc, and copper availability to the plant is limited, even if there are adequate amounts in the soil.

The widely used "pH Nutrient Availability Chart" shows how the availability of nutrients changes according to the pH. Image from Roques et al., 2013

Just as you would add lime to acidic soil to raise the pH, you can add sulfur to basic soil to lower it. This is one of the least labor-intensive ways to reduce the pH of your soil, though it takes time. Often, annual or biannual repeat applications are necessary to actually maintain a lower pH. The basic concept is: you can add sulfur to your soil, ideally in the fall, and the microbes in your soil will break it down into sulfuric acid. This process takes time, and so by applying sulfur in the fall, you can start to reduce your pH before the next growing season.

For fruit crops, it is much easier to apply sulfur before planting than once the plants are established. Therefore we recommend reaching the correct pH range the year prior to planting and only doing additional “tweeks” later as needed.

The Nutrient Management Guide for Commercial Growers has a full list of sulfur products available to fruit and vegetable growers. Organic growers have a few options including elemental sulfur, calcium sulfate, potassium sulfate, and langbeinite.

The following rates are recommended for reducing the pH of your soil by one full unit (e.g. from 8 to 7):

  • In sand, loamy sand, and sandy loam soils, add 0.8 lb / 100 sq. feet, 8 lb / 1000 sq. feet, or 350 lb / acre.
  • In loam or silt loam soils, add 2.4 lb / 100 sq. feet, 24 lb / 1000 sq. feet, or 1045 lb / acre
  • Lowering the pH of clay soils is difficult due to their high buffering capacity. Because of potential salt build-up with acidifying amendments and poor internal drainage, lowering the pH of clay soils with sulfur is not recommended.
  • Many websites, including this UMN Extension page, mention the use of sphagnum peat to quickly lower soil pH for blueberries. While sphagnum peat can be used effectively, it is mined from a fragile ecosystem. Given the potential environmental ramifications, please consider using something else before resorting to sphagnum peat.
  • Do not use pine needles or coffee grounds to lower soil pH. Coffee grounds are not always acidic, and even when they are, it would take massive amounts to amend the soil. The amount of pine needles needed to meaningfully lower pH is also not realistic or feasible in an agricultural setting.
  • We do not yet have good recommendations about the amount of sulfur needed to bring down the pH of a high tunnel, as the right rate can depend on soil texture, moisture and calcium levels. For now, stick to the above recommendations based on your soil type.

If you're working on changing the pH of your soil, conduct annual soil tests around the same time each year to track your progress.

Acidifying your irrigation water

Looking ahead to the next growing season, you can adjust the pH of your soil by acidifying your irrigation water. This approach is especially relevant for growers with basic water, or with "hard" alkaline water, and even more so for high tunnels, which do not have acidic rain water moving through them to wash out soluble salts. Acidifying irrigation water is not necessarily a substitute for using sulfur to acidify your soil, but rather a supplemental approach.

Your irrigation water should have a pH of between 5 and 7, and alkalinity between 37.5 and 130 ppm (but a maximum of 65 ppm for seedlings). In order to effectively change the pH of your water, you need to know both the pH and alkalinity. pH is a measure of acidity based on the concentration of hydrogen ions, and alkalinity is a measure of the solution's ability to neutralize acids. Knowing your alkalinity is key to calculating the amount of acid needed to bring down the pH of your water. The University of Minnesota Soil Testing Laboratory offers water tests for $10, which include both pH and alkalinity. Once you have this measurement, the AlkCalc tool to determine the amount of acid you’ll need to add to your water. Conventional growers typically use phosphoric, sulfuric, or nitric acid, depending on local availability and price. While more concentrated acids may be more economical, they are also more dangerous to work with and should be handled with caution. Organic growers can use citric acid, which is typically less concentrated.

Preventing your soil from becoming more basic

  • Over-reliance on composted manure can result in the excessive build-up of cations in your soil, which drives increasing alkalinity and pH. Test your manure and make applications based on soil tests.
  • Composted manure is very different from composted vegetative material like crop residues, food scraps, leaves, grass clippings, etc. If you're using deep compost mulch systems, or generally relying heavily on compost in your farming operation, try to stick mostly to vegetative compost.
  • Take regular soil tests so that you can see how your pH is changing over time.
  • Flooding your soil a few times per year can help to wash salts out of your soil. This is especially important in tunnels and other covered environments that do not have access to rainwater. While removing the plastic on tunnels is not practical on a regular basis, it does need to be changed from time to time. During years when you plan to apply new plastic, consider leaving the tunnel open without any plastic covering for a few months.

This post was adapted from the webpage: How to correct problems caused by using too much compost and manure

Image source: S. Roques, Kendall, S., Smith, K., Price, P. 2013. Review of the non-NPKS nutrient requirements of UK cereals and oilseed rape. Technical Report.

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