[NAFEX] pH question

dmnorton at royaloakfarmorchard.com dmnorton at royaloakfarmorchard.com
Wed May 2 08:05:15 EDT 2007

Options to lower soil ph include sphagnum peat, elemental sulfur, aluminum sulfate, iron sulfate, acidifying nitrogen, or organic mulches. Below is an article I had on hand that may help:

An excellent way to lower the pH of small beds or garden areas is the addition of sphagnum peat. (The pH of Canadian sphagnum peat generally ranges from 3.0 to 4.5.) Sphagnum peat is also a good source of organic matter. On small garden plots, add a one to two inch layer of sphagnum peat and work it into the top 8 to 12 inches of soil before planting. The addition of sphagnum peat to large areas would be cost prohibitive. 

Granular sulfur is the safest, least expensive but slowest acting product to use when attempting to lower your soil's pH. The table below shows the pounds of sulfur needed per 100 square feet to lower the pH to 6.0 in the top 6 inches of soil. 

      Soil pH Sandy Loam Loam Clay loam 
      5.0 8 10 15 
      5.5 6 8 10 
      6.0 3 4 6 

To avoid plant injury, don't exceed 2 pounds of sulfur per 100 square feet per application. Wait at least 3 months to make another application. 

Aluminum sulfate and iron sulfate react more quickly with the soil than elemental sulfur. However, aluminum sulfate and iron sulfate must be applied at a 5 to 6 times greater rate. Do not apply more than 5 pounds per 100 square feet of aluminum or iron sulfate at any one time. Excessive amounts of these two sulfates can also injure plants. 

Some types of fertilizers can help to acidify the soil and most of them are safe to apply. Acidifying fertilizers include ammonium sulfate, diammonium phosphate, monoammonium phosphate, urea, and ammonium nitrate. Read the label on the fertilizer bag to determine if it is an acidifying fertilizer. 

Research suggests that wood chips as a surface mulch may actually allow greater nutrient absorption by some trees. Spread a layer about three inches thick at least out to the dripline. Each spring add more mulch to keep the depth at about three inches. 

If the pH of your soil is greater than 7.5, then the soil may contain a large amount of free calcium carbonate. This compound strongly resists changes in soil pH. Lowering the pH becomes difficult or impractical on soils that have a pH above 7.5. 

The pH of highly acidic soils can be raised by incorporating limestone into the soil. Hydrated lime works quicker, but over liming is more likely. The table below shows pounds of ground limestone needed per 100 square feet to raise the pH to 6.5 in the top 6 inches of soil. 

      Soil pH Sandy loam Loam Clay loam 
      5.0 8 10 15 
      5.5 6 8 10 
      6.0 3 4 6 

Wood ash will also raise the soil pH and make the soil more alkaline. Do not apply wood ash, limestone, hydrated lime, or other liming materials to alkaline soils. 

Modifying a soil's pH is usually a slow process and may require repeat treatments. It is often most effective to use a combination of treatments. However, don't expect a quick fix or a miracle cure. 

This article originally appeared in the April 6, 1994 issue, pp. 1994 issue, pp. 42-43. 


      Prepared by Eldon Everhart, Department of Horticulture
      Iowa State University, Ames, Iowa. 

Dennis Norton
Royal Oak Farm Orchard
  ----- Original Message ----- 
  From: Jerrydana5 at aol.com 
  To: nafex at lists.ibiblio.org 
  Sent: Tuesday, May 01, 2007 1:43 PM
  Subject: [NAFEX] pH question

  I have put a lot of horse manure on my garden spot, with the result that it is very fertile, but the pH is too high, due to (I was told) the amount of horse urine, which produced Ammonium hydroxide, which raised the pH too much.  Besides sulfur any suggestions for what to put on it?  I know pine needles is a possibility.  How about chicken manure, since I suddenly have a source of some?  How long should it be composted before spreading?  Jerry in southern Indiana

  See what's free at AOL.com. 
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