Reducing soil pH in landscapes

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Alkaline soil conditions can result in chlorosis (yellowing) in many landscape trees and shrubs. Typically this situation is due to reduced uptake of key nutrient elements as soil pH increases. This commonly occurs with pin oak (reduced iron uptake) and red maples (reduced manganese uptake) and many conifer species. The obvious solution, of course, is to follow the mantra of ‘right tree-right place” and avoid planting alkaline sensitive trees on high pH soils. But what if you’re dealing with an existing tree that is chronically chlorotic? Lowering soil pH is possible, within limits, and alkaline-induced chlorosis can be reduced by addition of elemental sulfur or ammonium sulfate (see table). Elemental sulfur is available in most garden centers; finding ammonium sulfate may require a trip to a fertilizer co-op.

In looking at the table, there are several key factors that we need to note about reducing pH:

  • Small pH changes (0.5 pH unit or less) are feasible in many cases. Larger pH changes require exponentially more product to achieve the desired result. If you go back to your high school chemistry, you’ll recall that pH is the logarithm of the reciprocal of hydrogen ions in a solution. Don’t remember that? Well, put it this way, you’d need to spread three 50 lb bags of ammonium sulfate over an 11 yd x 11 yd area to reduce the pH of a loam soil by 2 pH units. Do not try this at home.
  • Clay soils are more resistant to change than sandy soils. As we’d expect, clay soils are more chemically active and buffered than sands.
  • Ammonium sulfate will produce a faster shift than sulfur. Ammonium sulfate will also provide nitrogen which may also result in a faster green-up.
  • Lastly, and most importantly, the pH reducing effect of both products is transitory. Due to the inherent chemical properties of the parent material, alkaline soils will gradually return to their initial pH. This means soils will likely need re-treatment every two or three years.

Amount of elemental sulfur or ammonium sulfate required to reduce carbonate-free soil to a target pH of 6.5 to a 7-inch depth

Elemental sulfur
Approx. lbs of product per 1,000 sq ft
Initial pH
Sand
Loam
Clay
8.5
45
60
70
8
30
35
45
7.5
12
18
23
7
2.5
3.5
7
 
 
 
 
Ammonium sulfate
Approx. lbs of product per 1,000 sq ft
Initial pH
Sand
Loam
Clay
8.5
120
150
180
8
70
90
120
7.5
30
47
60
7
6
9
18

Adapted from A & L Labs Fact Sheet No. 28 http://al-gl.com/pdf/factsheets/ALGLFS28_Reducing_Soil_pH_Field_Crops.PDF

So, to summarize, small pH shifts can be achieved by additions of sulfur or ammonium sulfate. Fortunately small reductions in pH can often make a big difference. MSU Extension Educator and Master Gardener Coordinator Mary Wilson notes that she eliminated chlorosis in a red maple in her yard with regular applications of sulfur that have resulted in a pH drop from 7.7 to 7.2. (Note: when asked why she planted a red maple at that starting pH, Mary was quick to blame her husband, “He really wanted that tree.”).

Related Events

Related Articles

Related Resources