HORTICULTURE

 

 

Lowering the Soil pH with Sulfur

Mark Longstroth, MSUE District Extension Educator, SW Michigan

 

 

With the popularity of blueberries many people are interested in quickly adjusting the soil pH downwards.  Blueberries prefer acid soils with a PH of 4.5 to 5.5.  When blueberries are grown in soils with a pH of 6 or more they do poorly.  The plants are often yellow due to cholorosis, an iron deficiency induced by the high soil pH.  Often new blueberry fields will have areas where the plants are yellow due to the soil conditions in that part of the field.   This yellowing may be due to wet soils or the soil pH is too high. 

The cheapest way to lower the soil pH is to add elemental sulfur to the soil and allow soil bacteria to change the sulfur to sulfuric acid, lowering the soil pH.  The MSUE publications; E-2006 Hints on Growing Blueberries and E-2011 Managing Highbush Blueberry Nutrition both discuss adjusting soil pH before planting blueberries.   In this article I discuss it a little more in depth because most people do not understand what is happening and expect the soil pH change to be immediate.  Acidifying soil is not an exact science; this posting is just a guide.

 

If soil pH is greater than 5.5, apply adequate elemental sulfur (S) to decrease the soil pH to 4.5 (see Table 1).  Spring application and incorporation work best.  Soil bacteria convert the sulfur to sulfuric acid lowering the soil pH.  It is important to note that this is a biological process (slow) and not a chemical reaction (rapid).  This process occurs when the bacteria are active, when the soil is moist and warm.  The soil temperature needs to be above 55F.  The bacteria are not active in the winter so fall applications of sulfur have no affect on the soil pH next spring.  In addition, the soil must not be flooded (anaerobic) or the sulfur is converted to hydrogen sulfide (rotten egg smell) by anaerobic bacteria.  Hydrogen Sulfide will kill the roots of plants including blueberries.  Irrigate if the field becomes dry to moisten the soil but do not over irrigate the soil.  This causes flooding and anaerobic conditions.  Most Michigan irrigation water is high in alkalinity (dissolved lime) and will gradually raise the soil pH.

 

The amount of sulfur required is very dependent on soil texture.  This is because clay and organic matter act as a buffer, absorbing and releasing mineral ions.  Relatively little sulfur is needed on sands, whereas soils high in clay or organic matter require much more.  It is important to apply and incorporate sulfur at least a year before planting to allow the sulfur time to react and lower the soil pH before planting. 

Large amounts of Sulfur cannot be incorporated to change pH quickly after plants are present.  If large changes in pH are needed, do not assume that the change can be completed in a short time.  Check soil pH again just before planting and apply additional sulfur if needed.  Do not apply more than 400 lb sulfur per acre to established plantings at one time.  When large amounts are needed, spread the application out over several years. 

Ferrous sulfate also decreases soil pH, but it is more costly to use than sulfur.  Eight times as much ferrous sulfate is needed than elemental sulfur since we are not creating an acid in the soil but adding a salt that disassociates into iron and sulfuric acid.  The iron binds to the clay or precipitates out of the soil solution leaving the sulfuric acid behind. Aluminum sulfate can also be used but it too is expensive and the are reports of aluminum toxicity when too much is applied.  Many people want to change the soil pH right now!  To do this you need to add acid to the soil.  Some growers have even irrigated with dilute sulfuric acid to lower the soil pH.

 

 

Many potential growers have a soil sample from their potential site that shows a high pH and need to know what type of soil they have to determine the amount of sulfur to apply to the site.  Soil samples give a value for the CEC or Cation Exchange Capacity.  The CEC can be use to estimate soil texture.  The table below gives typical CEC ranges for different soil texture types.

The CEC or Cation Exchange Capacity can be use to estimate soil texture. 

For example if your soil sample had a soil pH of 6.5 and a CEC of 12-meq/100 g, this suggests a loam soil. Table 1 in indicates over 2000 lb of S per acre would be needed to reduce pH to 4.5, lowering the soil pH 2 pH points.  This is a large shift in pH and unlikely to be accomplished quickly.

Ideally, 2,000#/A of sulfur would be applied early in the season the year before planting and the pH would be checked again in the Fall and the following Spring.  If the soil pH was below 5.5 then blueberries could be planted in the Spring.  If the soil pH was high in the Fall and still high in the spring more sulfur should be added before the fall planting of blueberries.

Since more than 400 lb S should not be applied at one time to established plants, correcting the pH here will require 5 years of 400 lb applications in the spring if the pH was not corrected before planting.  I doubt the poor growth of the planting while the pH is being corrected would justify the quick planting before the pH was in the 4.5 to 5.5 range.