What is pH? pH is the measure of the amount of H+ ions in the soil solution. It is measured on a scale that ranges from 1 (VERY acid) to 14 (Very alkaline). This is a ‘log scale’, which means each increase in pH of 1 is a 10-fold increase in the concentration of H+ ions. A low pH soil (acid) has greater amounts of H⁺ ion compared to OH^– whereas a high pH soil has the opposite with high amounts of OH^– and lower amounts of H⁺.

pH = log 1/[H+] 

Soil pH= 5                 0.00001 H+ in 1L of water 

Soil pH= 6                  0.000001 H+ in 1L of water 

An increase in pH of 1 is a ten-­fold increase in the amount of H+ ions in the soil! Even though the number may seem small, the impact on soil chemistry is huge! 

Why does it matter? The soil pH directly affects how chemicals (including fertilizers and nutrients released from organic matter) will react in the soil. Each nutrient has a pH range in which it is most available for plants to take up, therefore, as the pH of a soil changes, the availability of nutrients changes. The ideal pH for cranberries is 4.2-5.5, which is quite acidic com- pared to other crops. If the soil is higher than 5.5, the availability of nutrients dramatically changes. Calcium, magnesium and potassium become much more available and can become ‘toxic’ by creating nutrient imbalances in the soil (Figure 1).

High pH can also affect availability of nitrogen (N). Cranberries take up N primarily in the form of ammonium (NH4⁺), which is the form of N present in an ideal cranberry soil (pH 4.2-5.5). As soil pH increases, the ammonium in the soil is converted to nitrate (NO3^–) which is not taken up by cranberries as effectively and is leached out of the soil more quickly.

Transformation of ammonium (NH4+) to nitrate (NO3-) with increasing soil pH. 

If the pH of your soil is not in the appropriate range for cranberries, it will be very difficult to meet the nutritional demands of the plant. In a high pH soil, the plant does not receive appropriate amounts of nutrients because they are in the wrong form, not because they are not present. Adding nutrients will have little to no effect until the pH has been corrected. Managing soil pH should be the first step in managing your crop. 

Lowering soil pH with sulfur. It is possible to adjust the soil pH by addition of amendments. Lime can be added to the soil to raise the pH and sulfur can be added to lower the soil pH. When sulfur is added to the soil, bacteria in the soil convert the sulfur to sulfate. There are few key factors that should be considered when adding sulfur to adjust soil pH.

1. It takes time! The speed at which the soil pH decreases depends on how many bacteria are in the soil and how quickly they are working. When you first add sulfur to the soil, the amount of bacteria in the soil is relatively low and it will take time for populations to increase. The rate at which the conversion happens will also depend on the soil temperature. Bacteria are not very active in cool soils, therefore sulfur applications in the fall and spring will take longer to effect soil pH due to cool soil temperatures. Do not wait to address pH issues, this should be done as soon as possible. 

2. Oxygen is needed. Bacteria require oxygen to convert the sulfur. If conditions are very wet, or you are planning to flood, you should delay the sulfur application until after the flood. 

3. This is usually not a one-time fix. Often, marshes that require sulfur applications to lower the soil pH will need to be adjusted on a regular basis. This is just one more good reason to make soil sampling part of your regular routine. It is also important to consider the pH of the water being applied to the marsh. High pH water will continuously add carbonates to the soil which will raise the soil pH over time.

4. Apply no more than 100-500 lb granular elemental S/a annually, and if you have never used the material before, use lower rate, 100-200 lb/a. In sandy soils, a small amount of S can produce large reduction in pH, so splitting the application into 2 or 3 is a good strategy to avoid excessively lowering the soil pH. 

This article was posted in Cranberry and tagged CCMJ, Cranberries, Soil pH.