Blueberry fields with pH ranges out of the optimum (4.2-5.5) are common in many areas of blueberry production. I know you all feel confident about applying elemental sulfur to reduce soil pH to the desired range, but what should you do if your soil pH drops below 4.5, and how that will affect blueberry plant growth?
Let’s first understand why soil pH keeps decreasing every season with fertilization in blueberry fields. We all know blueberries prefer ammonium-based fertilizer — among the largest percentage of cations taken up — as their nitrogen source,. This actually is the primary reason your soil pH keeps decreasing every season.
During the uptake of ammonium ions, blueberry plant roots excrete hydrogen ions, which accumulate in the soil to increase soil acidity. When urea is used, it will first break down to ammonium and carbonate by urease. This process takes one hydrogen ion away from the soil, so urea is less acidifying than ammonium sulfate fertilizer.
Then, ammonium may form ammonia (NH3), so soil pH close to the urea particles could increase temporarily. Because of the eventual uptake of ammonium by blueberry plants, the long-term effect of urea on soil pH will still be a reduction in soil pH.
Just Add Lime
What can you do to reduce the rate of soil pH drop in blueberry production? We can calculate the liming factor (or acidifying potential) of different fertilizers and add lime to neutralize it.
Unfortunately, the acidifying potential of these various nitrogen fertilizers is calculated based mostly on their nitrification in the soil and because blueberry plants take up most of the nitrogen fertilizer in the ammonium form, this traditional way of determining lime requirement may be of limited use for blueberries.
Since the majority of soil acidity in blueberry production is mainly due to root exudation of hydrogen ions into the soil by removal of cations, such as ammonium ion, calcium, magnesium, and potassium from the soil under low soil pH conditions, the soil acidification or the amount of lime required can be estimated by the ratio of base cation to nitrogen in plant tissues.
I estimated this ratio for blueberry plants is around 0.7-1.0. So take the high number of the ratio, for one acre of blueberries with a 20,000 lb. yield, about 170 lbs. of lime is needed to neutralize the soil acidification potential due to fertilizer uptake. This means it’s safe to apply about 150 to 200 lbs. of lime every fall before the next spring to stabilize your soil pH.
Apply In Autumn
The best time to apply lime is in fall or when winter starts. Also, the amount of lime mentioned so far is for band applications within the planting row. It is not the broadcast application rate. Remember, do not over-lime and do not add lime in the spring when applying fertilizers.
I know liming blueberry fields sounds really strange because blueberry plants love acid soils and are calcifuge (do not tolerate alkaline soil) plants. However, the benefits of adding some lime in the fall (not in the spring) to maintain a desired soil pH range have been practiced in Oregon’s Willamette Valley by some growers. The calcifuge nature of the blueberry plant really means its inability to survive in high pH soils and has nothing to do with lots of calcium in the soil.
If the soil pH in your blueberry field drops below 4.5 already, especially in upland or mineral soils, free soil aluminum could interfere with nitrogen, phosphorus and iron uptake, which leads to iron chlorosis. You can do two things to relieve this problem.
First, you will need to start bringing the soil pH up gradually. This could be a long process since lime moves into the soil profile slowly when surface applied. You can add 200 lbs. lime/acre in addition to the lime you need to neutralize the potential acidity produced by fertilizer uptake during the growing season. You can do this for a few seasons, and then wait a few more years to test the soil pH to see if more lime is needed.
Second, you can find ways to reduce the concentrations of free soil aluminum in the soil. Increasing soil organic matter such as sawdust mulch, compost material, and/or organic acids will reduce free soil aluminum concentrations over time.
Finally, I’d like to mention the difference between sulfur (S) and sulfate (SO4=) on soil acidity. There is a misconception about sulfate that it reduces soil pH. The reality is sulfate does not reduce soil pH.
When potassium sulfate or ammonium sulfate are applied as potassium or nitrogen fertilizer, the sulfate group in both fertilizers will either be taken up by the plants or lost to leaching, a process which does little to change soil pH.
When elemental sulfur is applied to the soil, however, it will be oxidized by sulfur oxidizers (bacteria) such as Thiobacillus species or other heterotrophic sulfur oxidizing fungus such as Aspergillus niger and Trichoderma harzianum. During this oxidation process, sulfuric acid is produced and soil pH is lowered. Interestingly enough, the rate of elemental sulfur oxidation increases with soil pH. This is because soil bacteria are more active at higher soil pH.
In summary, the reason for the gradual soil acidification in blueberry production is due to the action of blueberry roots, not the nitrification processes that happen in soil as usually assumed because the nitrification process is limited at low pH soils.