In November, my soils tested near a pH of 8 due to the limestone parent and its alkaline calcium contributions. I figured that laying down partially composted horse manure with its pine bedding, mulching it, and letting it compost for another 6 months would help buffer the pH into more manageable levels. Knowing that the blueberry bushes were set to arrive later in the week, I used my quick and imprecise soil kit:

Uh oh!

It is apparent that more action is required.

What acidifying soils actually does:

For some reason landscape designers absolutely love putting Pin Oaks in the soil islands in parking lots. No consideration is given to the requirement of these trees to grow in low pH soils. As a result, the trees can’t obtain iron through its roots and wind up chlorotic, or yellowing sick leaves year round. This is the same condition that blueberries will face in too alkaline of soil.

Remember anions and cations from high school chemistry? I’ll spare the most gruesome of details regarding what is going on in soils with varying pH. An alkaline soil is high in cations like calcium or magnesium effectively ties up necessary nutrients in soil like iron due to a low anion exchange capacity.

Further exacerbating the iron uptake issues is that iron is immobile meaning that within a plant, iron cannot be moved from old growth to new growth. The iron requirement in every bit of growth of new leaves, fruit, buds etc. must come from the soils. That is why pH is such a big deal with plants like blueberries!

Acidifying soil strategies:

Attempts to acidify soils can be broken into two categories: Temporary and Slightly Less Temporary. Biologically healthy and active soils have tremendous buffering capabilities in regards to pH. Whatever the motivation to grow a plant like blueberries in alkaline soils like I am doing, the gardener needs to understand that the soil will need constant maintenance to maintain a low pH.

Slightly less temporary acidification means

Slow release soil amendments will release acid as the soil microbes process the parent material of the amendment. The only organically-approved material here is elemental sulfur. While certainly not biologically organic as one would intuitively assume, it still gets organic certification because is is nothing more than a basic element off the periodic table that is found in soils. Application rate depends on the current pH and the structure of the soil. The below table provides a guide with units of pounds per acre. The table below contains the figures once I converted to pounds per 1000 square feet (source):

 

Table 1. Rates of elemental sulfur required to decrease soil pH to a depth of 6 inches.
Desired change in pH	Application rate based on soil texture1
	Sand	Silt loam	Clay
	———————– lb S/A ———————-
8.5 to 6.5	370	730	1460
8.0 to 6.5	340	670	1340
7.5 to 6.5	300	600	1200
7.0 to 6.5	180	360	720
8.5 to 5.5	830	1660	3310
8.0 to 5.5	800	1600	3190
7.5 to 5.5	760	1530	3050
7.0 to 5.5	640	1290	2580
1 Assumptions—cation exchange capacity of the sandy loam, silt loam, and clay soil are 5, 10, and 20 meq/100 g, respectively; soils are not calcareous.

 

Table 1. Rates of elemental sulfur required to decrease soil pH to a depth of 6 inches.
Desired change in pH	Application rate based on soil texture1
	Sand	Silt loam	Clay
	———————– lb S/1000 sq. ft ———————-
8.5 to 6.5	8.5	16.8	33.5
8.0 to 6.5	7.8	15.4	30.8
7.5 to 6.5	6.9	13.8	27.5
7.0 to 6.5	4.1	8.3	16.5
8.5 to 5.5	19.1	38.1	76.0
8.0 to 5.5	18.4	36.7	73.2
7.5 to 5.5	17.5	35.1	70
7.0 to 5.5	14.7	29.6	59.2
1 Assumptions—cation exchange capacity of the sandy loam, silt loam, and clay soil are 5, 10, and 20 meq/100 g, respectively; soils are not calcareous.

 

 

As sulfur takes months to have an effect of soil pH, I regret not applying it in November with the installation of the compost. I did rent a tiller and worked 8 pounds into my 80 square foot blueberry bed. I will continue to topdress with elemental sulfur (under the mulch layer). Now I’ll have to work slowly and carefully with the living plants using temporary means while the sulfur goes to work.

Peat moss is another amendment that has been proven to significantly boost blueberry yields. Personally, I don’t think peat moss deserves a place in horticulture as it is a not renewable resource and is extracted from fragile ecosystems. Here is a good article on the subject.

Temporary acidification means

Acidifying fertilizers are one method to maintain soil pH. As there is a large livestock feedmill nearby, I use 50+ pounds sacks of cottonseed meal ($12) as a fertilizer that both provides nitrogen and acidifies soil as it breaks down. It is pelletized which I initially did not like, but after the first rain since applying 200 pounds of it, the pellets explode in volume and hold quite a bit of moisture.

Acidic irrigation is another part of my strategy. I use 2 tablespoons of the cheap distilled white vinegar per gallon of water to irrigate the root zones. I use a much more aggressive rate to apply between plants where no roots yet exist. I repeat this weekly as vinegar only temporarily ties up calcium! Oak tea is an experiment I am currently trying to find a more biological approach than distilled vinegar.

Acidic mulching: mulching materials like pine straw/needles have long been recommended to help acidify soils. While acidifying soils has tossed some obstacles to my biological approach to raising food crops, I still want to mulch in a manner that promotes mycorrhizal fungal dominance that all woody species need. Therefore I mulch with shredded wood from tree tops but apply elemental sulfur below the mulch.

 

Conclusion

The best time to acidify soil is at least 2-3 months before planting. I missed the boat here and now have that added complications of working with living roots that will be burned by a direct application of strong acid. I hope to be able to gradually lower the pH of the soils before the bushes begin producing berries.