Raising and Lowering pH Values Based on a Soil Test
Professor DeBacco
Know What Your Plant Prefers
The optimum pH is based on the plant species you are growing.
Most garden plants prefer a soil pH between 6.0 and 6.8.
This includes cannabis, however hydroponic growers tend to favor 5.5-6.5
Notable exceptions…
Acid-loving blueberries prefer a pH of 4.5 to 5.3.
Potatoes varieties without scab resistance are also grown at a lower pH (5.2 to 5.4) to inhibit the growth of this disease organism.
Region Impacts pH
Geographic area can also influence native soil pH
General soil pH by region…
East Coast: 4.8 - 5.5
Midwest: 6-6.5
SW: 7.5-8.5
Getting to Your Plants Optimum pH
You will likely need to raise or lower your soils pH to reach the optimum levels for your plants.
Raising Soil pH (lime)
Ground limestone is commonly used to raise a soils pH
Soil testing labs will offer recommendations that is based on the actual pH, plants being grown and the buffering capacity of the soil
Typically the lower the pH and the more clay and organic matter in the soil the greater the amount of limestone required to raise the soil pH to the targe level.
Limestone Applications
Limestone recommendations by the soil lab are made in tons/acre or pounds/100 or pounds/1000 square feet, so know the square foot area of your growing space.
Limestone needs to react with the soil so it is highly recommended to be tilled into the soil.
If you are not going to incorporate the lime than apply no more than 50 - 75 lbs/1000 square feet (5 -7.5 lbs/100 sq. ft.) to the soil surface at one time.
Reapply at one- to six-month intervals until the total recommended amount is administered.
Limestone Options Powdered vs Pelletized
It will take several months to a year for the soil pH to increase.
However, it is possible to move soil pH a full point even over winter months is powdered lime is applied and tilled into the soil.
Powdered vs Pelletized
Powdered lime will be quicker acting, but is more difficult to apply
Pelletized lime is easier to apply, but may take longer to adjust pH
This is essentially the same lime, but is inside a small coating to make application much easier, so there is a slight delay as this coating dissolves.
Limestone Options Dolomitic vs Calcitic vs Hydrated
Many growers think there is just lime, but there are three types…
Dolomitic limestone: (Calcium carbonate + Magnesium carbonate)
Most commonly found
Contains elevated levels Mg
Which can be an added bonus
Calcitic limestone: (Calcium carbonate)
Often called “hi-cal lime”
Lacks high levels of Mg
Used if Mg levels are determined to be optimum
Hydrated Lime: (Calcium hydroxide)
Quick acting lime
May change pH quickly, but does not have a long residual change as the other lime options
Mucus membrane irritant, so be careful if you are applying it
Lowering Soil pH
Sulfur or Ferrous sulfate is often recommended is used to lower pH
The recommendations by the soil lab are made in tons/acre or pounds/100 or pounds/1000 square feet, so know the square foot area of your growing space.
Typically the higher the pH and the more clay and organic matter in the soil the greater the amount of material is required to lower the soil pH to the target level.
Both of these compounds need to react with the soil so it is highly recommended to be tilled into the soil.
If you are not going to incorporate the sulfur than apply no more than 400lbs/acre to the soil surface at one time to established plants per year.
Sulfur vs Ferrous sulfate
Elemental Sulfur (S)
Spring application and incorporation work best since the acidification process is based on soil biology and not a chemical reaction.
Soil temperature should be above 55F and have good aeration so the bacteria are active.
If soil is anaerobic (water logged) the sulfur gets converted to hydrogen sulfide and can damage plants.
Ferrous sulfate
More costly to use than sulfur
Eight times more ferrous sulfate is needed than elemental sulfur.
It is quicker acting than sulfur
Avoid Aluminum Sulfate and Sulfuric Acid
Aluminum sulfate can be toxic to certain plants (ex. Blueberries) if high rates are applied.
Irrigating with dilute sulfuric acid can lower the soil pH, but it can damage plants and also corrode metal pipes and fittings in irrigation systems.
Follow Lab Recommendations
The soil lab you select will know not only the pH of your sample but the buffer capacity (resistance to ph change) and soil type.
Utilizing all of these factors will help produce a good starting point for correcting your soils pH.
Remember the lab is only as good as the sample you provide them so make sure it is representative of the entire growing area and not just a single site to ensure the pH adjustment will maximize plant performance.
Link to Lecture Slides:
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