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September 2017
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Episode 170 Soil Ph Demystified

What is PH? – Power of hydrogen

 What’s really being measured is the concentration of hydrogen (H) ions — the more hydrogen ions there are, the more acid the thing being measured is.

An anion (−) (AN-eye-ən), from the Greek word ἄνω (ánō), meaning “up”, is an ion with more electrons than protons, giving it a net negative charge (since electrons are negatively charged and protons are positively charged).

A cation (+) ( KAT-eye-ən), from the Greek word κατά (katá), meaning “down”, is an ion with fewer electrons than protons, giving it a positive charge. Since the charge on a proton is equal in magnitude to the charge on an electron, the net charge on an ion is equal to the number of protons in the ion minus the number of electrons.

Sources of Acidity

Acidity in soils comes from H+ and Al3+ ions in the soil solution. While pH is the measure of H+ in solution, Al3+ is important in acid soils because between pH 4 and 6, Al3+ reacts with water (H2O) forming AlOH2+, and Al(OH)2+, releasing extra H+ ions. Every Al3+ ion can create 3 H+ ions.

  • Rainfall: Acid soils are most often found in areas of high rainfall. Excess rainfall leaches base cation from the soil, increasing the percentage of Al3+ and H+ relative to other cations. Additionally, rainwater has a slightly acidic pH of 5.7 due to a reaction with CO2 in the atmosphere that forms carbonic acid.
  • Fertilizer use: Ammonium (NH4+) fertilizers react in the soil in a process called nitrification to form nitrate (NO3), and in the process release H+ ions.

Na+, K+, Ca2+, Mg2+ and Cl cause salinity.

  • Plant root activity: Plants take up nutrients in the form of ions (NO3, NH4+, Ca2+, H2PO4, etc.), and often, they take up more cations than anions. However plants must maintain a neutral charge in their roots. In order to compensate for the extra positive charge, they will release H+ ions from the root. Some plants will also exude organic acids into the soil to acidify the zone around their roots to help solubilize metal nutrients that are insoluble at neutral pH, such as iron (Fe).
  • Weathering of minerals: Both primary and secondary minerals that compose soil contain Al. As these minerals weather, some components such as Mg, Ca, and K, are taken up by plants, others such as Si are leached from the soil, but due to chemical properties, Fe and Al remain in the soil profile. Highly weathered soils are often characterized by having high concentrations of Fe and Al oxides.

Sources of Basicity

Basic soils have a high saturation of base cations (K+, Ca2+, Mg2+ and Na+). Alkaline soils are characterized by the presence of carbonates.

Too Much Acidity

Plants grown in acid soils can experience a variety of symptoms including aluminium (Al), hydrogen(H), and/or manganese(Mn) toxicity, as well as potential nutrient deficiencies of calcium (Ca) and magnesium(Mg).
Aluminium toxicity is the most widespread problem in acid soils. Aluminium is present in all soils, but dissolved Al3+ is toxic to plants; Al3+ is most soluble at low pH, above pH 5.2 little aluminum is in soluble form in most soils. Aluminium is not a plant nutrient, and as such, is not actively taken up by the plants, but enters plant roots passively through osmosis. Aluminium damages roots in several ways: In root tips and Aluminium interferes with the uptake of Calcium, an essential nutrient, as well as bind with phosphate and interfere with production of ATP and DNA, both of which contain phosphate. Alminium can also restrict cell wall expansion causing roots to become stunted.

Below pH 4, H+ ions themselves damage root cell membranes.

In soils with high content of Manganese (Mn) containing minerals, Manganese toxicity can become a problem at pH 5.6 and below. Manganese, like aluminum becomes increasingly more soluble as pH drops, and Manganese toxicity symptoms can be seen at pH’s below 5.6. Mn is an essential plant nutrient, so plants transport mangnese into leaves. Classic symptoms of manganese toxicity are crinkling or cupping of leaves.

Why is pH is so important in gardening? Because soil acidity or alkalinity directly affects plant growth. If a soil is too sour or too sweet, plants cannot take up nutrients like nitrogen (N), phosphorus (P) and potassium (K). And plants need specific amounts of those compounds–just like we need proteins, carbohydrates and vitamins to grow–to thrive and fight off disease and stress. Let’s look at it another way…

Most nutrient deficiencies can be avoided between a pH range of 5.5 to 6.5, provided that soil minerals and organic matter contain the essential nutrients to begin with.

Plant Preferences for pH

Very acid
(pH 5.0 to 5.8)

Moderately acid
(pH of 5.5 to 6.8)

Slightly acid
(pH 6.0 to 6.8)

Very alkaline
(pH 7.0 to 8.0)

Irish potato
spinach beet
wild strawberry
sweet potato
white birch
Brussels sprouts
carrotcollard greens
lima bean


beetbok choy
Swiss chard
Chinese cabbage
date palms
dusty miller
salt cedar

 Many environmental factors, including amount of rainfall, vegetation type and temperature can affect soil pH. Here are some general guidelines:

  • Areas with heavy rainfall and forest cover have moderately acid soils.
  • Soil in regions with light rainfall and prairie cover tend to be near neutral.
  • Areas of drought and desert conditions tend to have alkaline soils.
  • The pH of cultivated and developed soils often differ from that of native soil. During construction, for example, the topsoil may be removed and replaced by a different type. Hence, your garden soil pH could be very different from your neighbor’s.

To RAISE the soil pH
(Translation: If you have acidic soil)

If your soil is too acid, you need to add alkaline material. The most common “liming” material is ground limestone. Ground limestone breaks down slowly, but it does not burn plants like “quick lime” does. Apply it to the garden and lawn in the fall to allow time for it to act on soil pH before the next growing season. A rule of thumb for slightly acid soils: apply 5 pounds of lime per 100 square feet (say a 5 x 20-foot raised bed) to raise the pH by one point.

Apply limestone: 5 pounds per 100 square feet

Applying wood ashes also will raise soil pH. Wood ashes contain up to 70 percent calcium carbonate, as well as potassium, phosphorus, and many trace elements. Because it is powdery, wood ash is a fast-acting liming material. Be careful, a little goes a long way. Limit your application to 2 pounds per 100 square feet and only apply it every other year in a particular area.


To LOWER the soil pH
(Translation: If your soil is too alkaline)

In this case, you need to add a source of acid. Options include pine needles, shredded leaves, sulfur, sawdust and peat moss. Pine needles are a good source of acid and mulch. Peat moss with a pH of 3.0 is often recommended as a soil additive. Before you use it though, consider the other options, because peat moss is nutrient-poor, expensive, and it’s a nonrenewable resource.

7 comments to Episode 170 Soil Ph Demystified

  • Jim

    Adding onto the pH discussion, I agree it is important to understand pH as a stand alone measure for plant success, but understanding the relationship between plants, bacteria, and fungi is argueably more important in the soil make-up.  All three participate in the ionic exchange (both hydrogen and nutrient) and the cumulative relationship helps contribute to the pH and affects what microorganisms are present in the soil.  The sum total plus soil texture dictates the success of a given plant.  It’s too bad there is no quick measure to gauge that ‘web’.  There is the lesser known measure of fungal to bacterial biomass (F:B ratio) that can hone in specific types of plants, but that has to be shown through lab soil tests.

    As for acidifying soil, you’re right about the mythology.  Many folks really keep propagating that pine needles in all formats when used as mulch or amendments instantly move the pH needle into acid land significantly.  I don’t think there are any peer-reviewed studies that show pine needles/straw/mulch as ammendment or mulch work to lower pH significantly at all.  This study ( http://www.ars.usda.gov/research/publications/publications.htm?SEQ_NO_115=223401 ) using both pine straw and pine bark show no effect on pH in field tests, but still recommends using pine straw for container gardening to some extent. There is some thought that since limestone is constantly dissolving in most local soils, that offsets most application rates of pine mulches.  Also, if green needles are used, you introduce not only the terpenes discussion, but also resin acids.  All of it again is hard to find any good peer-reviewed data, and again more myths.

    Using Peat Moss as a general acidifying ammendment is bio-sterile, so it adds less value (beyond what you already listed). 

    There really is no self-sufficient amendments to significantly acidify local soils if they need to be.  Best to container grow if possible. The only way to lower pH significantly is sulphur powdered or pelletized sulphurs.  There are wet formulations, but can be harder to apply significantly.

    My experience (and as you also say) has been that most every vegetable works well if you just use homemade leaf/grass/manure compost as an amendment over time.  The compost adds to the bacteria, fungus and microorganism populations, which manipulate the pH. Your soil ends up in the 5.5-7.0 range on its own, with a majority at 6.5, able to grown almost any garden vegetable adequately. 

    I agree with your conclusion.

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  • Blitz1976

    What is the net negative for blueberries if the soil insnt acidic enough? Will it just be lower blueberry yield, gree growth, root growth..all of the above? I transplanted 6 blueberries last year into my soil which is probably in the 6 for the most part, we had a dry summer and I really didnt water them so I think that killed some of them, but the ones that remain havent really gone crazy from a growth standpoint. Any ideas? Good show Jason.


  • Blitz1976

    Forgot to add to email notification…sorry.

  • Jason

    Thanks Jim.


  • Jason


    The net negative as I understand it is yes reduced growth rate, reduced fruit set.  If my assumption is correct the acid is there to help the plant dissolve the minerals it needs.  Hot and dry climates don’t really do well with blueberries.  I’ve tried to limit my plantings in this arena.  Keep it mulched add compost and if you really can’t get that to work try some aluminum sulfate. 
    Thanks Bro!Jason

  • Blitz1976

    Kinda what my game plan was, I just hate watering 🙂 Im stubborn like that!