You’ve got options for measuring and adjusting your soil’s pH: part II

Last post, I wrote about measuring your soil’s pH on your own. If you successfully measured pH, congratulations! Now you have to decide what to do with this information. For purposes of this post, I will focus on what to do when the pH is lower (more acidic) than you want it to be. If the pH is perfect for the crops you want to grow, sit back, relax, and wait until next year to measure again.

Lime (dolomitic or calcitic) is the most common amendment used to raise soil pH. The smaller the size of the lime (powder vs. pellets) the faster the reaction, though the reaction is not instantaneous. Photo credit: Wikipedia.

Lime (dolomitic or calcitic) is the most common amendment used to raise soil pH. The smaller the size of the lime (powder vs. pellets) the faster the reaction. The complete reaction may take months or years. Photo credit: Wikipedia.

Unfortunately, determining how much material to add to raise the pH the desired amount is trickier than just measuring the pH; a recommendation cannot be made without more information about your soil. Soils will resist, or buffer, changes in pH, and each soil has a different “buffer capacity.” This is largely a function of soil texture and organic matter, but because of the variability among soils, it is best to get recommendations based on lab results of your soil (from a lab in your region) or by talking with a knowledgable advisor or extension agent who knows the soils in your area.

The lab or advisor will likely tell you how much lime to add to raise the pH the desired amount, but what they probably won’t tell  you (and what I want to tell you here) is that there are options beyond lime for raising the pH of your soil or lessening the effects of acid soil.

Calcium carbonate is the traditional liming material. So much, in fact, that other “liming” materials are measured in terms of their “calcium carbonate equivalents” or CCE.  Depending on the calcium (Ca) and magnesium (Mg) levels in your soil, a lab or advisor may recommend dolomitic lime (Mg & Ca-carbonates) or calcitic lime (Ca-carbonate) to raise the pH of your soil. Both Ca and Mg are plant macronutrients that tend to be lost via leaching in acid soils, so adding them back in the form of lime also has a fertilizing effect.

Estimating lime requirements yourself

If you really don’t want to send in a sample and want to stick to the do-it-yourself method, you can approximate the amount of lime (or other material) needed based on the texture of your soil using a graph like the one below. The depth to which you want to change the pH is very important in determining how much lime (or other material) to add. In no-till soils and perennial crops or pastures, most acidification takes place near the soil surface. Get our your pH meter to figure out how deep into the soil you actually want to change the pH!

If you don’t know your soil texture, you can try estimating it by feel. I can’t tell you how many times someone has said to me “I’ve got a lot of clay in my soil” when in fact it’s a lot of silt… be careful about accurately judging soil texture.

lime by texture

If you know the starting pH of your soil, you can estimate how much lime you need based on the texture. This is to lime the soil to a depth of six inches. Source: The Nature and Properties of Soils, Brady and Weil.

Other “liming” products: wood ash

In states like Maine, soil labs may give additional recommendations for using wood ash instead of lime because it is a common byproduct. Using byproducts is a great form of recycling, but there are always considerations in doing so. For wood ash, the most significant consideration is increasing potassium (K) levels too much. If you collect your own wood ashes for amending your farm fields or garden, be sure to keep them out of the rain, as rain will wash away the active compounds. The CCE of wood ash varies and if you’re interested in using it, definitely read the bulletin from the University of Maine.

Calcium Silicate

Wollastonite is the natural mineral calcium silicate. It has been used for industrial applications, but has promise as an agricultural amendment.

Wollastonite is the natural mineral calcium silicate. It has been used for industrial applications, but has promise as an agricultural amendment.

Another byproduct that has use as a liming agent is calcium silicate (CaSiO3). It is a byproduct of steel mills, but it is also naturally found as the mineral wollastonite, much of which is mined in New York state. I first heard Joseph Heckman, from Rutgers University, talk about his research using calcium silicate a few years ago and I have been wanting to try  it out ever since. I even got a quote on a pallet of wollastonite last year (the price of the material itself is comparable to dolomitic lime, but you have to consider shipping too), but I never followed through because there wasn’t anywhere on our farm that needed liming.

Heckman has written an excellent fact sheet on silicon (Si) in agriculture in which he delves into the research showing increased disease and pest resistance in crops like wheat and pumpkin (also rice and sugarcane, which are not as relevant to our region). Although the liming effect of calcium silicate has been known for decades, the role of silicon in plant nutrition/health has been largely ignored until recently because it is not considered a necessary nutrient.

These photos of pumpkin show the importance of raising the pH for crop health (upper left vs. lower left), but also the potential for calcium silicate, both as a byproduct and the natural mineral, to reduce disease incidence. In his most recent research, Heckman reported to me that wollastonite was the best source of Si for increasing plant uptake in pumpkins and suppressing powdery mildew. This is good news for organic growers, because wollastonite (as a mined substance) should be allowed by certifiers (always check with your certifier first).

Calcium silicate in both natural (wollastonite) and slag forms raises the pH of the soil AND decreases disease severity in cucurbits like pumpkins. Photo credit:  Joseph Heckman, Rutgers University.

Calcium silicate in both natural (wollastonite) and slag forms raises the pH of the soil AND decreases disease (powdery mildew) severity in cucurbits like pumpkins. Note the apparent difference in plant size is because of distance of the photograph. Photo credit: Joseph Heckman, Rutgers University.

Researchers at University of Maryland also tried using calcium silicate as an alternative liming material for pumpkin production and found that in a very wet year, the presence of silicon (as calcium silicate) increased pumpkin resistance to powdery mildew, though it did not increase yields by itself. In that study, the combination of silicon and Stimplex (a seaweed extract) did increase yields. New Jersey, where Heckman is, and Maryland have similar, highly weathered soils for the most part. Whether the same effects would be observed in the generally younger soils of New England has not been researched (to the best of my knowledge). The CCE of calcium silicate is close to 90%.

Sources for wollastonite include Canadian Wollastonite and sources for the byproduct calcium silicate include Crossover and AgroSil. (If you have another product, please contact me and I can add it to the list).

Combining practices: no-till AND calcium silicate?

Since this is the no-till veggies blog, it is worth mentioning that no-till production of pumpkins using high-residue cover crops has also been shown to reduce disease severity, thereby reducing the need for fungicide applications. The combination of calcium silicate and no-till production might produce even better results than either alone!

Changing the impacts of acid soil with organic matter

Organic matter can ameliorate some of the effects of soil acidity because it ties up aluminum, which is actually what is toxic for many crops in low pH soils. If you live in a state with some natural organic soils (like New York or Michigan), you may have noticed that the pH recommendations are quite different for these soils (peat or muck soils, technically called histosols). Building organic matter to a level that it truly impacts soil acidity for most soils takes time, however, and should not be considered an alternative to liming.

Adding various forms of manure and compost can also raise the pH of soil (fresh manure may actually lower the pH, however), but adding manure in quantities that substantially change the pH generally results in overloading of nutrients like phosphorus (P). This can lead to long-term nutrient imbalances and pollution risks.

A note on gypsum

Finally, a note on gypsum. Gypsum is calcium sulfate, and can have many beneficial effects  on soil, but it is not actually a liming material. The Agronomy Society has a nice publication on gypsum. It is important to remember that for lime, it is the carbonate, not the Ca, that changes the pH. For calcium silicate, it is the changes in silicate, not Ca that change the pH. Gypsum can, however, reduce the effects of acid soils by reducing aluminum toxicity. This can be an effective way to ameliorate acid soils in deeper soil horizons because the gypsum is more soluble than limestone and the Ca and sulfate move relatively easily through the soil profile. With your own pH meter, it’s easy to measure pH at multiple depths! Gypsum also has the added benefit of contributing sulfur (S) to the soil in the readily available form of sulfate. I discussed the increasing problem of S deficiencies and the ability of Brassica cover crops to recycle S in a previous post.


Lime, both calcitic and dolomitic, are tried and true for raising soil pH, and raising soil pH to an optimal level has innumerable benefits on crop production and soil health. The research on other “liming” materials like calcium silicate is still relatively new, but it’s certainly something to consider when you’re making plans. As with all farming decisions, the important considerations are: price, availability, ease of application/use, and functionality. If one product can provide multiple benefits (increase the pH and increase disease suppression thereby lowering fungicide cost for example), this must be factored into the functionality part of that equation.

As always, contact me if you have any questions and if I can’t answer them, I will put you in touch with someone who can!

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