I refer to agricultural limestone or “ag lime” as a “sweet” subject because some farmers consider soils with a pH of at least 7.0 – which is neutral – to be “sweet,” as opposed to soils with a pH below 7.0, which are known as “sour:” slightly to strongly acidic.

There’s nothing unique about the use of finely pulverized limestone to increase soil pH; wood ashes will have a similar effect, and homeowners often spread ash on their flower and vegetable gardens for this purpose. Wood ash is about one-half as efficient as ag lime in its soil neutralizing ability, and sometimes is blended with lime to be used as a commercial liming product. The neutralizing power of the combined product is slightly less but wood ash has the advantage of providing a modest amount of nutrients, particularly potassium, but also smaller amounts of phosphorus, sulfur and trace nutrients. When I was an agronomist at Miner Institute, we seldom needed ag lime because most of our fields were naturally high in pH. But when we did need to increase soil pH we used a lime-ash product available from a local lime company.

A timely topic

This is a good time to be discussing ag lime since most crops have been harvested from fields, and it’s “prime time” for soil sampling. As noted in previous columns, fall is the best time to take soil samples because there’s plenty of time to get the results back and plan fertilizer programs for next spring. As you review the results of your soil tests, pay attention to pH so if lime is needed you can have it applied before winter sets in. It’s okay to leave lime sitting on the soil surface over winter, including top-dressed applications on hay fields; it has very slow solubility on the soil surface so unless there’s soil erosion, lime will stay where put.

Pay particular attention to corn and grass fields that get annual applications of nitrogen fertilizer. Most nitrogen fertilizers are acidifying to the soil, and several years of application during corn or grass production can decrease soil pH. The source of nitrogen can make a big difference: Dairy manure is almost neutral in pH, so applying dairy manure to supply nitrogen has much less impact on soil pH than does urea, ammonium sulfate or UAN (urea and ammonium nitrate) solutions. Ammonium sulfate is more acidifying than most other nitrogen sources, and with dramatically lower concentrations of atmospheric sulfur (due to EPA clean air regulations) and therefore less “free” sulfur via precipitation, much more ammonium sulfate is now being used on farm fields.

Soil pH and nutrient availability

The plant nutrient most sensitive to low soil pH is phosphorus. The availability of phosphorus to plants begins to decline when the pH drops below 6.5. We often state that corn and grasses will tolerate soil pH levels as low as about 5.8, but “tolerate” doesn’t mean thrive, especially if soil fertility is low. There are two factors affecting nutrient uptake by plants: having enough nutrients and then having the soil conditions right for their uptake. Low soil pH isn’t the only factor limiting nutrient uptake – there’s also high pH (affecting several heavy metals including copper and iron) cold soil (phosphorus and zinc) and dry soils.

If alfalfa isn’t included in your crop rotation it may not be necessary to raise soil pH to 7.0. However, it’s not a good idea to try to maintain the minimum pH for the crops you grow. For instance, a pH of 6.2 is often the recommended minimum for fields growing corn or grass. But consider how you determine soil pH – not the laboratory analysis but how you take a composite sample. Small cores are extracted from a number of places in a field. If some of these areas have a pH of 7.0 and others have a pH of 5.8, the composite sample submitted to the lab might test 6.4 while parts of the field are low enough to significantly impact nutrient uptake. (The same holds for soil fertility, which is why I like to maintain soil test levels of potassium “medium – high” range.) Meaningful within-field differences in soil pH are much more common in the northeastern United States with its variability in soil type. For instance, one 40-acre field at Miner Institute (Chazy, New York) includes at least eight different soil types.

A real-life example: The farmer asked me to troubleshoot one of his fields; the alfalfa was growing well in most places but poorly in others. The soil analysis showed the field to have a pH of 6.5. But when I soil sampled and tested just the places where the alfalfa wasn’t growing well the pH was under 6.0.

Although this column focuses on soil pH I’ll note that I found a similar situation in another alfalfa field with spotty growth just on the knolls. The composite soil analysis on that field looked fine – acceptable pH and fertility, but when we analyzed the soil from just the knolls the potassium level was extremely low. This could have been the result of a different soil type on the knolls, or perhaps over the years crops yielded especially well there, decreasing the available potassium supply. A soil analysis is an average of all the cores you or your crop consultant took to make the composite sample. This is also why if there are two distinctly different soil types in a field you should consider sampling the field by soil type, even though this involves the (albeit modest) cost of testing an additional sample. Soil analysis is cheap – lime and fertilizer are not!

Calcite and dolomite lime

There are two types of ag lime: calcite and dolomite. Dolomite lime contains magnesium, an essential nutrient that’s occasionally deficient in the eastern United States. Dolomite lime sometimes sells at a premium because of this magnesium content. The percentage of magnesium in ag lime is available from the company from which you purchase it: State laws require that each source of ag lime must be tested for its neutralizing ability, and this includes the percent of magnesium in the product. If your soils are low in magnesium then dolomite lime is often the least expensive way to supply this nutrient.