The optimal pH range varies between plant species. Many vegetable and field crops do best when soil pH levels are between 6.0 and 6.5 on the scale of 0 to 14. However, several forage species like alfalfa grow best at slightly higher levels like 6.8 to 7.4. This may not seem like much difference, but the difference does impact yield.

Agricultural fields are typically too low due to the cost of liming large acreages whereby producers try to save costs by delaying lime applications. The primary reason for soils to become more acidic or lower in pH over time is due to the nitrogen applications that occur in fields for corn production.

Since rock phosphate (0-46-0) is virtually non-existent now that dealers have chosen to handle only diammonium phosphate (DAP) (18-46-0), there is now a slight nitrogen application in legume forages to provide for phosphorus needs in the forage crop.

To a lesser extent and in non-cropped pasture areas, decomposition of organic material like manure and plant decomposition will cause pH to gradually decrease. These areas also are areas that typically have a lime application very infrequently, if ever. Lime is more difficult to understand than other soil nutrients so let’s take a look at lime from the plant nutrition perspective.

Interestingly, soil pH affects the availability of other nutrients. For example, high soil pH levels will result in one being more likely to find manganese, iron, boron, magnesium and zinc deficiencies.

Low soil pH values will result in one being more likely to find aluminum toxicity or phosphorous, calcium and magnesium deficiencies. One should first consider soil pH levels prior to considering other nutrient levels when plant deficiencies are observed. Soil testing every two to three years is critical to know what is going on with soil fertility.

Meters for pH are more accurate than other indicator kits are for other nutrients. However, it is economical and typically best to submit a sample for a basic soil test with a certified lab that typically costs anywhere from $10 to $15 per sample.

Visit www.ag.purdue.edu/agry/extension/ and click on “Certified Soil Testing Laboratories” at the bottom right for a list of certified labs. For sampling instructions and to borrow a probe to collect samples, most soil and water conservation district or extension offices will assist with loaning probes or helping with information.

Another difficult concept to understand about lime is the fact that different amounts of lime are needed to change the soil pH from 6.0 to 6.5 due to differences in soil types. Clay and organic matter function as nutrient exchange sites in soil, allowing soil its ability to hold nutrients. Soils with higher levels of these two materials are more highly buffered.

Being “more highly buffered” means that the soil is more resistant to “change” – thus more volume of the amending material is needed. The more clay and organic matter a soil has characteristically, the greater the amount of lime that is needed to change the soil pH, relative to soils with lesser clay and organic matter.

For this reason on a soil test report, one will see the column titled buffer pH in addition to soil pH. Use the recommendations with the soil test report or contact your local extension office if you need help in calculating the amount of lime needed based on the buffer index.

Further complicating matters, not all lime is created equal. Lime varies in quality in terms of its neutralizing value and the lime also functions more quickly and efficiently according to how finely it is ground.

Visit www.aglime.org/aglimekey.htm to view the 2009-10 Indiana Lime Council report for quarries listed by county in Indiana. One should price lime and determine vendors according to its relative neutralizing value, which factors in both the quality and the fineness of the lime.

Calcitic lime is typically the type of lime found in southern Indiana, whereas dolomitic lime is typically found in northern Indiana. Dolomitic lime is also a source of magnesium. Locally, it is typically not necessary to have the expense of transporting northern Indiana lime, as we have enough magnesium naturally available in our soils.

Lime, like potassium, moves and reacts very slowly in soil, thereby not providing instantly for the soil environment changes needed for plants. When 3 tons per acre or less are required, surface application will generally work well for the top three to four inches of soil.

When more than 3 tons per acre of lime is required, it is best to split the rate in half and apply the total in two applications over two years. When forages are in a rotation with row crops, it is best to apply half of the lime twice as often when the overall rotation is managed within a no-till system.

Furthermore, when pH- compromised soils or soils possessing lower pH levels for row crops have a planned forage use, it is best to apply the lime six months to a year ahead of the planned forage establishment while also adhering to the previous quantity guidelines mentioned.

Thus if more than 3 tons per acre of lime was needed, at least one-half should be applied six months to one year ahead of the forage seeding establishment. Corn and soybeans may be performing well with a pH of 6.2 but it is imperative to start a year ahead of an alfalfa and/or orchardgrass establishment to boost soil pH levels.

Producers must consider lime when needed as worth the expense in this time of very high fertilizer prices to most optimize the efficiency of both applied and inherent soil fertility.

Historically landlords have typically paid lime costs, as lime is a long-term investment. Recent trends though have shown that lime costs are being shifted towards the tenant more than in the past or at least are part of the cash rent equation. The margin squeeze occurring with relatively higher fertility input and land costs justifies the cost of liming to get another one to three tons of forage production from each acre.  FG

Photo courtesy of Kevin Caldwell, AET Consulting

 Mark Evans is an extension educator in the Purdue Extension-Putnam County office located at Greencastle, Indiana and a member of the Indiana Forage Council.