Let’s Talk About Lime
--Stepping Up your Management Game: Let's Talk About Lime--
Between all this snow and cold weather, liming is probably not at the top of your list right now. However, liming can be completed any time this winter or early spring as soon as we can get some more favorable conditions. So, lets take a few minutes to talk about soil pH and lime. In this post, I’ll cover some of the base definitions we need to understand to talk about pH and lime, when to lime, what factors affect our liming considerations, and what liming materials to use. I understand that this isn’t the most glamorous topic, but our pH levels are perhaps the most impactful segment of our operations. pH directly affects:
· Nutrient availability
· Metal toxicity to plants and microbes
· Efficacy of soil applied herbicides
· Ability of legumes to fix nitrogen
· Increase yield potential
This covers a wide swath of our crop management and is important to get right. Let’s dive in.
What is soil pH?
To set our discussion up, we need to spend some time going over definitions. First of all, what is pH? pH is a value representing the acidity and alkalinity of a solution. This is measured based on how much hydrogen atoms present on the soil and in the soil water. Each change in pH is extremely small units of hydrogen atoms, consequently a negative logarithmic scale is used to represent the changes. For example, a soil that has a pH of 4 would have a hydrogen concentration around 0.0001 moles per liter. The negative log of 0.001 is 4, a much easier value to work with. We have to remember though, since we are working with a negative log scale, that the differences between each value represent a 10x difference. So, a pH of 6 is 10x more acidic than 7. However, a pH of 5 is 100x more acidic than 7. We consider 7 to be a neutral value on the pH scale. Values below this have higher amounts of hydrogen atoms and are acidic, while values higher than 7 have less H+ atoms and more OH- atoms and are basic of alkaline. Water is generally the closest substance to a neutral 7. Water out of our tap may not be quite neutral, but pure water used for lab purposes would be a 7. Hydrochloric acid is near 0, lemon juice has a pH around 2, while coffee has a pH around 5.0. Eggs have a pH of 8, and oven cleaner near 14.
Active and Reserve Acidity
There are two segments of acidity that need to be discussed as individual units. The first is active acidity. This is the amount of hydrogen atoms measured in the soil solution when we do a soil test. This will indicate the need for lime. However, this will not tell the amount of lime needed. The second measure of acidity is the reserve acidity. Reserve acidity represents the amount of hydrogen and aluminum atoms that are bound to soil particles. As liming neutralizes the active acidity, atoms from the reserve acidity move off these soil particles into the soil solution and maintain the active acidity of pH. Imagine we apply enough lime to neutralize 5 hydrogen atoms. As soon as these atoms are neutralized, 5 more hydrogen atoms will release into the soil solution to replace those that were neutralized. Reserve acidity is sometimes called buffer pH because it buffers the reaction of pH to lime. It is essentially a measure of the soil’s resistance to chance. Some soils have a higher reserve acidity and require much more lime to neutralize than others. An example of this is noticeable when looking at different soil textures. Imagine two soils with the same exact active acidity with a pH of 6.1. One of the soils is a clay loam soil with lots of binding sites for hydrogen atoms to bind to. This soil has a high reserve acidity. It will be very resistant to change and require a higher lime amount to correct for both the hydrogen atoms in the soil solution, as well as those on binding sites that will release for each neutralized hydrogen atom. Imagine now the other soil is a silt loam or sandy loam. This soil would have much fewer binding sites due to the larger particle size. A much smaller amount of lime will be needed to correct for both the active and reserve acidity.
When we receive soil tests, we get a value for the active acidity, soil pH. This tells us if we need to lime or not. It will not tell us the amount. The soil tests also give us a value, usually called buffer pH representing the reserve acidity, that can be used to figure out how much lime we need to apply to a soil. Each buffer reading below 7.0 represents a higher resistance to change for that soil. The lower the buffer pH, the more lime that will be required to neutralize the hydrogen atoms.
Several soil and environmental factors affect both your active and reserve acidity. First off, how the soil was formed affects how acidic the soil naturally is. Soils formed from parent materials such as limestone will be naturally more alkaline, while soils formed in shale or sandstone will be more acidic. However, these factors are often masked by another factor: rainfall. Higher amounts of rainfall will result in a more acidic soil. That is why soils in the south, like in Georgia or Alabama, are much more weathered due to high amounts of rainfall, and are consequently much more acidic. Rainwater will leach basic ions like calcium or magnesium down the soil profile leaving acidic ions like aluminum near the soil surface.
Amount of soil organic matter and amount of clay particles can also affect soil buffer pH as previously discussed. Decomposition of OM can also result in creation of acidic organic acids.
Finally, the largest source of acidity comes from nitrogen. As ammonium-based nitrogen is applied, bacteria convert this form of nitrogen to nitrates. As this happens, H+ ions are released into the soil and creates an acidic layer at the depth of nitrogen application. This can result in a stratification effect for soil pH. From 0-8 inches, when nitrogen is typically applied, soil pH will be more acidic. Below that point, pH will be much more neutral. However, as the majority of roots are isolated within that upper soil layer, the pH levels we are most concerned with are in that more acidic layer.
When to Lime
We’ve discussed that our decision to lime is based on our active acidity, and our reserve tells us how much to apply. Amounts and thresholds vary from different sources as to the initial level to start applying or how much to apply to move the pH incrementally. The University of Nebraska recommends applying lime when the pH is less than 6.3. The amount of lime applied is based on the buffer pH and recommends applying 1000 -1200 pounds per acre for each 0.1 pH buffer below 7.0. If a buffer pH reading was around 6.1, that would be 0.9 below 7.0, so between 9000-10,800 pounds per acre would need to be applied. Other sources recommend different values, so take a look at several sources to find which equation you are most comfortable with.
When we are calculating lime requirements, we cannot fail to calculate based on the quality of the lime we have available to us. Lime quality is based on two factors, the calcium carbonate equivalent, and the particle size. Calcium carbonate equivalent (CCE) is referring to the neutralizing capacity of a particular liming source. It is relating all lime sources to the same standard so we have a comparative value with which to adjust our recommendation rates. For instance, comparing a lime that neutralizes 75 percent as much acid as a pure calcium carbonate would have a CCE of 75. Fineness of particle size is also necessary to consider, as smaller particle sizes have a larger surface area increases the neutralization effect by putting the lime in greater contact with the acid. A sample of lime will be categorized into three different particles size and then factored together to give a final fineness factor. Many sources combine these two factors into one variable by multiplying the fineness factor by the CCE and then dividing by 100. This is often referred to as the Effective calcium carbonate equivalent or ECCE or sometimes as effective cilium carbonate factor or ECC.
If our ECCE was 70% and our calculated application rate based on our buffer pH was 6000 pounds per acre, we would want to adjust for the effectiveness of our lime source by dividing by the percent ECCE. In this case, to get the equivalent of 6000 pounds per acre, we would need to apply 8571 pounds per acre.
Various liming materials are available to use, but most common in this area is aglime or pel lime.
Aglime is the most commonly used source and is crushed and ground limestone. Aglime would typically have a lower CCE, but is usually the cheapest source to use.
Pel-lime is lime that has been ground then compressed into granules to reduce the dust levels. This has a much higher ECCE due to finer particle size put does come with a higher price tag.
Weigh the pros and cons of each source to select the best option for your operation.
Before liming, take time to consider some additional factors affecting liming.
· Variable rate lime: do you have the option to do this? If so, this is probably the most effective use of variable rate technology. The pH levels change across your field. Using VRA allows your lime rates to change to match the variability. This can be one of the greatest sources of cost savings overall for application.
· Tillage system: mixing lime with the soil allows for the neutralization process to occur much faster. In a no-till system it will take several rains and a longer period of time to have the same effect. Surface applied lime in this situation will move down about .5 inches a year. After four years it will only have neutralized to about 2 inches depth. If using a no-till system, cut the rates to a third or half of your total and apply across several years, particularly when applying high rates. If pH values are very low, it is recommended tilling in the first liming application in order to put the lime to work as soon as possible and attempt to raise the pH quicker.
· Economics: Liming to a full rooting zone depth is a long-term investment. If renting land, work with your landlord to share the cost of the application. Increased yields from several years of crops will be needed to break even on the cost of lime. However, increased yield is likely, often times coming more with an increase in soybean yields than corn yields.