To understand why reserve acidity can be a problem, we need to introduce another concept in soil science, the cation exchange capacity (CEC). Simply, the concept is exactly what it states: the ability of a soil to 'hold' cations (positively charged ions) on negatively charged surfaces—clays and organic matter make up the negatively charged surfaces in soil. Using our swimming pool analogy, let's say the maximum number of people allowed into the pool at one time is 20, that number represents our cation exchange capacity. The pool can't safely hold more than that because it has a capacity. A larger pool could hold more people, or, a better built pool could hold more in the same area. These differences represent differences that exist in soils. A soil that has more organic matter and a higher clay content, both of which lend 'cation exchange sites' to soil, will have a higher cation exchange capacity. Therefore, a soil with a higher cation exchange capacity has a greater ability to buffer itself from pH change via liming due to the greater number of sites where hydrogen ions can reside and constitute the reserve acidity.
Cation exchange capacity is measured in millequivalents per 100 g of soil (meq/100 g); suffice to say that this is simply a way to quantify the number of available sites within 100 g of soil that can be extrapolated out to an entire soil profile. So in our example earlier, a CEC value of 20 equates to 20 meq/100 g soil.
Now that we understand how acidity and CEC work together, we can try to get down to brass tacks and make some liming recommendations. Another part of this equation is the buffer pH of your soil. The CEC of a soil has a large impact on what the buffer pH of the soil will be; again, a soil with a low CEC doesn't have as great an ability to maintain acidity—via reserve acidity—as one with a larger CEC does. Once buffer pH and actual soil pH are determined, lime recommendations are made based on their difference.
However, there is another way that lime recommendations can be made and that is dependent on the base saturation of a soil (don't get confused, all of these soil characteristics and resulting pH, buffering capacity, etc. are all related, these are just different ways to arrive at the same end point). Base saturation is simply the amount of basic (meaning basic in pH, not fundamental) cations that occupy the CEC sites in a soil. Just as H+ can attach to cation exchange sites in soil, so do other cations found in soil.
The basic cations are: K+, Na+, Ca2+ and Mg2+. Base saturation is measured as the percentage of the total CEC the sum of basic cations occupy; they can also be broken down individually. For example, let's say our CEC is 5.0 meq/100 g and our total base saturation is 80%. That means 4.0 out of the 5.0 meq/100 g of soil (our CEC sites) are occupied by basic cations. But what about the remaining 1.0 meq/100 g you ask? The amount of meq/100 g remaining after base saturation has been calculated, in this case 1.0 meq/100 g, is the amount of acidity in the soil. If the basic cations add up to 100% base saturation, then there is no acidity in the soil.
After the acidity of the soil has been determined, we can use this number to make a recommendation for lime. The old standard — we're still trying to figure out exactly where these numbers come from — is that it takes 1000 lbs of CaCO3 (calcium carbonate/lime) per acre to neutralize 1 meq of acidity. So, in this case, the recommendation would be 1000 lbs lime/acre to neutralize the acidity in that soil. This is a very basic guideline, however, and you should always consult with your lime dealer to figure out your exact needs based on your soil type, starting pH, and lime source.
Speaking of lime source, our calcitic limestone comes out of our quarry at least 95% pure and is ground finer than any other product on the market, increasing the effectiveness of it above published standards. We have seen an average 0.1 unit pH change in a typical Iowa soil per 100 lbs of our SuperCal 98G applied per acre. Due to the ease of application with 98G pelletized lime and the fineness of grind, our product can be applied at a lower rate per acre than ag lime, saving you money in the long run. Give it a try yourself and start down the road to improved yields now!