Calcium or Lime? Which raises soil pH?

Soils become acidic for different reasons, but the primary reason in Kentucky’s production agriculture is nitrogen (N) fertilizer application. Managing soil pH is a crucial part of your crop production program and can be monitored with regular soil sampling and testing. With the soil test report, you know the active acidity (water or salt pH) and the buffer pH (Sikora buffer) values that guide the rate of liming material needed to adjust soil pH to the range desired for a given crop. Soil testing is important for soil pH management.

By definition, an acidic soil has a higher concentration of hydrogen ions (H+) than hydroxyl ions (OH-) in the soil solution. However, a soil pH of ~6.5 is considered ideal for Kentucky row crops. Liming agents such as ag lime, pelletized lime, and other materials that consume hydrogen ions (acidity) are used to raise soil pH in agricultural fields. Ag lime consists of either calcitic (CaCO3) or dolomitic (CaMg(CO3)2) limestone, in a range of particle sizes, and is bulk spread over the soil to neutralize soil acidity. Pelletized lime is typically calcitic limestone and consists of smaller particles that are pressed into a “pellet” and held together using a chemical binding agent. This reduces dust and improves spreading performance.

Below is the generalized acid neutralizing reaction using calcitic limestone.

The reaction shows that acidity (H+) is consumed by carboxyl ions (CO32-) to form water (H2O) and carbon di-oxide (CO2). Although calcium (Ca2+) is often thought to be the cause of soil pH change with limestone addition, Ca2+ actually has nothing to do with the neutralizing reaction. An example of this is shown below, when gyp-sum (CaSO4) is applied to the soil.

The reaction shows that hydrogen ions (2H+) are still present after dissolving the added gypsum because gyp-sum is a simple salt. The dissolved sulfate (SO42-) present is not a base and cannot react with, and neutralize, the acid hydrogen ions (2H+) in the reaction. The same is true for other salts, such as calcium chloride (CaCl2) or calcium nitrate (Ca(NO3)2), where there is no liming ability in either product - as shown in the following re-action for CaCl2.

In short, hydroxides (OH-), oxides (O2-) and carbonate (CO32-) ions are required to neutralize H+ ions, effectively raising soil pH. Calcium (Ca2+) and magnesium (Mg2+) ions have nothing to do with soil pH change.

The effectiveness of limestone is determined by the purity of the material, referred to as the calcium carbonate equivalence (CCE), and the particle size of the material. The smaller the particle size of limestone the more quickly it will react with the soil when applied. The combination of particle size and CCE is used to calculate the relative neutralizing value (RNV) of the product in the following equation.

RNV (%) = CCE/100 x [0.5 x (% passing 10 mesh + % passing 50 mesh)]

Now that we have a basic understanding of how acidic soils are neutralized, we will share the preliminary findings of a liming study being conducted across the state. The objectives of this study were to compare the effective ness of liquid calcium, pelletized lime and agricultural lime in raising soil pH in both the laboratory and the field.

The experiment was conducted at 16 locations across the state in forage production fields (pastures or hay-fields). The target soil pH for site selection used for this experiment was 6.0, but this target was not always met. Once the site was identified, plots (5 ft by 5 ft) were established, an initial soil sample was collected, and treatments were applied. Treatments included an untreated check, liquid calcium (Advanced-Cal, AgriTec International) at 5 gallon per acre, pelletized lime (RNV of 83) at 2.4 ton/A, and agricultural (ag) lime (RNV of 77) at 2.6 ton/A. The rate of lime used at all locations was 2 ton/A with an RNV of 100 and both pelletized and ag lime rates were adjusted upward according to their RNV values. Soil samples were collected again, later in the season, typically when the producer harvested hay 2 to 3 months later. A laboratory soil incubation study was conducted in conjunction with the field study. Soil with an initial pH of 5.2 was incubated in specimen cups and maintained at 80% water-filled pore space. Treatments equal to those used in the field study were applied and mixed into the soil in the cups. Soil pH was then measured at 1 and 3 months of incubation (Table 1 – first three columns).

The soil samples from the field study sites were collected approximately three months after treatments were applied. The average pH prior to treatment application was determined and then determined again on the samples taken later (Table 1 – fourth column). This data shows that there was little to no change in soil pH in the untreated check and liquid calcium (Advanced-Cal, AgriTec International) treatments in the laboratory incubation (Table 1). In the field, both these treatments actually resulted in a decrease in soil pH, relative to the initial field soil pH. Both pelletized and ag limes caused a positive change in field soil pH, between 0.30 and 0.40 pH units. Similar trends were observed in the laboratory study, which shows that pelletized and ag lime amended soils exhibited increased soil pH with time while the check and liquid calcium treated soils did not. The soil pH changes with time show the natural progression of soil pH decline when liming agents are not used and soil pH increase when high quality liming agents are used.

Both pelletized lime and ag lime have increased soil pH during the measurement time frame in these experiments. Both are effective liming agents. The liquid calcium product has not raised soil pH and is not an effective liming agent. This comes back to the liming reactions shown above. There must be something present to consume the soil acidity, such as carbonate, hydroxyl or oxide, and the liquid calcium product has none of these. The pH of the liquid calcium product was measured in-the-jug and found to be 4.5, which means that this product would actually lower the pH of most field soil if a large quantity were applied. Fortunately, a 5 gallon/A use rate is not enough to alter soil pH one way or the other in most any agricultural field.

Another factor to consider is the cost of the products. Prices vary from location to location and should always be checked prior to making any decision on input purchases. In western Kentucky at the time this study was initiated ag lime was roughly $15 per ton or less. There is an additional delivery/spreading fee associated with this. Pelletized lime was between $200 and $300 per ton and still has an associated spreading fee. We purchased the liquid calcium for approximately $30 per gallon with a recommended use rate of 2-4 gallons per acre. Ignoring application fees, this works out to about $30/A for ag lime, $400-600/A for pelletized lime and $60-120/A for liquid calcium. An advanced degree in mathematics is not needed to determine which is the better route to go when trying to neutralize soil acidity, especially when one of the products doesn’t actually raise soil pH.

In closing, there are many products on the market that make great claims. Some even work. However, when a person is deciding on the best way to lime a production field there are two primary questions that need to be answered. Does the product work? What does it cost? Pricing the proven products will go a long way towards making good agronomic and economic decisions for soil pH management.