The question answered in this section is "How does one determine the amount of lime to apply for efficient crop production?"
One of the early approaches employed by soil testing laboratories to determine the lime requirement was based on the relationship between soil pH and percent base saturation and knowledge of the cation exchange capacity of the soil. The cation exchange capacity was often estimated based on the texture and organic matter content and knowledge of the predominant clay mineral present.
The knowledge that acid soils contain exchangeable Al brought about a new concept for making lime recommendations. The concept was to apply the amount of lime required to neutralize the Al which was extractable with a neutral unbuffered solution such as KCl. The amount of lime required to neutralize the exchangeable Al is given by the following equation:
Tons CaCO3 per acre = (2) (milliequivalents Al per 100 grams of soil)
The factor of 2 takes into account the pH dependent charge of the soils which results in a portion of the non-exchangeable acidity being ionized as the pH increases and reacting with the added lime.
The procedure for measuring exchangeable Al is not well suited for routine soil testing procedure. For this reason many soil testing laboratories now use a soil buffer pH procedure for determining the lime requirement. These procedures must be rapid and must provide reliable estimates of the lime requirement. Four methods for lime requirement are now being used in the United States. They are the Woodruff, the Shoemaker, McLean, and Pratt (SMP), the Adams-Evans, and the Mehlich.
The Woodruff method was developed for Mollisols (high organic matter mineral soils of the Midwest) for the purpose of liming soils to pH 6.5. Although in use by some laboratories in the Midwest it is not as widely used as the SMP buffer.
The SMP buffer was developed for soils having a relatively high lime requirement and significant reserves of exchangeable Al. The SMP buffer is suited for Alfisols having large amounts of three layer clays and high organic matter content. The majority of laboratories in the Midwest use the SMP buffer. The SMP is not well suited for low exchange capacity soils of the Southeast and can give inaccurate results on these soils.
The Adams and Evans buffer (used by Alabama, Florida, Georgia, Tennessee, South Carolina, and Virginia) was developed for soils with low cation exchange capacity and containing primarily kaolinitic clays. These soils usually have relatively low lime requirements and the possibility of overliming exists. The Adams and Evans buffer is very reliable for soils with relatively small amounts of exchangeable acidity and provides a fairly high degree of accuracy of estimating lime requirements to reach pH 6.5 or less. Sensitivity of the lime requirement by this method is within 500 lb/A of limestone. The Adams and Evans lime requirement method is based on separate measurements of soil pH in water and a buffer solution (pH 8.0) The soil pH determination is used as a measure of acid saturation of the soil (H-sat1) according to the following equation:Measured soil pH = 7.79 5.55 (H-sat1) + 2.27 (H-sat1)2
Where H-saturation is expressed as a fraction of the CEC. Buffer pH is used as a measure of soil acids (Soil H) according the following equation:Soil H = 8(8.00 buffer pH)
The desired soil pH is expressed in terms of acid saturation of the soil (H-sat2) according to the following equation:Desired soil pH = 7.79 5.55 (H-sat2) + 2.27 (H-sat1)2The Adams-Evans buffer method assumes that agricultural-grade limestone is about 2/3 effective in neutralizing soil acidity up to a soil pH of about 6.5, and allows for this by using a correction factor of 1.5. Thus, the lime requirement is the product of the following equation:
Soil H/H-sat1 x (H-sat1 (H-sat2) x 1.5or for 10 cm3 soil in 10 ml water + 10 ml buffer solution, the lime recommendation is:
Limestone (lbs/A) = 8000[(8.00 - buffer pH)/H-sat1] x (H-sat1 - H-sat2) x 1.5As noted above lime requirement calculations using the Adams-Evans method is based on two pH determinations, soil pH and buffer pH. With this information and the formulas above the lime requirement of a soil can be calculated. The following table is an abbreviated example of the lime requirement for a soil depth weighing 2,000,000 pounds per acre to increase soil pH to 6.5.
Soil pH in Buffered Solution Soil pH
7.9 7.80 7.70 7.60 7.50 7.40 7.30 6.3 183 366 549 732 915 1098 1281 6.1 324 648 972 1295 1619 1943 2267 5.9 436 872 1308 1744 2180 2616 3052 5.7 528 1056 1584 2112 2641 3169 3697 5.5 605 1211 1816 2422 3027 3633 4238 5.3 672 1344 2016 2689 3361 4033 4705 5.1 731 1462 2193 2924 3655 4386 5117 4.9 785 1569 2354 3138 3923 4707 5492 4.7 836 1672 2507 3343 4179 5015 5850 *Note: The depths of soil it takes to weigh 2,000,000 pounds will vary with the bulk density of the soil. For example, a sandy loam soil with a bulk density of 1.472 would weigh 2,001,232 pounds per 6 inch-depth, whereas, a loam soil with a bulk density of 1.299 would weigh 2,001,504 pounds per 6.8 inch-depth.
Without the aid of a computer, calculating the lime requirement for a soil by the previous method can be quite time consuming. That is why we rely on soil test laboratories to provide this information. There are occasions, however, when county agents, specialists, researchers, and consultants need to modify or change limestone recommendations. Dr. Owen Plank, from The University of Georgia, has developed a spreadsheet that enables you to make refinements in limestone recommendations. You can select the target pH and modify the depth to which the limestone will be incorporated. Simply by inputting these two values, along with the soil pH and Adams Evans buffer pH (Lime Index), a new recommendation can be calculated instantaneously. The spreadsheet can be used with Excel and Quattro. For more information contact Dr. Plank at firstname.lastname@example.org.
The Mehlich buffer method (used by North Carolina) was developed to determine the amount of lime required to neutralize the acidity extracted with an unbuffered salt solution. This approach was based on neutralizing the acidity that is limiting crop growth and not trying to achieve a given pH. A buffer solution of pH 6.6 is used to measure extractable acidity (Ac). The buffered solution extracts both exchangeable Al and the pH dependent acidity (H) which becomes ionized up to pH 6.6. The lime rate to apply is calculated with the following equation:
CaCO3 tons/acre = Ac (desired pH - soil pH)
(6.6 - soil pH)
The desired pH for a soil is the pH at which the activity of Al is neutralized. The effect of soil organic matter in decreasing the activity of Al has been taken into account by establishing desired pH's for three classes of soils based on their organic matter content: mineral, mineral-organic and organic. The desired pH at which exchangeable Al is essentially neutralized is 6.0 for mineral soils, 5.5 for mineral-organic soils and 5.0 for organic soils.
Many years of high-input management, including liming, of Ultisols in the Southeast has improved the chemical properties of the subsoil. Increases in exchangeable Ca and corresponding decreases in exchangeable Al create a favorable environment for root growth in the subsoil. This can result in better utilization of subsoil moisture and reduce the detrimental effects of short-term moisture stress.
Return to Part III.
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