Soil pH Management

Soil pH

This is a measure of the soil acidity or alkalinity and is sometimes called the soil “water” pH. This is because it is a measure of the pH of the soil solution, which is considered the active pH that affects plant growth. Soil pH is the foundation of essentially all soil chemistry and nutrient reaction and should be the first consideration when evaluating a soil test. The total range of the soil pH scale is from 0-14. Values below the mid-point (pH 7.0) are acidic and those above pH 7.0 are alkaline. A soil pH of 7.0 is considered to be neutral. Most plants perform best in a soil that is slightly acid to neutral (pH 6.0-7.0). Some plants like blueberries require the soil to be more acid (pH 4.5-5.5), and other, like alfalfa will tolerate a slightly alkaline soil (pH 7.0-7.5).

The soil pH scale is logarithmic; meaning that each whole number is a factor of 10 larger or smaller than the ones next to it. For example if a soil has a pH of 7.0 and this pH is lowered to pH 6.0, the acid content of that soil is increased 10-fold. If the pH is lowered further to pH 5.0, the acid content becomes 100 times greater than at pH 7.0. The logarithmic nature of the pH scale means that small changes in a soil pH can have large effects on nutrient availability and plant growth.

Buffer pH (BpH)

This is a value that is generated in the laboratory; it is not an existing feature of the soil. Laboratories perform this test in order to develop lime recommendations, and it actually has no other practical value.

In basic terms, the BpH is the resulting sample pH after the laboratory has added a liming material. In this test, the laboratory adds a chemical mixture called a buffering solution. This solution functions like extremely fast-acting lime. Each soil sample receives the same amount of buffering solution; therefore the resulting pH is different for each sample. To determine a lime recommendation, the laboratory looks at the difference between the original soil pH and the ending pH after the buffering solution has reacted with the soil. If the difference between the two pH measurements is large, it means that the soil pH is easily changed, and a low rate of lime will be sufficient. If the soil pH changes only a little after the buffering solution has reacted, it means that the soil pH is difficult to change and a larger lime addition is needed to reach the desired pH for the crop.

The reasons that a soil may require differing amounts of lime to change the soil pH relates to the soil CEC and the “reserve” acidity that is contained by the soil. Soil acidity is controlled by the amount of hydrogen (H+) and the aluminum (Al+++) ions that are either contained in, or generated by the soil and soil components. Soils with a high CEC have a greater capacity to contain or generate these sources of acidity. Therefore, at a given soil pH, a soil with a higher CEC (thus a lower buffered pH) will normally require more lime to reach a given target pH than a soil with a lower CEC.

The following analogy (adapted from University of Nebraska, Bulletin G74-153) may give a simple explanation. Consider two-coffee pots (figure to right) one 50-cup capacity and one 10 cup, both having the same size indicator tube and spigot. Coffee in the indicator tube represents the active acidity (measured by regular pH) and that coffee in the pot represents the reserve acidity (measured by buffer pH). Let the large pot represent a clay soil high in organic matter while the small pot represents a sandy soil. Both pots have equal amounts of coffee in the indicator tube; i.e., same active hydrogen, so same soil pH. Now open the spigot and remove one-cup of coffee from each pot (figure B). Removing one cup of coffee from each pot could be equated to the addition of small amount of limestone to an acid soil. Opening the spigot will cause the level of coffee in the indicator tube to drop below the level in the pot, but will return to almost the original level (clay soil) when the spigot is closed. The momentary drop of coffee in the indicator tube represents the initial increased in pH when lime is added (affects active hydrogen), but reserve hydrogen (similar to coffee in the pot) soon equalizes the effect from the lime and the pH returns to essentially its original level (clay soil, figure C). Thus, if the pH is 6.5 or lower, a buffer pH is run to measure the reserve acidity. The result of the buffer pH shows the amount of lime required to neutralize a major portion of the reserve acidity. The relative amounts of coffee in the two pots (figure C) show why a sandy soil and clay soil with the same pH result in different lime requirements. For example, the small addition of limestone (equivalent to removing one cup of coffee from each pot) reduced the total coffee (reserve acidity) by 10% in the small pot (sandy soil), but only 2% of the large pot (clay soil). In a similar manner, 1 ton of agricultural limestone will make a greater difference in the pH of a sandy soil than of a clay soil.

Soil pH Management

Lime tables Mineral Soil, High Organic Matter Mineral, and Muck Soil Organic matter 20% or more are selected portions of the lime recommendation calculations. Actual recommendations include many more combinations of Original pH, Target pH, and Buffer pH. Spectrum lime recommendations are expressed in pounds per acre of pure calcium carbonate (CaCO3) per 7 inch depth and typical fineness of grind. Make appropriate adjustments for the local lime source (see adjustments following lime tables).

Mineral Soil: Typical soils with organic matter between 0% and 10%
Sample Lime (CaCO3) Recommendations
Original pH Target pH Lb./Acre Original pH Target pH Lb./1000 ft.2
Soil Buffer pH (BpH) Soil Buffer pH (BpH)
5.0 5.5 6.0 6.5 7.0 5.0 5.5 6.0 6.5 7.0
4.5 5.0 11982 9427 6511 3776 1040 4.5 5.0 275 216 149 87 24
4.5 5.5 16827 12981 9136 5290 1444 4.5 5.5 386 298 210 121 33
4.5 6.0 20227 15602 10987 6353 1728 4.5 6.0 464 358 252 146 40
4.5 6.5 22891 17656 12420 7185 1950 4.5 6.5 526 405 285 165 45
5.0 6.0 16795 12949 9104 5258 1412 5.0 6.0 386 297 209 121 32
5.0 6.5 20195 15570 10946 6321 1696 5.0 6.5 464 357 251 145 39
5.5 6.0 0 9205 6470 3735 999 5.5 6.0 0 211 149 86 23
5.5 6.5 0 12940 9095 5249 1403 5.5 6.5 0 297 209 121 32
6.0 6.5 0 0 6468 3732 997 6.0 6.5 0 0 148 86 23
High Organic Matter Mineral - Soil O.M. Between 10% and 19.9%
Sample Lime (CaCO3) Recommendations
Original. pH Target. pH (Lb./Acre)
Soil Buffer pH (BpH)
4.5 5.0 5.5
4.5 6.0 24852 20227 15602
5.0 6.0 0 16795 12949
5.5 6.0 0 0 9205
Muck Soil Organic Matter 20% or more
Sample Lime (CaCO3) Recommendations
Original pH Target pH Lb./Acre
Soil Buffer pH (BpH)
4.5 5.0
4.5 5.2 17422 14183
5.0 5.2 0 7302

Steps to Convert CaCO3 Recommendations to Local Ag Lime

Spectrum lime recommendations are expressed in terms of 100% pure Calcium carbonate (CaCO3) equivalent (CCE). They assume a 6.75 inch tillage depth and a lime fineness of 50% - 70% through a 60 mesh screed. If local lime does not meet these criteria, use the following steps to adjust final recommendations.

Step 1: Tillage Depth Adjustment Factors: (Multiply CaCO3 rate by
the factor listed across from appropriate plow depth)
Effective Tillage Depth (inches) Multiplying Factor
0 - 3 0.40
6 0.86
7 1.00
8 1.14
9 1.29
10 1.43
11 1.57
12 1.71
Step 2: Lime Type/Purity Adjustment Factors: (Select most appropriate lime type or purity and
multiply the results of step 1 by the factor listed across from appropriate type or purity).
Lime Type, Purity or Analysis Factor
90% to 110% (CCE or TNP) 1.00
80% to 89% 1.17
70% to 79% 1.33
60% to 69% 1.54
50% to 59% 1.81
100% pure CaCO3 (40% Ca) 1.00
Dolomitic Lime Factor
50% CaCO3 + 50% MgCO3 (22%Ca + 15%Mg) 0.92
75% CaCO3 + 25% MgCO3 (31%Ca + 7% Mg) 0.96
Other Materials Factor
*Calcium oxide (burnt lime) 0.56
*Calcium hydroxide (hydrated lime) 0.74
Granulated slag 1.00
*These materials may achieve the target soil pH in 1 to 12 days after application.
Step 3: Adjustments For Lime Grind Fineness: (Select the multiplying factor across from
the applicable screen size, and multiply the results of step 2 by that factor).
% Passing Through Screen Size Multiplying Factor
100 Mesh 60 Mesh
80-100 95-100 0.80
60-79 70-94 0.85
40-59 50-69 1.00
30-39 50-69 1.25
20-29 30-39 1.45
10-19 20-29 1.70
0-9 0-19 2.00

Acidifying Soils

Acidifying a field soil is normally an expensive process and only economical for the highest value crops, or very small areas. Because of this, the recommendation program does not automatically make recommendations for acidifying soils.

The following tables, adapted from various sources, list typical rates of elemental S required to reduce soil pH, plus conversion factors for common acidifying materials. The values listed are approximate, and individual soils may behave somewhat differently. Re-test the soil annually to monitor the amount of pH change caused by applications. Information from North Carolina indicates that low CEC soils that have an acid sub-soil could experience an excessive initial pH drop from rates of S higher than 300 lb./acre (0.69 lb/100 sq. ft.). Their guidelines indicate that crop damage has resulted from this effect. On such soils, it would be wise to make multiple, small applications of S, and monitor the pH change to avoid problems.

Sulfur Effect on Soil pH (lb.-S/acre)
Original pH Target pH Soil CEC
1 5 10 15 20 25 35
5.0 4.5 88 175 353 530 665 800 1120
5.5 4.5 175 350 700 1050 1325 1600 2234
6.0 4.5 265 530 1035 1540 1925 2310 3228
6.5 4.5 330 660 1340 2020 2525 3030 4251
7.0 4.5 420 840 1695 2550 3190 3830 5368
7.5 4.5 501 1002 2004 3006 3758 4509 6317
6.0 5.0 110 220 335 450 550 650 885
6.5 5.0 274 548 943 1337 1644 1951 2701
7.0 5.0 374 747 1257 1766 2168 2569 3547
7.5 5.0 469 937 1547 2157 2642 3127 4309
8.0 5.0 845 1689 2024 2357 2641 2924 3849
6.0 5.5 109 218 272 327 382 436 576
6.5 5.5 218 436 545 654 763 872 1151
7.0 5.5 327 654 818 981 1145 1308 1726
7.5 5.5 436 872 1090 1308 1526 1744 2301
8.0 5.5 763 1526 1799 2071 2344 2616 3414
8.5 5.5 1090 2180 2507 2834 3161 3488 4526
7.0 6.0 189 377 472 566 685 804 1051
7.5 6.0 343 686 870 1054 1213 1372 1825
8.0 6.0 682 1363 1575 1786 2047 2308 2980
8.5 6.0 1045 2090 2379 2667 2956 3244 4199
7.0 6.5 50 100 125 150 225 300 375
7.5 6.5 250 500 650 800 900 1000 1349
8.0 6.5 600 1200 1350 1500 1750 2000 2545
8.5 6.5 1000 2000 2250 2500 2750 3000 3871
8.0 7.0 519 1037 1126 1215 1453 1692 2111
8.0 7.5 437 874 901 929 1156 1384 1676

Adapted from the Western Fertilizer Handbook 7th ed.: Nursery Management, 2nd ed. H. Davidson, et al., 1988; the Highbush Blueberry Production Guide (NRAES-55), Northeast Regional Agricultural Engineering Service, M. Pritts and J. Hancock ed., 1992.; and Vegetable Growing Handbook, W. E. Splittstoesser, 1979.

Sulfur Effect on Soil pH (lb.-S/100 ft.2)
Original pH Target pH Soil CEC
1 5 10 15 20 25 35
5.0 4.5 0.20 0.40 0.81 1.22 1.53 1.84 2.57
5.5 4.5 0.40 0.80 1.61 2.41 3.04 3.67 5.13
6.0 4.5 0.61 1.22 2.38 3.54 4.42 5.30 7.41
6.5 4.5 0.76 1.52 3.08 4.64 5.80 6.96 9.76
7.0 4.5 0.96 1.93 3.89 5.85 7.32 8.79 12.32
7.5 4.5 1.15 2.30 4.60 6.90 8.63 10.35 14.50
6.0 5.0 0.25 0.51 0.77 1.03 1.26 1.49 2.03
6.5 5.0 0.63 1.26 2.16 3.07 3.77 4.48 6.20
7.0 5.0 0.86 1.71 2.89 4.05 4.98 5.90 8.14
7.5 5.0 1.08 2.15 3.55 4.95 6.07 7.18 9.89
8.0 5.0 1.94 3.88 4.65 5.41 6.06 6.71 8.84
6.0 5.5 0.25 0.50 0.62 0.75 0.88 1.00 1.32
6.5 5.5 0.50 1.00 1.25 1.50 1.75 2.00 2.64
7.0 5.5 0.75 1.50 1.88 2.25 2.63 3.00 3.96
7.5 5.5 1.00 2.00 2.50 3.00 3.50 4.00 5.28
8.0 5.5 1.75 3.50 4.13 4.75 5.38 6.01 7.84
8.5 5.5 2.50 5.00 5.76 6.51 7.26 8.01 10.39
7.0 6.0 0.43 0.87 1.08 1.30 1.57 1.85 2.41
7.5 6.0 0.79 1.57 2.00 2.42 2.78 3.15 4.19
8.0 6.0 1.57 3.13 3.62 4.10 4.70 5.30 6.84
8.5 6.0 2.40 4.80 5.46 6.12 6.79 7.45 9.64
7.0 6.5 0.11 0.23 0.29 0.34 0.52 0.69 0.86
7.5 6.5 0.57 1.15 1.49 1.84 2.07 2.30 3.10
8.0 6.5 1.38 2.75 3.10 3.44 4.02 4.59 5.84
8.5 6.5 2.30 4.59 5.17 5.74 6.31 6.89 8.89
8.0 7.0 1.19 2.38 2.58 2.79 3.34 3.88 4.85
8.0 7.5 1.00 2.01 2.07 2.13 2.65 3.18 3.85

Adapted from the Western Fertilizer Handbook 7th ed.: Nursery Management, 2nd ed. H. Davidson, et al., 1988; the Highbush Blueberry Production Guide (NRAES-55), Northeast Regional Agricultural Engineering Service, M. Pritts and J. Hancock ed., 1992.; and Vegetable Growing Handbook, W. E. Splittstoesser, 1979.

Common Acidifying Materials
Material Chemical Formula Percent Sulfur Lbs of Material to Equal 100 Lbs of Sulfur
Sulfur* S 99.0 100
Sulfuric Acid H2SO4 32.0 306
Sulfur Dioxide SO2 50.0 198
Iron Sulfate FeSO4.7H2O 11.5 896
Aluminum Sulfate Al2(SO4)3 14.4 694
Ammonium Sulfate (NH4)2SO4 23.7 422
*NOTE: The acidifying effect of elemental sulfur is caused by sulfur oxidizing bacteria. These bacteria must be present in the soil, in sufficient amounts, in order to have the desired effect. If a soils pH is above 7.2 in its natural state. it may not have a large population of sulfur oxidizing bacteria. In these cases it may be helpful to inoculate it by adding some soil from another source that is naturally acid. Also, the pH change caused by the bacterial oxidation of sulfur may be relatively slow (12 months or more) since they are dependent on sufficient soil moisture and temperature to accomplish efficient sulfur oxidation. The other products listed produce a chemical acidifying effect, independent of soil organisms and may be faster and more dependable than elemental sulfur.
Calculated Equivalent Acidity of Common Nitrogen Materials
N Source % N Chemical Formula 100 Lb of Nitrogen* 100 Lb of Fertilizer*
Ammonium Sulfate 21 (NH4)2SO4 535 151
Anhydrous Ammonia 82 NH3 180 295
Ammonium Nitrate 34 NH4NO3 180 122
Urea 46 CO(NH2)2 180 166
UAN 28-32 CO(NH2)2+NH4NO3 180 101-115
Calcium Nitrate 15 Ca(NO3)2 135B 20B
Sodium Nitrate 16 NaNO3 180B 29B
Potassium Nitrate 13 KNO3 200B 26B
Adapted from the Potash and Phosphate Institutes Soil Fertility Manual
* Pounds of calcium carbonate (CaCO3) needed to neutralize the acidity formed from 100 pounds of nitrogen, or nitrogen containing fertilizer. The "B" denotes a basic (pH increasing) effect. These are theoretical values and may differ somewhat in actual soil.
 
library/articles/soil_ph_management.txt · Last modified: 2016/04/28 15:12 by bill