Soil pH And Liming In KS: Part 2 -- Is My Soil Acid And How Much Lime Is Needed?

 

In Part 1 of this series of Agronomy eUpdates articles we discussed how acids and soil acidity are defined, the concept of buffering and how the balance of cations on the soils cation exchange sites maintains a relatively constant pH in the soil solution -- especially in the common range of pH’s seen in agricultural soils, roughly 5 to 7. In this article we will discuss different methods of soil sampling to measure soil pH and acidity, how lime recommendations are made, and different approaches one might take to applying lime today.

 

 

Soil acidity and resulting lime needs can vary widely across a field or across the landscape. A number of factors impact lime needs, such as past soil erosion, previous soil management including N application rates, and differences in soil organic matter and clay content. Thus lime needs will often vary within a field, making a precision or site-specific liming program economical and something that should be considered by many farmers.

 

The role of soil buffer capacity in liming. The amount of lime needed to obtain a required change in soil pH, varies with the soil’s cation exchange capacity (CEC) and buffering capacity. Cation exchange capacity refers to the amount of the soil’s negative charge and the amount of positively charged ions such as calcium and magnesium or hydrogen which can be held. Cation exchange capacity is a function of soil organic matter and clay content. The higher the organic matter content and the heavier the soil texture, the higher the soil’s CEC. Since soil pH is a reflection of the percent acids on the exchange sites, acidic soils with a high CEC contain more acidity than low-CEC soils with the same pH. Thus, more lime will be required to get the same change in pH in a high-CEC soil than a low-CEC soil.

 

Fortunately or unfortunately depending on your perspective, soils are most highly buffered -- or resistant to change in pH -- at the extremes: very acid, and very basic. Soil organic matter is by nature a weak organic acid, and has a number of reactive chemical groups such as carboxyl and phenolic hydroxide structures which can donate hydrogen ions to the soil solution as the pH goes up. This serves to prevent a rapid rise in pH. The reverse happens as the pH of the soil begins to go down. The hydrogen ion re-associates with the carboxyl group, removing it from the soil solution, reducing the hydrogen/acid concentration in the soil solution, and in effect maintaining or raising the pH. This process effectively provides a bottom limit of around pH 4 for most soils. 

 

Calcium carbonate (limestone) provides a similar upper limit on pH in most soils where calcium is the dominant exchangeable cation. Calcium carbonate tends to precipitate at a pH of 8.2. So when lime is added to the soil, it will react with the hydrogen in the soil water at low pH and dissolve, releasing calcium into the soil solution. This will replace both the hydrogen in the solution and that on the CEC. As the pH increases, the lime becomes less soluble and no longer dissolves. So unless large quantities of salts, especially sodium, are present the pH stabilizes around that 8.2 point. Adding more lime will not increase the pH further.

 

Variation in pH and buffer requirement across a field. Many fields vary in organic matter content and soil texture. It is important that soil samples are taken in a manner that captures the variations in pH and buffering capacity rather than mixing everything together into one sample and obtaining an average. This process may require carefully selecting sample locations within a field to minimize the variation and more efficiently use lime. 

 

A detailed soil survey is available online for all Kansas counties and provides a helpful guide to soil difference that can be expected in a field. Out in the field, soil samples should be separated primarily on the basis of soil color differences, which reflect variations in organic matter. Separate soil samples should be taken also where the texture of soil surface varies widely or when historical differences in management between parts of a field are known to have occurred. Old aerial photographs can provide excellent information on previous farming activities.

 

With the advent of Geographic Positioning Systems and yield monitors, a number of other methods to assess soil variability have also become widely used.  Management zone sampling has become popular among farmers who utilize yield monitors.  This system entails identifying management zones, or areas with similar yields and soils, and using these as soil sampling areas. 

 

Grid sampling has also been used in some areas as a routine way to estimate soil variability. Grid sampling systems are not new, and have been recommended by the University of Illinois since the 1920's. Grid cells will routinely vary from 1 to 5 acres in size. Lime and fertilizer may be applied based directly on the sample results from the grid, or the data may be analyzed using geo statistical techniques to develop nutrient maps. Smart zones utilizing grid samples, soil maps, and yield maps provide very good ways to build lime application maps.

 

Use the appropriate sampling depth. Soil samples for routine pH, P, and K recommendations for grain crops are normally taken using a 6- to 7-inch sampling depth. However, sample depth needs to be adjusted when working with conservation tillage systems or in forage fields. If a moldboard plow or twisted shank chisel plow is used at least once every four or five years, take samples for fertilizer and lime recommendations at a 6- to 7-inch depth. While most of the soil mixing occurs in the upper one-half to two-thirds of the chisel depth (depending on the type of chisel points used), and nutrient stratification is known to occur, enough mixing occurs to get lime, and lime effects, throughout most of the surface soil.

 

Where fields are in continuous no-till for four years or more, especially where nitrogen fertilizer is broadcast or sprayed on the soil surface, or in pastures and hayfields, soil samples for pH and lime requirement should be taken from the top 3 inches of soil. This will identify the acidity stratified near the surface, which is also the acidity which a surface application of lime will neutralize. An adjustment of lime rate recommendations will also be necessary in these situations. Most soil testing laboratories make lime recommendations assuming that the lime will be mixed with 6 to 7 inches of soil. Since lime is relatively insoluble, the effects are limited to the top 3 to 4 inches of soil when applied on the surface. Thus normal lime rates should be cut in half when surface applying lime to no-till fields, pastures, or hay fields.

 

 

Soil testing labs routinely use a two-step process to measure soil acidity and determine lime requirement. The first step is to mix a sample of dry soil with deionized water and measure soil or water pH. If the soil:water pH falls more than 0.2 pH units below the recommended minimum pH level for the crop being grown, lime is normally recommended. The recommended pH varies with both crop being grown and soil characteristics. Subsoil depth and pH are important factors that influence response to lime. The recommended pH’s for different crops grown in different areas of Kansas are summarized in Table 1.

 

The pH measured in water reflects the environment plant roots are exposed to and is routinely used to determine if lime is needed. Water pH is not a good measure of the amount of lime needed to change soil pH, however. The water pH is a reflection of the percent base saturation of the soil. But it does not give any indication of the quantity of acids or bases that are present or the buffer capacity of the soil. To determine the amount of lime that will be required to neutralize the acidity on the exchange sites, a buffer test is used. Buffers are solutions designed to resist change in pH. By mixing a buffer of known pH with an acid soil and measuring the drop in pH that results, one can calculate the amount of lime needed to reach the desired pH on that particular soil.

 

In most of the Midwest, the SMP buffer, or the Sikora modification of the SMP, is used to estimate the lime requirement of mineral soils. The SMP buffer is adjusted to an initial pH of 7.5, and upon mixing the buffer with an acid soil the pH drops. The more the pH drops, the more lime will be required. Most soil test reports will give the measured pH of the soil:buffer mixture as either the SMP Buffer pH or the Lime Index. Lime index is created from buffer pH by removing the decimal point, i.e. a buffer pH of 6.4 is a lime index of 64.

 

The lime recommendations for mineral soils based on the SMP buffer test used in Kansas are summarized in Table 2. The rates are given as pounds of Effective Calcium Carbonate as ag lime, and assume that primary tillage such as moldboard or chisel plowing will be used to incorporate the lime to a 6- to 7-inch depth. For no-till or conservation-till systems or established forages, mixing of lime with the soil is minimal, and reaction and change of pH only occurs in the surface 2-4 inches of soil. Therefore the lime rates should be reduced 50%. 

 

If the lime recommendation is for 6,000 lbs ECC per acre or less, the lime can be applied at any time in a cropping sequence. When the lime recommendation exceeds 6,000 lbs ECC per acre, apply half the lime before primary tillage to improve mixing with the total tillage zone, and then re-test in 12 to 18 months to determine the remaining amount needed.

 

Low CEC Soils. Sandy soils have a low CEC and are very weakly buffered. In some cases the water pH may be below the recommended target pH, but the buffer pH may not indicate a need for lime. This is because these soils do not have enough acidity to lower the pH of the buffer solution. When this occurs, apply 750 lbs ECC per acre if the water pH is more than 0.3 pH units below desired, or 1,500 lbs ECC per acre if the pH is more than 0.6 pH units below desired.

 

What if you knife-in N and have bands of acid soil below the surface? Our standard recommendation for liming and pH management is to take one set of samples to a 6-inch depth in conventional-till, or a set of soil samples 0-3 inches deep on continuous no-till or grass hayfields. When most or all of the nitrogen (N) is surface-applied without soil mixing, the most common way Kansas no-till farmers apply N, the resulting acidity is normally confined to the soil surface, and easily corrected with surface-applied lime. But what if you knife in UAN solutions or anhydrous ammonia as your primary N source? 

 

Where anhydrous ammonia or liquid UAN is knifed-in or coulter-banded below the surface, an acid zone will develop deeper in the soil. With ammonia, this will usually be 2-3 inches above the release point where the fertilizer is placed in the soil. So if the ammonia is injected 8 inches deep, there will be acid bands 5 to 8 inches below the soil surface. In controlled traffic systems these bands will expand over time as more and more N fertilizer is placed in the same general area. The graphic below illustrates the effect of a high rate of ammonia (200 lbs N/acre or more) placed in the same general area in the row middle on a high CEC soil for more than 20 years.

 

The actual depth of the acid zone in fields fertilized with ammonia gets tricky as application depth can vary depending on the tool used to apply the ammonia. Traditional shank applicators generally run 6 to 8 inches deep, so a sample for pH measurement could be taken at 3-6 inches or 5-8 inches deep, depending on how deep the shanks were run. The new low-disturbance applicators apply the ammonia 4-5 inches deep. A sweep plow or V-blade applies ammonia only 3-4 inches deep. So sampling depth for pH should really depend on where the acid-forming N fertilizer is put in the soil.

 

Where do you place the lime in continuous no-till? If you surface apply N, then surface apply the lime. That’s a simple but effective rule. But remember that surface-applied lime will likely only neutralize the acidity in the top 2-3 inches of soil. So if a producer hasn’t limed for 20 years of continuous no-till and has applied 100 to 150 pounds of N per year, there will probably be a 4-5 inch thick acid zone, and the bottom half of that zone may not be neutralized from surface-applied lime. So, if a producer is only able to neutralize the top 3 inches of a 5-inch deep surface zone of acid soil, would that suggest he needs to incorporate lime? Not necessarily.  

 

Research has shown as long as the surface few inches of soil is at an appropriate pH and the remainder of the acid soil is above pH 5, crops will do fine. In fact, in some situations acid bands of soil below the surface could be an advantage by increasing the availability of some essential metals such as zinc, iron, and manganese.

 

 

Liming benefits crop production in large part by reducing toxic aluminum, supplying calcium and magnesium, and enhancing the activity of some herbicides. Aluminum toxicity doesn’t normally occur until the soil pH is below 4.8. At that pH the Al in soil solution begins to increase dramatically as pH declines further. Aluminum is toxic to plant roots, and limiting root growth will reduce the plant’s ability to take up nutrients and water.

 

Yield enhancement is not the only concern with low-pH soils, however. Herbicide effectiveness must also be considered. The most commonly used soil-applied herbicide impacted by pH is atrazine. As pH goes down, activity and hence performance goes down. So in acid soils weed control may be impacted. We do see that in corn and sorghum production.

 

Acidity levels and resulting lime needs can vary widely across a field. So a soil sampling plan should be used which will define this variation and allow a farmer to apply lime efficiently where it is needed.

 

Making lime recommendations is a two-step process: Use the soil pH to determine if lime is needed, and a buffer pH to determine the rate needed to achieve the desired pH change.

 

Nitrogen fertilizer is the primary cause of soils becoming acid. The N application placement method and the tillage system used will determine where the acidity will accumulate in the soil. If the soil is tilled regularly, the acidity will be distributed throughout the tillage zone. Adding lime and incorporating it through tillage will also neutralize the acidity throughout the tilled zone. However in no-till systems or in established forage crops, lime incorporation is not possible and surface-applied lime will only react with acidity in the top 2-4 inches of soil. Soil sampling and lime rates are adjusted to reflect this limited zone of activity.

 

With the relatively high cost of liming and the variability in recommended lime rates across a field, development of variable-rate lime programs should be considered. By adjusting rates to fit the needs of specific areas within fields, costs can be reduced and the over-application of lime – which reduces the availability of some nutrients such as zinc and iron – can be avoided.