Soil Fertility Management & Soil Acidity and Liming

The goal of soil fertility management is to create soil chemical conditions that encourage plant growth and supply required nutrients in the amounts and at the times they are most needed. Liming materials and plant nutrients may be added to the soil in many forms and can be done so in a way that maximizes the economic benefits of nutrients while minimizing any environmental impact. The ways in which crops respond to these applications often are different because some soils have inherent physical limitations to plant growth. Soil testing is the best guide to soil fertility. Plant tissue analysis also may be helpful when used in conjunction with soil testing. Some highlights of soil fertility management are presented in the following sections.

SOIL ACIDITY AND LIMING

Liming an acid soil is the first step in creating favorable soil conditions for productive plant growth. Crops vary in their ability to tolerate an acidic (low pH) soil. In addition, evidence has shown that soil acidity may influence other crop management problems such as herbicide activity. Soil pH is a good indicator of the need for liming, but a buffer pH measurement is necessary to determine the quantity of soil acidity to be neutralized in order to change the soil pH. The general goal of liming agricultural soils continues to be a soil pH of 6.0 to 7.0.

Raising soil pH requires a quantity of agricultural liming material that is determined by the amount of acidity in the soil and the quality of the liming material. Soil acidity is measured by soil testing; the quality of agricultural liming material is determined by its purity and particle size distribution. In Pennsylvania, agricultural liming materials must, by law, meet minimum quality standards.

Actual liming material required = (soil test limestone recommendation ÷ % CCE of liming material) × 100

Example: Apply 4,000 lbs calcium carbonate equivalent/A Liming material label: Calcium carbonate equivalent (CCE) = 80 percent Actual liming material required: (4,000 ÷ 80) × 100 = 5,000 lbs liming material/A

·         Purity

The limestone recommendation on most soil test reports, including the Penn State soil test report, is based on the use of a liming material that is equivalent in neutralizing power to pure calcium carbonate—that is, a material with a calcium carbonate equivalent (CCE) of 100 percent. This does not mean that you have to use a liming material that is 100 percent CCE. However, you must adjust the recommended amount of any liming material not equivalent to pure calcium carbonate limestone in neutralizing power (higher or lower than 100 percent CCE) so that you actually apply the correct amount of liming material to neutralize the acidity in your soil. All agricultural liming materials sold in Pennsylvania must, by law, be labeled with their calcium carbonate equivalent. Using the CCE of your liming material, you can calculate the actual amount required to neutralize the acidity in your soil (as shown to the right), or read directly from below Table-1

·         Fineness

A liming material must be finely ground to be effective. This is important because (1) limestone’s solubility increases as it is ground finer, and (2) limestone affects only a very small volume of soil around each particle. Therefore, more finely ground limestone has more particles and, if there is adequate mixing, neutralizes more of the soil with which it comes into close contact. The effect of limestone fineness on changing soil pH is illustrated in Figure 1.2-1.

In each case in Figure 1.2-1, sufficient aglime was applied to neutralize the soil acidity to raise the pH to 7.0. Note that material larger than 20 mesh was of little value for increasing soil pH and that only 100-mesh material came close to achieving that goal in a short time. Therefore, it would seem desirable to use only limestone that is 100 mesh or smaller. However, this decision must be balanced against the high cost of grinding limestone to finer than 100 mesh. A compromise must be reached so that the material is fine enough to be effective agronomically but still economically sound.

Pennsylvania aglime regulations classify agricultural liming materials into the following three groups based on fineness:

1. Fine-sized

95 percent through a 20-mesh screen

60 percent through a 60-mesh screen

50 percent through a 100-mesh screen

2. Medium-sized

90 percent through a 20-mesh screen

50 percent through a 60-mesh screen

30 percent through a 100-mesh screen

3. Coarse-sized

All liming materials that fail to meet one of the above minimums for fineness.

A material meeting the standard for a “fine-sized” liming material is considered adequate for meeting soil test recommendations in most situations. It is assumed that fine-sized materials will react rapidly enough to effect a change in soil pH in the year of application and typically will remain effective for about three years. The medium- and coarse-sized materials are slower to react; thus they are less effective in changing soil pH during the year of application and take longer to react completely.

The actual fineness must be printed on the label. The calculations and table for adjusting your recommendations for the CCE of your liming material assume that the material meets the minimum fineness standards for fine-sized limestone. When selecting a liming material, there generally is little advantage in exceeding the minimum standards for fine-sized material. In emergency situations where a very rapid change in soil pH is required, paying extra for a finer material may be warranted; however, planning ahead by using a less expensive material and giving it time to react normally, generally will give better and more economical results.

·         Pellet and fluid lime

Pellet and fluid lime are forms of limestone that provide alternative application methods. Both materials are made up of very finely ground limestone. Pellet lime is manufactured by forming the limestone into water-soluble pellets that can be spread like fertilizer. This allows easier handling and more uniform spreading—after application, the pellets quickly break down and release the limestone. Fluid lime is made by suspending the finely ground limestone in a water and clay suspension. This material then can be applied with a sprayer. Fluid lime can be sprayed very uniformly. Both materials are excellent liming materials that will react very rapidly; however, the very fine nature of these materials does not increase the neutralizing value of the limestone. Thus, the same amount of calcium carbonate equivalent is required with these materials as with regular pulverized limestone. Because these materials are usually more expensive than regular pulverized limestone, people will sometimes use a lower rate, assuming the fineness will compensate. This is not true. You need the full recommended rate regardless of the fineness or physical form of the material.

For maximum effectiveness, incorporate the liming material. Remember that minimum tillage mixes to only one-third of tillage depth. Since liming material incorporation is not possible in no-till situations, the soil should be limed adequately before no-till crop production begins. Also, it is very critical in long term no-till management systems to maintain soil pH near to optimum. No special management is required and this can be accomplished with a normal liming program every three years or so. The important thing is to not let the soil pH become too low because it can be very difficult and take a long time to correct a very low pH in a continuous no-till system. This same recommendation also applies in long-term perennial forage fields.

o   Calcium and magnesium

Although limestone is applied primarily to neutralize soil acidity, it also serves as a source of essential calcium (Ca) and magnesium (Mg). When a soil test indicates the need for magnesium, limestone is the most economical way to meet that need. The magnesium content of limestone varies considerably. Unfortunately, there is no official trade classification of limestone according to magnesium content. Local classification schemes often create confusion. Therefore, to select the proper limestone material, it's best to rely on the actual magnesium analysis rather than a name (e.g., dolomitic lime, high-magnesium lime, or high-calcium lime). Generally, if soils are limed to the recommended pH, there will be more than enough available calcium for agronomic crops. This is true even when a high-magnesium lime is used.

Penn State soil test recommendations not only indicate the amount of magnesium the crop requires, but also include a magnesium analysis of the limestone recommended to supply the needed magnesium.

o   Additional considerations

Although the best practice is to apply the recommended amount of a liming material, a partial application should be considered rather than no application at all. If only partial applications are made, it is best not to attempt to grow acid-sensitive crops such as alfalfa.

When less than the full limestone requirement is applied, make the application after primary tillage and incorporate it by secondary tillage, thus mixing the limestone 4 inches into the surface. This ensures a better environment for germinating seeds and young seedlings, as well as more effective herbicide action in the surface soil layer.

When high rates of limestone are recommended (4 tons or more per acre) the applications should be split. See Table-1 for guidance on splitting high limestone recommendations. Separate the applications by 6 months’ time or at least by tillage operations. In a tillage based management system, plowing part of the limestone down and then applying the balance and incorporating with secondary tillage is an excellent way to correct the pH in the whole plow layer.

As soil acidity increases (soil pH becomes lower), the need to apply a liming material to any crop and the expected returns of applying it become greater. If soil acidity is limiting crop production, other production inputs such as fertilizer, seed, pesticides, machinery costs, and labor will not realize the kind of return that would be possible on a well-limed soil.