Soil Fertility Management for Forage Crops: Pre-establishment

Soil fertility management for forage crops is a continuous process that begins well before the forage crop is established. In the pre-establishment phase, soil conditions are adjusted to ensure optimum soil fertility when the crop is established. At establishment, only minor adjustments in soil fertility should be required (e.g., starter fertilizer). Finally, once the crop is established, management focuses on maintaining optimum soil fertility levels for the life of the forage stand. This soil fertility management timeline is illustrated in Figure 1.

This fact sheet deals with the pre-establishment phase of soil fertility management for forages. The goal should be to have the soil pH and Mehlich 3 extractable phosphorus (P) and potassium (K) levels in the optimum range, as indicated in the table below.

Soil pH and Nitrogen in Legumes

Crops vary in their sensitivity to soil pH. Generally, alfalfa is the most pH sensitive forage crop, followed by the other legumes and then by the grasses. In legumes, pH sensitivity is related primarily to the nitrogen fixation process. At low soil pH, the bacteria responsible for nitrogen fixation are not very active and thus the crop can suffer from nitrogen deficiency (Figure 2). This is a scenario where planning ahead is critical in stand establishment. Correcting soil pH at least six months before planting will give legume seedlings the best nutrient availability for plant growth and root nodulation. Applying nitrogen without correcting soil pH will only temporarily mask soil pH issues and is not economical. With nitrogen fertilizer applied, soil pH has little or no effect on alfalfa yield. However, with no nitrogen applied, there is a very dramatic decline in alfalfa yield as soil pH decreases below the optimum range. Therefore, it boils down to adding fertilizer to maintain nitrogen sufficiency or applying lime to maintain the proper soil pH. Applying limestone is much more economical than adding nitrogen fertilizer. A typical requirement of 2 tons of lime, including spreading costs, every three or four years would be about $25 per year. This compares favorably to 250 pounds of nitrogen—the amount of nitrogen alfalfa would normally be able to fix by itself if pH was in the optimum range—required every year, which would cost over $75 per year depending on fertilizer prices. Adding to the cost of the nitrogen fertilizer are the application costs. Efficient uptake and use of several hundred pounds of nitrogen by any forage crop requires split applications of nitrogen (see “Nitrogen" section, below), adding greatly to the cost of a nitrogen fertilization program for the production of forage legumes unable to fix their own nitrogen due to low soil pH.

Soil pH and Herbicides

Soil pH also has an impact on triazine, sulfonylurea (chlorimuron, chlorsulfuron, metsulfuron), and imidazolinone (imazaquin) herbicide residues, which can affect the forage seeding. Low soil pH causes triazine and imidazolinone herbicides to bind to the soil, which has several important implications. First, when these herbicides are bound this way, they are unavailable for controlling weeds in a previous corn crop. This often results in increasingly higher rates of herbicide(s) being used to get adequate weed control. These higher rates multiply the amount residual herbicide in the soil when the forage is seeded. Further, liming these soils releases bound herbicides; postponing liming until just before seeding enables the herbicides to control not only weeds but also many of the forage seedings one might wish to establish. This result is aggravated if Eptam is used when the forage is seeded. Conversely, in higher pH soils, the triazine and sulfonylurea herbicides persist longer and more is available for plant uptake, thus causing injury or death to establishing forage seedlings. The best way to avoid these problems is to maintain the soil pH above 6.2 throughout the crop rotation. This will minimize the adsorption of the herbicide to the soil and result in maximum effectiveness at minimum herbicide rates in the previous crop. In turn, this will reduce the residual herbicide in the soil and avoid large releases of bound herbicide just prior to seeding.

Also be aware that certain herbicides (e.g., Group 27 or HPPD-inhibiting herbicides and others) can negatively impact forage establishment under any conditions. Therefore, always refer to the product label for crop rotation guidelines.

Liming Practices

There are several important considerations for liming practices. The rate of limestone required must be determined by a soil test that includes both a soil pH and a lime requirement test. The soil pH measurement tells you whether or not limestone is needed, while the lime requirement test tells you how much limestone is required to neutralize the acidity in the soil and raise the pH to the correct level. Limestone should be applied at least six months to a year ahead of seeding. If the test calls for more than 4 tons of limestone per acre, the application should be split—with half of the limestone plowed in and the rest worked into the surface with secondary tillage. For low rates of limestone or if a split application is not possible, the limestone should be worked into just the top 2 inches of soil rather than plowed down. This ensures the surface soil, where the seedling is developing and nodulation begins in legumes, has the proper pH.

In no-till management systems, monitoring pH is particularly important—and sometimes challenging because limestone cannot be incorporated into the soil with tillage to neutralize acidity throughout the topsoil layer. Therefore, before initiating a permanent no-till system, be sure the whole topsoil layer is at the appropriate pH, and if necessary, make soil pH adjustments while limestone can still be incorporated into the soil with tillage. After commencing the no-till system, acidity will develop at the soil surface where crop residues accumulate and nitrogen fertilizer is applied. When taking soil samples in long-term no-till systems, if a soil sample taken to the regular depth of 6 inches indicates that lime is required, then apply the recommended amount of limestone to the soil surface. If no limestone is recommended based on the analysis of the regular soil sample, take an additional soil sample from 0-to-2-inch depth and check the soil pH with a field test kit or submit it to a lab to monitor for surface acidity. If the pH in the 0-to-2-inch depth is less than 6.2, then apply 2,000 pounds per acre of calcium carbonate equivalent (CCE).

Not all limestone is equal. The rate of the limestone applied should be adjusted for its neutralizing ability, which is given as the CCE of the limestone. All soil test recommendations are made on the basis of 100 percent CCE. This does not mean only 100 percent CCE limestone should be used. The rate of application for any limestone with a CCE higher or lower than 100 percent must be adjusted. The formula for making the adjustment is as follows:

Adjusted limestone recommendation = 100 × limestone recommendation ÷ CCE

An explanation and a table showing how to make this adjustment are provided in "Agronomy Facts 3: Soil Acidity and Aglime."

The fineness of the limestone is also an important quality consideration. The finer the limestone is ground, the more rapidly it reacts in the soil to neutralize the acidity. To be effective, a minimum of 95 percent of the limestone should pass a 20-mesh screen, 60 percent should pass a 60-mesh screen, and 50 percent should pass a 100-mesh screen. Generally, there is little practical advantage in using a liming material that exceeds these fineness standards. It will probably only be advantageous to pay more for a finer limestone in an emergency where the pH is very low and rapid neutralization is required.

Nitrogen

Nitrogen is a key nutrient in forage establishment. Legumes create their own nitrogen through a symbiotic relationship with rhizobia bacteria and therefore do not require supplemental nitrogen. However, grasses do require nitrogen for plant development. Mixed stands of grass and legumes that contain 50 percent or more legume obtain enough nitrogen through nitrogen fixation and recycling of roots and residues, and therefore do not need any nitrogen applied. Inorganic nitrogen fertilizers do not accumulate in the soil and can be lost to the environment if application timing does not match well with plant uptake. Therefore, any nitrogen applied during the pre-establishment phase should be minimal and timed as closely as possible to planting of the forage stand.

Phosphorus and Potassium

Forage crops have a high demand for phosphorus and potassium. Thus, it is critical that soil levels of these nutrients be built up into the optimum range in preparation for growing forage crops. Phosphorus and potassium are relatively immobile in the soil. For example, phosphorus may move less than 1/8 inch a year in soil. To bring the rooting zone of a soil into the optimum level for crops, the phosphorus and potassium must be thoroughly mixed with the soil. Therefore, it is very important that the soil levels be built up before the perennial forage crop is established because, once the crop is established, there is no way to effectively mix the phosphorus and potassium with the soil in the rooting zone. A similar issue with limited mobility of phosphorus and potassium can occur in permanent no-till systems. It is difficult to bring low soil phosphorus and potassium levels into the optimum range without some tillage to mix the nutrients throughout the primary rooting zone of the crop. In no-till cropping systems this is an especially difficult problem because there is no opportunity for mixing. If soil fertility is low in no-till, it is suggested that at some point in the crop rotation the recommended limestone, phosphorus, and potassium be applied and tilled into the soil to build the fertility levels. Once the soil levels are in the optimum to high range on the soil test, regular surface applications of the limestone and the fertilizer phosphorus and potassium in a no-till system can maintain these levels. When preparing to implement a permanent no-till system, be sure to raise nutrient levels into the optimum range while tillage is still an option.

Calcium and Magnesium

Calcium and magnesium are essential secondary nutrients required by crops. Legumes have an especially high demand for calcium and magnesium. A soil test is the best guide for determining the calcium and magnesium needs of a soil. Almost all calcium and magnesium is supplied to crops in limestone. In Pennsylvania soils in general, if the soil pH is in the optimum range, there will be adequate calcium for the crop. However, depending on the soil’s origin and the source of limestone that has been used in the past, magnesium may not be adequate. A soil test level of less than about 60 ppm is usually considered deficient in magnesium. When this occurs, a limestone should be used that contains magnesium to build the magnesium level in the soil into the optimum range. The amount of magnesium required depends on the CEC of the soil and the soil magnesium level.

Soil test recommendations for magnesium are given in several ways. The recommendation is sometimes given as the amount of magnesium to apply or as a minimum magnesium content in the recommended limestone. Recommendations on Penn State soil tests are given in both of these ways. Sometimes the recommendation is simply “Use a dolomitic limestone." This is less desirable because there is no set definition or minimum magnesium content for a limestone to be labeled as dolomitic. If the soil is optimum or high in magnesium, either a calcitic or dolomitic limestone can be used. The decision should be based on the availability and the cost of limestone.

High soil potassium can have a negative effect on calcium and magnesium uptake by crops. This antagonism does not usually affect the yield of the crop, but it can have a negative effect on animal performance. In particular, grasses that are low in magnesium and high in potassium when they are the primary feed for cattle can result in hypomagnesemia, or grass tetany. A similar problem known as milk fever or hypocalcemia can occur in animals when calcium levels in the forage are low relative to potassium. These are primarily problems in systems where cattle get most of their forage from the pasture. Pastures that have not been limed with a dolomitic limestone and that have had potassium fertilizer applied are the most susceptible to this problem, particularly early in the spring. When soil fertility samples are submitted to the Penn State Agricultural Analytical Services Lab and recommendations are requested for a forage grass, if the soil potassium level is above 200 ppm, the magnesium fertilizer recommendation will be increased dramatically to counteract the antagonism between potassium and magnesium. If the potassium and magnesium antagonism cannot be resolved through soil fertility management, consult with a nutritionist to determine ways to supplement magnesium in the livestock ration to maintain animal health.

Manure

On farms with livestock, manure applied to corn is a major contributor to meeting the phosphorus and potassium goals at establishment. When manure is applied to corn to meet its nitrogen needs, excess phosphorus and potassium are also applied (Figure 3). This excess phosphorus and potassium builds soil levels, often into the above optimum range, and can be used by a forage crop later in the rotation. When manure is not available, fertilizer must be applied to the corn to replace the nutrients removed in the corn crop and, if necessary, to build the soil levels into the optimum range before forage establishment. When applying manure to forages, grasses should take priority over legumes, as the grasses can benefit from nitrogen in the manure.

Keeping records and following soil test trends can be very helpful in finetuning the fertility program for a corn forage rotation (Figure 4). Notice the buildup during the corn part of the rotation when manure is being applied to meet a significant proportion of the corn’s nitrogen needs, and a drawdown of the soil nutrient levels in the forage part of the rotation. Also notice that over time the trend in soil test levels should be relatively flat and maintained with the low points in the trend toward the lower end of the optimum range.

Soil Testing

Soil test recommendations from the Penn State soil testing program are designed to gradually build soil phosphorus and potassium levels into the optimum range and then maintain them by replacing the nutrients that the crop removes. A regular soil testing program should be followed in fields by sampling at least every three years or when the cropping system in a field significantly changes. When submitting a soil sample to Penn State for a fertility analysis, the submission form requests information on the crops that will be grown for the next three years of the rotation, and fertilizer recommendations will be given for these three years based on the build-and-maintain philosophy. After three years of cropping, a new soil test should be conducted to assess current fertility levels and make recommendations for the future based on those nutrient levels.

A crucial part of a good soil testing program is to take good soil samples. Recommended guidelines for taking soil samples are provided below. Most of the errors associated with soil testing are a result of samples that are not representative of the fields being tested. Remember, the recommendations can only be as good as the sample and the information supplied to the lab.

The soil test prior to establishment of a forage crop is especially important. A regular soil testing program along with application of the recommended nutrients as fertilizer or manure is critical in the crops prior to forage crop establishment and should result in meeting the pre-establishment soil fertility goals.

Guidelines for Taking Soil Sample

1. Sample at the right time. The best time to sample from a logistical standpoint is in the fall. However, when establishing a forage crop, soil samples should be collected at least six months prior to establishment to allow for soil pH adjustments, if necessary.
2. Take cores from at least 15 to 20 spots randomly over the field to obtain a representative sample. One sample should not represent more than 10 to 20 acres.
3. Sample between old crop rows. Avoid old fencerows, dead furrows, and other spots that are not representative of the whole field.
4. Take separate samples from known problem areas.
5. Sample to plow depth in cultivated fields.
6. Take two samples from no-till fields: one to a 6-inch depth for lime and fertilizer recommendations, and one to a 2-inch depth to monitor surface acidity.
7. Sample permanent pastures to a depth of 3 to 4 inches.
8. Collect the samples in a clean container.
9. Mix the core samplings, allow them to air-dry, and remove roots and stones.
10. Fill the soil test mailing container.
11. Complete the information sheet, giving all information requested.

Summary

In the pre-establishment phase, the soil conditions must be adjusted to provide optimum soil fertility when the crop is established. Liming during the last year in corn and applying fertilizer and/or manure during the years preceding the forage seeding is important for building soil levels into the optimum to high range. A regular soil testing program plays a critical role in this process.

Revised by Charlie White, Hanna Wells, John Spargo, and Ron Hoover. Originally prepared by Douglas Beegle, Distinguished Professor Emeritus of Agronomy.