Forage Focus: Potassium & Manure On Alfalfa

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With high fertilizer potassium (K) prices, many farmers have not been applying the recommended K or are applying K at a reduced rate. As a result, K deficiencies have become common. This is especially a problem in forage systems because of the large amount of K removed by forage crops. For example, alfalfa removes 50 lb K2O/ton of dry hay equivalent harvested…that's 250 lb K2O/A for a 5 ton/A yield and corn silage removes a similar amount. Over time this K removal must be replaced to maintain soil fertility. This need can be met with 400 lb potash fertilizer (0-0-60) or with 10,000 gal/A or 30 ton/A of typical dairy manure. (Dairy manure on average contains 25 lb K2O/1000 gal or 8 lb K2O/ton, all available similar to fertilizer.) Because of the higher fertilizer prices, there is increased interest in manure to meet this requirement.

In the typical corn silage alfalfa rotation, manure is often applied to the corn silage which helps maintain soil K levels. However, manure is not usually applied to the alfalfa so that can rapidly deplete the soil K. Regular soil testing is important to make sure there is adequate K for all of the crops. Manure is a good source of P and K for alfalfa but the alfalfa does not need the N in manure like corn does thus there is no economic value to manure N applied to alfalfa. Therefore, the first priority economically is to apply the manure to the corn fields to get the value from all of the nutrients. Then, if there is more left over this can then be applied to the alfalfa fields to supply P and K.

If you have the choice of which alfalfa fields to spread manure on go to the older fields and use fertilizer on the newer seedings. There are several words of caution about spreading manure on alfalfa.

* Do not spread a large amount at one time to avoid smothering the alfalfa,

* During the growing season spread as soon after harvest as possible to avoid burning the new growth or injuring the new growth by manure spreader traffic.

* Avoid spreading when the soil is wet to avoid compaction.

* Realize that spreading manure on alfalfa has the potential to introduce or encourage weed growth. So good weed control is essential.

If you must spread manure in the winter, alfalfa fields may be the most acceptable fields. We all know that winter is not the best time to apply manure and should be our last choice. The best nutrient utilization will come from applying the manure as close to the time of crop uptake as possible, therefore winter is not a good time to apply. Also, because of no crop utilization and frozen/snow covered soils there is high potential for nutrient loss in the winter. So if we could avoid applying manure to frozen, snow covered soils that would be the best approach but this is not always practical.

In the winter when temperatures in the surface soil fall below 50°F the potential volatilization losses are less, however there is a significant potential for losses of surface applied nutrients in runoff from snow melt or winter rains under these conditions. Thus try to select fields like alfalfa fields or fields with cover crops that have good ground cover and will begin growing and taking up nutrients early in the spring. Following are important guidelines for winter manure application:

* Select fields with cover crops or at least good residue (PA regulations require a cover crop or at least 25% residue for winter spreading)

* Stay as far away from water as practical (PA regulations require staying 100 ft from water in the winter)

* Select the most level fields available and especially avoid significant slopes

* Avoid areas in fields were concentrated water flow is likely

* Avoid poorly drained fields

* Don't spread on snow unless it is unavoidable

* Try to avoid spreading when rain or melting conditions are expected.

* Stay away from roads and don't spread in road ditches

* For daily spreading, mark where you stop spreading in case fresh snow covers up the previous application to avoid skips and overlaps

* Keep application rates as low as practical. Less than 5000 gal/A of liquid manure, 20 ton/A of solid manure or 5 ton/A poultry manure.

Source: Douglas Beegle, Penn State CMEG Soil Fertility Specialist



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