By Stan Smith
When hay harvest gets into full swing, it’s also a good time to consider replacing the soil nutrients that are removed during harvest. Since hay is the basis for most Ohio winter beef cow rations, it’s common for cattle producers to occasionally pull soil samples from hay fields that don’t seem to be as productive as they once were. Often times they’re surprised to discover the fertility is low, especially in fields that have been in hay for some time.
It’s not uncommon to hear farmers suggest they didn’t realize the mechanical removal of forages took with it a significant amount of soil nutrients. From there, conversations sometime evolve into comments like, “But I always thought forages were good for the soil. Don’t we constantly hear that cover crops are good for soil health?” The response is simple: The plant material generated from a cover crop is seldom removed from the field; thus, it does not take with it the soil nutrients it utilized while growing.
Similar to each bushel of corn that removes 0.37 pound of phosphorus (P2O) and 0.27 pound of potash (K2O) when harvested, a mechanically harvested ton of forage takes with it 13 pounds of P2O and 50 pounds of K2O.
To put that into perspective, consider the total average annual hay yield in Ohio is, and has been for decades, a little less than 3 tons per acre. At the fertilizer crop removal rates mentioned above, that amounts to an annual removal of 39 pounds of P2O and 150 pounds of K2O per acre. Since corn grain only removes about 0.27 pound of K2O per bushel, it would take a yield of over 555 bushels of corn to remove the same amount of potash that an average Ohio hay yield removes annually from a field. This is regardless of the quality of the forage that's harvested.
Recognizing that phosphorus and especially potash make up a significant portion of the dry matter in a forage plant, it should be apparent that we can’t sustain production in the absence of either. It’s certainly possible to delay fertilizer application on a hay field, but those savings are short term. Never replacing the nutrients removed through harvest results in “mining” of the soil, and if the practice continues over a period of years, yields and stand quality decline.
Since nearly all the phosphorus sources we have available include some nitrogen, when replacing fertility immediately after the first cutting, we also enjoy the benefit for grass-based hay fields to utilize the nitrogen that comes along with the phosphorus.
The basics of fertilizing hay fields are simple:
• Soil Test, always soil test. In cases where manure nutrients have been utilized or fertilizer applied infrequently over the years, it’s the only way to know if fertility is a yield limiting factor. If we don’t know what we presently have, we can’t possibly know what we might need. Contact your local OSU Extension office for help finding a soil testing lab.
• Read the soil test carefully or get help reading it. I’d discourage anyone from blindly accepting the fertilizer recommendations that sometimes come back with a soil test. In some cases, I’m not even certain I believe the little graphs that are sometimes found on the soil test results that indicate a sample might be high, medium or low in a nutrient. As for how recommendations are generated, more than one Ohio testing lab said after establishing the nutrient levels in the soil through their laboratory procedures, the recommendations are typically generated based on the opinions of the company who might have submitted the sample for the landowner. That said, unless you send in the sample yourself, you may get back a recommendation based on data other than what Ohio State University or other Midwest university research might suggest is appropriate as published in OSU Extension Bulletin E-2567, “Tri-State Fertilizer Recommendations.” Ask your local Extension ag educator for help in developing a recommendation if you have questions.
HARVEST: A mechanically harvested ton of forage takes with it 13 pounds of P2O5 and 50 pounds of K2O.
If one insists on fertilizing without the benefit of knowing the current fertility levels of the hay field, or if one knows current fertility levels meet or slightly exceed critical minimum levels, then it’s prudent to base fertilizer application rates on actual or expected crop removal. As mentioned previously, we know every ton of hay removed (regardless of quality) takes with it 13 pounds of P2O and 50 pounds of K2O. No matter how you slice it, that’s a ratio of roughly 1 to 4, phosphorus to potash. Without benefit of a soil test to tell us otherwise, fertility needs to be replaced in that ratio based on how much hay is harvested.
• Hay harvested means soil nutrients are removed.
• Immediately after the first cutting, harvest is an excellent time to apply fertilizer to a hay field.
• One ton of hay removes P and K in a ratio of roughly 1 to 4, or 13 pounds P2O and 50 pounds of K2O per ton of hay.
• To maintain current fertility levels and productivity in your soils, it must be replaced in a ratio of 1 to 4, or 13 to 50, per ton of hay removed annually.
Smith, an OSU Extension educator from Fairfield County, is a member of the OSU Extension Beef Team, which publishes the weekly Ohio BEEF Cattle letter that can be received via email or found at its website, beef.osu.edu.