Lime applied after establishment will take longer to correct the low pH problem. Even when recommended amounts of these materials are applied before establishment, it will be necessary to apply phosphorous and potassium each year. A certain amount of phosphorus applied as fertilizer is absorbed by the soil and is not available for plants. In addition, large amounts of phosphorus and potassium are removed each year in hay. This amount is removed as part of the hay crop and must be replaced by fertilizer or soil weathering.
Soil weathering is a complex process whereby chemically bond minerals are released by the soil.
Regular soil testing is the best way of accounting for how well soil weathering restores crop removal of phosphorous and potassium. In addition to reducing yield and shortening stand life, as noted above, they can exaggerate the effects of pests. Low fertility conditions result in weak alfalfa plants that do not compete well with weeds. Plant stress caused by low fertility coupled with insect infestations can result in losing an alfalfa stand in a short time. In contrast, well-fertilized alfalfa plants without insects, compete with broadleaf and grassy weeds very well, resulting in very little weed encroachment for several years.