By Emma Matcham
Extension resources in both Ohio and Pennsylvania commonly recommend that farmers collect soil samples and make fertility management decisions in fall.
This article breaks down some of the practical reasons for that recommendation and some activities to consider on your own farm in the coming weeks.
Soil testing
Theoretically, you can sample soils at any time of the year, although I do strongly recommend you pick one time of year and stick with it. Fall is a particularly great option, though: there’s not usually a tall crop to walk through, soil is firm enough to drive over but has enough moisture that the soil probe won’t get bent in half and we’re all more likely to carefully review lab results over winter instead of when it’s time to plant or spray. Generally, soil samples should be collected every three to four years on loamy and clayey soils. Sandy fields are more prone to nutrient leaching and may benefit from more frequent testing (every two to three years). So, if your most recent lab report is dated 2023 or earlier, consider pulling new soil samples in the next couple weeks.
Lime
Lime application is another management activity that is particularly well-suited for fall. Depending on the particle size/fineness of the lime source, soil pH increases will occur over the six to 24 months following lime application. Lime applied in November will have at least partially reacted by April, and soil pH will continue to improve throughout the next growing season. The moisture from our region’s typical winter snowfall and rainy springs is also helpful, since water is required for lime to react.
Potash
Either spring or fall potash applications can make sense for fields that are already in the healthy maintenance range for potassium (120-170 ppm for agronomic crops in Ohio, when analyzed via Mehlich-3 extraction). But, a fall potash application can help improve winter survival in alfalfa fields with low potassium. Aim to apply potash at least a few weeks before growth stops for winter so that the fertilizer can dissolve into the soil profile and be taken up by plants.
Another situation where fall potash applications makes sense is in grass or grass/legume mix hayfields and pastures. Applying your potash in fall keeps you from needing to make a spring application, which is helpful since high potassium levels in spring can exacerbate grass tetany.
Phosphorus
The link between soil test phosphorous levels and winter survival for alfalfa is less clear than the link between potassium and winter survival. But, soil test phosphorus is very, very important for seedlings. Soil testing in fall can help you identify any fields with low soil test phosphorus levels so that you can make a plan to fertilize before planting. In some areas you can apply phosphorus in fall to build up fertility levels and save yourself a pass in spring, but be mindful of any restrictions that might apply in your watershed.
Phosphorous is expensive this year, and that always brings questions about where we can reduce applications without limiting yields. You are unlikely to see an increase in yield when your soil test values are in the maintenance range (20-40 ppm Mehlich-3 for phosphorous in Ohio corn fields). For fields close to the maintenance range (around 15-20 ppm), fertilizer applications may increase your yield next year, but average corn yield reductions are also fairly small (usually around 5% to 10%). There are lots of reasons to avoid letting your soil test levels continue to draw down, especially since we really don’t know what fertilizer costs will look like in future years. But, if you’re within 5 ppm of the phosphorus critical level, yield declines are typically small.
Soil testing is the backbone of the fertilizer recommendation system; consider testing this fall, and review your lab reports carefully while you make management plans over the coming months.
(Emma Matcham is an assistant professor in the Horticulture and Crop Science Department at Ohio State University. She can be reached at matcham.3@osu.edu.)
