How much N can cover crops capture?
Cover crops planted in late August of last year captured up to 300 kg of N per hectare (270 lb N per acre)! The experiment station in Clarksville, MD where they were planted is located in the fertile Piedmont region of where soils tend to have higher organic matter and different mineralogy than other parts of the state and the mid-Atlantic. Located on an active dairy farm, these fields also have a history of manure and compost application.
This is only one site year’s worth of data, but we have seen high N uptake from this site in years past as well. Here are two of the lessons we take from it:
- There can be a lot of N “hanging out” in the soil profile, especially in higher organic matter soils with a history of manure or compost. When we say profile we’re not talking about the top foot– the soil (and plant roots!) go deeper than that. If this N, especially in the form of nitrate, is not captured and stored in plant biomass over the winter, it can be lost from the system. This is a favorite topic of ours, and you can read more about deep nitrogen and cover crops in some previous posts.
- Cover crops can capture much of the N in the soil profile, but not all cover crops are created equal. We have been looking into phacelia as a low-residue winterkilled alternative to forage radish because many farmers have expressed concern over introducing another Brassica to their already Brassica-heavy rotations. Our first results (above) show that while phacelia captured nearly 200 kg N per hectare, it was not as efficient as radish at mopping up N. As for differences among radish varieties, when we combined root and shoot N uptake to get total N uptake, we saw no statistically significant differences between the five varieties we tested: Graza Radish, Tillage Radish, Toro Radish, Respect Radish, and an unnamed variety.