How Much Nitrogen Does Your Cover Crop Actually Produce?

0:10 So we did ask Ray this question but we get it a lot: How much nitrogen or fertilizer will this cover crop produce, and how much do I need to apply in order to get this cover crop started?

0:24 Oh my, that's a loaded question. It is one we get a lot, and like so many loaded questions the answer is it depends.

0:35 If you want maximum productivity out of a cover crop, just maximum biomass, I would probably go a mix that is very high in grasses with a lot of nitrogen fertilizer.

0:53 That's just the nature of the beast. If however you're wanting to stimulate your soil biology, increase your soil health and so forth, and do that with a minimum of synthetic inputs, the route I would probably take is to include a fairly high level of legumes and inoculate with the biaso.

1:19 That way you can make all the plants in the mix fix nitrogen. As far as how much you get, essentially you can eyeball cover crop mix.

1:35 Look at the biomass and if you look at the protein content in that biomass, each protein—one percent nitrogen equals 6.25 protein. And so if you look at this biomass that's 13 protein, that's two percent nitrogen. So if you convert biomass into that, so if you have two tons an acre of legume biomass, that's let's say four percent nitrogen.

2:21 When the bacteria break that down, half of it it's consumed by the bacteria. The other half will probably get kicked out so you can do the math and figure out about how much nitrogen you get out of that.

2:37 The standard rule of thumb is the microbes eat first and they're going to eat about half the nitrogen that's in that cover crop. If you're in that carbon nitrogen ratio where legumes are in that 20 to 30 range, about half of the total nitrogen content is going to be available for the next crop.

3:00 Keith, I didn't feel I did a very good job of explaining that. Then the gaps there. Well, it certainly depends on a lot of different things. And you know, if your goal is to produce a lot of nitrogen, then you really need to load up on.

3:16 Legumes and you know what I tell people is the only real way to know is you gotta pull a sample, send it in and have it tested. And again, that webinar with Dr. Ward, we kind of went through that, how you do that. You know, we did some videos on, and we'll have more information down the road about that. But you know, that came back 224 pounds of nitrogen that was in that biomass. And when Dale and I looked at it, we were guessing lower than that, but it really was cranking it out. A lot of mixes if

3:50 You have a diverse cover crop mix, you know, that has 30 percent legumes and 70 broad leaves, of which maybe half of that is grasses, you know, which is fairly typical.

4:02 It's not uncommon to see 60 to 80 pounds of nitrogen if you send that in for a test. That's pretty standard, pretty common. Some of that's being produced by the legumes, some of that is just being cycled by the grasses, but you're still keeping it from getting away from you. And so to me, that still counts. That still counts as nitrogen that's going to be available for the next crop.

4:24 That's a pretty decent rule of thumb as far as the fertilizer question. When people ask me should they fertilize their mix, how much should they put on, it kind of depends on your goals. If your goal is to graze or to hay and you're going to be removing biomass, then I think you need to grow as much as you can and so then I think it is worth some fertilizer investment. I generally tell people 40 to 50 pounds is probably all you need. You're not trying to produce a grain crop so you don't have to go crazy on how much nitrogen you put out there.

5:31 What your goals are and how much you're going to put out there after it. The other thing, especially like with the phosphorus, if your soils are really low in phosphorous, it may not be a bad idea to put some of that out there for the cover crop because if you're not hauling that cover crop off in a hay bale, that phosphorus is going to cycle and stay out there for the next crop.

5:50 So a lot of times what we would do is put the phosphorus out there in the summer after wheat harvest, let the cover crop utilize it. We know that those winter kill summer cover crops are going to cycle pretty completely by.

6:06 The time the corn is needing it, so we're essentially pre-fertilizing for our corn by putting it out there for the cover crop, but we're letting the cover crop use it first and then cycle and decompose it back through the system. I mean, so we don't feel like that was money that we were attributing as an input investment attributed to the cover crop. It's more for the next cash crop, but we're letting the cover crop use it first and then cycle it back through the system.

6:33 Right, you're fertilizing two crops with one application and one fertilizer.

6:38 And as far as the crop after the cover crop, so much of that depends on the carbon nitrogen ratio of the material and how long it has to decay. And a lot of it also depends on how many years you've been doing this cover crop thing.

6:58 I hear a lot of people tell me that they planted a cover crop and the first, the very first cover crop you grow, I would not give much if any nitrogen credit to it. I mean you'll get some but I wouldn't count on it unless you are doing some testing and you know where you sit.

7:21 But once you get the system going, I said half the nitrogen in that cover crop will be available to the next crop. Once you've done this three or four years, you start adding a half of this cover crop. You know, you'll get half of last year's, you get a quarter of the year before, you get an eighth of the year before that, the sixteenth of the year before that. The more times you do this, the bigger the pool of organic nitrogen you build up, and eventually you get to where you have a very large pool of organic nitrogen you can tap into.