Dennis Delaney – Fertilizing Cover Crops


I’m Dennis Delaney, an Extension Specialist in the Department of Crop, Soil and Environmental Sciences at Auburn University. I’m going to talk a little today about fertilizing cover crops to increase residue and the benefits from it. I want to give a lot of credit to Dr. Kip Balkcom at the USDA, National Soil Dynamics Lab here at Auburn. A lot of this research and a lot of the slides that will be presented today here are coming from him. So appreciate Kip’s input. What is a cover crop? Lot of people have different definitions. My definition is a crop whose main purpose is to benefit the soil or the next crop in one or more ways, but it’s not really intended to be harvested for fee or for sale or make it a cash crop. Some folks like to graze what they call cover crops but again, that would fall outside the definition and become a forage or grazing crop. It’s also used as cover crops, so a hybrid there. So again, there’s all kinds of definitions from all kinds of different people. We use all kinds of grasses, legumes, forbs, just a whole lot of different plants we can use as cover crops. They help improve the soil or benefit the next crop. Some potential benefits from cover crops, of course you’ve all heard about those I’m sure, erosion control, improving the soil and water quality, increased water infiltration which can be real important at certain times of the year, actually getting this heavy rainfall into the soil particularly in the summertime, minimize the nutrient loss, the runoff that take the soil and carries nutrients with it. Nitrogen, if you had legumes out there, can supply free nitrogen to the next crop. But that said, in order to really get advantage of these cover crops, we need to optimize the conditions and maximize the growth of them. We start out with optimum soil fertility and pH just like you would in your row crop, quality seed, inoculating any legumes with the right inoculant, fresh, make sure it’s alive. Plan as early as possible to take advantage of any warm Fall weather before it gets cold. Need good stand establishment just like a cash crop, and then terminate as late as possible and still be able to establish your cash crop on time. Take advantage of all that money already spent in time, in establishing the cover crop to get the maximum benefits from it. Cover crops do need fertility also just like a cash crop. There’s some differences between legumes and small grains. Usually, we can assume that there’s adequate P, K and soil pH already there from a well-managed cash crop unless a lot of biomass has been removed in which case you may be removing a lot of potassium in particular, and also phosphorous sometimes in that cash crop something like just corn silage crop. So you think it might be a question, it’s good idea to take soil test. But on low fertility sites, sites maybe being rowed into production, it can be important particularly phosphorus for legumes, looking back the old rotation here in Auburn University campus. In 1896, those sites were relatively low fertility. One set of treatments out there was splitting the P and K between the winter legumes and the cotton, half on each those crops because phosphorus in particular was so critical to getting good growth out of legumes. So it can be important most of the time is adequate P and K out there though. With legumes and the wide variation and the type of legumes achieving plant, wide differences in production but some of them like Hairy Vetch estimates for the nitrogen linked via to the next cash crop can be anywhere 40 to 200 pounds of nitrogen per acre, and that depends just how much growth we have out there, how ready you got plan it, how good the weather was, how late you wait to kill it. Now, all those variables into it and that’s why there’s such a wide range out there, how much nitrogen you can supply. But if there’s no need to apply nitrogen to legumes, what about all the others? The grasses for Nebraska, things like that. As you can see, there’s a huge difference in this slide between a zero grain that’s had 90 pounds per acre of nitrogen applied to it versus no nitrogen. You can see with zero nitrogen very little growth out there as compared to the fertilized. In lot of cases, it may be really hard to recover the money in which you already spent on seed if you don’t put some fertilizer out to boost it on along. Just makes a huge difference in how much mulch you can get, how much soil erosion you prevent, and how much water infiltration you’ll get later in the summer seasons. My rule of thumb is most of the time you want to have at least two tons per acre of cover crop biomass out there and really make it worthwhile. You can see this is a slide of some fertilization of cereal rye and with zero nitrogen, one year was able to barely make it over two tons per acre. Adding just 30 pounds of nitrogen per acre tremendously boosted the amount of biomass that was produced, and as you keep increasing more nitrogen out there, you’ve got more biomass produced. The biggest bank for the buck is that first 30 pounds of nitrogen really get it going, after that you could just keep on increasing the biomass and after really make a difference and add some mulch and organic matter back to the soil. There are some alternative sources that may be a little cheaper than commercial fertilizer such as poultry litter. This is the test Dr. Balkcom conducted several years ago, looking at timing of applying the nitrogen poultry litter Fall versus Spring and compare it with commercial nitrogen and it rates with 0, 30, 60, 90 pounds per acre or one, two, three tons of litter per acre. You’ll see this again this is a cereal rye and you can see the amount of rye biomass tend to increase to higher fertility rates taken with the commercial fertilizer, but also with litter it kept climbing. But again, the biggest climb per buck what I’ll tell you is at 30 pounds of nitrogen per acre or the first one ton per acre of litter. Again, you have to match and keep increasing it but you really want to make sure that rye cover crop is not stressed for nitrogen, you want to get your biggest return from it. As far as timing, in Fall applied and Spring applied, a lot of times with our weak crops, we put most of nitrogen out in the Spring but with something like rye, we just trying to get the biomass early, get good growth early, and then be able to terminate it in time to plant your Summer crop, cash crop. Fall applied is the best way to go. Again, just to get as much biomass as you can, take advantage of that warmer winter weather that we tend to have here in the South. Of course, the big question is, do you get that money back? It’s all nice and good to have more cover crop in improving soil quality but does it actually increase yield? This is a set of graphs of the cotton lint yield after the 0, 30, 60, 90 pounds of commercial nitrogen versus the chicken litter rates and you can see with no nitrogen-side risk, typical cotton that the cover crop does start releasing some of that nitrogen back to the cotton crop and will slightly increase yield especially at higher rates. But it still takes some nitrogen on cotton, the top center line is 90 pounds per acre of nitrogen in addition to what you put on the rye cover crop. So it does tend to boost yield. Again, that first 30 pounds per acre or one ton per acre of litter is what really gets you that boost in yield after boosting amount of cover crops out there. One thing I was talking to farmers, I know there’s a lot of cost involved between getting the equipment and the crew out there to plant these cover crops, buying the seed, buying the fertilizer, managing it. There’s a lot of that goes into it, a lot of cost. So you always want to try to manage that to get the maximum amount of benefits from it. Sometimes it’s hard to do but again like in long run had have the maximum biomass out there is will really be to produce some benefit. This is a quote from Michael and Reese who’s here few years ago that soil carbon and crop residues are really the key to making conservation tillage work. It’s not really the lack of tillage or no tillage but production and conservation of crop residues that offers the most benefit productivity. So again, if we’re planning into a real sparse, or real thin unfertilized cover crop or even just the stalks of the previous year, we may not do a lot to really build that soil. But if we can make the most biomass, the best cover crop that we can and leave it out there without destroying it, strongly they are gaining better, they will give us the most benefit to our next cash crop. In lot of times, its difficult here in the South but I think in the long run it will be worthwhile. Thank you for your time and it’s all I’ve got. Thank you.

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