In mid-August we rotovated (shallow-tilled) our sorghum sudangrass and cowpea cover crop. This post will describe the learning curve we encountered with our first time rotovating and how we got it to work.
Good mix of brown and green plant material
We planted this cover crop at the end of April, let it grow to about 5 feet tall, and mowed it in July. Our bush hog made a lot of nice mulch. After a week or so, the sorghum sudan and cowpeas came charging back mean and green and grew about 18 inches. This is exactly what we wanted. We wanted to rotovate green plant material “green manure” to feed and jumpstart soil life, and brown mulch material to breakdown and transform into organic matter. These fields (our neighbor’s) were previously planted in no-till roundup-ready soybeans for at least a decade. Organic matter is very low, below 2%. Our goal is to stimulate all the soil critters to grow and multiply and bring life back to the fields.
our 7-foot rotovator
Rotovators are tillage tools, like a very wide garden tiller. Our rotovator is 7 feet wide. It has no wheels. Instead, it skids along the ground like a sleigh, and the tractor’s 3-point hitch keeps it level. We rotovated 25 acres of sorghum sudan and cowpeas (our neighbor’s fields) and two acres of our own grass fields. In addition to the rotovator, our tractor was carrying a homemade spray tank on the front. The spray tank is described here, and the spray mix (beneficial soil microbes and other goodies) is described here.
We want to decrease the time it takes for the fields to get healthy. Our own fields, which were previously under the same soybean farmer, have been in pasture grass for the past 3 years. We’ve been mowing 2 or 3 times per year, fertilizing, and applying compost. Unfortunately, we’ve seen very little improvement in the grass. After studying biological farming techniques, we’re convinced the soil needs some disturbance to break up the sandy, crusty topsoil and to get a good dose of plant material and biology into the root zone.
Incorporating a green manure crop is essentially chopping plants and feeding them to the soil. Five percent of a plant’s makeup comes from the soil (ash/minerals), and the remaining 95% comes “free” from photosynthesis of sunlight, water and air. Incorporating the minerals (now in plant form) and all the free stuff like carbon, vitamins, and plant metabolites, provides a huge benefit to the soil in terms of improved soil structure and energized soil life. We’re aiming for “chocolate cake” soil: very dark, loose, and crumbly with high organic matter and a very healthy soil ecology.
Our goal was to rotovate as fast as possible (to decrease time in the tractor) and still be satisfied with the results. In terms of results, we wanted to see most of the mulch incorporated into the top 4 inches of soil with a good mulch cover left on the surface for protection. We also wanted to see a relatively smooth seedbed, but not so smooth that our sandy soil was pulverized.
We quickly discovered that getting our desired results would take a lot of trial and error! On top of taking a large amount of time, rotovating includes many variables: tractor speed, rpm, soil moisture, rotovator blade depth, vegetation content, row coverage (overlapping), and the position of the rotovator’s back gate.
Rotovating too slowly- powdery soil with little surface mulch
Lesson #1: Don’t go too slowly. On the first day, we started rotovating in our tractor’s 2nd lowest gear at about 1900 rpm. (The rotovator manual said 1900 was the max). This is moving VERY slowly, about 2 mph. 2nd gear cut the plants off at ground level but left them all on the soil surface. The blades seemed to be bouncing a little instead of digging down. Then we tried 1st gear at 1900 rpm. Going slower allowed the blades to get into the soil, but the rotovator now chopped the soil way too much and turned it into powder. All of the vegetation was incorporated with hardly any protective mulch on top. It was getting dark, and we hadn’t even finished two acres. We decided to sleep on it and try again tomorrow. That night, it started raining heavily. I had nightmares recalling the old 1930s TVA movie clips showing drastic soil erosion! I didn’t feel good about the soil turning to powder. It felt like a giant step back, and I was worried.
Pile made by rotovator catching mulch from previous row and dragging it.
The next morning, we walked out to look at the fields. No erosion at all, everything was fine. Except now in the morning light, we could see all the big piles of soil and mulch the rotovator created. One side of the rotovator kept catching on the mulch in the previous row and dragging it until it balled up big enough to discharge out the side. It was discouraging because we didn’t want to bring our tractor back over those nice and fluffy rows to smooth out the piles. The tractor is heavy and would compact the soil and remove all the air that microbes need when they’re working. Looked like we’d be pitch forking them by hand! (We did.)
Lesson #2: Don’t rotovate bone-dry soil. We waited a few days for the rains to stop and the soil to dry out. We knew 1st gear was bad, so we tried 2nd gear again at a little lower rpm, 1800, and we also set the blade depth deeper. Voila! It worked! Most of the mulch was getting incorporated with some left on top. But the major lesson here was soil moisture. After the rain, the moisture level was now just right. The soil’s aggregates stayed together in nice, small clumps. It was too dry on the first day, and that’s why it got pulverized so easily.
Uneven seedbed from rotovating in 3rd gear with gate up
Lesson #3: Leave the back gate down if you need a smooth seedbed. We were still making the big piles though, and we couldn’t figure it out. We decided to go faster, in 3rd gear, and lift the rotovator’s back gate up so all the mulch could leave without balling up. This just helped make the piles a little smaller. Also, keeping the back gate up left the soil surface very uneven. That was another concern because we didn’t know how the drill (planter) would perform in a seedbed that bumpy. We put the gate back down.
Lesson #4: Don’t overlap rows. It wasn’t too much longer when we discovered what was making the big piles. I was bringing an iced tea out to my hubby in the tractor. When he saw me to his far left, he turned his head and waved. I saw the tractor and rotovator steer to the left far into the previously rotovated row, and a giant pile came out the side! From then on, we concentrated hard on keeping the rotovator squarely on unrotovated ground. This was hard to do continuously, but it worked! We ended up leaving very narrow strips of the cover crop standing just so we’d avoid the piles. This didn’t look very good, but we’re fine with it because the sorghum sudangrass and cowpeas will both die at frost.
Just right - moist soil, most of plant incorporated with plenty of protective mulch left on top
Lesson #5: Do lots of test runs on the day you’re rotovating. We ended up going back down to 2nd gear at 1800 rpm for most of the job. The cover crop was very lush and thick in some places, and we needed a slower speed for most of it to get incorporated. However, when we rotovated our 2 acres of pasture grass, we could go back up to 3rd gear. The density of the vegetation makes a difference.
With all the variables involved, it’s key to test before you start to find the happy medium for that day. Make sure your soil has some moisture (but not anywhere near wet) and start testing to see what speed and rpm gets most of the plant material below the surface and still leaves a protective mulch cover on top. And if you start creating big piles, stop overlapping the rows!
Hurricane Irene Side Note: Irene came two weeks later with 85 mph winds and 10 inches of rain. The next morning, there was surprisingly little water standing and no sign of erosion. The large quantity of mulch, both below and above the surface, protected the soil well.