With sandy soils dominating much of Florida, soil health has not been an area of much interest or research over the years. This is especially true in citrus. HLB has changed that calculation as growers have placed a bigger emphasis on supporting the roots of citrus trees affected by the disease. Many are trying different approaches to build organic matter (the foundation of soil health) in their groves. This past November, UF/IFAS hosted a field day on the topic to take a look at two different approaches — the use of cover crops and compost to build healthier soils. The field day took place in groves owned by Ed James and Ben Krupski in Howey-In-The-Hills. The two growers, separated by a very sandy field road, have different approaches to improve soil health.
Mastering Cover Crops
Ed James’ grove was among the first in his area to begin to rapidly decline as citrus greening took hold. The grove had a history of Diaprepes root weevil problems, which exacerbated the decline. He made the decision to push the grove about nine years ago to make way for an alternative crop.
“I started with the cover crops to help rebuild the soil in the grove,” James says. “At first, the cover crops would not even grow, the soil was so worn out.”
After some time, he noticed the trees were beginning to respond to the cover crops. No inputs were being applied other than the cover crops and a compost application meant to help them germinate. He attributes the comeback to a change in the soil’s biology.
“I didn’t measure the soil back then because the intention was to push the grove, but there was zero organic matter,” James says. “It was sugar sand. You could not drive around the grove without bagging your tires while in four-wheel drive. In the past, I paid attention to the physical and chemical aspects of the soil and ignored the biological aspect. Biology is just as important, if not more, in the greening environment.”
Over time, James also has changed the seed bank in the grove from weeds to the mix of beneficial cover crops he has planted. “For years, the weeds managed me,” he says. “I spent all my time as a caretaker fighting the weeds. Now, it is much more fun as I manage the cover crops.”
James planted a mix of cover crops with the goal of changing the seed bank from perennials to annual crops. He mixes species with legumes, Brassicas, and grasses. Buckwheat, hairy indigo, sunn hemp, along with various greens like mustard, turnips, and Daikon radish are among the crops planted. With the seed bank built up, he doesn’t need to plant every year. He lets seed from the previous crops do the job.
“It helps to have a blend because if you have one species that doesn’t take, you aren’t left without any germination,” he says. “As the buckwheat begins to play out, the hairy indigo and sunn hemp start to come on. As that begins to play out, the brassicas are coming. We already have a monoculture with the trees, so the mix of cover crops makes the soil feel like it is a getting a crop rotation.”
James cautions that it takes some time to begin seeing the benefits of cover crops. As time passes and biology builds in the soil, the trees respond. Yields in his grove are profitable again. He says for the tree size and more traditional wide spacing and density, it is about as good a yield as one could hope for with very little fruit drop.
“Today, I have good organic matter in the soil, and this year applied virtually zero commercial nitrogen fertilizer other than the small amount in a foliar bloom starter,” James says. “I am usually short on potassium, magnesium, and sulfur, which I will supplement with applications to the grove. But I am getting plenty of nitrogen from the cover crops.”
In fact, James says he is running a little too high in nitrogen in the grove, so he has planted some grass cover crops to help remove some of the nitrogen.
After the cover crops have grown, James will take a heavy roller to destroy them and press down the plant debris into the soil. As that crop residue decomposes, it continues to benefit the soil improving organic matter and releasing nutrients.
Dr. Sarah Strauss, a Microbiologist with UF/IFAS, gave a presentation during the field day on the work she and her colleagues are doing to study the impact of cover crops in citrus. She noted that among the research community, there is a clear understanding that soil microbial activity benefits crop production, but the complex interactions among soils, organic matter, roots, and microbes are still not fully known. The vast amount of diversity adds to the complexity.
“In 1 gram of soil, you could have more than 1 billion microorganisms,” she said. “Microbiologists think there could be as many as 50,000 different species of bacteria in that tiny sample of soil.”
Strauss conducted a study over the summer in Southwest Florida to determine how certain mixes of cover crops perform and germinate. She noted germination rates were pretty good and similar across the board, but brown top millet was the clear winner when it came to providing biomass.
“We also are very interested in seeing how combining compost and cover crops will work, so we will be studying this as well,” she said. “The compost could benefit the cover crop germination, and both could have a really positive impact on microbial communities and plant biomass.”
James says what he is doing may not be for everyone. Getting beyond having a grove that appears to be weedy is a big leap for many growers. A trial conducted by Strauss, which compared a low-input cover crop grove to a high-input, non-cover crop grove in Southwest Florida showed the high-input grove out yielded the cover crop grove.
“I like growing citrus, but had to find a way that I could afford to stay in it,” James says. “This approach will work in large groves like they work in my small grove here.”
Capitalizing on Compost
Across the field road, Krupski has been leasing and farming his grove for five years. When he started managing the grove, greening had battered it hard.
“When I started, you could look and see through the grove because the trees were so thinned out,” he says. “My first harvest was only 60 boxes per acre. So, we had a long way to come back.”
Krupski follows a more traditional fertilizer program. He applies two applications of slow release fertilizer per year for about 120 pounds of nitrogen, along with six liquid fertigations totaling another 25 pounds of nitrogen. He has been applying compost to the grove for four seasons and plans to continue.
“I would tell other growers to put out as much compost as you can afford, especially at planting,” he says. “I am making a yearly application of compost. We have been adding some microbes on top of the compost because it is a good medium to support them. This season, I am backing off on the commercial nitrogen (because of the market and economics of the season) but will maintain my compost applications.”
The program seems to be working, with yields rising from 60 boxes per acre to 309 boxes last season. Tests by UF/IFAS show the organic matter in the grove has risen to 8%.
During the field day, Strauss noted that compost can provide many benefits, but there is a lot of variability in the types of compost. There are biosolids and plant-based composts with varying rates of nitrogen content.
“It can be tricky figuring out the best timing for applications,” she said. “This can be especially tricky in citrus. Do you apply just at planting? How many times do you apply if you applying beyond planting? A lot of this is tied to economics as well.”
Strauss just received funding to conduct a large-acre trial where compost will be applied at planting on a new block and will receive annual applications for two years after planting.
“There are four rootstocks in the trial, so we will see how they respond to compost and which ones are more vigorous,” she said. “We also will be looking at soil microbial communities in the soil and taking several measures of tree health. We are just getting started, so stay tuned for results.”