By Ronald G. Doetch
I am a lifelong farmer/agronomist and, at the age of 73, have worked with many different crops through the many changes in agriculture challenges and production practices. I moved to Florida four years ago to grow a few oranges and have a firsthand look and understanding of the relationship of soil health to tree decline and improvement.
In getting to know many of the successful growers in the top citrus-producing counties in Florida, I can appreciate all the innovation and effort that has been put into practice to reverse the continued decline in the citrus industry. Working with these resilient growers and our grove, I am confident that improving soil health is the key factor in restoring our citrus groves to acceptable production levels in the presence of HLB. The complication is how to effectively restore soil health while reducing input costs and maintaining a profit.
My orange grove consists of about half Hamlin and half Valencia oranges, similar to the many citrus groves I have walked in the Flatwoods of Desoto County. Ravaged by HLB, we replaced about 12 percent of our grove this summer with Vernia resets. We planted sunn hemp in the row middles to mine potash from the subsoil, but the prolonged dry weather prevented a successful cover crop. We are operating on the premise that HLB will stay with us, so we have to restore plant health to our groves while reducing input cost and risk. Searching for the silver bullet has been costly while tree decline has continued.
Many labs now conduct soil health evaluations. All are based on the amount of carbon dioxide expelled from the soil sample over a short time. The test is not perfect as it isn’t known if the microbes are responsible for the carbon dioxide or the contribution is from larger soil fauna. However, the test is an indicator of microbial activity in the soil. Using this test, we have been comparing soil nutrient profiles and soil health scores to yields. In our sampling, yields correlated better with soil health scores than with nutrient levels.
We then focused on every technique we could use to improve microbial activity or soil health. This included reducing or discontinuing certain pesticides, adding biologicals to the soil, improving irrigation, and nutrient planning. To accomplish the latter two goals, we use sap-flow meters inserted into the tree with a reading every 30 seconds of sap-flow volume and rate. We are using the tree as the remote sensor for soil moisture, solar radiation, humidity and daily activity.
We were quite surprised at the timing of activity in the tree and the sensitivity of the tree to changes in environmental conditions that are both above ground and below ground. We thought that tree activity came up with the sun, but the tree starts up rapidly around 8:30 a.m. in the summer season. The factory (tree) runs hard until shortly after noon, then may decrease gradually or slam shut. An afternoon rain shower causes the sap-flow to turn off. Using feedback from the trees, we modified four techniques to improve soil health:
1. PESTICIDE REDUCTION
Most herbicides are compatible with improved soil health. Very few chemicals used in citrus production cause major disruptions of the soil microbial communities. However, we switched to a curative approach instead of a preventive approach. With increased scouting, tissue testing and soil sampling, we cut back both rates and the need for several pesticides.
We use diatomaceous earth (DE) to control soft-bodied insects in the soil and to reduce some of the flying insects such as psyllids. Because the mode of action in DE is mechanical, there is not a build-up of immunity nor toxicity to soil microbes. Using DE allows us to reduce insecticides, not eliminate them. DE also improves water infiltration as a secondary benefit.
2. SOIL AMENDMENTS
There are many ways to improve microbial activity in the soil. Time is a great healer, but we need to accelerate this improvement. Mature, screened compost contains living microbes and provides a food source and habitat that is favorable to microbes. We lab-test and credit our compost for part of the nutrient package for the trees. Compost is highly variable depending on the source, so it needs to be tested before application to be a part of our nutrient plan.
We also use other sources of microbes and accelerants as are needed and available. In tight soils, we are disking the middles at the dripline to get oxygen in the soil and prune necrotic tree roots.
3. PRECISION IRRIGATION
The sap-flow meter calculates the water pulled from the soil by the trees. Using Florida Automated Weather Network weather monitoring stations, we can calculate the amount of water we need to replace in the soil.
Overirrigation washes away the potash and nitrogen. One hundred boxes of oranges per acre remove 12.5 pounds of nitrogen. With HLB, nutrient uptake is inhibited, but we have cut our nitrogen rates substantially without decreasing nitrogen levels in the tissue test.
4. NUTRIENT PLAN
The only nutrients important to the tree are the available nutrients. We focus on placing the nutrient selection within the reach of the tree. We feed the roots at the calculated rate when the tree is taking up water. First-year compost will help infiltrate water in the soil and hold moisture but does not contribute much for nutrients until the second year.
One of our interesting discoveries is that the trees treated with our soil health system showed nighttime water use of about a quart of water per hour all night, while the check trees completely shut down. We commissioned an independent researcher to compare this system over 24 months to the methods we were using previously and in the checks. The trial will be completed this December. However, we know that the soil health system trees took up 20 percent more water throughout the season than the check trees.
The sap-flow meter has given us a new tool to see immediate feedback on agronomic practices and changes in the tree’s performance well ahead of harvest. In the past two years, we have consistently increased yields and reduced costs with higher pound solids. However, Florida citrus harvest on a per-acre basis was up on average for the past two years as well.
We are trying to understand the tree and help it tolerate HLB rather than trying to eliminate the disease or the psyllid. We are encouraged by the new bud and rootstocks that are available, but there is great potential in restoring our existing trees to 75 to 90 percent of historical production levels.
Ronald G. Doetch is a citrus grower and consultant in Bradenton, Florida.