By Brad Turner
In the 1970s, the majority of my family’s groves in Hillsborough, Polk and Hardee counties remained on a 25-foot by 25-foot setting or wider. Most of these “old school” blocks received several light cultivations in both directions, two fertilizations and two foliar sprays per season. These groves contained some of the most grand and bountiful trees in the region. Before the freezes in the 1980s, most all our groves were grown this way.
Through the 1980s, micro-sprinkler irrigation was added, additional trees were interset and a chemical herbicide program was implemented on most all of the blocks. Our caretaking practices became more mainstream.
POSITIVE RESULTS WITH LESS INPUTS
Even though statewide production was rebounding and increasing during the 1990s and early 2000s, imported frozen concentrated orange juice still poured into the country, driving down price per pound solids as low as 55¢ for growers. My father and I debated how we could reduce inputs and remain profitable. (Read more of this history in the September 2004 issue of Citrus Industry in the profile of George Turner.)
Before my time, dad, as well as his father and grandfather, had experience with reducing soil-applied fertilizer by 50% or more during lean years without loss of production or quality. I would reluctantly cut out one of the two fertilizer applications and reduce the herbicide band width from its usual 50% to 100% of the floor to a mere narrow band of Paraquat herbicide down the tree drill.
When this was done, the native plants would take over nearly trunk-to-trunk across the groves, getting weedier than I had been accustomed to tolerating over the previous decade or two. Before harvest, we would shallow disc, mow or chop the native plants down. Even with this reduction of inputs, production would remain consistent. I would always shake my head and wonder how production and quality were being maintained after reducing fertilizer applications and allowing “fertilizer-robbing” weeds to grow wild.
Although I don’t consider myself a scientist, I am an experienced citrus grower and caretaker who has come to realize that some management practices and their results provided lessons on how soil and plants function. Combining my experience with the evolving science and monitoring tools available today, I have come to understand that during those times of input reductions we were giving the soil a break from excessive salts and synthetic inputs. This allowed the diverse native plants to cover the grove and promote indigenous soil biology. In turn, these microbial communities could make previously unavailable nutrients available for the trees to utilize. The mowed or chopped cover crops served as plant-available nutrition and a food source to support microbial activity.
ACHIEVING MINERAL BALANCE
Using the adage “You can’t monitor what you can’t measure,” I use various types of soil testing and sap analysis. The goal with soil testing is to measure mineral levels and then balance minerals to achieve the best air/water exchange through improved soil structure.
Based on William Albrecht’s research from the 1940s, mineral balance has nothing to do with soluble fertilizer but everything to do with nutrient availability and uptake. Minerals in the correct balance enable the physical, chemical and biological systems of the soil to function as a whole — even in sandy citrus soils. Regular sap analysis results enable me to monitor nutrient levels in the plant and focus fertility applications on what the plant needs to improve photosynthesis.
WILD VS. CULTIVATED
I have compiled four years of soil testing data along with three years of sap analysis data on my research block. I compared that to data from wild citrus trees growing in the neighboring forest. These citrus trees have limited available sunlight and no applied fertility, yet they show no visible signs of insect or disease pressure and produce high Brix fruit. The soil test results are very similar to most of the citrus-growing soils in Florida: adequate levels of calcium and phosphorus, but with limited solubility and low levels of magnesium, potassium and many micronutrients.
One glaring difference is the soil organic matter (carbon) levels in the wild citrus soils are 5% or more, while most conventional citrus soils I have tested are typically less than 1%. What I have found most eye-opening is the sap analysis data on these wild trees have levels of specific minerals like calcium, phosphorus and nitrogen in the correct form, all at optimal levels. Trace mineral levels are balanced, and while not always at optimal levels, they are better than sap analysis results from my research block and other commercial groves I have reviewed. The wild trees, with help from the surrounding plants, have “worked out a deal” with nature to deposit carbon into the surrounding soil system to stimulate biological activity and function. In return, the trees receive the mineral nutrition in the right form, at the right time, in the right place.
More comprehensive total nutrient digestion soil-testing results indicate there are luxury levels of locked up minerals in the sand, silt and clay. These minerals are not in the correct form for plant uptake, and this is where a highly functioning biological system shines. Given an adequate food source (carbon) and good air/water exchange (soil structure), the bacteria, fungi/yeast, protozoa and nematodes solubilize and release plant nutrients in the right form. This is what is happening with these wild citrus trees. They are not only healthy but remain resistant to insect and disease pressure while producing high-quality fruit.
Out of necessity to grow healthier trees, growers need to better understand and adopt nutritional practices that support:
- The relationship between healthy soil and citrus trees
- The nutrient demands and timing
- The interactions (synergism and antagonism) between nutrients
- The importance of the correct forms of nitrogen at all stages of growth
When we improve the soil, we increase a plant’s capacity to photosynthesize and push carbon into the soil and feed the biology. In return, these microbes supply needed nutrition to the plants. This results in healthier trees that are more resistant to pests and disease. Once we get our trees healthy and holding fruit, volume will follow.
Brad Turner is a citrus grower and operator of Sand to Soil Services in Lithia, Florida.
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