Drench Applications in Citrus Trees: Soil–Plant Boron Dynamics and Fertilizer Recovery

In the world of agriculture, feeding a tree is a delicate balancing act. Plants require a suite of nutrients to grow, but not all of them are needed in large quantities. Some, known as micronutrients, are required in such small amounts that a tiny error in dosage can mean the difference between a healthy crop and a sickly one. Boron is one such essential element. It acts as a structural glue for plant cells, helping to build strong walls and ensuring that tissues divide and grow correctly. Without enough boron, citrus trees struggle to develop properly, leading to poor fruit set and reduced yields. However, boron is also a tricky nutrient to manage. In the soil, it moves easily with water, which means it can wash away quickly before the tree has a chance to use it, especially in the sandy or weathered soils common in tropical citrus regions.

For years, farmers have tried to solve this by spraying boron onto the leaves or mixing it into herbicides sprayed across the orchard floor. But these methods have flaws. Spraying leaves is often inefficient because citrus trees do not easily move boron from old leaves to new growth. Spraying the ground with herbicides spreads the nutrient over a wide area, increasing the risk that rain will wash it away before the young, shallow roots can drink it up. As orchard managers look for better ways to care for young trees, particularly in the early years when root systems are still small, they have begun to explore a more targeted approach: pouring nutrient solutions directly onto the soil right next to the trunk. This method, known as a drench, aims to place the fertilizer exactly where the roots are most active, minimizing waste and maximizing uptake.

A team of researchers at the Sylvio Moreira Citrus Research Center in Brazil set out to test whether this drenching strategy could effectively feed young citrus trees. They focused on a specific question: does pouring boron directly onto the soil near the trunk actually improve the tree's nutrition, and does it matter which type of boron fertilizer is used? To find the answer, they established a new orchard of 'Valencia' sweet orange trees and conducted two carefully controlled experiments. In the first, they applied different amounts of two common boron sources—boric acid and zinc borate—directly to the soil around the base of the trees. In the second experiment, they used a special version of boron that could be tracked. By using a rare, stable form of the element that is not naturally common in the soil, they could distinguish between the boron the trees absorbed from the fertilizer and the boron that was already sitting in the ground or inside the tree from previous growth.

The researchers applied their treatments by pouring 200 milliliters of the solution onto the soil within a circle about 20 centimeters wide around each tree trunk. They did this three times over the course of the growing season, mixing the nutrient solution with an insecticide to save time and labor. After the treatments, they waited for the trees to grow and then took samples of the soil and the trees themselves. They dug up soil from different depths and distances to see how far the boron had traveled. They also harvested entire trees, cutting them into pieces—roots, trunks, old leaves, new shoots, and flowers—to measure exactly how much boron ended up in each part.

The results showed that the drench method worked exactly as hoped in terms of placement. The boron stayed where it was poured, increasing the nutrient levels in the soil right next to the trunk, but it did not spread out to the edges of the tree's canopy. This confirmed that the method successfully targeted the zone where the young tree's roots are most concentrated. When the researchers looked at the trees, they found that the amount of boron in the leaves increased steadily as they applied more fertilizer. Specifically, for every 100 milligrams of boron applied to a single tree, the concentration of the nutrient in the older leaves rose by 12.6 milligrams per kilogram. The highest dose they tested, 900 milligrams per tree, boosted the nutrient levels in the leaves significantly without causing any harm or toxicity to the plant.

Perhaps the most revealing part of the study came from the tracking experiment. By measuring the special boron isotope, the team could calculate exactly how much of the nutrient in the tree came from the fertilizer they applied. They found that, on average, only about 6.7 percent of the boron accumulated in the plant came from the fertilizer they added. The rest of the boron the trees used was already present in the soil or had been stored in the tree from before the experiment began. This low recovery rate might sound inefficient, but it tells an important story about how citrus trees manage their resources. The trees were so effective at holding onto the boron they already had that they did not need to rely heavily on the new supply to grow. In fact, despite the increase in leaf nutrient levels, the researchers observed no significant change in the overall size or growth rate of the trees. This suggests that the trees were not suffering from a severe deficiency to begin with, and the extra fertilizer simply raised their nutrient reserves rather than triggering a burst of new growth.

The study also compared the different types of boron sources. One source was highly soluble, dissolving quickly in water, while another was less soluble and released its nutrients more slowly. The researchers wondered if the faster-dissolving source would be absorbed more efficiently. However, the data showed no real difference between the two. Whether the farmers used the quick-release boric acid or the slower zinc borate, the trees absorbed the nutrient at similar rates, and the amount of fertilizer recovered in the plant biomass was the same. This finding is significant for growers because it means they have flexibility in choosing their fertilizer without worrying that one type will be wasted while the other works better.

Ultimately, the research confirms that pouring boron directly onto the soil near the trunk is a practical and effective way to manage this tricky nutrient in young citrus orchards. It places the food exactly where the roots can reach it, avoiding the waste associated with broad spraying. While the trees did not grow larger in response to the extra food, their nutritional status improved, and the highest dose tested proved safe and effective. The study suggests that for young trees, applying 900 milligrams of boron per tree via a soil drench is a promising strategy to ensure the trees have a strong nutritional foundation as they establish themselves. This approach offers a way to combine pest control and fertilization into a single, efficient operation, helping farmers maintain healthy orchards with less waste and greater precision.