Boron is a micronutrient required for healthy plant growth and development, though plants need it in very small quantities. Its primary roles involve strengthening cell walls, regulating the transport of sugars and carbohydrates, and ensuring successful pollination and seed set. Because boron is mobile in the soil but often immobile within the plant, it must be continuously available for new growth. This article provides practical guidance on how to safely and effectively supplement boron levels in your soil.
Recognizing Boron Deficiency in Plants
Boron is directly involved in cell division and the formation of new tissues, meaning deficiency symptoms first appear in the youngest growth areas. A lack of boron can cause the tips of shoots and roots to become stunted or distorted, often leading to the eventual death of the growing point. Plants may also show visual signs such as abnormally thick, brittle leaves, and the stems and petioles might become cracked or hollow.
In root crops, deficiency can manifest as “brown heart,” characterized by soft, dark spots in the center of the root. Reproductive failure is also common, resulting in poor flower production, reduced pollen viability, or the failure of flowers to set fruit. Crops with particularly high boron needs, such as alfalfa, sugar beets, and brassicas like cauliflower and cabbage, are the most susceptible to these deficiency symptoms.
Soil Testing and Calculating Application Rates
Boron has a notoriously narrow range between the amount needed for healthy growth and the amount that causes toxicity in plants. Applying boron without knowing the existing soil level is risky, as over-application can easily damage crops. Professional soil testing is the only reliable way to accurately diagnose a deficiency and determine the necessary application rate.
The most common method used by laboratories is the Hot Water Soluble Boron (HWB) test, which measures the form of boron available for plant uptake. Results are reported in parts per million (ppm); a level below 0.5 to 1.0 ppm typically indicates a deficiency for sensitive crops. Your local extension office or soil lab will provide a specific recommendation based on your current ppm, the target ppm for your crop, and the soil type.
Calculating the exact amount of fertilizer needed requires knowing the percentage of actual boron in your chosen product. For example, if a recommendation calls for one pound of actual boron per acre, and your fertilizer contains 20% boron, you must apply five pounds of the product to deliver the required nutrient amount. This precise calculation prevents both under-fertilization and the risk of over-application and toxicity.
Methods for Applying Boron Fertilizers
Boron fertilizers are available in several forms, including highly soluble sodium borates, granular products, and boric acid. Highly soluble products, such as those with 20% actual boron, are often used for foliar applications or fertigation due to their rapid dissolution. Granular or prilled forms, which contain lower concentrations, are suitable for blending with other dry fertilizers for soil application.
The two primary application techniques are soil application and foliar spraying. Soil application, often done pre-plant, involves either broadcasting the fertilizer uniformly across the soil surface or applying it in a narrow band near the seed row. Broadcasting is considered safer because it dilutes the boron concentration, reducing the risk of toxicity near delicate seedling roots.
Foliar application involves spraying a diluted boron solution directly onto the plant leaves during the growing season. This method provides a quick supply of the nutrient and is effective for correcting acute deficiencies. Because leaves have a limited capacity for absorption, foliar treatments use significantly lower doses than soil treatments and may require multiple applications throughout the season.
Timing and Safety Considerations
The timing of boron application is crucial for maximizing effectiveness and minimizing toxicity risk. Soil-applied boron is best applied before planting and incorporated into the topsoil, ensuring the nutrient is available throughout the season. Foliar applications are most beneficial just before the reproductive stage, such as before flowering, to support pollen viability and fruit set.
The most significant safety concern is boron toxicity, often called “boron burn,” which occurs when too much is applied. Symptoms typically appear as a yellowing of the leaf margins and a scorched appearance on the edges of older leaves. To avoid this, ensure uniform spreading of soil-applied products and never place high concentrations directly in contact with seeds or seedlings.
Soil characteristics affect boron availability; the nutrient is less available in alkaline soils with a high pH. Boron is also highly mobile and easily leaches from sandy soils with low organic matter, necessitating more frequent applications in those environments. Maintaining a proper soil pH, typically between 6.0 and 7.0, and improving organic matter content will help keep the applied boron available.