Which soils and crops are prone to boron deficiency and how do you apply boron fertilizers?
In southern China, granites with low boron content, along with other acidic igneous rocks, gneiss, and sandstones, are commonly found. Due to intense leaching processes, these regions suffer from widespread boron deficiency. Soils such as red soil, laterite, yellow earth, and purple soil are typical examples. In contrast, the Loess Plateau features soils derived from loess deposits and alluvium from the Yellow River, which often contain mostly insoluble forms of boron, leading to similar deficiencies. Soils like bauxite soil, cotton soil, and yellow loam also exhibit this issue. Additionally, calcareous soils containing free calcium carbonate are prone to boron deficiency. Over-application of lime on acidic soils can further reduce boron availability, while poorly drained meadow soils may experience boron deficiency due to high organic matter adsorption.
Different crops have varying requirements for boron. Cruciferous plants like rapeseed and legumes such as soybeans are highly sensitive to boron deficiency and respond well to its application. Root crops like sugar beets, potatoes, and fiber crops, as well as fruit trees, also require significant amounts of boron. On the other hand, certain woody plants in the elm family tend to need less boron.
Boron fertilizer should be applied when water-soluble boron levels fall below 0.5 ppm. Although the total boron requirement for crops is relatively small, the margin between adequate and toxic levels is very narrow. Therefore, it's crucial to apply boron carefully and evenly to avoid localized toxicity. The most common method of application is direct soil incorporation. Crops with higher boron needs typically require about 0.25–0.5 kg per mu, while general crops may only need 0.25 kg per mu (equivalent to borax). Foliar sprays are also effective, with concentrations ranging from 0.05% to 0.2%. Soluble borax is commonly used, and some studies suggest that foliar application can be as effective or even better than soil application in highly leached sandy soils. However, seed treatments with boron are generally reserved for areas with severe deficiency due to the risk of seed damage.
The best time to apply boron is before the growth period and during the transition from vegetative to reproductive growth. To prevent over-application or uneven distribution, slow-release fertilizers like boron-containing glass-based or boron-magnesium fertilizers can be used. Boron tends to be less effective in soil and often requires annual applications to maintain optimal levels.
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