Boron Deficiency in Coffee
Coffee is one of the most susceptible crops to boron deficiency. In coffee cultivation, boron is necessary for:
- Cell division and cell wall formation in interaction with calcium
- Root growth and water uptake
- Growth of internodes
- Fruit size and setting
- Calcium uptake
- Decreasing aluminum toxicity
- Drought and disease resistance
Boron deficiency causes the death of the terminal growing point. The later development of secondary branches (sometimes as many as seven at the same node) below the dead terminal bud has a typical fan-like effect. In severe cases, the secondary branches quickly die resulting in dieback of terminal sections of the new shoots.
In a recent study published in the Journal of Environmental Management, Muhammad Riaz, et al. demonstrated the importance of boron’s role in decreasing aluminum toxicity in citrus. Boron acted in the regulation of multiple physiological processes and has the potential to increase dry matter weight, plant height, root length, and number of leaves. This is applicable for coffee and other plant species.
Studies have also shown that with increased leaf boron content, there was a reduction in red mite infestation (Tetranychus pioroei). There was a correlation between boron and cyanidin production. Cyanidin is a polyphenol that is toxic to the mites. Another recent study showed that leaves with higher rates of boron concentration had lower rates of damage caused by coffee borer (Hypothenemus hampei).
What does boron deficiency look like?
When a plant is boron deficient, the leaves are typically misshapen and smaller in size. They are often narrow and twisted with irregular edges and have a leathery texture. The internodes are short, leaf tips may fail to develop properly, and the apical portion may then turn a pale olive/green color. In contrast, the basal portion of the leaf remains a deep dark green. It has been suggested that this apical chlorosis is due to a local calcium deficiency which may be the result of boron deficiency reducing calcium translocation.
Defoliation can occur. The underside of the midrib of both chlorotic and otherwise healthy older leaves may become suberized.
Production will be seriously reduced because of poor fruit formation. In nutrient solution studies, it has been demonstrated that boron deficiency does not seem to affect flowering but that, in contrast, fruiting is markedly reduced. In controlled experiments, no fruit was produced even though flowering was regular. This is in accord with work demonstrating that coffee shows a peak demand for boron (and for calcium) just after flowering and when the formed cherry is developing.
When do symptoms appear?
Terminal dieback and the development of crinkled leaves towards the end of a dry period and at the start of the rainy season (due to reduced boron absorption from the dry upper soil layers) are often the first signs that the coffee plant is suffering from boron deficiency.
Symptoms are also particularly noticeable at flowering and after liming due to the reduced availability of soil boron. Boron is mainly used on coffee to prevent the occurrence of the transient deficiency symptoms rather than to correct severe deficiencies which result in considerable branch dieback.
How much boron is enough?
Coffee removes significant amounts of boron from the soil each year (73 grams per ton of grain produced). Once a plant shows signs of deficiency it cannot fully recover. Flowering may be regular however there may be no fruit production due to lack of pollen tube development.
In soils containing more than 4% organic matter, boron may not be available to the plant because it can form complexes with diols contained in organic matter. In addition, affinity of organic matter for boron may affect boron uptake by plants due to changes in the boron concentration in the soil solution. Boron uptake by plants is controlled by the level of boron in the soil solution and by the total boron content in the soil.
Always consult your local department of agriculture or crop advisors to check the proper dose rates. Rates of boron fertilization should be based on yield goals along with soil tests and/or plant tissue analyses obtained from young leaves found the terminal section of top branches.
Timing your boron application
- High demanding periods for boron are after harvest, blooming, fruit setting, and fruit growth.
- Boron should be applied to the soil alone or preferably blended with NPK and Mg.
- With dry bulk blend fertilizer, broadcasting before initiation of floral bud is recommended, using Granubor® 2.
- Boron in liquid fertilizers may be applied by fertigation using Solubor®.
- Solubor may also be foliar applied just to amend when it is suspect that the uptake of boron through the roots could be disrupted. Application can be done during the spraying of insecticides or fungicides.