In coffee cultivation in Vietnam, experience shows that besides supplying the three primary macronutrients—nitrogen (N), phosphorus (P), and potassium (K)—farmers also need to pay close attention to providing adequate secondary and micronutrients if they want consistently high yields and good quality coffee. This is especially important on the red basalt soils of the Central Highlands.
Alongside the basic macronutrients, coffee trees require several secondary and micronutrients:
1. Secondary nutrients
These are nutrients the coffee tree needs in relatively large amounts: calcium (CaO), magnesium (MgO), and sulfur (S).
Calcium (CaO): After N, P, and K, calcium is taken up by coffee trees in the greatest quantity, but farmers often overlook it because they assume the soil contains enough. Research shows the amount of calcium removed from the soil by coffee is nearly three times that of phosphorus. Replacing this calcium is important to raise soil pH and encourage beneficial soil microbes. In fact, the soils of the Central Highlands are typically low in calcium and sulfur.
Calcium can be supplied through lime (good limestone lime contains about 90 % CaO; shell lime around 50 %), superphosphate (about 30 % CaO), or fused phosphate (about 38 % CaO). If using single fertilizers, about 300–400 kg of lime per hectare per year is usually sufficient; if using mixed fertilizers, apply about 1.5 times more.
Magnesium (MgO): Essential for photosynthesis and nutrient metabolism, magnesium promotes vigorous growth, dark green foliage, large firm cherries, and helps the tree withstand dry-season stress. Coffee absorbs a significant amount—about half as much as calcium.
It can be supplied through fused phosphate (about 18 % MgO), dolomite (10–18 % MgO), or magnesium sulfate (about 15 % MgO). Research and field practice suggest 60–80 kg MgO per hectare per year is enough.
Sulfur (S): Sulfur deficiency causes leaf bleaching and clearly reduces yield and quality. Although the amount of sulfur taken up is lower than other secondary nutrients, it is essential, and Central Highlands soils are particularly deficient.
Recommended rates are about 40–60 kg S per hectare per year. Sulfur can be supplied via ammonium sulfate (SA, about 24 % S), superphosphate (about 14 % S), fused phosphate (about 12 % S), or certain compound fertilizers that contain sulfur.
Note: calcium and magnesium are less prone to leaching than sulfur. Application rates should reflect not only the crop’s nutrient uptake but also losses and the tree’s absorption efficiency.
2. Micronutrients
These are needed only in trace amounts: copper (Cu), zinc (Zn), boron (B), manganese (Mn), molybdenum (Mo), iron (Fe), chlorine (Cl), etc.
For micronutrients, it is generally recommended to spray coffee trees 4–5 times per year with a solution containing boric acid (0.3 %) and zinc sulfate (0.6–0.8 %). Copper can be supplied incidentally when applying copper-based fungicides—such as copper oxychloride or Bordeaux mixture—for coffee leaf rust control; this is typically enough to meet the plant’s needs. Other micronutrients are usually abundant in nature and generally do not require supplementation.
Because nutrients can be lost through leaching, erosion, volatilization, or chemical reactions that make them unavailable to roots, fertilization should at least compensate for these losses. Meeting this minimum replacement level is essential for sustainable coffee cultivation.