Jeju Citrus Orchards Show Contrasting Nutrient Levels: Three Nutrients Deficient in Volcanic Ash Soils, Two Excessive in Non-Volcanic Ash Soils

【Financial News, Jeju = Reporter Yong-bok Jeong】 A survey of 88 citrus orchards in Jeju found that volcanic ash soils were deficient in potassium, calcium and magnesium, while non-volcanic ash soils had phosphate and potassium levels above the appropriate range. Soil acidity has improved, but nutrient levels vary, indicating that fertilizer use needs to be adjusted for each orchard.
According to the Agricultural Research and Extension Services of Jeju Special Self-Governing Province, a soil chemical properties survey conducted this year at 88 citrus orchards across the province found clear differences in nutrient levels between volcanic ash and non-volcanic ash soils.
The survey was conducted as part of the Rural Development Administration’s (RDA) agricultural environment change survey project. Soil chemical properties at citrus orchards are surveyed every four years. Seven indicators—including soil acidity (pH), organic matter, available phosphate, potassium, calcium and magnesium—are analyzed to track long-term changes in soil conditions.
The clearest differences were in the excess or deficiency of key nutrients needed for citrus tree growth. In volcanic ash soils, potassium levels were 0.48 cmolc/kg, below the appropriate range of 0.5–0.7 cmolc/kg. Calcium and magnesium levels, at 4.43 cmolc/kg and 1.32 cmolc/kg, respectively, were also below the appropriate levels.
By contrast, potassium levels in non-volcanic ash soils were 0.88 cmolc/kg, exceeding the appropriate upper limit. Calcium was 5.77 cmolc/kg and magnesium was 1.65 cmolc/kg, both within the appropriate range.
Potassium, calcium and magnesium are nutrients needed for tree growth and fruit development. The survey measures the amounts present in forms that roots can absorb from the soil. Excessive levels of one nutrient can interfere with the absorption of others.
Differences were also found in phosphate. Available phosphate—the form of phosphate plants can use—declined in volcanic ash soils from 382 mg/kg in 2022 to 291 mg/kg this year. That puts it within the appropriate range of 200–300 mg/kg.
Available phosphate in non-volcanic ash soils fell from 736 mg/kg to 469 mg/kg over the same period. Although it declined considerably over four years, it remained above the appropriate upper limit of 300 mg/kg.
The acidity of both soil types showed a long-term improving trend. The average pH of volcanic ash soils rose from 4.9 in 2002 to 5.0 in 2022 and 5.2 this year. Non-volcanic ash soils also rose over the same period, from 4.8 to 5.0 and then 5.6.
A lower soil pH indicates greater acidity. The recommended range for citrus orchards is pH 5.5–6.5. Non-volcanic ash soils have entered the appropriate range, but volcanic ash soils remain more acidic than the benchmark despite the improvement.
Organic matter content was measured at 105 g per kg of soil in volcanic ash soils and 40 g per kg in non-volcanic ash soils. Both soil types remained within their respective appropriate ranges.
Differences in the soils of Jeju’s citrus orchards are related to the island’s volcanic geology. Volcanic ash soils, formed under the influence of volcanic ash, are characterized by high organic matter content. Non-volcanic ash soils have been relatively less affected by volcanic ash. Soil properties determine how well nutrients are retained or kept in forms that plants can use.
Existing citrus cultivation guidance from the Agricultural Research and Extension Services of Jeju Special Self-Governing Province also explains that high rainfall in Jeju can wash nutrients such as potassium, calcium and magnesium out of the soil, and that severe soil acidification can make phosphate harder for plants to absorb.
The issue is that nutrient levels can differ even within a single citrus orchard, depending on soil type and the kinds and amounts of fertilizer and compost used over time. It may not be appropriate to apply more potassium or calcium across the board simply because an orchard has volcanic ash soil, or to cut phosphate fertilizer across the board simply because it has non-volcanic ash soil. Since these figures are averages by soil type for the soils surveyed, each grower should apply fertilizer based on the test results for the specific plot.
In citrus orchards where phosphate and potassium are already sufficient, it is important to account for nutrients in compost as well as those in chemical fertilizers. For orchards with low pH, calcium and magnesium levels should also be checked rather than applying lime-based soil amendments without first testing the soil.
Adjusting fertilizer use is also linked to production costs for growers. Repeatedly supplying nutrients that are already sufficient can lead to unnecessary fertilizer expenses and worsen nutrient imbalances.
Kim Tae-gyun, head of the Jeju Special Self-Governing Province Agricultural Research and Extension Services, emphasized, “Nutrient levels vary not only by soil type but also according to fertilizer-use history, so appropriate fertilization based on soil tests for each plot is important.”


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