Calcium application for Robusta plantations: Soil improved but yield did not increase immediately

In many coffee plantations in the Central Highlands, farmers focus on supplementing NPK fertilizers to maintain yield from one crop to the next. However, there is a less-often-discussed issue: the soil is becoming increasingly acidic.

Acidic soil is not just a story about a low pH number. As acidity increases, aluminum and iron in the soil can become more easily soluble, while the ability to provide certain nutrients such as calcium and magnesium is reduced. The root environment also changes, affecting the activity of soil microorganisms.

Calcium application is one of the measures often used to improve this situation. However, a practical question needs to be raised: does good soil mean coffee plants will increase yield immediately in the next crop?

Acidic soil does not just deprive the plant of a nutrient

pH is an index that shows the level of ion hydro activity in the soil solution. When pH decreases, the soil environment becomes more acidic. However, the biological impact of this process does not stop at the change in pH.

In acidic soil, exchangeable aluminum can increase and be detrimental to root development. Roots that are limited in development will reduce the ability to exploit water and nutrients from deeper soil layers. At the same time, the imbalance between cations such as Ca²⁺, Mg²⁺, and Al³⁺ can affect the plant's ability to absorb nutrients.

One of the factors contributing to acidic soil in intensive cultivation systems is the use of ammonium-based fertilizers. When ammonia is converted through nitrification, hydrogen ions are released and can contribute to a decrease in soil pH over time. The degree of impact depends on the type of fertilizer, the amount applied, the soil's buffering capacity, the amount of organic matter, and the irrigation regime.

This explains why a coffee plantation can be fully fertilized but the plants still do not effectively exploit nutrients if the soil environment and root zone are not favorable.

Experiment at Đắk Đoa, Gia Lai: What changes after applying lime?

A study published in 2025 in the Soil Research journal surveyed the impact of dolomite lime on Robusta and black pepper systems in Đắk Đoa District, Gia Lai Province.

The research group implemented the experiment on 10 Robusta plantations and 10 black pepper plantations, all using the TRS1 variety, with an average age of around 8-10 years. Each plantation was divided into two areas: one area was treated with dolomite lime and the other was a control area without lime.

The tested dosage was 2.5 tons of dolomite lime per hectare, evenly spread on the soil surface. The coffee experiment lasted 15 months; soil parameters were evaluated after 12 months.

Notably, the average initial pH of the soil at the coffee plantations was around 4.7. After 12 months, the area treated with lime reached an average pH of 4.9, while the control area decreased to around 4.4.

The results show that the soil in the lime-treated area has significantly improved pH, organic matter content, potassium, calcium, and magnesium exchangeable levels. The exchangeable iron and aluminum levels are also lower than those in the control area.

Notably, the study recorded an increase in the level of arbuscular mycorrhizal fungi (AMF) root endophytic symbiont in the lime-treated coffee garden. This group of microorganisms can support plant nutrient uptake, particularly phosphorus, through symbiotic relationships with roots.

The notable point: Coffee yield has not increased significantly.

If only looking at the positive changes in the soil, readers may expect coffee yield to increase correspondingly. However, the harvest results in the study do not show that.

After 15 months, the average yield of green coffee beans in both the lime-treated and non-lime-treated areas was approximately 2.7 tons/ha. The difference between the two treatments was not statistically significant.

This is an important result because it shows that improving soil conditions and increasing yield are two processes that can occur at different times.

Lime can adjust pH, add calcium and magnesium, reduce exchangeable aluminum, and improve some root zone biological conditions. However, coffee yield also depends on many other factors such as tree age, canopy structure, fruit-bearing branch quantity, irrigation, nutrient supply throughout the growth cycle, weather during flowering, and fruit set rate.

This study does not demonstrate that lime has no value for long-term yield. It only shows that, in the time and conditions of the experiment surveyed, no statistically significant increase in yield was recorded.

What is dolomitic lime different from ordinary lime?

Dolomitic lime mainly contains calcium magnesium carbonate, which can neutralize soil acidity and add both calcium and magnesium simultaneously.

For coffee gardens, the choice of lime should not be based solely on the product name. Consider the actual soil condition:

• If the soil is acidic and lacks both calcium and magnesium, dolomite may be a suitable choice.
• If the soil lacks calcium but magnesium is at a suitable level, consider whether to use a different calcium source or not.
• If the soil has a suitable pH but lacks a specific nutrient, do not assume that increasing lime is the correct solution.

Over-liming can also lead to nutrient imbalance, reduce the ability to absorb some micronutrients, and increase unnecessary costs. The goal of soil improvement is not to raise pH as high as possible, but to bring the soil environment to a suitable state for coffee plants and cultivation conditions.

Before deciding to apply lime, know what the soil is lacking.

For a Robusta coffee garden, at least pH, organic matter, calcium, magnesium, potassium, phosphorus, and exchangeable aluminum should be evaluated when possible. These indicators help distinguish between acidic soil and soil that is acidic with nutrient deficiencies or cation imbalance.

The 2.5 tons/ha dose in the Gia Lai study is a specific experimental dose, not a default formula for all coffee gardens. Soil type, cultivation depth, buffering capacity, lime quality, and initial nutrient status can all affect the need for lime application.

A coffee garden may need soil improvement before increasing fertilizer application. However, soil analysis results are more reliable than relying on leaf color or using the previous year's fertilizer experience.

Reference

1. Nguyen-Van, L. et al. (2025). Liming effect on soil chemical and biological properties, pests and diseases, and crop yields in robusta coffee and black pepper in Vietnam. Soil Research, 63(3), SR24143. https://doi.org/10.1071/SR24143
2. Alliance of Bioversity International and CIAT. Information and summary of field research in Gia Lai. View research file

Knowledge Library: SAVINA Coffee Knowledge