Understanding the Impact of Soil Balances

Soil fertility, the capacity of soil to provide essential nutrients for plant growth, hinges on a delicate balance of macronutrients (e.g., nitrogen, phosphorus, potassium, calcium, sulphur, magnesium) and micronutrients (e.g., iron, zinc, manganese, boron, cobber). In coffee cultivation, maintaining optimal nutrient levels is paramount to sustain vigorous plant growth and high-quality yields. Nutrient dynamics in coffee soils are influenced by various factors, including soil type, climate, land management practices, and microbial activity. Understanding these dynamics is crucial for formulating effective soil fertility management strategies. 



1. Nutrient availability

Proper soil fertilisation is fundamental to maximising coffee yield and quality. Nitrogen, a primary macronutrient, plays a pivotal role in promoting vegetative growth, flower initiation, and overall productivity. Phosphorus supports root development and flowering, while potassium enhances fruit quality and resilience. Micronutrients are equally vital, serving as catalysts for enzymatic reactions essential for plant metabolism and defence mechanisms, while also being principal for flower quality and fruit set. Deficiencies or imbalances in soil nutrients can result in stunted growth, reduced yields, and inferior cup quality, underscoring the critical importance of targeted fertilisation practices tailored to the specific needs of coffee plants.

While adequate levels of nutrients are essential for healthy plant growth and optimal crop yield in coffee cultivation, excessive availability of these nutrients can lead to various issues and challenges, such as imbalances, toxicity, reduced cup quality, and increased susceptibility to pests and diseases. To mitigate these issues, coffee growers should adopt nutrient management practices that emphasise soil testing, precise fertilisation, and integrated nutrient management approaches tailored to the specific needs of coffee plants. A rough estimate of the NPK requirements for coffee cultivation per hectare per year is commonly used by coffee farmers and agronomists. For a typical commercial coffee plantation, the recommended NPK application rates are in the range of:

• Nitrogen (N): 60-80 kg/ha/year

• Phosphorus (P): 20-30 kg/ha/year

• Potassium (K): 60-80 kg/ha/year
  
  

These are general guidelines and will vary based on specific yields, soil conditions, coffee variety, and local agricultural practices. Farmers should adjust these rates based on soil test results and expected yields to optimise fertiliser application for their particular circumstances. Optimal nutrient availability ranges in soil samples for the majority of coffee varieties can be seen in section 3.

2. Soil fertility

Beyond nutrient provision, soil fertility management encompasses a broader spectrum of considerations, including soil pH, organic matter content, and cation exchange capacity, which are often somewhat interdependent and equally important.

Soil pH profoundly influences nutrient availability and microbial activity, with coffee plants generally thriving in slightly acidic soils (pH 6-6.5), where the availability and uptake of nutrients from the roots are optimal. Regular soil testing and pH monitoring are important practices for coffee growers to ensure proper soil pH management and optimise growing conditions for coffee plants.

Organic matter, derived from decomposed plant and animal residues, enhances soil structure, water retention, and nutrient cycling - fostering a favourable environment for root development and nutrient uptake, and microbial diversity. It serves as a binding reservoir of essential nutrients due to its high retention properties. 

As organic matter decomposes, nutrients are released into the soil through mineralization, making them available for uptake by coffee plants. It improves soil structure and aggregation by binding soil particles together and promoting the formation of stable aggregates. This enhances soil porosity, water infiltration and drainage, and root penetration – this is important as coffee plants thrive in well-drained soils with good structure and porosity. Additionally, organic matter contributes to the soil's Cation Exchange Capacity by providing negatively charged exchange sites for cations. Humic and fulvic acids, components of organic matter, possess high cation exchange capacities.

Organic matter provides a substrate and energy source for soil microorganisms, including bacteria, fungi, and earthworms, which play vital roles in nutrient cycling, organic matter decomposition, and soil fertility. 

Aiming for an organic matter content of around 3% to 5% in the soil is often considered beneficial for coffee cultivation.

Cation Exchange Capacity (CEC), the soil's ability to retain and exchange positively charged ions, governs nutrient availability and plant uptake, highlighting its importance in sustaining soil fertility and productivity over time. The CEC complex holds onto essential nutrient cations such as calcium (Ca²⁺), magnesium (Mg²⁺), and potassium (K⁺) preventing them from leaching away with rainfall or irrigation water. This retention ensures that nutrients remain available for uptake by coffee plants, promoting healthy growth, and optimal crop yield. The higher the CEC, the better the capacity to hold onto cations. Soils with higher clay content typically have higher CEC because clay minerals have a high surface area and negative charge, allowing them to adsorb and retain more cations. Organic matter can also significantly increase CEC due to its high content of negatively charged functional groups.

The different cations compete for exchange sites within the CEC complex. Understanding the competitive interactions between cations helps coffee growers manage nutrient availability and uptake. For example, high levels of certain cations, such as calcium, can push away and inhibit the uptake of other essential nutrients like magnesium and potassium. By balancing cation levels and ratios, growers can optimise nutrient uptake and prevent deficiencies or imbalances in coffee plants. The CEC complex also acts as a buffer against changes in soil pH, helping to maintain a stable and favourable pH range. In regions where coffee soils tend to be acidic, an increased CEC can help mitigate the effects of acidification and prevent nutrient deficiencies.

Optimal ranges for soil pH, organic matter content, cation exchange capacity, and the relationship between cations in the cation exchange capacity complex can be seen in section 3.

3. Management strategies

To optimise fertilisation strategies, and improve the nutrient- and soil fertility status, it is recommended to continuously take soil and leaf samples for analysis. The soil samples will tell you what is the nutrient content in the soil and how readily available it is for the plants. The leaf sample will give an idea of what is the actual uptake of the plants.

Soil samples (full spectrum) should optimally be performed once every three years as a minimum to put together a three year fertilisation plan. The best time to take the soil sample is just after harvest. Leaf samples and N-mineralisation soil samples should be taken every 6 months to follow the actual uptake of the plants during the season and what they might lack or are not capable of taking up through the root system. 

Soil Samples Optimal Values

A fertilisation plan should be based around which organic- and inorganic fertilisers that are allowed to be used in different countries and regions. To increase the CEC, applying organic manures is quite effective and beneficial. The application of humic and fulvic acids will also increase the CEC in the soil. For acidic soils, liming is the preferred action of deacidification. It is always recommended to use a mix of organic- and inorganic fertiliser for conventional growers to preserve the health of the soil.

It should be noted that changes in soil takes time. One cannot just add all the missing nutrients at once and expect the values to return to normal immediately. A dramatic application of fertiliser will like result in quite some leaching. The solution is to slowly increase applications, both during the season and year-on-year, until target values are reached. This will typically take anything from 2-4 years dependent on the deficiency.

NB: The uptake of several micronutrients (Zn, B, Mn etc.) is often better done as foliar nutrition, as uptake through the roots can often be difficult for coffee plants. 

4. Case Study

This case study focuses on a coffee plantation in Alajuela, Costa Rica, that has been experiencing issues like slow growth, low yields, and poor quality of its fruit. Historically, the plantation hasn't conducted soil or leaf analysis before applying fertilisers. Instead, they've relied on a mixture of composted fruit flesh, liquid inorganic nitrogen, and excess manure from their cattle farm for fertilisation.

We performed a soil analysis, in which we found that there was a noticeable deficiency in nitrogen (N), potassium (K), calcium (Ca), magnesium (Mg), manganese (Mn), and iron (Fe). Relations between cations in the CEC complex was within range, but the pH was too low. Leaf samples collected during the growing season further still revealed deficiencies in manganese (Mn) and iron (Fe). Surprisingly, there were also high levels of phosphorus (P) present, indicating excessive application, which might be toxic to the plants.

To demonstrate the importance of informed decision-making based on soil and leaf analyses, we carried out an experiment. We divided the plantation into two halves: one received the usual fertiliser regimen, while the other half was treated with fertilisers tailored to meet optimal nutrient levels over a 2-year period. This involved providing extra nitrogen, potassium, calcium (lime to pH-regulate simultaneously), and magnesium to the soil, and supplementing manganese and iron through foliar nutrition once after the soil analysis, and once more after the leaf analysis that still showed deficiency. Notably, we withheld phosphorus application for that year to reduce its excessive levels.

The impact of implementing a proper fertiliser plan on nutrient balances, growth, yield, and fruit quality over a 2-year period is summarised in the table below, which represents a soil samples after 2-years and an average across 50 trees per treatment for the yield and growth results. The results speak for itself and underlines the importance of being aware of the soil health and balance in your orchard. All elements in balance after 2-years of fertilisation planning based on soil samples. pH regulated succesfully. The result is more growth, higher yields, and higher cup quality. Proper nutrient management results in increased vigour, which means more fruiting wood and good development of both vegetative and reproductive organs, while proper pH management means improved uptake and utilisation of nutrients.

All elements in balance after 2-years of fertilisation planning
based on soil samples. pH regulated succesfully.

More growth, higher yields, and higher cup quality
with precision fertilisation.

Are you unsure about the nutrient- and soil balance status of your soil and want to explore your growing conditions, need help with fertiliser plans, or have any questions related to nutrient- and soil balances? Reach out to info@pomacoffee.com and send a soil and/or leaf sample to get analysed. Our team of agricultural advisors are happy to help.