Which Soil Is Good for Growing Tea and Coffee and Why: Unearthing the Perfect Ground for Your Brew

Unearthing the Perfect Ground: Which Soil is Good for Growing Tea and Coffee and Why?

For years, I’ve been fascinated by the journey from a humble seed or cutting to the comforting warmth of a morning cup of coffee or the nuanced flavors of a finely brewed tea. It’s a transformation that hinges on so many factors, but one that’s often overlooked by the casual consumer is the very foundation of it all: the soil. I remember visiting a small coffee plantation in Costa Rica, the rich, dark earth clinging to my boots. The farmer, with a weathered hand, pointed to the vibrant green bushes. “This,” he’d said, gesturing to the soil, “is where the magic begins.” It’s a sentiment that has always resonated with me. Understanding the ideal soil for tea and coffee isn’t just academic; it’s the secret ingredient that unlocks their full potential. So, which soil is good for growing tea and coffee and why? The answer, in a nutshell, involves a delicate balance of drainage, acidity, nutrient content, and a healthy microbial ecosystem, with specific preferences often dictated by the unique needs of each plant.

The Foundation of Flavor: What Makes Soil Ideal for Tea and Coffee?

At its core, the quest for the perfect soil for tea and coffee is a quest for an environment that allows these sensitive plants to thrive, develop their complex flavor profiles, and produce abundant yields. It’s not simply about the dirt beneath our feet; it’s about a living, breathing medium that nourishes and supports the entire life cycle of the coffee cherry and the tea leaf. For both tea and coffee, the paramount characteristic of good soil is its ability to drain well. This might seem counterintuitive, as plants need water, but waterlogged roots are a death sentence for both. Stagnant water suffocates root systems, preventing them from accessing oxygen and making them susceptible to fungal diseases. Beyond drainage, acidity plays a crucial role. Both tea and coffee plants are acid-loving, meaning they perform best in soil with a lower pH level.

Understanding Soil pH: A Crucial Factor for Tea and Coffee Cultivation

Soil pH, a measure of its acidity or alkalinity, is a critical determinant of nutrient availability for plants. Tea plants (Camellia sinensis) generally prefer a slightly acidic soil, typically ranging from 4.5 to 6.0. Coffee plants (Coffea spp.), particularly the widely cultivated Arabica variety, are even more particular, thriving in soil with a pH between 5.5 and 6.5. If the soil is too alkaline, essential nutrients like iron, manganese, and zinc become locked up and unavailable to the plant, even if they are present in the soil. This leads to nutrient deficiencies, stunted growth, and reduced crop quality. Conversely, overly acidic soil can release toxic elements like aluminum, which can also inhibit growth and damage root systems. Maintaining the correct pH is, therefore, paramount. Farmers often amend their soil with materials like sulfur to lower pH or lime to raise it, depending on the existing conditions. This careful management ensures that the plants can efficiently absorb the nutrients they need to produce those sought-after beans and leaves.

The Nuances of Coffee Soil: Where Flavor is Born

When we delve into the specifics of coffee cultivation, the soil requirements become even more nuanced. The world’s most prized coffee, Arabica, is notoriously demanding. It flourishes in volcanic regions, a clue to its preferred soil composition. These soils are often rich in minerals and possess excellent drainage due to the porous nature of volcanic rock and ash. However, it’s not just about volcanic origin. The ideal coffee soil is deep, well-drained, and fertile, with a good amount of organic matter. This organic matter acts as a slow-release fertilizer, providing essential nutrients like nitrogen, phosphorus, and potassium, while also improving soil structure and water retention (without waterlogging!).

Key Characteristics of Ideal Coffee Soil

• Excellent Drainage: This is non-negotiable. Coffee roots need air. Waterlogged soil leads to root rot, a swift killer. Volcanic soils, with their inherent porosity, are often ideal.
• Acidity: As mentioned, a pH between 5.5 and 6.5 is generally optimal for Arabica coffee.
• Rich in Organic Matter: Decomposing leaves, compost, and other organic materials provide a steady supply of nutrients and improve soil structure. This also supports a healthy population of beneficial soil microbes.
• Sufficient Depth: Coffee plants develop extensive root systems, so the soil needs to be deep enough to support this growth, typically at least 1.5 to 2 meters (about 5-6.5 feet).
• Moderate Fertility: While richness is good, overly fertile soil can sometimes lead to excessive leafy growth at the expense of bean development. A balanced fertility is key.
• Good Aeration: Similar to drainage, roots need oxygen. Soils that are too compacted will hinder aeration.

My personal observations from visiting coffee farms in Colombia further solidified this understanding. The highest quality beans often come from farms situated on the slopes of mountains, where gravity assists with drainage. The soil there is a dark, crumbly loam, frequently enriched with compost made from coffee pulp and local vegetation. The farmers spoke of the importance of “mineral richness,” a direct link to the volcanic origins of many prime coffee-growing regions. They also emphasized the ‘living’ aspect of the soil, the presence of earthworms and beneficial fungi, which they actively cultivate through organic practices.

The Role of Organic Matter in Coffee Soil

Organic matter is the unsung hero of healthy coffee soil. It’s not just about feeding the plant directly; it’s about creating a robust soil ecosystem. When organic matter decomposes, it releases nutrients slowly, providing a sustained food source for the coffee plants. Furthermore, it acts like a sponge, helping the soil retain moisture during dry spells, but crucially, it also improves the soil’s physical structure, making it more porous and thus enhancing drainage and aeration. A soil rich in organic matter is typically darker in color, has a pleasant earthy smell, and is crumbly and friable, meaning it breaks apart easily. This texture is vital for root penetration and gas exchange. Farmers often use cover crops, which are planted and then tilled back into the soil, to continuously replenish organic matter. This practice not only feeds the soil but also helps suppress weeds and prevent erosion.

Volcanic Soils: A Coffee Lover’s Dream (with caveats)

Volcanic soils are often lauded as being ideal for coffee, and there’s a lot of truth to that. The minerals released from volcanic rock, such as potassium and phosphorus, are essential nutrients for coffee plants. The porous nature of these soils, formed from ash and lava, inherently promotes excellent drainage and aeration. Think of regions like the Kona district in Hawaii, the Sidamo region in Ethiopia, or the highlands of Central America – all renowned coffee-producing areas with significant volcanic influence. However, it’s not a simple case of “if it’s volcanic, it’s perfect.” Even volcanic soils can become compacted over time, or their pH might drift. The presence of other soil types mixed with volcanic material, along with proper management practices, still plays a significant role in determining the ultimate quality and health of the coffee plants.

The Delicate World of Tea Soil: Nurturing the Fragrant Leaf

Tea, on the other hand, while sharing some fundamental soil requirements with coffee, has its own unique preferences. Tea plants (Camellia sinensis) are typically grown in mountainous or hilly regions, where the elevation and slope contribute to the well-drained conditions they need. The ideal tea soil is often described as a friable loam or sandy loam, rich in organic matter, and, importantly, acidic. The specific varietal of tea and the climate also play a role, but the core principles remain consistent.

Key Characteristics of Ideal Tea Soil

• Excellent Drainage: Just like coffee, tea plants cannot tolerate waterlogged roots. Hilly terrain is naturally advantageous.
• Acidity: A pH between 4.5 and 6.0 is generally considered optimal for tea cultivation.
• High Organic Matter Content: This is crucial for providing nutrients and improving soil structure. Tea plants are heavy feeders, and a constant supply of organic nutrients is vital.
• Good Aeration: Compacted soil inhibits root respiration.
• Sufficient Depth: Similar to coffee, tea plants have root systems that benefit from deep, well-structured soil.
• Moderate Fertility: While fertile soil is beneficial, excessive nitrogen can lead to rapid, leafy growth that may not yield the best quality tea leaves.

My experiences visiting tea estates in Darjeeling, India, and the Cameron Highlands in Malaysia highlighted the importance of elevation and soil composition. The air is cooler at these altitudes, which slows the growth of the tea plant, allowing it to develop more complex flavor compounds. The soil, often a reddish-brown loam, was visibly rich with decomposed plant matter, and the tea bushes seemed to hug the steep slopes, a testament to their need for rapid drainage. The estate managers emphasized that the subtle variations in soil composition, influenced by the local geology and the type of vegetation that previously occupied the land, contribute to the unique terroir of their teas – the very essence of what makes a Darjeeling “muscatel” or a specific single-origin tea so distinctive.

The Importance of “Terroir” in Tea Soil

The concept of “terroir” is deeply intertwined with the soil in tea cultivation. Terroir refers to the environmental factors that shape the character of a crop, and soil is a major component. The mineral content of the soil, its drainage properties, its pH, and the type and amount of organic matter all contribute to the unique flavor profile of the tea leaves. For instance, tea grown in mineral-rich soils, particularly those with a good supply of trace elements, may develop more complex and nuanced flavors. Soil that is slightly deficient in certain nutrients can sometimes lead to more concentrated flavor compounds in the leaves. This is why a tea from one region can taste vastly different from a tea grown in another, even if they are the same varietal. The soil is, in essence, imprinting its characteristics onto the plant.

Why Acidic Soil is So Important for Tea

Tea plants are naturally adapted to thrive in acidic environments. This acidity influences the uptake of various nutrients. For example, in acidic soils, iron and manganese are more readily available to the tea plant. These micronutrients are vital for photosynthesis and overall plant health. Furthermore, the acidic environment helps to suppress certain soil-borne diseases that can affect tea plants. While the ideal pH range is 4.5 to 6.0, it’s important to note that within this range, there can be subtle differences that impact the tea. Extremely low pH levels can lead to aluminum toxicity, so a balance is always sought. Soil testing is a common practice to ensure that the pH is within the desired parameters, and amendments like organic matter (which can naturally buffer pH) or specific fertilizers are used to maintain it.

Comparing and Contrasting: Tea vs. Coffee Soil Needs

While both tea and coffee are tropical/subtropical plants that benefit from well-drained, acidic soil rich in organic matter, there are subtle but significant differences in their optimal conditions. These differences often dictate where each crop can be successfully cultivated on a commercial scale.

Key Differences in Soil Preferences

• Acidity Level: Tea generally tolerates a slightly wider range of acidity (4.5-6.0) compared to coffee (5.5-6.5). This means tea plants can sometimes be grown in soils that are a bit too acidic for optimal coffee production.
• Nutrient Requirements: While both need essential macronutrients (N, P, K) and micronutrients, the specific balance and availability can differ. Coffee, especially Arabica, is known to be quite sensitive to nutrient deficiencies.
• Soil Structure: Both require good aeration and drainage, but the specific texture that best facilitates this can vary. For coffee, particularly in volcanic regions, porous soils are highly prized. For tea, a friable loam or sandy loam is often favored.
• Elevation and Climate Interactions: While not strictly a soil characteristic, the ideal soil for tea is often found at higher, cooler elevations, which also influences soil development and nutrient cycling. Coffee also benefits from elevation, but the specific soil characteristics remain paramount.

A Table of Comparison

To better illustrate the similarities and differences, consider this comparative table:

Soil Characteristic	Ideal for Tea	Ideal for Coffee	Commonality
pH Level	4.5 – 6.0 (Slightly acidic to acidic)	5.5 – 6.5 (Moderately acidic)	Both prefer acidic conditions.
Drainage	Excellent	Excellent	Crucial for both to prevent root rot.
Organic Matter	High content essential for nutrients and structure	High content essential for nutrients and structure	Key for fertility, water retention, and soil health.
Aeration	Good	Good	Roots need oxygen for respiration.
Soil Texture	Friable loam or sandy loam	Loam, sandy loam, often enriched by volcanic material	Loose, well-structured soil is preferred.
Depth	Sufficient for root development	Sufficient for robust root systems	Deep soil supports healthier plants.

It’s fascinating to see how these two beloved beverages, originating from different parts of the world and cultivated for centuries, have such similar fundamental needs from the soil, yet with nuanced distinctions that contribute to their unique qualities. For instance, while both need good drainage, the slightly more acidic tolerance of tea allows it to thrive in certain subtropical highland soils where coffee might struggle to achieve its optimal flavor potential. Conversely, the specific mineral compositions found in many volcanic soils lend themselves exceptionally well to the complex flavor development required for high-quality coffee beans.

Beyond the Basics: Factors Influencing Soil Quality for Tea and Coffee

While drainage, acidity, and organic matter are the cornerstones, several other factors contribute to the overall quality of soil for growing tea and coffee. These elements often work in synergy, creating an environment where the plants can truly flourish.

The Role of Soil Microbes and Mycorrhizal Fungi

A healthy soil is a living ecosystem teeming with microorganisms. Bacteria, fungi, and other microbes play a vital role in nutrient cycling, breaking down organic matter, and making nutrients available to plant roots. Mycorrhizal fungi, in particular, form a symbiotic relationship with plant roots. These fungi extend their thread-like hyphae far into the soil, effectively increasing the root’s surface area for nutrient and water absorption. In exchange, the plant provides the fungi with sugars produced during photosynthesis. For tea and coffee plants, a robust population of mycorrhizal fungi can significantly improve their ability to absorb phosphorus and water, especially in soils that might be less fertile or prone to drought. This is why sustainable farming practices that focus on building soil health, rather than depleting it, are so crucial.

Topography and Its Impact on Soil

As we’ve touched upon, topography plays a significant role, especially in areas where tea and coffee are traditionally grown. Hilly and mountainous regions offer natural advantages. The slopes facilitate drainage, preventing water from pooling around the roots. This also helps to reduce the risk of soil erosion, particularly on steeper gradients, when combined with good ground cover from the plants and appropriate farming techniques like terracing. The elevation also influences temperature and rainfall patterns, which in turn affect soil temperature, moisture levels, and the rate of organic matter decomposition, all contributing to the unique characteristics of the soil and the final product.

Climate and its Interplay with Soil

The climate in which tea and coffee are grown directly influences the soil. Consistent rainfall, temperature, and sunlight all affect the rate of weathering of parent rock material, the decomposition of organic matter, and the activity of soil microbes. For example, warm, humid climates can accelerate the decomposition of organic matter, leading to soils rich in humus, but they can also increase the risk of fungal diseases if drainage is poor. Cooler, high-altitude climates, often favored for certain types of tea and coffee, slow down decomposition, leading to soils that might have a slightly different nutrient profile and structure. Understanding this interplay is key to selecting the right soil management strategies for a particular region.

Steps to Assessing and Improving Soil for Tea and Coffee Cultivation

For aspiring growers or those looking to optimize their existing plantations, a systematic approach to soil assessment and improvement is essential. This isn’t about guesswork; it’s about informed decision-making.

1. Soil Testing: The Foundation of Understanding

The very first step is to get a professional soil test. This is non-negotiable. A reputable lab can provide crucial data on:

• pH Level: This is the most critical initial measurement for tea and coffee.
• Nutrient Content: Levels of macronutrients (Nitrogen, Phosphorus, Potassium, Calcium, Magnesium, Sulfur) and micronutrients (Iron, Manganese, Zinc, Copper, Boron, Molybdenum).
• Organic Matter Content: This indicates the soil’s fertility and structure.
• Cation Exchange Capacity (CEC): A measure of the soil’s ability to hold onto positively charged nutrients, indicating its fertility potential.
• Texture: The proportion of sand, silt, and clay particles, which influences drainage and water-holding capacity.

Based on these results, you can develop a targeted strategy for improvement. Don’t rely solely on visual cues, though they are helpful indicators.

2. Addressing pH Imbalances

• If the pH is too high (alkaline): Incorporate sulfur or elemental sulfur. This gradually lowers the pH as soil microbes convert it to sulfuric acid. Organic matter, like peat moss or composted pine bark, can also help acidify the soil.
• If the pH is too low (acidic): Apply agricultural lime (calcium carbonate) or dolomitic lime (calcium and magnesium carbonate). The amount needed will depend on the soil’s buffering capacity and the target pH.

Always follow recommended application rates from your soil test report to avoid over-liming or over-acidifying.

3. Enhancing Organic Matter

• Composting: Use well-rotted compost made from plant residues, coffee husks, tea prunings, or other organic materials.
• Cover Cropping: Plant nitrogen-fixing legumes (like clover or vetch) or nutrient-accumulating crops, then till them into the soil before they mature.
• Mulching: Apply a layer of organic mulch (e.g., wood chips, straw, compost) around the plants. This helps retain moisture, suppress weeds, and slowly adds organic matter as it decomposes.
• Manure: Use well-composted animal manure, ensuring it’s properly aged to avoid burning plant roots or introducing weed seeds.

Increasing organic matter improves soil structure, water retention, drainage, and nutrient availability—a win-win-win for both tea and coffee.

4. Improving Drainage and Aeration

• Incorporate Organic Matter: As mentioned, this is key to improving soil structure.
• Raised Beds: For areas with consistently poor drainage, consider planting on raised beds. This allows excess water to drain away from the root zone.
• Aeration: In heavily compacted soils, mechanical aeration (using specialized equipment) can be beneficial, though this is more common in larger-scale operations.
• Proper Tilling Practices: Avoid over-tilling, which can destroy soil structure and lead to compaction. When tilling is necessary, do it when the soil is at the right moisture level (not too wet, not too dry).

5. Nutrient Management

• Balanced Fertilization: Based on soil test results, apply fertilizers that provide the necessary nutrients in the right proportions. Slow-release fertilizers can be beneficial.
• Micronutrient Supplementation: If soil tests reveal deficiencies in micronutrients, specific chelated micronutrient fertilizers can be applied.
• Organic Fertilizers: Fish emulsion, bone meal, and blood meal are examples of organic fertilizers that can provide a range of nutrients.

The goal is not just to add nutrients but to create an environment where the plant can efficiently absorb them. Healthy soil biology is crucial for this.

Frequently Asked Questions About Soil for Tea and Coffee

Q1: Can I grow tea and coffee in the same soil?

In theory, yes, you could grow both in soil that meets their overlapping requirements. However, their optimal conditions are not identical. Tea plants tend to be a bit more tolerant of slightly higher acidity (lower pH) than coffee plants. Coffee, particularly Arabica, is quite sensitive and thrives in a specific range that might be too neutral for optimal tea production. If you were starting a new planting, it would be best to choose one or the other, or to manage two separate areas with slightly different soil amendments to cater to each plant’s precise needs. Trying to find a single soil sweet spot that maximizes the potential of both simultaneously would be challenging and likely result in suboptimal performance for both crops.

Furthermore, the management practices might differ. For instance, some coffee cultivation methods might involve practices that could be detrimental to the delicate tea leaves, and vice versa. From a practical standpoint, especially for commercial viability, focusing on one crop and tailoring the soil to its absolute best requirements is usually the most effective approach. If you are a hobbyist with the space and time to experiment, you might be able to find a happy medium, but understanding the distinct preferences is key to success.

Q2: How important is the parent material of the soil for growing tea and coffee?

The parent material, which is the underlying rock or sediment from which the soil is formed, is incredibly important because it dictates the initial mineral composition and texture of the soil. As mentioned earlier, soils derived from volcanic activity are often highly prized for coffee because they tend to be rich in essential minerals like potassium and phosphorus and have a porous structure that promotes excellent drainage and aeration. These inherent qualities provide a fantastic starting point. Similarly, soils derived from ancient sedimentary rocks or metamorphic rock can also offer desirable characteristics for tea cultivation, contributing to the unique mineral profiles that define regional “terroir.”

However, it’s crucial to understand that parent material is just the beginning. Over time, climate, topography, vegetation, and human activity all contribute to soil development and modification. While a volcanic origin might give coffee a head start, a volcanic soil that has become compacted or leached of nutrients through poor management will not perform as well as a well-managed soil derived from non-volcanic parent material. Conversely, a tea grown in soil that originated from less mineral-rich parent material can still produce excellent quality if it is diligently managed to enhance organic matter and maintain the correct pH and drainage.

Q3: Can I use sand or clay soil for growing tea or coffee?

Pure sand or pure clay soils are generally not ideal for either tea or coffee cultivation without significant amendments. Sandy soils drain very quickly, which can lead to rapid drying out and a lack of water retention, making it difficult for plants to access moisture consistently. They also tend to be low in nutrients. Conversely, clay soils have very small particles that pack together tightly, leading to poor drainage and aeration. Plants in clay soil can easily suffer from root rot because water sits stagnant, and roots struggle to get oxygen. They can also become very hard and compacted when dry.

However, these extremes can be improved. If you have sandy soil, you can dramatically improve its water and nutrient-holding capacity by incorporating large amounts of organic matter (compost, aged manure). This will help bind the sand particles together and create a more stable soil structure. If you have clay soil, incorporating organic matter is also the primary solution. Organic matter will help to loosen the clay structure, improving drainage and aeration. You can also consider adding gypsum, which can help flocculate (clump together) clay particles, further improving drainage. In essence, while pure sand or clay might pose challenges, they can often be amended into a more suitable growing medium with diligent effort and the addition of organic matter.

Q4: What are the signs of unhealthy soil for tea and coffee plants?

Observing your plants and the soil itself can provide clues about underlying soil health issues. For both tea and coffee, unhealthy soil often manifests in several ways:

• Stunted Growth: Plants that are not growing vigorously, with small leaves and short internodes (the space between leaf nodes), may indicate nutrient deficiencies or poor root development due to compacted or poorly drained soil.
• Yellowing Leaves (Chlorosis): This is a common sign of nutrient deficiency. If the veins of the leaves remain green while the tissue between them turns yellow, it often points to iron or magnesium deficiency, which can be exacerbated by incorrect pH.
• Wilting: While wilting can be a sign of drought, it can also occur in waterlogged soil. If the soil is constantly wet and the plant wilts, it’s a strong indicator of root rot due to poor drainage.
• Increased Susceptibility to Pests and Diseases: Plants growing in stressed conditions due to poor soil are more vulnerable to attack by pests and diseases. You might see signs of fungal infections on leaves or stems, or notice more insect activity.
• Poor Root Development: If you were to excavate a struggling plant, you might find that the root system is shallow, dense, or shows signs of rot. Healthy roots should be white or light-colored and spread widely.
• Compacted Soil Surface: If the soil surface appears hard, crusted, and water struggles to penetrate, it’s a sign of compaction, which limits oxygen to the roots.
• Unpleasant Odor: Soil that has a sour, rotten, or sulfurous smell, particularly when wet, often indicates anaerobic conditions (lack of oxygen) due to waterlogging and the activity of anaerobic bacteria.

Addressing these signs promptly through soil testing and appropriate amendments can help restore plant health and productivity.

Q5: How does the altitude and microclimate affect the ideal soil for tea and coffee?

Altitude and microclimate are intricately linked to soil characteristics and the overall suitability for tea and coffee cultivation. As altitude increases, temperatures generally decrease, and rainfall patterns can change. For coffee, particularly Arabica, cooler temperatures at higher altitudes slow down the ripening process of the coffee cherry. This slower maturation allows for the development of more complex sugars and acids within the bean, contributing to a richer, more nuanced flavor profile. The soil at these higher altitudes often has different microbial activity due to the cooler temperatures, and the parent material might be influenced by different geological processes. The decomposition of organic matter might also be slower, leading to a soil rich in humus but with a different nutrient release rate.

For tea, high altitudes are also favored, especially for producing some of the most prized varieties. The cooler temperatures slow down growth, leading to smaller, denser leaves that are packed with flavor compounds. The soil in these elevated regions often develops a distinct profile due to the specific climate. For example, mist and cloud cover common at high altitudes can increase soil moisture and affect the rate of organic matter decomposition. The types of vegetation that grow at these altitudes also contribute to the organic matter content and nutrient cycling in the soil. Essentially, the ideal soil is not just about its physical and chemical properties in isolation; it’s about how those properties interact with the unique environmental conditions of a specific location—the altitude, temperature, rainfall, and sunlight—to create the perfect “terroir” for these plants.

Conclusion: The Groundwork for Greatness

In conclusion, understanding which soil is good for growing tea and coffee and why is fundamental to producing high-quality beans and leaves. Both plants share a need for well-drained, acidic soil rich in organic matter, with good aeration. However, subtle differences in their optimal pH ranges and specific nutrient needs mean that the ideal conditions, while overlapping, are not identical. Coffee, particularly Arabica, often favors deep, fertile soils, frequently found in volcanic regions, with a pH between 5.5 and 6.5. Tea plants, while also requiring excellent drainage and organic matter, can tolerate slightly more acidic conditions, typically between pH 4.5 and 6.0, and thrive in friable loams or sandy loams.

The journey from soil to cup is a testament to nature’s intricate processes. The health and composition of the soil directly influence the plant’s ability to absorb nutrients, resist disease, and ultimately develop the complex flavors and aromas that we cherish. By paying close attention to soil testing, managing pH levels, enhancing organic matter, and ensuring proper drainage, growers can lay the groundwork for truly exceptional tea and coffee. It’s a continuous process of understanding and nurturing the living ecosystem beneath our feet, a partnership between the farmer and the earth that yields some of the world’s most beloved beverages. The magic, as that Costa Rican farmer suggested, truly begins in the soil.