Understanding Tucson’s Soil

What is Soil?

Soil is a complex, life-sustaining medium composed of mineral particles, organic matter, water, air, and countless microorganisms. It acts as a habitat for plant roots and various organisms while playing a crucial role in the global ecosystem.

Components of Soil

Soil is composed of four primary components:

1. Mineral Particles (45% of soil): Derived from the weathering of rocks, they determine soil texture and structure.
2. Organic Matter (5%): Decaying plant and animal materials contribute nutrients and improve soil fertility.
3. Water (25%): Essential for plant life, water moves nutrients through the soil and maintains soil stability.
4. Air (25%): Air provides oxygen for root respiration and for soil-dwelling organisms.

Types of Soil and Their Differences

Soils vary based on the proportions of sand, silt, and clay, each providing unique characteristics:

1. Sandy Soil:
   • Characteristics: Sandy soil has large particles, giving it a gritty texture and excellent drainage but low water and nutrient retention.
   • Structure: Sand particles are large and loosely packed, creating large pore spaces that enable quick drainage and minimal water retention.
   • Improvement Tips: Add organic matter (e.g., compost) to increase water retention and nutrient content.
2. Clay Soil:
   • Characteristics: Clay soil has very fine particles that are dense and sticky when wet, resulting in poor drainage and high water retention.
   • Structure: Clay particles are tiny and compact, leaving small pore spaces. This limits air flow and slows drainage.
   • Improvement Tips: Add organic matter and, in some cases, gypsum to break up the clay structure, improving drainage and aeration.
3. Loamy Soil:
   • Characteristics: Loam is an ideal soil type for most plants, with a balanced mix of sand, silt, and clay. It retains moisture well while allowing sufficient drainage and air exchange.
   • Structure: Loam has a crumbly, stable structure that balances water retention with good drainage.
   • Improvement Tips: Adding organic matter helps maintain its nutrient levels and structure over time.

Water Retention, Air/Gas Exchange, Insulating Properties, and Stability

1. Water Retention: This varies significantly by soil type. Clay retains the most water but may become waterlogged, while sand drains quickly. Loam provides a balance, holding adequate moisture without saturation. This is why a one size fits all irrigation method doesn’t work.
2. Air/Gas Exchange: Soil needs to allow gas exchange to provide oxygen to roots and release carbon dioxide. Sandy soils allow easy gas exchange, while clay can restrict it, which can suffocate roots. Loamy soil provides the best balance.
3. Insulating Properties: Soil insulates roots against temperature fluctuations. Loamy soil and soils with organic matter provide the best insulation due to their moisture content and structure. A three inch layer of mulch around a plant can have a drastic cooling effect on its root zone.
4. Stability: Soil stability impacts plant support. Sandy soil is prone to erosion, clay is more stable but may crack, and loam has an ideal balance of stability and flexibility, supporting roots effectively.

Root Health and Root Rot

Roots ideally function within a temperature range from near freezing to 85°F. Above or below this range, roots struggle to absorb nutrients effectively. Root rot is primarily caused by poor oxygenation, not excessive water alone. In poorly drained soil, water fills air spaces, depriving roots of oxygen, leading to decay. Good soil aeration is essential for preventing root rot.

Role of Mycorrhizae in Nutrient Uptake

Mycorrhizae are symbiotic fungi that form a network with plant roots, extending far into the soil. They help plants absorb nutrients, particularly phosphorus, by increasing the surface area for nutrient uptake and allowing roots to access nutrients beyond their reach. Mycorrhizae are also beneficial for plant resilience, helping to protect plants from drought and nutrient-poor conditions.

Potting Soil vs. Native Soil

Potting soil differs from native soil in several ways:

• Composition: Potting soil is often made from lightweight organic materials (e.g., peat, coconut coir, perlite) to ensure proper drainage and aeration in containers. Native soil has a natural mix of minerals and organic matter that varies based on location.
• Water and Air Balance: Potting soil is designed to drain easily and retain moisture at a balanced rate, while native soil may need amending to achieve this balance.
• Nutrient Content: Potting soils often come pre-fertilized. Native soil can have variable nutrient levels and may require amendments to support plant growth.

Structural Differences Between Sand, Loam, and Clay

• Sand: Large particles that don’t stick together, resulting in quick drainage and limited water retention.
• Clay: Small particles that stick tightly, holding water well but often restricting drainage and air movement.
• Loam: A mix of particle sizes that balances water retention, drainage, and nutrient availability. Loam is considered ideal due to its stable, crumbly structure.

Making Different Soils More Loam-like

To improve various soil types and make them closer to loam:

1. Sandy Soil: Add organic matter, like compost or aged manure, to increase water retention and nutrient content.
2. Clay Soil: Add organic matter and coarse sand or perlite to improve drainage and reduce compaction.
3. Silt Soil: Similar to clay, silt soils benefit from organic matter additions to prevent compaction and enhance structure.

With this understanding of soil types, composition, and properties, you can cultivate a productive environment for plant growth by amending soil according to its unique needs.

Understanding Tucson’s Soil

Tucson is part of the Sonoran Desert, which means its soil has unique properties compared to more temperate regions. The area’s soil is typically composed of sandy and clay components, with varying levels of organic matter, pH, and mineral content.

Soil Types Common in Tucson

• Sandy Soil: Quick-draining and low in nutrients but easy to work with. It often has a gritty texture and can feel loose.
• Clay Soil: Dense and nutrient-rich but poorly draining, making it prone to compaction and challenging for root penetration. It feels sticky and clumpy when wet and hard when dry.
• Loamy Soil (rare in Tucson): This is an ideal balance of sand, silt, and clay, providing good drainage, structure, and nutrient content. Loam is typically soft, crumbly, and feels balanced in your hands.

Identifying Your Soil Type

To understand your soil type, try these tests:

1. Squeeze Test: Moisten a small amount of soil and squeeze it in your hand.
   • Sandy Soil: Falls apart and feels gritty.
   • Clay Soil: Forms a tight, sticky ball.
   • Loamy Soil: Forms a ball that crumbles easily.
2. Jar Test: This test provides a visible breakdown of soil components.
   • Fill a jar halfway with soil, add water, and shake. Let it settle for 24 hours. Layers will form, with sand at the bottom, silt in the middle, and clay on top.
3. Percolation Test: Dig a 1-foot deep hole and fill it with water, then see how long it takes to drain.
   • Quick Drainage (1-2 hours): Sandy soil.
   • Moderate Drainage (3-4 hours): Loamy soil.
   • Slow Drainage (4+ hours): Clay soil.

Amending Soil in Tucson

Soil amendments are essential to improve the structure, nutrient content, and moisture retention of desert soils. Here’s how to amend each type:

1. Sandy Soil Amendments:
   • Add Compost: Organic matter like compost helps retain moisture and provides nutrients. Spread a 2-3 inch layer of compost over your soil and mix it in.
   • Use Mulch: A layer of mulch will help keep moisture in and reduce soil temperature fluctuations. Rock heats up too much in our intense heat and isn’t an appropriate choice for non-natives in our harsh climate.
   • Incorporate Organic Fertilizers: These can improve nutrient availability, which sandy soil often lacks.
2. Clay Soil Amendments:
   • Add Organic Matter: Compost and well-decomposed manure can help break up clay particles, improve drainage, and add essential nutrients.
   • Use Gypsum Sparingly: In clay soils, gypsum can help break down compacted soil without altering pH. Use it moderately, as overuse can cause nutrient imbalances.
   • Avoid Over-Tilling: Frequent tilling can further compact clay soils over time.
3. Loamy Soil Amendments (if applicable):
   • Maintain Organic Matter: Even though loam is ideal, keep the soil structure balanced by adding a yearly layer of compost or organic material.

Soil pH

Soil pH affects nutrient availability for plants. Tucson’s soil tends to be alkaline (high pH), often between 7.5 and 8.5, which limits the availability of essential nutrients like iron, phosphorus, and manganese.

Testing Soil pH

Testing your soil’s pH will give you a baseline understanding of its current state. You can:

• Use a DIY pH Kit: These are available at garden centers and provide quick results.
• Send a Sample to a Lab: This gives a precise reading and a full breakdown of your soil’s nutrient content.

Amending Alkaline Soil to Lower pH

Lowering pH in alkaline soil helps make nutrients more accessible for plants. Here are a few methods to temper Tucson’s alkaline soil:

1. Sulfur (Elemental Sulfur or Soil Sulfur):
   • Sulfur is the most effective long-term solution for lowering soil pH.
   • It works slowly, usually taking several months to a year to have a significant effect.
   • Application: Use about 1-2 pounds of elemental sulfur per 100 square feet for each point you want to decrease pH (from 8.5 to 7.5, for example). Mix it thoroughly into the top 6 inches of soil.
2. Ammonium-Based Fertilizers:
   • Ammonium sulfate and ammonium nitrate can help acidify the soil over time. These fertilizers provide nitrogen while also lowering pH gradually.
   • Application: Follow package instructions carefully to avoid over-fertilizing, as ammonium can lower pH significantly with repeated use.
3. Incorporate Organic Matter (Compost, Manure):
   • Organic matter doesn’t drastically lower pH but buffers soil pH changes and encourages microbial activity that can aid in nutrient uptake.
   • Application: Add a 2-3 inch layer of well-composted organic matter and mix it into the top 6-8 inches of soil. This can be done annually to maintain benefits.
4. Iron Sulfate:
   • This is another option for lowering soil pH but acts faster than sulfur, though its effects are more short-term.
   • Application: Use about 1 pound per 100 square feet, but avoid excessive use as it can create an iron imbalance.
5. Mulch with Pine Needles or Bark:
   • Acidic mulch materials like pine needles or pine bark release small amounts of acid into the soil as they break down. While not a primary method for pH adjustment, they help maintain acidity over time.

Caliche: What It Is and How to Overcome It

Caliche is a hard layer of calcium carbonate (lime) cemented with clay and sand particles found below the soil surface in desert areas like Tucson. It creates a tough barrier that can prevent root penetration and limit water drainage. Caliche layers vary in thickness from a few inches to several feet and are often encountered about 6-18 inches below the surface.

Identifying Caliche

• Visual and Physical Clues: When digging, caliche appears as a hard, often white or off-white layer. It’s tough to break through with a shovel.
• Drainage Issues: Soil above caliche layers may become waterlogged because water can’t penetrate through the caliche, creating a shallow “pan” effect.

Methods to Overcome Caliche

1. Breaking Through Caliche:
   • Mechanical Tools: Use a pickaxe, trenching tool, or digging bar to break up small areas of caliche. For larger areas, consider a jackhammer or power auger.
   • Hydraulic Drilling: For gardens and larger plantings, you can drill through caliche with specialized equipment that uses pressurized water, although this requires hiring professionals.
2. Raised Beds and Container Gardening:
   • Raised beds avoid the caliche layer altogether. Raised beds allow for better soil control and root expansion.
   • Container gardening is also effective for plants with deep root systems or that need highly amended soil.
3. Amend and Build on Top:
   • In areas where breaking through the caliche isn’t feasible, you can improve soil quality above the caliche layer by adding several inches of organic matter, compost, and topsoil to create a fertile layer for shallow-rooted plants.
4. Incorporate Gypsum to Improve Drainage:
   • Gypsum doesn’t dissolve caliche but can improve soil drainage in clay soils above the caliche layer by helping to loosen and flocculate clay particles.
   • Application: Add 1-2 pounds per 10 square feet and mix it into the top 6-8 inches of soil.
5. Soil Acidification for Caliche Reduction:
   • Vinegar Treatment (Small-Scale Use): For potted plants or small garden plots, you can use a diluted vinegar solution to dissolve caliche over time. Mix one part vinegar with three parts water and pour it over the caliche layer. This method is not suitable for large areas, as it can alter soil pH quickly and should be used cautiously.
6. Improving Water Drainage Above Caliche:
   • Because caliche prevents water from percolating deeply, add organic matter and sand to increase the soil’s moisture-holding capacity and avoid waterlogging near the surface.

Key Tips for Garden Success in Caliche and Alkaline Conditions

• Select Alkaline-Tolerant Plants: Many Mediterranean, desert, and native plants are adapted to alkaline soils, such as rosemary, lavender, sage, mesquite, and palo verde.
• Drip Irrigation: Drip irrigation systems can prevent waterlogging above caliche by delivering water directly to roots.
• Mulch Regularly: Mulch conserves soil moisture and reduces heat stress, promoting healthy root systems and lessening the impact of poor drainage from caliche.

Working with Tucson’s unique soils can be challenging, but with proper amendments and adjustments, you can create a productive, thriving garden despite the alkaline soil and caliche layers.