Soil types and what they mean to the Treasure Valley

All About Soil Types: Sandy, Clay and Silty plus Why Loam Is The Ideal Growing Medium

Soil isn’t just “dirt” — it’s one of the most important natural resources for plants, water filtration, and healthy ecosystems. One way soil is classified is by the size of its mineral particles: sand, silt, and clay. These tiny particles might seem simple, but their proportions make a huge difference in how soil behaves.

1. Sandy Soils

• Particle size: Largest of the three.

• Feel: Gritty and coarse.

• Properties:

  • Excellent drainage — water flows through quickly.

  • Poor nutrient and water retention.

  • Easily worked, but often too loose for many plants.

• Typical use: Ideal for drought-tolerant plants or to improve drainage when mixed into other soils.

✅ Pros: Drains quickly, warms sooner in spring.

❌ Cons: Doesn’t hold nutrients or water well.

2. Clay Soils

• Particle size: Tiny; packs together very tightly.

• Feel: Sticky when wet, hard when dry.

• Properties:

  • Holds water and nutrients well.

  • Slow drainage — can stay soggy and cause root stress.

  • Hard to cultivate when dry.

✅ Pros: Fertile and nutrient-rich.

❌ Cons: Poor aeration and drainage; heavy and compact.

3. Silty Soils

• Particle size: Between sand and clay.

• Feel: Smooth or silky.

• Properties:

  • Holds moisture better than sand.

  • Softer and more fertile than pure sand.

  • Can compact and form crusts.

✅ Pros: Holds water better than sand, relatively fertile.

❌ Cons: Can be muddy, prone to compaction.

Why Loam Is Often the Ideal Soil

Loam is a balanced mixture of sand, silt, and clay — typically roughly 40% sand, 40% silt, and 20% clay (though there’s a range). Loam has:

• Good drainage without drying out too fast.

• Excellent nutrient and water retention.

• A crumbly texture that’s easy to till and plant.

• The best structure for most garden crops and lawns.

For many plants, ranging from vegetables to ornamentals, loam provides the “Goldilocks” combination of water, air, and nutrient holding capacity. When you’re looking for “Topsoil” containing this formula, I can only wish you the best of luck.  As far as I know, when you buy topsoil, you are buying soil that has been scrapped from another lot, without and additives or alternations being done to the original condition of the soil. This is true no manner which nursery you buy topsoil from. And it will vary from load to load, even from the same nursery.

Soil Texture Triangle — A Tool for Understanding Soil

A soil texture triangle is a graphic tool that helps you determine the soil type based on the percentage of sand, silt, and clay. Each side of the triangle represents 0–100% of one type of particle, and where your sample lands shows its classification.

How to Use the Triangle

1. Measure the percent of sand, silt, and clay in a sample.

2. Find each value on the triangle’s axes.

3. Draw lines inward from each percentage.

4. The area where the three lines intersect determines the soil texture (e.g., sandy loam, clay loam, silty clay, etc.).

This tool reveals 12 possible textural classes — with loam at the center giving that “ideal balance” most gardeners seek.

The images above show multiple versions of the soil texture triangle so you can visualize how soil blends are plotted and classified.

Understanding Soil pH — What It Means And Why It Matters

Soil pH is a measure of how acidic or alkaline the soil is, on a scale of 0–14:

• pH < 7 → Acidic

• pH = 7 → Neutral

• pH > 7 → Alkaline

The pH greatly affects how well plants can absorb nutrients. Most vegetables and ornamentals in our area prefer acidic to neutral soil (about 5.8–7.0) because essential nutrients are most available in that range.

Treasure Valley — Soil pH And Alkalinity

In the Boise area and the broader Idaho region, soils tend to be near neutral to alkaline, often due to parent rock material (e.g., limestone) and low rainfall that limits leaching of alkaline minerals.

Many soils in the Treasure Valley and around Boise show pH levels above 7.5; and in some cases such as West Boise, up to 8.0–8.5 or higher, meaning they can be highly alkaline.

• Alkaline soils can make iron, manganese, and phosphorus less available to plants.

• Some ornamental and edible plants prefer more acidic conditions, so gardeners often amend alkaline soil with sulfur, organic matter, or raised beds with imported soil to achieve desired pH levels. We often recommend Ferti-lome “GreenMaker” since this fertilizer contains high amounts of iron and sulfur.

Tip: Before amending, we recommend testing your soil’s pH through the Ada County Extension Office, so you know whether adjustments are actually needed.

Final Thoughts

Understanding your soil’s texture and pH is one of the most important steps in successful gardening or landscaping. Whether you’re dealing with sandy, clay, silty, or a wonderfully balanced loam soil, knowing what’s beneath your feet lets you make smart choices about plants, amendments, and water management.

If you’re in Boise or other parts of the Treasure Valley with generally alkaline soils, pH can be a challenge, but with testing and appropriate amendments, you can create a productive soil environment that will meet your gardening goals.

Soil Amendment Plan for Treasure Valley Gardeners

In and around the Treasure Valley, native soils commonly:

• Lean clay to clay-loam

• Test alkaline (pH 7.5–8.5)

• Drain slowly when compacted

• Contain free lime (calcium carbonate)

• These conditions affect nutrient availability — especially iron, manganese, and phosphorus.

  
  

STEP 1️⃣: Start With a Soil Test

Before adding anything, test for:

• pH

• Organic matter %

• Phosphorus & potassium levels

• Texture (sand/silt/clay %)

Ideal targets for most garden plants:

• pH: 5.8–7.3

• Organic matter: 4–6%

• Well-balanced nutrients

  
  

STEP 2️⃣: Improve Soil Structure (Most Important in Boise)

Regardless of pH, structure comes first.

✔ Add Organic Matter (Every Year)

• 2–3 inches of compost worked into top 6–8 inches

• Aged manure (well composted only)

• Shredded leaves

Why?

• Loosens clay

• Improves drainage

• Increases microbial activity

• Slowly buffers pH

⚠ Avoid adding straight sand to clay — it can create a concrete-like mix.

STEP 3️⃣: Address Alkaline pH (If Needed)

Many Treasure Valley soils are naturally alkaline. Typically, you only adjust pH if:

• You're growing acid-loving plants (blueberries, azaleas)

• You see nutrient lockout symptoms (yellow leaves with green veins = iron chlorosis)

Lowering pH Safely

• Sulfur and Iron (best long-term option)

• Acid-forming fertilizers (Ferti-lome GreenMaker)

• Heavy compost incorporation (Cowgirl Compost)

Sulfur and iron take time to breakdown — apply 3 times per year: spring, summer, fall.

STEP 4️⃣: Correct Nutrient Lockout (Common in the Treasure Valley)

Iron Deficiency (Very Common Here)

Symptoms:

• Yellow leaves

• Green veins

Fix:

• Chelated iron (EDDHA type works best in high pH soils)

• Ferti-lome GreenMaker

• Improve organic matter with Cowgirl Compost

• 
  

STEP 5️⃣: Build Toward Loam

Here’s the ideal soil composition:

Soil Texture Triangle

Loam typically contains:

• ~40% sand

• ~40% silt

• ~20% clay

You likely won’t completely change our Treasure Valley native soil to true loam, but you can certainly improve the structure so it behaves like loam.

Organic matter is the key.

STEP 6️⃣: Watering Strategy for our Treasure Valley Soils

Clay-heavy alkaline soils:

• Hold water longer

• Need deep, infrequent watering

• Benefit from mulch (at least 2–3 inches)

Mulch:

• Reduces evaporation

• Improves soil life

• Gradually improves structure

If You’re Growing…

Vegetables

• Raised beds help

• Add compost annually

• Target pH: 6.5–7.2 (many vegetables like near nuteral soils)

Lawns

• Aerate annually

• Top-dress with compost

• Watch for iron deficiency

Trees & Shrubs

• Mulch heavily

  
  

Sample: Treasure Valley Soil Improvement Plan

Early Spring

• Soil test

• Add 2–3 inches compost

• Apply sulfur and iron if pH is above 7.8

Late Spring

• Mulch beds

• Apply chelated iron if needed

Fall

• Add shredded leaves

• Light compost top-dressing

Repeat annually for soil improvement. This is a long-term process in Idaho because of the high pH of soil.

Bottom Line for Treasure Valley Gardeners

In the Treasure Valley:

✔ You’re usually working with alkaline soil

✔ Organic matter is a great tool for improving soil structure

✔ Don’t fight the soil — improve it gradually

✔ Test before adjusting pH