Altadena, California, located in the foothills of the San Gabriel Mountains, presents a diverse landscape with varied soil types influenced by both natural geology and human development. With elevations ranging from 1,200 to 1,800 feet above sea level and an average annual rainfall of 21 inches, Altadena’s soils support a wide range of native and cultivated plant species. Understanding these soils can significantly improve gardening success and water efficiency.
1. Geological and Climatic Influences on Soil
Altadena’s soils have formed over millennia through a combination of alluvial deposition from the mountains, weathered granite, and sediment layering. Its proximity to the Raymond Fault and historical flood plains contributes to a mosaic of textures—from coarse sand to heavy clay. The region lies in USDA Hardiness Zone 10a and has a Mediterranean climate, which also affects soil development and structure. Annual evaporation rates exceed 40 inches, which significantly impacts soil moisture retention and nutrient cycling.
2. Major Soil Types in Altadena
Loam Soil
Loam—a balanced mix of sand, silt, and clay—is common in older residential and well-managed areas. Loam in Altadena typically has a neutral to slightly acidic pH (6.2–7.0), moderate organic matter (2–4%), and good fertility. Loamy areas tend to support citrus trees, roses, tomatoes, and herbs with minimal amendments. Loam has a moderate water-holding capacity and good infiltration, making it ideal for most garden plants.
Clay Soil
Clay-heavy soils are often found in lower-elevation areas and older flood plains. These soils are compact, have poor drainage, and may become hard when dry. They usually test high in potassium and magnesium but are deficient in nitrogen and phosphorus. Their pH tends to be on the alkaline side (7.0–8.0). Adding organic compost, gypsum, and cover crops can improve aeration and structure. These soils retain water well but may need drainage amendments to prevent root rot in sensitive species.
Sandy Soil
In canyon edges and higher elevations, sandy soils dominate. These drain quickly and are low in organic matter, calcium, and phosphorus. The pH of sandy soils in Altadena often falls between 6.0 and 6.8. Gardeners in these areas should focus on soil building with compost and mulch to increase water and nutrient retention. Sandy soils warm quickly in spring, making them suitable for early planting of annual crops.
3. Specialty Soils and Decomposed Granite
Decomposed granite (DG) is common along trails and hillside lots. While visually appealing for hardscaping and paths, DG offers low fertility and compacts easily. Gardeners should not rely on DG soils for planting without substantial amendment. Nearby, areas with alluvial fans may display high variability in texture and drainage. Loamy-sand mixes and rocky outcrops are not uncommon, especially where erosion has exposed subsoils.
4. Soil pH and Fertility Ranges
Typical soil pH in Altadena ranges from 6.0 to 8.0 depending on the neighborhood. A 2020 study by the USDA NRCS found that residential gardens in Altadena averaged a pH of 7.2 and organic matter content between 2–4%. However, localized variations can be significant due to previous land use, imported fill, or irrigation practices. Electrical conductivity (EC) values are usually low to moderate, indicating generally low salinity, although localized hotspots exist near older urban areas.
5. Common Soil Challenges in Altadena
- Compaction: Especially prevalent in clay soils and areas with foot traffic or machinery use.
- Alkalinity: Elevated pH levels can lock out essential micronutrients like iron and zinc.
- Low Organic Matter: Most soils need regular organic input to support healthy microbiology.
- Salinity: Excessive irrigation or poor drainage can cause salt buildup, stressing plants.
- Erosion: Slope areas are at risk, especially during winter rains or wildfire aftermath.
- Shallow Topsoil: Some areas, especially on slopes, have limited topsoil depth due to erosion.
6. Local Resources for Soil Sampling Supplies
Residents interested in learning more about their soil should begin with a soil test. Soil sampling supplies can be found at these local garden centers and hardware stores:
-
Plant Material
Address: 3081 Lincoln Ave, Altadena, CA
Phone: +1 626-345-5750 -
Nuccio's Nurseries
Address: 3555 Chaney Trail, Altadena, CA
Phone: +1 626-794-3383 -
Altadena Hardware
Address: 849 E Mariposa St, Altadena, CA
Phone: +1 626-794-4393 -
Lincoln Ave Nursery
Address: 804 Lincoln Ave, Pasadena, CA
Phone: +1 626-792-2138 -
Armstrong Garden Centers
Address: 352 E Glenarm St, Pasadena, CA
Phone: +1 626-799-7139
7. Recommended Practices for Local Gardeners
- Test soil every 2–3 years for pH, NPK levels, and texture.
- Apply 2–4 inches of compost each season to maintain organic matter above 3%.
- Use mulch to reduce evaporation and control soil temperature.
- Grow cover crops like clover or vetch to add nitrogen and reduce compaction.
- Choose deep-rooted California native plants to stabilize and improve soil health.
- Use biochar in poor soils to improve microbial habitat and water retention.
- Implement drip irrigation to minimize salt accumulation and conserve water.
8. Native Plants Suited for Altadena’s Soils
- California Buckwheat (Eriogonum fasciculatum): Tolerates sandy and gravelly soils.
- Coast Live Oak (Quercus agrifolia): Prefers loamy-clay soils with deep rooting zones.
- Manzanita (Arctostaphylos spp.): Ideal for well-drained sandy or DG soils.
- Cleveland Sage (Salvia clevelandii): Suited for dry, nutrient-poor soils.
- California Poppy (Eschscholzia californica): Thrives in poor soils with full sun exposure.
- Toyon (Heteromeles arbutifolia): Does well in clay and loam soils, drought-tolerant.
- White Sage (Salvia apiana): Adapted to sandy, rocky soils with low fertility.
Conclusion
Altadena’s soil types reflect its rich geological history and varied topography. By understanding the specific characteristics of local soils—including texture, pH, and fertility—gardeners and landscapers can create vibrant, drought-resilient, and ecologically beneficial landscapes. Soil health begins with knowledge, and residents are encouraged to test, amend, and care for their soil year-round for the best long-term outcomes.
Sources:
- USDA NRCS Soil Survey for Los Angeles County
- University of California Cooperative Extension (UCANR)
- California Soil Resource Lab (https://casoilresource.lawr.ucdavis.edu/)
- Altadena Historical Society: Landscape and Topographic History
- Local interviews with Plant Material and Lincoln Ave Nursery staff, 2024
- California Native Plant Society (https://www.cnps.org/)
