By Ron Alexander
Healthy landscapes begin below the surface. Yet soil testing is often treated as a last-minute requirement rather than one of the first design decisions. The result? Poor plant performance, unnecessary soil replacement, inappropriate plant choices, higher costs, and avoidable maintenance problems.
Whether you’re reviewing an existing soil test, evaluating a soil amendment, or comparing contractor submittals, understanding what the numbers actually mean can lead to better design decisions and more successful projects.
Over the past four decades, I’ve worked with landscape architects, contractors, municipalities, compost manufacturers, and soil amendment producers across North America. One lesson has remained constant: successful landscapes begin with understanding the soil and not making assumptions about it.
Why Soil Testing Matters
One of the biggest mistakes I continue to see is failing to test soils early enough in the design process.
A comprehensive soil test helps answer critical questions before plant selection or specification writing begins:
- Can the existing soil support the proposed landscape?
- Can the site soil be improved instead of replaced?
- Which soil amendments are most appropriate?
- Will the existing soil support the desired plant palette?
Answering these questions early often reduces project costs while improving long-term landscape performance.
What Does a Soil Test Tell You?
Many landscape architects ask, “How do I read a soil test?”
Although laboratory reports vary by testing facility, most provide the same core information. Understanding a few key measurements allows you to evaluate both existing soils and manufactured soils with confidence.
The most important measurements include:
- Soil texture (sand, silt, and clay)
- Organic matter
- Soil pH
- Electrical conductivity (soluble salts)
- Nutrient levels
Equally important is selecting a laboratory that provides practical recommendations and is willing to explain the results. A knowledgeable lab can be just as valuable as the test itself.
Understanding Soil Test Results
Soil Texture
Soil texture measures the content (dry weight) of soil’s mineral components; sand, silt and clay.
In most cases, landscaping projects desire a sandy loam soil, a soil that 43 – 85% sand, 0 – 50% silt and 0 – 20 clay. In the DMV, most soils contain 60 – 70% combined silt and clay, which means they are slow to drain and easy to compact.
Organic Matter
Organic matter is often the single most valuable measurement on a soil test.
It improves soil structure, increases water-holding capacity, supports beneficial microorganisms, reduces compaction, and creates healthier growing conditions over time. Fresh Bloom and Bloom’s Woody Blend contain substantial organic matter, often higher than that of commercially available compost.
Unlike fertilizer, organic matter cannot simply be added later to correct every problem. Building healthy soils begins with incorporating stabilized organic matter from the start.
Soil pH
Soil pH affects how efficiently plants can absorb nutrients.
Many soil amendments have their own pH values, but what matters most is how they influence the finished soil pH after incorporation, not simply the pH listed on a product data sheet.
Electrical Conductivity (Soluble Salts)
Electrical conductivity measures soluble salts.
Excessive salt levels can stress or injure plants, making this one of the most important values to review on both soil tests and soil amendment analyses. Of course, Bloom products and compost rarely have a high soluble salt content. Interestingly, trials have shown that Fresh Bloom can elevate soil ammonia content and sometimes its electrical conductivity. However, this is quite temporary, with research illustrating that Bloom has a limited (short-term) effect on soil electrical conductivity and not concerning for plant growth.
Nutrients
Nutrients content of an existing soil matters, but often not as much as people think.
Unlike soil structure or organic matter, nutrient deficiencies can usually be corrected with fertilizer. I encourage designers to focus first on the characteristics that are much harder to change after installation, including soil texture, organic matter, salinity, and pH.
How to Read a Soil Amendment Test Report
Reading a compost or soil amendment analysis follows many of the same principles as reading a soil test.
Quality soil amendments are commonly evaluated for:
- Organic matter
- Moisture content
- pH
- Electrical conductivity
- Nutrient content
- Stability
- Maturity
- Heavy metals
- Pathogens
- Physical contaminants
Today’s standardized testing methods make it much easier to compare products from different manufacturers using consistent testing procedures and reporting units.
Why Stability and Maturity Matter
Some of the most valuable, and often least familiar, tests were developed by the commercial composting industry.
Stability measures how actively microbes are still decomposing organic matter. Highly stable products have already completed most of that decomposition, making them less likely to compete with plants for available nitrogen. While some composts are more stable than Bloom products, because of its higher nitrogen content, Bloom has never been shown to compete with plants for nitrogen (it supplies them with it).
Maturity testing evaluates whether an amendment contains substances that could inhibit seed germination or root growth (e.g., certain herbicides, ammonia, soluble salts, etc.).
Together, these tests help ensure a soil amendment supports plant establishment rather than working against it.
Modern Soil Amendments Do More Than Improve Soil Structure
Today’s organic soil amendments provide benefits that extend far beyond improving soil structure.
High-quality products contribute stabilized organic matter, support beneficial microbial activity, improve water-holding capacity, reduce compaction, and can even improve drought resilience.
Products such as Bloom, which is produced through thermal hydrolysis and anaerobic digestion, represent a newer generation of recycled organic soil amendments that contribute physical, chemical, and biological benefits to soils while helping recycle valuable resources that would otherwise be wasted.
Soil Testing Can Save Money
Many projects automatically assume imported topsoil is the only solution.
However, in reality, soil testing often reveals that existing soils simply need improvement rather than replacement.
By adjusting soil structure and incorporating high-quality organic amendments, designers can often manufacture planting soils on-site that meet project requirements while dramatically reducing trucking, imported materials, and overall construction costs.
Healthy Soil Leads to Better Landscape Performance
Healthy soils do much more than support plant growth.
Increasing stabilized organic matter helps:
- Reduce soil compaction
- Improve moisture infiltration and stormwater performance
- Increase drought resilience
- Support beneficial soil microbes
- Improve long-term plant health
- Reduce maintenance requirements
I’ve found that investing in soil health at the beginning of a project consistently produces landscapes that establish more quickly, require fewer inputs, and perform better over time.
The most successful landscape projects don’t simply begin with selecting the right plants. They begin with understanding and improving the soil those plants will depend on for decades.
Ron Alexander is a horticulturist, author, educator, and one of North America’s leading experts on compost, recycled organics, and manufactured soils. With more than 40 years of experience consulting with landscape architects, municipalities, manufacturers, and the green industry, he has helped advance the science and practical application of compost and organic soil amendments. Ron has contributed to numerous books and authored hundreds of technical publications and is a frequent speaker on soil health, sustainable landscapes, compost utilization, and soil amendment technologies.
