In a webinar with the Potomac Chapter of the American Society of Landscape Architects, recycled organics expert Ron Alexander discussed how landscape architects can utilize soil nutrient analyses to improve their projects. If you missed it, you can watch it here! 

Here are a few key takeaways:

Fundamentals of Soil Testing in Landscape Projects

  • Starting with proper soil evaluation is crucial for developing successful long-term landscape projects, noting that major mistakes occur when soil isn’t appropriately evaluated first.
  • Early soil testing helps determine whether site soil can be used as-is, upgraded or remanufactured on site, or if new soil must be brought in, and whether the planned plant palette will work long-term.
  • Basic soil tests should include soil texture (sand, silt, clay quantities), organic matter, pH, and electrical conductivity (salt content), with testing occurring early in the design process.
  • When selecting a soil testing lab, choose one that provides good recommendations specific to landscape projects (not just agricultural applications) and is willing to spend time discussing results.
  • Finding a good soil testing lab that will spend time explaining results is crucial, and professionals shouldn’t get stressed about the learning curve of soil data interpretation.

Evolution and Types of Soil Amendments

  • The soil amendment industry has changed dramatically with more recycled products now available, moving beyond traditional materials like mined peat moss or pine fines. The 2010 definition from the American Association of Plant Food Control Officials expanded soil amendments’ purpose to improve physical, chemical, biochemical, biological or other characteristics of soil.
  • Two main categories of soil amendments exist: organic matter-based (more common in typical landscaping) and mineral-based (used in specialized, higher-cost landscape areas).
  • Newer products like Bloom represent the evolution of soil amendments that provide multiple benefits beyond traditional amendments. Modern soil amendments supply organic matter, nutrients, beneficial soil microbes, and some (like Bloom) offer drought resistance characteristics, expanding their functionality beyond traditional amendments.

Testing Methods and Data Interpretation

  • Growth of the composting industry has led to better standardized testing labs for soil amendments, making it easier to understand test results and reducing market confusion.
  • The national testing program uses uniform test data sheets for compost products, enabling direct comparison between different facilities and composters with consistent units of measure.
  • Technical data sheets for quality compost products follow standardized formats despite different lab appearances. Once familiar with these sheets, users can effectively compare different products like Bloom that use similar test packages.
  • Different labs have specialized testing focuses – agronomic/horticultural labs for nutrient data and environmental labs for heavy metal testing. Ron recommends using specialized labs for their respective testing areas. Ron also emphasized the importance of lab interpretation data quality, noting that automated interpretation data sometimes conflicts causing confusion. Working with labs willing to explain results is recommended.

Key Parameters for Evaluating Soil Amendments

    • pH: Plants have specific soil pH ranges in which they can extract nutrients more efficiently, though they can survive outside their optimal range.
    • Electrical conductivity (salt content): Most plants have maximum salt tolerance before injury occurs. Bloom can have a higher salt content than some composts, but this is related to its content of ammonia. Since the ammonia in Bloom is rapidly converted in the soil, it does not have a damaging effect on plants, Ron said. 
    • Stability: This test measures CO2 expelled by microbes breaking down organic matter in a closed vial. Lower CO2 production indicates a more stable product with less available food for microbes.
    • Maturity: This test identifies harmful substances like volatile fatty acids, salts, and ammonia and involves a growth screening where cucumber seeds are planted in a mixture of the amendment and vermiculite, then compared to pure vermiculite growth.
    • Nitrogen: Soil amendments should have 90% or more of total nitrogen in organic form, which is released over a 2-3 year period rather than all being immediately available.
    • Organic Matter: When you buy a compost, you want the organic matter to be 40 or 50%. “This Bloom product is a little bit higher (61%). That’s nice. That’s a good number,” said Ron. 

Case Study

  • When specified compost was unavailable for the Frederick Douglass Bridge project, soil testing revealed an alternative approach: manufacturing soil on-site by combining sand with Bloom. 
  • On-site soil manufacturing like in this case can reduce costs by up to 50% on large projects compared to transporting new soil, though it requires technical expertise that landscape architects may need to source externally. 
  • The project was successfully completed with the alternative approach, lowering costs while maintaining soil quality requirements and keeping the project on schedule despite supply challenges.
  • Remanufacturing soil on-site by adding components like sand and organic matter is a cost-effective alternative, potentially saving about 50% of costs .

Soil Compaction and Remediation Strategies

  • Soils with over 65-70% combined silt and clay are highly susceptible to compaction problems, particularly in the DMV area, Pennsylvania, and North Carolina regions.
  • Climate change is exacerbating compaction issues by reducing soil’s ability to assist with stormwater management when compacted.
  • Aim for 5% organic matter in landscape soils (higher for garden beds) specifically to slow down compaction in heavy clay and silt soils.
  • “Things like pine bark blended with compost or with Bloom, all of which have disease suppressive characteristics, can make a really cool media,” Ron said.

Soil Health and Environmental Benefits

  • Ron said the study of microbes in soil using DNA sequencing is an ongoing science, and Bloom is digging into some of this pretty cool research around microbial changes in soil blends containing compost and biosolids products.
  • Erosion and loss of organic matter were identified as major problems across the US, with adding stabilized organic matter forms immediately resulting in higher populations of beneficial microbes in the soil.
  • The key aspect of soil health was identified as high microbial population and high microbial diversity. Even materials made from societal waste provide benefits by adding stabilized organic matter to improve soil health.
  • Healthy soils with rich organic matter show multiple benefits: better plant growth, more diverse plant growth, plants growing outside typical pH ranges, and improved recovery from environmental injuries.
  • Organic matter serves dual purposes in the environment: helping feed plants and dealing with contamination through breakdown or binding processes, providing both practical and environmental benefits.
  • A major education push in landscape architecture was emphasized, with professionals needing to educate clients about investing more upfront in soil improvement for long-term benefits.