Understanding the difference between compost analysis and soil tests
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EmailNot all soil tests are equal, and compost requires an analysis of its own. Understanding the differences between the two is an important step to providing the best amendment for your soil.
Compost analysis tests and soil tests are essential tools for understanding the composition and health of growing mediums, but they serve different purposes and provide distinct insights. Whether you're an agricultural professional, a backyard composter or a sustainability advocate, knowing the differences between these tests and why compost analysis is necessary can help optimize soil health and plant productivity.
Whether you are producing your own compost or purchasing it from a vendor, it is important to know that not all composts are created equally, even the ones available for purchase at garden centers and home improvement stores. The quality of compost can vary significantly based on its source materials, processing methods and level of decomposition. Some commercially available composts may contain high levels of salts, heavy metals or synthetic chemicals that can negatively impact soil health and plant growth. Understanding the composition of the compost you apply to your soil is crucial to ensure it supports rather than hinders plant development. Testing purchased compost before widespread application can help verify its quality and suitability for your gardening or agricultural needs.
Compost analysis vs. soil testing: Understanding key differences
Compost analysis tests: Compost analysis focuses on the physical, chemical and biological properties of finished compost. These tests evaluate factors such as:
- Nutrient content: Measures nitrogen (N), phosphorus (P), potassium (K) and other essential micronutrients.
- Moisture content: Determines the percentage of water in the compost, which affects microbial activity and decomposition.
- pH Levels: Indicates acidity or alkalinity, influencing plant compatibility and microbial activity.
- Organic matter: Evaluates the amount of decomposed plant and animal material, essential for improving soil structure and fertility.
- Soluble Salts (EC - Electrical Conductivity): Measures salt concentrations to prevent plant toxicity.
- Maturity and stability: Determines if the compost is fully decomposed and ready for application.
- Microbial activity: Assesses beneficial microorganisms, including bacteria and fungi, crucial for soil health.
Soil testing: A soil test, on the other hand, evaluates the composition and fertility of soil where plants will grow. Key soil test components include:
- Soil texture: Analyzes the proportions of sand, silt and clay.
- Macronutrient and micronutrient levels: Determines deficiencies or excesses in essential nutrients.
- Cation Exchange Capacity (CEC): Measures soil’s ability to hold and exchange nutrients.
- pH Levels: Similar to compost analysis but focuses on the soil's capacity to support plant growth.
- Soil Organic Matter (SOM): Evaluates the presence of decomposed material that contributes to soil health.
Why compost analysis is important
Getting a compost analysis done ensures that your compost is of high quality and safe for use in gardens, farms or landscaping projects. While there are similarities between soil analysis tests and compost analysis tests, a soil test simply won’t look for all the key indicators that you have a well-balanced compost. Here are a few key reasons why testing your finished compost is beneficial:
- Ensures nutrient balance: Understanding nutrient levels helps prevent deficiencies or toxicities in soil when compost is applied.
- Confirms maturity and stability: Unfinished compost can deplete nitrogen in soil and harm plant roots. Compost maturity is one of the most important indicators of a high-quality product. Ensure the maturity of your compost by giving it 3-12 months to cure after the active phase.
- Detects contaminants: Testing helps identify heavy metals, pathogens, or excessive salts that can damage plant health.
- Optimizes application rates: Knowing compost composition allows for precise application, maximizing benefits without overuse.
- Improves soil health: Ensures compost amendments contribute positively to soil structure and microbial activity.
Where and when to send compost for analysis
The best time to send your compost for analysis is after the active composting phase has ended, it has cured for 3-12 months depending on the feedstocks to allow temperatures to cool and microbial life to settle, and always prior to land application. The United States Compost Council has a seal of testing assurance program (STA) which provides a trusted network of certified labs that analyze compost quality and meet industry standards. Test prices may range from $50 to $500 depending on what lab provider you choose, and the array of tests being performed. For more information on finding a lab for analysis, visit: https://www.compostingcouncil.org/page/CertifiedLabs
By regularly testing your compost, you can ensure it remains a beneficial addition to soil systems, fostering healthier plants and a more sustainable environment. Whether you're producing compost for personal use or large-scale application, compost analysis is a valuable step in achieving optimal soil health and fertility.
For more information regarding soil tests, compost use and production, visit MSU Extension and learn more today. For compost related inquiries, you can reach out to MSU Extension’s Compost Systems Educator, Eliza Hensel at hensele1@msu.edu.
