Purchasing Compost & Mulch
When writing a procurement specification, there are certain elements you should consider. This chart was developed to help you select the elements that are appropriate for your end-use application. It is advisable to talk with potential suppliers about your intended end use and include the elements that are necessary for your needs. They may be able to provide you with information about how much material you need and how to apply it.
| Characteristic | Associated Value | Comments |
| 1. Particle Size | < 1"; 2"; etc. | Porosity affects air and water infiltration. Smaller particles have more available nitrogen. |
| 2. Salt Concentration | Mmhos/cm | High salt concentrations, > 4.0 mmhos/cm, can be harmful to seeds and plants. |
| 3. Stability/Maturity | Stable or mature (i.e. when the organic material stops decomposing) | In mature compost, nitrogen is available to plants; and there is less
potential for odor problems. The CIWMB is currently developing a maturity index through a
contract with an industry association to help define what constitutes mature compost. This
index should be available by summer 2000. Contact: , CIWMB |
| 4. Feedstock Materials | Specify ingredients | The type of feedstock used can help you decide what product best suits your needs. Typical feedstock's include landscape/yard trimmings; grass clippings; food scraps; bio-solids; and agricultural crop residues. |
| 5. Nutrient Content | N-P-K; Ca; Mg; S; Bo; & others | Compost provides slow-release nutrients, more efficient plant uptake; and much lower rates of fertilizer leaching. |
| 6. Trace Contaminants | Metals (Lead, Mercury, Etc.) | Product should meet US EPA, 40 CFR 503 regulations. Compost also binds up heavy metals. |
| 7. pH | Acid/base | Helps balance the pH of your soil. Compost helps buffer soil toward neutral (pH=7). |
| 8. Visible Contaminants | Specify inert:
|
Amount of glass, paper, plastic, etc., visible in the final product; ideally should be none visible. Cal Trans specification requires < 0.1 % by weight or volume. |
| 9. Moisture Content | 35-55% (40-50% preferred) | If you purchase by weight, wet compost means you're paying to haul excess water. Very wet compost can cause odor problems, while dry compost can be dusty and irritating to work with. |
| 10. Organic Matter Content | 30-70% by dry wt. (50-60% preferred) | Compost improves soil structure and water holding capacity. |
| 11. Certifications | California Compost Quality Council (CCQC) | Requires that registered suppliers disclose feedstock and specified parameters. The supplier must also have a quality assurance/quality control program. Buyers can have greater confidence regarding the consistency and appropriateness of the compost product they buy for intended end uses. |
| 12. User Guidelines | Application rates Vol/area |
Ask suppliers to provide guidelines on how to apply their product. CIWMB is developing informational fact sheets for specific landscaping applications; these should be available by Spring 2000. Check the Board's web site at www.ciwmb.ca.gov/organics/. |
| 13. Bulk Density | 800 lbs./cubic yard | Depends on feedstock and moisture content, typically in range of 700--1200 lbs./cubic yard. Affects product handling, transportation and application. |
| 14. Carbon and Nitrogen Ratio | C:N less than 20 | C:N ratio is sometime used as a measure of stability. Ratio of less than 20:1 is likely to indicate that the compost is stable. |
| 15. Other | Color, smell | Should have an "earthy" odor that is not unpleasant. |
Procurement Specifications Guidelines
Last updated: August 29, 2008