Purchasing Compost [tip sheet]

better compost

Compost, the end product of a controlled decomposition of plant and animal wastes, makes an excellent addition to lawn or garden soil. While compost supplies nutrients to the soil, these nutrients aren’t always readily available for use by the plants. Therefore, compost should be viewed as an amendment rather than a fertilizer because it can’t always be relied on to supply an adequate amount of nutrients within a growing season like a fertilizer is guaranteed to do. While not a fertilizer, adding compost to your soil helps improve both its drainage and water-holding properties. Compost stores plant nutrients and prevents them from leaching. It “opens up” the soil so plant roots can penetrate easily. Some recent research also indicates that compost may help prevent many common plant diseases. 

Although most gardeners can produce their own compost from kitchen scraps and waste from the yard and garden, making enough compost is a tough job for gardeners with thin or poor topsoil and for gardeners using compost as mulch. On a small scale, achieving appropriate temperatures to kill weed seeds and pathogens can be a challenge, which means that the quality of home-made compost may not be the same as a commercial product. Learn how to compost at home with this resource:  https://extension.unh.edu/resource/composting-home-gardener-fact-sheet 

Current Legislation and Commercial Enterprises 

Legislation that went into effect in 1993 made it unlawful for New Hampshire municipalities to incinerate or landfill leaves and yard wastes. Some municipalities currently collect and compost these wastes at town solid waste facilities; check to see if your town is among them. 

The legislation also created opportunities for commercial composting operations to take in leaves and other organic waste products and process them into compost for sale. Some New Hampshire farms, garden centers, landscapers and dedicated businesses sell bulk compost. 

Compost Products 

Many different types and percentages of organic materials may go into any given batch of compost, so no two batches – even from the same supplier – are likely to be uniform with respect to plant nutrients, pH and texture. Compost cannot be returned if it doesn’t perform as expected, so it’s important to ask questions and scrutinize the product before you make a purchase. 

Manure typically refers to livestock waste mixed with bedding, wood shavings or feed waste, and is only composted if the material has been decomposed to the point where its nutrient content is stable. Aged manure is not composted manure. Consult our Guidelines for Using Animal Manures and Manure-Based Composts in the Garden: https://extension.unh.edu/resource/guidelines-using-animal-manures-and-m...

Rates of application should be based on soil test results but should not exceed 4 cubic yards per thousand square feet. Compost products can be used as a soil amendment, fertilizer supplement or mulch in a home yard and garden setting. If your soil test results show high levels of phosphorus, you may want to avoid using compost or manure until phosphorus levels are at low or optimum levels. 

Assessing Compost Quality 

When purchasing bulk compost, it’s important to visually inspect the material. Look for trash debris such as plastic, metal or glass in the pile. Also look for undecomposed materials such as leaves, stems, seeds, chunks of bark or eggshells.  

In addition to looking for trash debris, also assess the texture of the material. The compost should appear light and fluffy, not damp or wet. Finally, compost should not have an off-odor, such as a strong ammonia-like smell or the smell of rotting food. While finished quality compost may contain worms, it should not be covered in flies or maggots. Finished compost has a rich, sweet, earthy smell.  

Note: experts recommend wearing gloves when handling compost or at least scrubbing thoroughly after handling compost with bare hands. 

The Compost Analysis 

Ask the vendor for the compost recipe, which means asking what materials went into the compost and how it was made.  

Biosolids might be included on the ingredient list. These are recyclable organic materials produced in municipal wastewater treatment plants. Only Class A biosolids are available for use in compost products for the home garden and landscape. To meet Class A requirements, biosolids must be treated by a process that reduces pathogens to low levels, and that process must be monitored to show compliance and must be tested to show low pathogen levels. Class A biosolids must also meet a strict standard for trace element pollutant levels, along with other requirements. Using composted biosolids is shown to have some benefits, but it’s up to gardeners to decide whether they are comfortable with biosolids in their compost. New Hampshire law requires any compost containing biosolids to be tested and sold with a label that lists the product’s concentration of heavy metals and nutrients, along with recommendations for use of the product. Organic gardeners should know that compost made from biosolids is not approved for organic production. 

You can ask if the vendor has tested the compost and if the laboratory analysis is available. Keep in mind that a laboratory test of the vendor’s compost is not a guaranteed analysis for compost you purchase because of the lack of uniformity in compost products. Some of the parameters may be pH, soluble salts, nutrient content, water-holding capacity, stability, organic matter content, moisture content, particle size and bulk density. A knowledgeable vendor should be able to talk about the product’s benefits and potential uses, as well as application rates and procedures based on the characteristics of their product.  

For example, a compost with a small particle size (less than a 1/4”) is most suitable for top dressing, whereas a particle size between a 1/4” and 1/2” is suitable for use as a soil amendment or potting media. A compost with particle sizes less than 1/2” in diameter, a pH between 6 and 7.8, a soluble salt level less than 2.5 mmhos per cm, a low respiration rate, no weed seeds, and contaminant concentrations below EPA and state standards will have almost unlimited use. 

If purchasing compost from a farm, you may want to ask if the feedstock material in the compost was collected from other producers or if it’s from their own farming operations. There are additional regulations for composting feedstock from a separate farm. Sourcing may be important to avoid herbicide contamination. If your vendor isn’t knowledgeable about the source of the manure, you can ask if the producer has conducted a plant assay to ensure the safety of using the material on broadleaf plants, like tomatoes. 

Gardeners can also do a simple bioassay test at home before adding compost to the garden. A method developed by Washington State University suggests filling three three-inch pots with potting soil, then filling three more pots with a mixture of two parts compost and one part potting soil, marking the pots accordingly. Then plant three pea or bean seeds per pot, putting the pots in a sunny, warm location, keeping them well watered and capturing water that drains through the pots to prevent contamination of soil in other pots. Once the seedlings have three sets of leaves, compare the plants growing in the compost mix with the plants growing in the potting mix (the control group). If the plants growing in the compost mix display unusual cupping, thickening or distortion of leaves, there may be herbicide contamination in the compost. 

Desirable Attributes of a Typical Compost Analysis 

Temperature - If the compost is still steaming or feels hot there is still biological activity cranking along. This process creates organic acids that are harmful to plants. However, it is okay if it’s a little warm. Wait to use it or use it cautiously if you think it feels hot. 

Color - Dark brown to black 

Consistency - Generally homogeneous, sometimes includes some wood chips and/or bones that don’t completely break down.  

pH - Typically around 7 (neutral), although that is not always the case. The pH of the compost may inform how or where it is applied in the home landscape. 

Salts - Salts are listed as EC or mmhos/cm or ds/m. Levels greater than 2.5 can be detrimental to plants, depending on plant species, initial soil salinity and amount applied. 

Bulk density - Generally speaking, compost should have a bulk density (BC) of 1000 pounds per cubic yard, give or take a few hundred pounds. 


Photo credit: "better compost" by normanack is licensed under CC BY 2.0