CORVALLIS, Ore. — Gardeners hear it often: Add compost to improve the soil, boost beneficial microbes and grow healthier plants. But what if too much organic matter does more harm than good?
“There are environmental consequences to adding too much organic matter. Fertilizer leaching, runoff, plant burn and wasted time and money are all possible outcomes.”
Research by Oregon State University suggests that might be the case — especially in urban gardens. The results surprised even the scientists conducting the study.
“There are environmental consequences to adding too much organic matter,” said Mykl Nelson, instructor in the OSU Department of Horticulture. “Fertilizer leaching, runoff, plant burn and wasted time and money are all possible outcomes.”
Nelson began to suspect that many urban gardens were over-enriched with compost. A two-year study confirmed his hypothesis: Soil samples from 27 urban gardens averaged 13% organic matter. The recommended minimum, according to Cornell University Extension and other sources, is just 3% to 5%.
“Most soil studies focus on native or agricultural soils,” Nelson said. “There wasn’t much research on garden soil — especially in raised beds in urban settings.”
Nelson surveyed more than 80 gardeners and selected 27 who lived within the urban growth boundary near his research site. All were OSU Extension Master Gardeners, making them easier to contact and likely to participate.
He examined both in-ground and raised beds, focusing on those used for vegetables or annuals. Core soil samples were taken at multiple depths, combined and analyzed in a lab. As expected, raised beds — often filled with imported soil mixes — had the highest levels of organic matter.
“Every bed was at least above the recommended minimum,” Nelson said. “In one case, organic matter reached 30%. That gardener used only compost — and their pepper plants burned and died by the end of the season. That’s an example of going too far.”
Too much phosphorus, too little balance
High levels of organic matter are often linked with elevated phosphorus and microbial activity. Excess phosphorus can run off if the soil erodes, and nitrogen may leach out — especially if quick-release fertilizers are used.
“Phosphorus levels can build up over time, particularly with repeated use of manure or manure-based composts,” said James Cassidy, senior instructor in OSU’s Department of Crop and Soil Science. “If soils are left uncovered and erode, that phosphorus can contaminate waterways.”
Cassidy, who wasn’t involved in Nelson’s study, acknowledged that his own enthusiastic promotion of compost may have contributed to the trend.
“I’m afraid I’m the source of many gardeners gone astray,” he said. “I’ve spent years telling people to add organic matter — and maybe I was too successful.”
What to put in built beds
Nelson recommends filling the bottom of raised beds with carbon-based materials — wood chips, limbs or straw — and topping with a mix of loamy soil and compost. Local bark mulch facilities often carry appropriate blends. To save money, consider sharing a truckload with neighbors or friends. Always smell the product before buying — if it doesn’t smell fresh, skip it.
The key: Test your soil
The most important step for any home gardener? Get a soil test.
“If your organic matter is in the 3% to 5% range, hold off adding more,” Nelson said. “Test again in a year or two. You may not need additional compost or fertilizer.”
In heavy clay soils, some organic matter can improve structure and boost water and air movement to roots. But adding too much — especially without testing — can be counterproductive.
“Most people want a recipe,” Cassidy said. “But when it comes to soil, there isn’t one. Testing is the closest we can get.”
Resources to get started
Learn more about soil testing and interpretation through OSU Extension:
You can also bring your soil test results to your local OSU Extension office for help from trained Master Gardeners.
Previously titled Study shows some urban gardens contain too much organic matter