In October 2022, Extension educators visited 20 community gardens and urban farms in Ramsey County to conduct soil tests. All 20 sites used county compost and many of the gardens had applied compost for years.
While compost is an important soil amendment, and using municipal or county compost in gardens is a great way to recycle organic waste, we suspected that some gardeners had been using too much compost. Over-applying compost can result in the buildup of phosphorus, which can negatively affect the health of lakes, rivers and other freshwater ecosystems. This is a significant issue in Minnesota.
- All 20 gardens had too much phosphorus. 27 parts per million (ppm) was the minimum phosphorus reading. The median was 133 ppm, and the maximum was 244 ppm.
- A soil phosphorus reading of 25 ppm is very high. At a reading of 25 ppm, you should not add any additional phosphorus to grow garden vegetables.
- When asked if they had applied any additional fertilizer, almost none of the gardeners had applied anything except for compost.
- While compost is generally low in nutrients, phosphorus and potassium can build up over time when gardens use a lot of compost (many inches each year).
Take a soil test this spring before adding more compost
The best way to know whether or not your garden could benefit from additional compost is to take a soil test. If your soil has high levels of phosphorus, take a break from applying compost for a few years. But if your soil test indicates that your soil phosphorus levels are not excessive, then go ahead and add compost. We typically recommend about an inch or less of compost over your garden beds each year.
Download a form and find instructions from the U's soil testing lab, and watch this video to learn how to test your soil.
What to do if you have excess soil phosphorus
- If your soil phosphorus levels are above 25 (Bray test) or 18 (Olsen test), do not use a phosphorus-containing fertilizer for a few years, until your levels drop to a point where your soil test specifically recommends more phosphorus.
- If you have enough phosphorus in your garden but still need nitrogen or potassium, consider options like feather meal or blood meal (nitrogen), and potassium sulfate or langbeinite (potassium).
- Remember that compost is a source of nutrients. While concentrations are low, large volumes of compost can still contribute to soil nutrient build-up.
- Consider reducing compost inputs or stopping applications altogether if your soil phosphorus levels are well above the recommended amounts.
- Try to prevent soil erosion. Phosphorus can move into storm drains, lakes and rivers when soil moves over the surface of the garden during times of heavy rainfall or strong winds. Surround your gardens with perennial vegetation to absorb runoff.
Soil health practices beyond compost
- Use cover crops in your garden. Legume cover crops can provide nitrogen and build your soil health without adding extra phosphorus to your soil. Cover crops also provide a range of benefits including erosion control, nutrient retention, weed management and pollinator support. Watch this video about cover crops for gardeners.
- Reduce tillage to improve soil health.
- Keep your soil covered as often as possible to prevent erosion. You can do this with cover crops, mulches (straw, leaves, etc.), and even tarps.
- Learn more about soil health for gardeners on this website, or at the UMN Extension Yard and Garden YouTube channel.
- Your local Master Gardeners may be able to help you interpret soil tests and provide guidance about soil health strategies for your garden.