Let us practice a bit of the Socratic method. If a gardener ready to plant tomatoes or peppers learned the soil was deficient in nitrogen, what could he or she do to rectify the situation?
Now assume he or she wants to protect the microbial life in the soil by avoiding burning chemicals. What are the options here?
Organic nitrogen amendments are commonly in 3 forms:
- Manure: great source of nutrients but it requires a complete composting process to be useful in the garden. Examples: any animal poop, worm castings can be applied with no composting but require a few month to build up a harvest.
- Animal products: highest nitrogen levels that don’t need composting but would benefit from dilution in soil and topdressing to protect plants from burning. However these may attract unwanted varmints to the garden. Examples: bone-/blood-/fish-meals:
- Plant products: lower concentration and more balanced nutrients but can safely be applied at higher rates or frequently. Requires active microbes to release nitrogen, thus 50 degree Fahrenheit (10 degree Celsius) or higher soil temperatures. Examples: cottonseed meal, soy meal, etc.
Lets look at #3. It can be applied right at the time of planting and promotes a healthy, vibrant soil microbe population. Depending on the conditions, these will break down in 2-3 months (The Intelligent Gardener: Growing Nutirent Dense Food by Steve Solomon, Erica Reinheimer, page 74) and should be reapplied as long as the plants need nitrogen. What do these organic soil amendments have in common?
Most plant-based organic nitrogen fertilizers are seedmeals. What is coffee exactly? Coffee is the seed in the berry of the Coffea plant. Therefore coffee grounds are simply seedmeal where the grinding is already done in food preparation. Used grounds are happily given away by all Starbucks locations (and most coffee shops at that) for free. Just call ahead a few hours as some shops need warning to collect them.
Thus coffee grounds can confidently be used in the same manner as any other organic seedmeal:
- Added directly into soil as an amendment (recommended rates given later)
- Top dressing plants for a slow release of nitrogen
- Green/nitrogen matter to turbocharge a compost pile
- I’m hoping they will deter rabbits from damaging my trees (no scientific or even observational evidence on this one though!)
- Despite some contention, they can be used as bedding/food for vermicomposting
Note: coffee grounds are pH neutral as the acidic compounds are water soluble and mostly leached out in the brewing of coffee. People commonly assume they will acidify the soil for plants like blueberries, pines, rhododendron, etc. but at 6.2 pH, they cannot be used in a quantity to acidify soil to the levels that the listed plants require. While acidifying soil is not a benefit of coffee grounds, they are a beneficial amendment for these and most plants.
The organization Sunset experimented with starbucks coffee grounds sending them to Soil and Plant Laboratory Inc. for analysis. The full report. Here are the summarized results.
Nitrogen: 2.28 percent
Phosphorus: 0.06 percent
Potassium: 0.6 percent
pH of dried grounds was slightly acidic: 6.2
Salinity shows problematic ions are insufficient to pose a problem (chloride and sodium)
Phosphorus, Potassium, Magnesium, and Copper are all bioavailable in the grounds meaning plants can use them immediately
Only 0.09% of nitrogen is bioavailable (10.31 pounds per cubic yard). The remaining nitrogen is slowly released by soil microbes.
442 pounds of organic matter per cubic yard
Starbucks grounds passed readily through a 1mm screen. Long term, this fine texture significantly improves soil structure.
Recommended to use at 25-35% when amending soil.
Not only do gardeners have a free source of nutrient-filled organic matter, they also prevent entirely compostable material from being sent to a landfill where it won’t ever get a chance to undergo aerobic decomposition resulting in the permanent removal of vital nutrients from cycling through the food system. If possible, I would take grounds by the truckload and spread it on the pasture. Best of all, it is free!