Used coffee grounds make good soil amendments. That's the buzz among gardeners lately. But what do your coffeepot's leftovers really add to the soil?
To find out, Sunset sent a batch of Starbucks' used coffee grounds ― the company gives them away for free ― to a soil lab for analysis. Turns out the grounds provide generous amounts of phosphorus, potassium, magnesium, and copper.
They also release nitrogen into the soil as they degrade. And they're slightly acidic ― a boon in the Western climate.
Dig or till them into the soil to a depth of 6 to 8 inches.
The following information was developed for Sunset by Soil and Plant Laboratory Inc., Bellevue, WA.
Summary: Use of Starbucks coffee grounds in amending mineral soils up to 35 percent by volume coffee grounds will improve soil structure over the short-term and over the long-term. Use of the coffee grounds at the specified incorporation rates (rototilled into a 6- to 8-inch depth) will substantially improve availabilities of phosphorus, potassium, magnesium and copper and will probably negate the need for chemical sources of these plant essential elements.
The nitrogen, phosphorus, potassium "guaranteed analyses" would be as follows for the coffee grounds:
Nitrogen: 2.28 percent
Phosphorus: 0.06 percent
Potassium: 0.6 percent
Available nutrient levels: The pH or reaction of the coffee grounds is considered slightly acidic and in a favorable range at 6.2 on the pH scale.
Salinity (ECe) is a measurement of total soluble salts and is considered slightly elevated at 3.7 dS/m. The primary water-soluble salts in this product are potassium, magnesium, sodium and chloride. The potentially problematic ions in sodium and chloride are each sufficiently low as to be inconsequential in terms of creating problems for plants.
The availabilities of nitrogen, calcium, zinc, manganese and iron are quite low and in some cases deficient. Thus, the coffee grounds will not supply appreciable amounts of these essential plant elements when used as a mineral soil amendment.
However, the availabilities of phosphorus, potassium, magnesium and copper are each sufficiently high that there will be a very positive impact on improving availabilities of these elements where the coffee grounds are used as a mineral soil amendment. The coffee grounds will negate the need for additional sources of phosphorus, potassium, magnesium and copper when blended with mineral soils.
In summary, the available plant essential elements which will be substantially improved where the coffee grounds are used as a soil amendment, include phosphorus, potassium, magnesium and copper.
Total nutrient levels: Each cubic yard of these coffee grounds contains a total of 10.31 lbs. nitrogen, of which 0.01 lb. (0.09%) are available. Thus, even though available nitrogen is considered deficient in this product, there still remains over 10 lbs. of total nitrogen per cubic yard of coffee grounds. Thus, nitrogen is primarily bound in the organic fraction and is unavailable to plants until soil microorganisms degrade the organic fraction. Through this process, the nitrogen is converted to plant available forms. Over the long term the coffee grounds will act like a slow release fertilizer providing long-term nitrogen input which can then be utilized by plants.
Nearly all potassium and all magnesium are in the available forms. This means that immediate availability improvements for these two elements will take place when the coffee grounds are blended with mineral soils. About half of the copper and calcium are in their immediately available forms.
All other plant essential elements are primarily bound in the organic fraction and will thus be subject to slow release over time as soil microbes continue to degrade the organic fraction.
Physical properties: Virtually all particles passed the 1 millimeter (mm) screen resulting in a product which is very fine textured. Each cubic yard of the coffee grounds will supply an excellent amount of organic matter, measured at 442 lbs. organic matter per cubic yard. At the use rates indicated in this report, the input of organic matter will be substantial and will result in considerable short-term and long-term improvement of mineral soil structure.
Carbon/nitrogen ratio: On the basis of dry matter bulk density (452 lbs. per cubic yard), organic matter content (97.7%) and total nitrogen (2.28%), the estimated carbon/nitrogen ratio is about 24:1. This means that there is more than sufficient nitrogen present in the coffee grounds to provide for the nitrogen demand of the soil microorganisms as they degrade the organic fraction.
Use rate: Based on the overall chemistry and physical properties of the coffee grounds, they can be utilized at rates similar to other organic amendments when used in amending mineral soils. These data indicate that 25-35 percent by volume coffee grounds can be blended with mineral soils of any type to improve structure of those soils.