The researchers conducted the experiment in a greenhouse, with the lettuce planted in pots. They used from zero to 20% coffee grounds as fertilizer in the Lactuca sativa L. var. capitata variety of lettuce – also called butterhead lettuce.
Once the potted lettuce was mature, the researchers tested and compared the nutrient content of several key phytonutrients. They found that lutein content, beta-carotene content and chlorophyll content were all significantly greater among the lettuce fertilized with the coffee grounds. The highest levels were produced with 2.5% to 10% spent coffee grounds. Beta-carotene levels, for example were up to 72% higher, and lutein levels were up to 90% higher.
However, they also found nitrogen levels were lower among the coffee-ground fertilized lettuce, illustrating that coffee grounds can stress plants somewhat. The researchers noted that the caffeine levels may have increased the stress among the plants – just as they do among humans.
The research is useful not only to gardeners and farmers, but to consumers.
The study illustrates the value of using organic composting methods to produce our fruits and vegetables. Organic biomass produced from aged plant matter, biomass and compost has the effect of increasing the nutrient levels of the foods we eat.
Organic growing methods require fertilizing plants with composted plant matter and other types of natural biomass. Organic certifiers require farmers to provide proof of fertilization methods, and they disallow the use of chemical fertilizers.
Conventional foods are usually grown using chemical fertilizers. These render a narrow range of nutrients available to the plant, and thus produce a limited nutrient content. This limits the nutrients those consumers can absorb from those conventionally-grown foods.
What this means is that choosing organic foods is not simply an altruistic endeavor for the liberal minded. It is a practical concern of nutrition. Lutein and beta-carotene, for example, are critical to the health of our eyes. One of the reasons vision conditions have increased over the past few decades has to due with our reduced intake of these important nutrients over the last half century. This relates directly to the rise of chemical fertilizer use in modern agriculture.
Cruz R, Baptista P, Cunha S, Pereira JA, Casal S. Carotenoids of Lettuce (Lactuca sativa L.) Grown on Soil Enriched with Spent Coffee Grounds. Molecules. 2012 Feb 7;17(2):1535-47.