Should you use coffee grounds and tea leaves as fertiliser for your garden?

In terms of soil pH, both coffee grounds and tea leaves are mildly acidic. Photo:

Some gardeners swear by them; they say spent coffee grounds and tea leaves should be reused as fertilisers or added to compost heap. They are often touted as green, eco-friendly options for garden fertilisers because of their nutrient content and potential benefits for soil health. These organic materials are abundant and contain nutrients that are important for plant growth.

Spent coffee grounds contain approximately 2% nitrogen (N), 0.1% phosphorus (P), 0.3% potassium (K), 0.2% calcium (Ca), and 1.9% magnesium (Mg). Spent tea leaves offer an even richer source of nutrients: 4% N, 0.2% P, 1.3% K, 0.7% Ca, and 3% Mg. These values compare favourably with those of other organic waste products traditionally used in composting and soil amendment.

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In terms of soil pH, both coffee grounds and tea leaves are mildly acidic, registering pH levels between 5 and 6. For acidic soils, such as those in Malaysia, which typically range from pH 4 to 5, the slight acidity of these materials is not expected to exacerbate soil conditions. Additionally, the high carbon (C) content, constituting approximately 50% of the weight of these spent materials, can be beneficial for enhancing soil organic matter when applied consistently over time.

Despite these positive attributes, there are significant concerns regarding the caffeine content in these waste products. Coffee grounds contain approximately 1% caffeine, while tea leaves can contain as much as 3.5% caffeine.

Caffeine poses a two-pronged problem for soil health and plant growth. It exhibits allelopathic properties and can inhibit plant growth and seed germination.

Furthermore, the antibacterial qualities of caffeine may detrimentally affect soil microbial populations, disrupting the soil ecosystem, even at concentrations as low as 1g per litre of soil.

Research has underscored these concerns. A 2016 study by the University of Melbourne demonstrated that a mere 2.5% volumetric concentration of spent coffee grounds was sufficient to hinder the growth of five horticultural crops across three soil types.

Additionally, a 2011 study by the Nova Scotia Agricultural College in Canada indicated that incorporating spent coffee grounds into vermicompost could lead to an 85% reduction in earthworm populations, suggesting the emission of potentially toxic organic compounds from coffee grounds.

However, these materials have been identified as effective metal-chelating agents, which can be advantageous under certain soil conditions. Chelating agents bind metal ions such as iron, copper, manganese, and zinc, thereby enhancing the availability of these essential micronutrients, especially in alkaline soils where such nutrients become less accessible to plants.

In particular, alkaline soils with pH levels between 8 and 9, such as those used in a Japanese rice trial, showed improved iron and zinc uptake in crops treated with spent coffee grounds and tea leaves. This was corroborated by a 2008 study that demonstrated increased iron content in radishes grown with these materials.

Nevertheless, Malaysian soils are generally acidic and do not suffer from the same micronutrient deficiencies as those observed in alkaline soils.

The negative effects of coffee grounds and tea leaves can be mitigated by composting them prior to application.

Composting them will break down the caffeine and reduce its allelopathic effects. Even then, they can be applied in small quantities and do not depend only on them as your sole or main fertiliser source.

In summary, while the nutrient content and chelating properties of spent coffee grounds and tea leaves present potential benefits for soil amendment, their caffeine content and possible negative effects on soil life must be carefully considered.

The choice to use these materials should be informed by the specific soil conditions, the type of plants being cultivated, and ongoing scientific research.

Given the complexities of their impact, it is crucial to approach the use of coffee grounds and tea leaves in agriculture with a nuanced understanding, acknowledging both their potential benefits and limitations.

Assoc Prof Dr Christopher Teh Boon Sung heads the Department of Land Management, Faculty of Agriculture, Universiti Putra Malaysia. His field of specialty is in soil and water conservation.

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