Soil and Fertilizer

Soil Amendments & Fertilizer

The third in our April series on soil and fertilizer, this post explores the value of composting, manure – and humanure! – as a soil amendments, as well as looking at other kinds of soil amendments.

Waste not want not: Santiago’s poorest district plants recycling seed

Municipal staff collect organic material to be sent for a vermiculture recycling process in the commune of La Pintana in Santiago, Chile.

Every morning, trucks collect potato and avocado skins, orange peels and other food scraps that residents of Santiago’s poorest neighborhood leave hanging in bags on their front doors or in tree branches or place in special bins. For nearly two decades, the residents of La Pintana have been pioneers of recycling in Chile—South America’s largest garbage generator. Under a project started in 2005, the commune of 190,000 people enthusiastically gather their plant-based food waste, which is then turned into compost to help green their community.

In La Pintana, where 15% of people live in poverty, 50% of the community’s organic waste is collected for recycling—a figure that puts to shame the 0.8% achieved by Chile as a whole, according to environment ministry data. “They do a lot with it (the waste): they produce compost and it is used for the community itself, for the squares and gardens,” La Pintana resident Jose Vera told AFP as he left two large cardboard boxes filled with scraps on the sidewalk, proud of his contribution. “It is also a saving (for the municipality) because they no longer have to buy” fertilizer or pay landfill fees, he said.

Chile generates some 1.13 kilograms (about 2.5 pounds) of waste per person per day—the highest output in South America, according to World Bank data. (Note: for reference, a 2013 study suggests that Canadians produce more garbage per capita than any other country on earth. Canadians generate approximately 31 million tonnes of garbage a year and only recycle about 30% of it. Thus, each Canadian generates approximately 2.7 kg of garbage each day.)

The municipality estimates to be saving some $100,000 per year—money that can go to other community projects. “There has been a change in people,” since the project started, resident Vera said. “They are now concerned about recycling and no longer put the vegetables with the garbage.” La Pintana’s nursery, built on what used to be an unsightly landfill, yields some 100,000 plants of 400 different species every year. These are planted back in La Pintana, one of the areas of Santiago with the fewest green spaces per inhabitant.

La Pintana’s nursery, built on what used to be an unsightly landfill, yields some 100,000 plants of 400 different species every year.

Planting flowers outside a municipal sports center, municipal worker Jeanette Gonzalez told AFP the project “brings us… joy. The town is improving.” “It is a virtuous circle: people see that where there used to be a landfill there is now greenery and everything is flourishing, and they stop throwing garbage there,” she added. There have been spillover benefits too: more than half of the municipal nursery’s 15 staff are former inmates doing community work in lieu of serving prison time. Chile’s Environment Minister Maisa Rojas recently proposed a bill to reproduce the project in the rest of Chile.

(See also: Impoverished Chilean neighbourhood’s pioneering waste recycling scheme unites community; and In an impoverished Chilean suburb, a recycling drive flourishes)

Our toilets as alternatives for widespread polluting fertilizers

Notwithstanding concerns we saw in yesterday’s post about the impact of contaminants in manure, several recent articles have highlighted its potential as a replacement for expensive chemical fertilizers.

Image courtesy of The Humanure Handbook.

To tackle the climate crisis, biodiversity loss, and pollution, humanity will need to move to a circular economy, where all resources are recycled. Why not recycle our own body waste too as fertilizer, provided there is no risk that harmful microbes or traces from pharmaceuticals end up in the consumed crops? Most nutrients needed for plant growth occur in human urine and feces. Urine is especially rich in nitrogen and potassium, and also contains trace amounts of metals such as boron, zinc, and iron. Feces could in theory supply other nutrients such as phosphorus, calcium, and magnesium or valuable organic carbon to soils.

Now, a new study in Frontiers in Environmental Science has shown that modern ‘green’ products recycled from human excreta are excellent—and importantly, safe—fertilizers for agriculture. First author Franziska Häfner, a Ph.D. student at University of Hohenheim, Stuttgart, Germany, said, “Here we show that products derived from recycling human urine and feces are viable and safe nitrogen fertilizers for cabbage cultivation. The fertilizers from nitrified human urine gave similar yields as a conventional fertilizer product, and did not show any risk regarding transmission of pathogens or pharmaceuticals. The combined application of nitrified urine fertilizers and fecal compost led to slightly lower crop yields, but may increase soil carbon content in the long term, promoting climate-resilient food production.”

The researchers tested two so-called ‘nitrified urine fertilizers’ (NUFs), modern products synthetized from human urine that has been collected separately from feces, in which nitrogen-bearing compounds are converted by microbes into valuable ammonium and nitrate. These products were found to perform slightly better in field trials than plots fertilized by fecal compost alone. The authors also screened for the presence of 310 chemicals in the fecal compost, from pharmaceuticals to rubber additives, flame retardants, UV filters, corrosion inhibitors, and insect repellants. Only 6.5% of these were present above the limit of detection in the compost, albeit at low concentrations, including 11 pharmaceuticals. Among the latter, only the painkiller ibuprofen and the anticonvulsant and mood-stabilizing drug carbamazepine were detectable in the edible parts of the cabbages, at markedly low concentrations (between 1.05 and 2.8 μg per kg). This means that that more than half a million cabbage heads would need to be eaten to accumulate a dose equivalent to one carbamazepine pill.

Lead author Dr. Ariane Krause, a scientist at the Leibniz Institute of Vegetable and Ornamental Crops in Großbeeren in Germany, said, “If correctly prepared and quality-controlled, up to 25% of conventional synthetic mineral fertilizers in Germany could be replaced by recycling fertilizers from human urine and feces. Combined with an agricultural transition involving the reduction of livestock farming and plant cultivation for fodder, even less synthetic fertilizer would be necessary, resulting for example in lower consumption of fossil natural gas.”

Brown gold: the great American manure rush begins

Hundreds of dairy farms across California have sold the rights to their manure to energy producers. Illustration: Ricardo Cavolo/The Guardian

On the other side of the pond, Jessica Fu reports for The Guardian UK on how US farms are selling their manure to energy firms. The energy industry is transforming mounds of manure into a lucrative “carbon negative fuel” capable of powering everything from municipal buses to cargo trucks. To do so, it’s turning to dairy farms, which offer a reliable, long-term supply of the material.  

Algae as sustainable fertilizer

Credit: Pixabay

Current chemical fertilizers often used in the agricultural industry are not all absorbed by the plants due to the quantities used, leading to some of them being washed away into water bodies such as lakes when it rains. This then encourages algae to grow, which can cause other plant life in lakes to die due to a lack of sunlight and oxygen. New research, published in the Chemical Engineering Journal and led by The Department of Chemical and Biological Engineering’s Dr. Seetharaman Vaidyanathan, found that different strains of algae from a similar habitat can absorb varying amounts of phosphates and nitrates—key nutrients in fertilizers that also encourage algae to grow—potentially from wastewater streams before they get to lakes.

Coir, peat or pine bark – which is best?

A flow chart depicting the wetting up and drying down cycles of substrates evaluated in this study. The wettability of each substrate was tested at each moisture content framed within hydration cycles 1 and 2. Credit: HortScience (2022). DOI: 10.21273/HORTSCI16698-22

Gardeners seeking alternatives to peat will be interested by this work. The objective of a recent study published in HortScience was to quantify the sorptive effects on substrate wettability and water-holding capacity. Inferences into the effectiveness of the substrate to capture water have been difficult to demonstrate statistically. To assist in this, researchers used a monomolecular exponential model to quantify water holding capacity and the irrigation volume required to reach that capacity. Because the wetting behavior of peat can be greatly affected by hydrophobicity, a second objective was to determine the effectiveness of hydrophilic coconut coir in mitigating the initial hydrophobicity of a peat substrate.

The substrate materials tested were a 6-month aged loblolly pine bark, sphagnum peat moss, and coconut coir. Data from these experiments provide evidence that the moisture content and preconditioning of a substrate can lead to differences in initial water capture efficiency. This information can be critical to growers, growing media manufacturers, and researchers alike. The wettability of peat was most affected by moisture content and the initial wetting and drying cycles. Hydration efficiency was improved in peat by blending in as little as 15% coir by volume.

Image from an article by Green Lawn Fertilizing.

Two other articles dealt in some detail with the science of amendments for specific soil deficiencies.

An article called A nifty trick to help plants thrive in iron-poor soils describes how scientists at RIKEN have determined the structure of a key transporter protein that helps plants gather iron from soil. This work may help to formulate more targeted fertilizer products. The paper was published in the journal Nature Communications. Meanwhile, researchers at Michigan State University are Helping plants grow as phosphorus levels in soil deplete. Plants absorb phosphorus from the soil. When soil doesn’t contain enough phosphorus, plants will take up more iron from the soil, which becomes toxic at increased levels. Previous research supported the idea that iron toxicity caused a plant’s roots to stop growing. Now, for the first time, researchers at MSU and the Carnegie Institution for Science have found evidence that the plant roots stop growing early, without any evidence of iron. This changes the way researchers look at this problem. Their research was published in the journal Current Biology.

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