Mounds, mud and trees

At a time when biodiversity is collapsing, conventional agriculture is exhausting the soil and climate change is imposing new rhythms, it is becoming urgent to farm differently. But how can we reconcile sustainable agriculture with a growing world population? By scientifically studying new techniques that can be transposed directly to the heart of cities. Caroline De Clerck from Gembloux Agro-Bio Tech takes a closer look.

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hile agricultural activities in towns and cities (smallholdings, vegetable gardens) have a long history for practical reasons of food supply, population growth has gradually relegated fields to the periphery. But in the age of megacities, the possibility of urban agriculture is once again attracting interest as a vector for ecological transition: a small plot of land can provide several kilos of vegetables a year, not to mention contributing to the well-being of the population and the environment. Researchers are taking an interest. At Gembloux Agro-Bio Tech in particular.

Behind the Terra unit's large building, five hectares of cultivated land are home to various plots growing lettuce, cabbage, spinach, onions and other vegetables. These long-term experiments are part of the ‘Walloon Platform for Innovative Systems in Urban Agriculture and Biodiversity’ - known as ‘Wasabi’ - founded in 2020. Among the various experiments being carried out are the ‘Terra-Terre’ gardens: 57 ares divided into three plots cultivated without pesticides and managed by Caroline De Clerck, first assistant at Gembloux Agro-Bio Tech. They are all designed to evaluate different urban market gardening techniques. ‘Their principle is to test growing systems that can easily be transposed to market gardening in an urban or peri-urban environment, in order to accumulate statistically valid scientific results’, explains the biologist. Three plots? More like four, in fact, because the last one, a community garden subsidised by the town of Gembloux and soon to be operational, ‘ will be there for all members of the University, students and citizens who want to take over the site to plant and cultivate a little piece of land’, she reveals. We're really committed to the idea of a living space, open to everyone and to all kinds of knowledge.

Growing on hillocks

Caroline De Clerck and her teams therefore identified three types of permacultural mound, rich in organic matter, which they decided to compare to a conventional plot of land, as well as to a simple mound of earth without any particular additions, in order to assess ‘ the influence of the shape of the mound itself on the temperature, humidity and life of the soil’. ‘ The first mound, known as a ‘lasagne’, is made up of alternating ‘brown’ layers rich in carbon, such as straw or dead leaves, and ‘green’ layers rich in nitrogen, where you can find grass clippings, green leaves or kitchen waste’, she explains. A lot of people are familiar with it because, unlike the others, it's not very intimidating: it's easy to set up, fits in well with a vegetable garden and is easy to remove, especially as it sinks quickly and almost disappears after a year ‘.

The other two permaculture-inspired mounds are standards of their kind. ‘ First, we wanted to test Robert Morez's ‘sandwich’ mound, which is very rich in carbon and features both small-diameter wood in the centre and faggots planted at the top of the mound to facilitate the entry of water,’ continues the researcher. As for the last mound, it really is the flagship mound of permaculture. It is called ‘forest’ because its centre is occupied by large logs of wood, which are supposed to decompose slowly and release nutrients available to the plants for 10 or 15 years.

Four years after their installation, a number of conclusions can be drawn. ‘ We had to wait until the third year to obtain good yields from the ‘sandwich’ mound,’ reveals the researcher. This delay is probably due to the micro-organisms that had to use soil resources to decompose the rich organic matter, thus depriving the plants of these resources. But today, these same mounds stand out clearly from those grown in simple soil, as well as from the plot cultivated in the open ground. Our yields are higher than the average for market gardeners in Wallonia, and the quality of the soil is very good, which is very positive.

Planned to last at least ten years, these trials are designed to accommodate all researchers wishing to test different methods. Because of the very wet weather, slugs have been multiplying’, says Caroline De Clerck. To remedy this, and thanks to funding from Ares, we have launched a small project to house Indian runner ducks in a small pond. They are predators of slugs, and their presence will increase biodiversity, which is one of Wasabi's major objectives.

Fertile market gardening

Not far from the mounds, spinach and lettuce flourish in a more conventional-looking plot. But don't be fooled by appearances, because this is where Caroline De Clerck's team is experimenting with Small Plot INtensive Farming (Spin Farming). ‘ This is a method of intensive market gardening on a small surface area, quite specific to urban environments’, explains the biologist.

Theorised and developed by Canadian Wally Satzewich in the early 2000s, Spin Farming is as much a method of cultivation as it is an economic model. ‘ On an area of just 0.1 ha, ‘Spin Farming’ is profitable for 1 full-time equivalent, with little mechanisation and six to eight crop rotations a year’, says the scientist. The idea is really to produce where the crops will be consumed, by the neighbours or ideally by a restaurant that buys in the produce. In fact, that's what we want to do, by supplying the university restaurant directly ’. This year, the spent grains from the brewery had a very positive impact on the life of the soil.

As this is a very intensive system on a minimum surface area, the method requires significant inputs of organic matter to avoid exhausting the soil. ‘ We are looking to make the most of readily available organic fertilisers. To achieve this, Caroline De Clerck's teams only had to cross the road. The Beerfac brewery, which has been brewing Abbaye de Gembloux beer for a number of years now, produces an abundance of a waste product - spent grain - a filtration residue amounting to 20 kg per hectolitre. As a major beer producer, Europe has 3.4 million tonnes of spent grain every year, a ready-to-use fertiliser rich in nutrients that could be of interest to many urban market gardeners. What's more, this spent grain can also be used to make bokashi, ‘ an oxygen-free composting method that uses lactic acid bacteria to produce a liquid residue that can be applied directly to plots of land to fertilise them “, explains the biologist, although she admits that ” the impact of bokashi has been mixed this year. But the spring and early summer were particularly wet, and the liquid inputs may have been washed away by the rain.

Not forgetting the trees

Other ideas are still germinating in his head. We're thinking of compost made from poultry droppings, this time from the biopony experiments,’ she muses. And I'm also interested in vermicompost and plant purins like nettle purin. Not forgetting aquaponics, which is currently being tested on the Wasabi platform. This is a circular economy system in which vegetables and fish live side by side. The water enriched by the fish's faeces is used to feed the plants and, in return, the water purified by the plants is returned to the fish pond. The residual aquaponic sludge, which is rich in nutrients, is recovered and applied to the crops,’ explains Caroline De Clerck. Last year, we also tested the same process with the residues from insect breeding, but the smell was particularly nauseating, and the availability was rather uncertain, which are obvious obstacles to their acceptance by market gardeners.

Finally, to complete the Terra-Terre gardens, the biologist is also supervising a third plot where two double rows of local apple varieties are already growing. We're soon going to start experimenting with agroforestry for market gardening,’ she says. Vegetables will be planted on either side of the fruit trees, to study the impact of their presence on the vegetables.Because there will certainly be impacts, both positive and negative. Positive, because the root network provides organic matter (as do dead leaves and fallen fruit), as well as a reduction in disease and pests, due to greater biodiversity’, predicts the researcher. Negative, because there will be competition for water, nutrients and light due to the shade provided by the trees. And on one side of the line, we're going to work the soil so that the roots go deeper and no longer under the vegetables, and so study the impact of their presence ‘.

The agroforestry plot is also home to large solar panels that can be moved around an axis to follow the sun, proving that the Faculty's different experiments can sometimes blend together in a whirlwind of ideas. On the Wasabi platform, there's an agrovoltaics experiment that's trying to determine whether, at our latitudes, it's possible to produce electricity in the same place as vegetables without the latter being hindered by the shade of the panels’, explains the scientist. Some studies have shown that the panels conserve soil moisture during the summer months, while protecting crops from frost or hail during the winter.

This openness to others is one of the key features of the Wasabi platform and the Terra-Terre gardens. It‘ s vital that these trials remain open to everyone, including researchers from all over the University,’ insists Caroline De Clerck. The greater the number of participants in the experiments, the more complete and relevant the results will be for everyone.And there are many opportunities for collaboration. ‘ While our projects are often quantitative (yield or soil quality), it is entirely possible to carry out qualitative projects, and compare the nutritional and taste values of the vegetables on the different mounds or the different ‘Spin Farming’ amendments, for example’, she imagines. It might also be interesting to study the carbon dioxide and nitrous oxide emissions from our crops, because it's likely that our work has an environmental impact that we don't know about. The call goes out.