Nut trees in fields of potatoes

Video: Wageningen University & Research


Higher yields and better resistance to diseases and pests: that is what agroecologist Wijnand Sukkel expects from his agroforestry project in the Flevoland polder. It is a project that combines the cultivation of annual crops such as potatoes with hedgerows of nut trees. “If you consider the price of a kilo of nuts, this should be an interesting business model.”

There is a strong wind blowing across the Flevoland polder, including across the plot of land belonging to Wageningen University & Research, which looks no different from all the other plots of land stretching out towards the horizon, except perhaps for the tall bamboo poles that the young trees are strapped to. WUR agroecologist Wijnand Sukkel points to a row. “If I have anything to do with it, we will be harvesting hazelnuts there in a few years’ time, along with potatoes, grain, carrots and onions.” Wageningen Plant Research is studying mixed cultivation, an arable farming system in which various annual crops are grown at the same time in a field. Strip cropping is a well-known form of mixed cultivation; in this system, the crops are grown in alternating rows six or twelve metres wide. Mixed cultivation is much better for biodiversity than monocultures, it provides better resistance to pests and yields are higher. WUR recently started research on a new form of mixed cultivation, termed agroforestry. This involves growing annual crops in combination with woody crops such as fruit trees and nut trees.

Higher yields

“It isn’t actually new,” says Wijnand Sukkel. “The apple orchards of the past in the Betuwe region, where cows would graze under the trees, were actually agroforestry as well.” According to Sukkel, research into the effects of agroforestry is interesting for several reasons. “Based on know-how from countries such as the UK and France, we expect a higher yield from mixed cultivation than from the sum of the crops cultivated separately. The sum of the yields from one hectare of potatoes and another separate hectare of hazelnuts is less than from two hectares of combined cultivation with a mix of potatoes and hazelnuts.”

Blooming land borders and young trees on the farm land. Photo: WUR Open Teelten

Another positive effect of mixed cultivation, according to Sukkel, is the increased biodiversity. “Going from a monoculture to two species is already a significant boost to diversity. It enhances biodiversity and improves resistance to diseases and pests. There’s quite a lot of scientific evidence for that.”

More distance

The problem with the way most arable farms operate at the moment is that large tracts of land are used for growing genetically identical crops. Sukkel draws a comparison with the measures to combat Covid. “We walk around with our facemasks on, keeping our distance from one another, because if we didn’t, the virus would spread fast throughout the population. That is essentially what happens now in arable farming. To prevent diseases and pests from spreading fast across the crop, you either need pest control and protection using pesticides or you make sure there is more distance between plants, as in mixed cultivation.” When setting up the agroforestry experimental site, Sukkel and his colleagues drew inspiration from international literature. Studies from countries including Britain, France and Belgium show that there is an optimum relationship between the height of the trees and the distance between them. If you have rows of trees at least 17 metres tall that are about 95 metres apart, you get a slight increase in the yield of the crops growing between the rows.

The positive impact of agroforestry has already been proven in the tropics, but what about agroforestry in temperate climates? WUR researcher Maureen Schoutsen investigates how this approach could work in the Netherlands. She believes that agroforestry, meaning agriculture that incorporates the cultivation of trees, offers a solution for current agricultural problems, such as bouts of extreme drought and excess water.

Sukkel: “We will be investigating whether this also applies to the situation in the Netherlands. We have 15 hectares in which the usual annual arable crops — potatoes, cabbages and wheat — are grown in rotation, and now we have planted rows of trees 60 and 120 metres apart. We opted for fast-growing species such as alder, poplar and white willow, so that we will soon have a row of trees that we can keep about six metres tall. They have also been planted in lines running north to south to minimise the shadow effect, and we have installed a drip irrigation system to make sure we have hedgerows that will have grown enough in four years to let us measure the effects properly.”

Fields with a microclimate

Sukkel is mainly interested in how this form of agroforestry affects the microclimate between the rows of trees. “How does the soil fertility change, what happens to the water ecology, and what impact does the reduction in the wind have on the crops? It’s also interesting to look at how the biodiversity changes, and what happens to yields, the incidence of pests and the spread of diseases.” That is all very well, but poplars and potatoes don’t count as mixed cultivation. Sukkel: “That’s true, which is why next year we will be planting hazelnut trees next to the rows of trees. In six years’ time, we will take out the original rows of trees and leave the hazelnut hedgerow. That will then start producing yields. Only then can we assess the overall yields of the annual crops and hazelnut trees combined. In the next six years, we will probably also have solid results on the effects of such rows of trees on arable crops, the incidence of pests, biodiversity, and so on.”

We expect a higher yield from mixed cultivation than from the sum of the crops cultivated separately

The researcher is not worried by the criticism expressed by potato growers who argue that the rows of trees will result in stationary air and therefore increase the risk of the notorious potato disease Phytophthora. “It’s possible — we don’t know. We have a hypothesis based on the literature and it is also a question of thinking things through logically. Based on that, we arrived at a combination that could be good for resistance, biodiversity and yields. I think the risk is small when you consider there is a distance of at least 60 metres between the rows of trees. But as I just said, we don’t know for certain, which is why we are doing this research.”

Wheat combined with trees. Photo: WUR Open Teelten

Farmers in the UK experiment with agroforestry as well. Photo: Wakelyns Agroforestry, Suffolk UK

These days, not a single country in Europe combines annual crops with the cultivation of woody crops on any scale. Sukkel: “Given current market prices for nuts and arable crops, this approach is too labour intensive. The problem with combining crops is that the machinery is not yet designed to cope with smaller harvesting plots. The machines available at the moment are too big.”

The rows of trees are planted north to south to minimise the effect of shadow on the crops

Additionally, mixed cultivation systems such as agroforestry have to compete with a system of crop protection, fertilisation and large-scale mechanisation that has been perfected over the course of a century. “It isn’t realistic to expect us to come up with all kinds of combinations of annual crops and woody crops that generate 20 per cent higher yields and require less labour,” says Sukkel. Even so, he is optimistic about the prospects for growing nuts. At present, little is known about how to harvest hazelnuts entirely mechanically, but walnuts and hazelnuts are very well suited to the Dutch climate. “If you consider the price of a kilo of nuts, cultivating hazelnuts in combination with annual crops should be an interesting business model.”

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