BIO In de Polder

Groenendijk 35

4926 RE Lage Zwaluwe

+31 6 421 561 25 Janine Keller – den Dunnen

+ 31 6 288 438 18 Paul den Dunnen

BIO In de Polder is a mixed organic farm with arable crops and livestock (oxen and suckler cows) and a campsite. The farm and campsite are situated in a village called Blauwe Sluis, which lies in between the villages of Lage Zwaluwe and Zevenbergschen Hoek. Blauwe Sluis is part of an area called West-Brabant in the province of Noord-Brabant, and is located a short distance from the Biesbosch National Park. The farm has been in the family for many generations. The current owners, Paul and Janine and their two kids Eline and Jochem are the seventh generation at the helm.

BIO In de Polder: Arable farming

Company details

The soils around our arable farm consist of sabulous clay and marine clay.  The lutum percentage in our soils varies between 20 to 35 percent, making them suitable for growing a wide range  of  arable crops.

Arable farm 86 acres
organic 74 acres
in conversion 12 acres
partridge field edges 4,9 acres
field edges 3,7 acres
wildflower field edges 0,62 acres
Groene camping ‘In de Polder’ 4,9 acres


Conversion to organic farming

On the 16th of April 2015, we started the conversion from conventional arable farming to organic arable farming. So far all our fields have been registered at Skal, the control authority which supervises organic production in the Netherlands. We aim to harvest our first fully organic crops by 2020 (about 86 acres). The name of our arable family farm was changed to “Bio In de Polder” on the 1st of January 2017.


Crop rotation plan

To keep the soil healthy and control pests it is important to make a crop rotation plan. This is a strategic plan, which helps to change the type of crop grown on a particular piece of land from year to year. For example, at Bio In de Polder we only allow the same crop to grow on the same piece of land once every seven years. In agricultural terminology this is called “a seven-year crop rotation”

How to decide which vegetables to grow?

You need to start by examining the soils you are going to use to grow your vegetables. The soils around our farm mainly consist of young marine clay, which is a light type of soil. This means that the soil is easy to till and suitable for growing most kinds of arable crops. It is important to look at plant families and common fungi and bacteria (both in the soil and in the plants) to help prevent pests and suppress diseases, which can attack crops.

To be able to harvest healthy and tasty vegetables the weather plays an important part. This is especially the case when dealing with fungal diseases. This because, for instance, wet soils can cause such diseases to spread quickly, resulting in poor crop quality and a bad harvest.

Our crop rotation plan consists of the following vegetables/crops:

  • yellow onions
  • carrots
  • sweetcorn
  • cauliflower
  • green beans
  • sugar beets
  • grass-clover
  • green manure such as barley, rye, fodder radish etc.

Soil life

Clover is a so-called nitrogen fixer as it takes nitrogen from the atmosphere and puts this into the soil. Apart from being a nitrogen fixer, clover roots run rather deep (16-17 inches). This helps to create many pore spaces or empty spaces between the soil particles. This in turn helps to transport oxygen to the soil and improves the soil’s ability to absorb and hold moisture, which is beneficial during periods of heavy rainfall. We mix the clovers seeds with grass seeds to serve as a cover crop and fully cover the soil to suppress weeds.

The clover-grass mix is high on protein and provides a good diet for the cows. The clover-grass mix is left to grow on the same piece of land for about 1.5 to 2 years. During this period the clover-grass mix will have provided the soil with enough nitrogen and oxygen for us to grow five different kinds of vegetables for 5 consecutive years on this piece of land. Planting clover and green manure helps to improve soil fertility and helps to fight climate change as they help to remove carbon dioxide from the atmosphere and convert it into carbon.  The plants use part of this carbon to grow, but send the excess carbon down into the soil to store it there.   A fertile soil means a higher number of earthworms and organism, a better soil quality, increased crop yields, more workable days and less need to work the soil and damage it.


About 20 years ago we created arable field edges around the edges of our fields. These edges are located between our cultivated fields and the ditches. These edges along with the wild-flower field edges, the partridge field edges, the greenery on our campsite and the two toad ponds, are all valuable assets to our company. They not only add colour to the landscape, remove carbon dioxide from the atmosphere and help to store carbon in the soil, but also provide an essential habitat for numerous wildlife species, such as partridges, beetles and insects. The hiking path which runs alongside our fields, is open to the public. All these measures have made the polders around our farm more attractive to humans and wildlife.

Barn swallows

Through the years our farm and campsite have become attractive places for a wide range of birds like partridges, common kingfishers, letter spotted woodpeckers, European green woodpeckers, little owls, barn owls, white wagtails, western yellow wagtails, European goldfinches, finches, hen harriers, barn swallows and so on. Barn swallows seem to love BIO In de Polder as about 60 young barn swallows were raised here last season.

Spring at Bio In de Polder

Summer at Bio In de Polder

Autumn at Bio In de Polder

BIO In de Polder at work

BIO In de Polder: oxen and suckler cows

We usually let our oxen out to graze on the grass-clover crops around the beginning of April.  Our meadows are dry enough by then to prevent our cows from causing damage to the soil. For when the soil is too wet, their hooves will sink into the mud and compact the soil. Bio “In de Polder” is a mixed farm as our cows graze on our own meadows.

Our suckler cows are let out a bit later in the season as we wait for all of them to have calved first. When this is the case, our suckler cows and their calves are let out to graze together on our meadows as well. We try to have our calving season in the spring. In this way our spring-born calves can join their mothers and enjoy the fresh grass on our meadows.


All our horned cows are allowed to keep their horns at Bio In de Polder. Some of our cows, such as the Hereford and Angus cows, are naturally hornless though, so they were born without horns. Horns are known to play a role in the digestive system of cows. How this works exactly, isn’t clear yet.

You can see how often a cow has calved by looking at her horns. The number of horn rings indicates the number of calves a cow has had so far. This is probably due to the fact that the growing calf uses up calcium, so there is less available for the mother to develop new horn substance.

Cattle breeding

We aim to have a relaxed and healthy herd (oxen and suckler cows). To achieve this Paul not only checks the bloodlines of a specific bull, but also checks how this bull responds to certain distractions. We buy a new one-year-old bull each year. We often choose between one of the following breeds: Hereford, Angus or Charolais.

Deep litter barn

We have made a conscious decision to house our animals in a deep litter barn during wintertime. This means that instead of removing old layers of litter-bedding , fresh layers of litter-bedding are added on top of the old layers throughout the winter. In this way a thick layer of bedding and manure can build up and keep our cows warm during wintertime and keep their hooves healthy. This also allows the deep-litter layers to decompose in place and turn into a solid compost. As this kind of solid compost contains a lot of organic matter and nutrients, it serves as one of the best types of fertilisers to increase the amount of organic organisms and improve soil fertility. Moreover, by using this solid compost on our soils we increase the soil organic carbon content, and ensure that our soils remain healthy and fertile to meet the needs of future generations.

Apart from that, by returning this natural compost to our fields, essential nutrients are replenished in the soil. It is a sustainable and efficient way to keep nutrition flowing and to maintain a healthy soil and grow healthy crops. This process is called closed-loop farming. This because the cover-crops, like the grass-clover mix, which grow on these fields, are grazed by our cows, which deposit their cow dung on the fields again. Their urine and the plant nutrients in the manure also help to replenish the soil. Since the fungi and bacteria in their faeces are familiar nutrients to the soil, the soil quality will improve, leading to healthy crops and an increased productivity.