Three paths to plant-based crop rotation with food crops

Disclaimer: This article has been machine translated from Danish to English, so please note that there may be errors in the translation.

The climate footprint of plant-based food grown with plant-based fertiliser is expected to be significantly lower than with animal manure. Therefore, it is interesting to ask the question whether it is possible to create realistic crop rotations and supply crops with sufficient nutrients using only self-produced or purchased plant fertiliser.

According to Chief Consultant Sven Hermansen, the answer is yes. Based on practical examples, he has developed three models for how such decoupled, plant-based production can look.

- "With plant fertiliser, we have the advantage of avoiding methane emissions from livestock, but if we are to ensure a low climate impact, it is important that yields do not fall and that the soil is not depleted. In the three scenarios, we therefore change the crop rotation and replace animal manure with green manure, composted garden/park waste and organic fertiliser," explains Sven Hermansen.

Memo on plant-fertilised crop rotations

The no-input system

In a no-input system, crops are only supplied with nutrients from the soil pool, green manure and nitrogen fixed via legumes.

In this situation, Sven Hermansen suggests a six-field rotation with clover grass, cereals and legumes, catch crops where possible, and where winter crops are fertilised with silage or fresh green manure.

-Without year-round green manure on part of the land, it's not feasible. On some soils you can use up the soil pool for a while, but it won't be possible to maintain yields in the long term and there is a risk of depleting the soil," says Sven Hermansen.

The compost-based system

When fertiliser needs to be plant-based, garden/park waste and similar products are an obvious source of nutrients. Some organic plant growers have such a fertilisation strategy, which can illustrate the opportunities and challenges of large amounts of garden/park waste. The Innovation Centre for Organic Farming is currently testing garden/park waste enriched with clover grass silage and seaweed in an attempt to lower the C/N ratio of the compost and achieve a higher first-year effect of the compost.

-"The starting point is the same crop rotation as in no-input, but the result is more robust due to carbon build-up and good nutrient balances for P and K. If you want to base crop production on compost, experience shows that the effect comes when you add compost several times over a number of years," says Sven Hermansen.

Systems based on commercial fertiliser

There are various fertilisers on the market that are produced without manure in any part of the process and can be combined to achieve an optimal ratio of N, P and K.

-With commercial fertilisers, you can largely meet the needs of the crops and grow more demanding special crops such as grass seed," says Sven Hermansen, who has calculated nutrient balances for a crop rotation with seed grass, legumes, and three years of cereals, including bread wheat, as an example. The nutrient supply comes from a combination of compost and vinasse. The fertilisation plan is close to the limit of 170 kg total N per hectare.

-At the same time, winter wheat and seed grass are on the edge of getting the recommended supply. You can compensate for this by growing a less demanding crop, such as winter rye, as a food crop," says Sven Hermansen.

The memo was prepared in the project 'Plant-based fertiliser for plant-based foods', which is supported by the Foundation for Organic Agriculture.