Organic area growth cannot be based on livestock manure

With the goal of increasing organic acreage and transitioning to a more plant-rich diet, organic farmers of the future will have to adapt to plant-based fertilisers as a new normal.

By Sven Hermansen, Karen Munk Hansen

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

Since the first farms were converted, organic farming has largely been based on nutrients from animal manure - both organic and conventional. With a political goal of doubling the organic area and a parallel transition to a more plant-rich diet, this can hardly be the model for the future.

The Innovation Centre for Organic Farming is therefore currently working to find and describe sustainable models for organic crop production without the input of animal manure. The nutrients for crops in such a system can come from sources such as ensiled green manure, composted garden/park waste and source-separated organic waste.

7-year model crop rotation

In the model crop rotation below, the input/output of nutrients is balanced through the application of mobile green manure and composted garden/park waste.

The crop rotation is seven years with clover grass for two years, from which there is no sales revenue. The grassland provides only mobile green manure and crop value for cash crops, in this example grain and potatoes.

The assumption of exclusively plant-based fertiliser is deliberate. In practice, there will be all sorts of intermediate scenarios where you can sell grass for feed or import small amounts of animal manure, but in this case the goal is to generate knowledge about a 'new normal' and investigate what the system can perform and withstand when you go to the edge.

Crop rotation with plant-based fertiliser

The crop sequence in a seven-year crop rotation, timing of fertiliser application and placement of catch crops.

No room for winter crops

The plant fertilisers chosen - composted garden/park waste and ensiled clover grass - limit the crops that can be grown. There is no liquid fertiliser available, and as fertiliser regulations make it virtually impossible to apply organic fertiliser in the autumn for cash crops other than winter oilseed rape, it is difficult to fit winter cereals into the crop rotation.

Strategic use of compost

Both fertilisers are nitrogenous, but only green manure has a significant first-year effect. Composted garden/park waste has no measurable first-year effect in practice, which is why it is used strategically for the nitrogen-collecting crops clover grass and field pea. The ensiled green manure is applied to potatoes, spring rye and spring wheat in years 2, 3 and 5 after clover grass. The oats feed solely on the fertiliser value of clover grass.

In trials, good first-year effects of mobile green manure have been achieved when the biomass has a low C:N ratio and it is comminuted and carefully incorporated into the soil before sowing.

Biogas gives more degrees of freedom

If the scenario includes selling biomass from clover grass, straw and cover crops to biogas plants, and you can also get the degassed manure back, there are better opportunities to vary crop rotations and include winter cereals. However, this requires that the degassed manure is separated into a wet fraction and a fibre fraction, as it is difficult to utilise degassed manure with a high dry matter percentage in winter cereals in the spring.

Gross nutrient supply in plant fertilised crop rotation, average

Source: Source	Total-N, kg/ha	P, kg/ha	K, kg/ha
Composted garden/park waste	93	15	60
Clover grass silage	50
Legume forage	29
In total	172	15	60

Expected yields in crop rotation

Crop	Hkg/ha	Kg DM/ha
Clover grass, 2 years		6000*
Clover oats	50
Potatoes, marketable	180
Spring rye	30
Field pea	30
Spring wheat	35

*18 per cent protein

Back to the organic roots

The crop rotation is to some extent an example of the organic 'school' that celebrates the principle of fertilising the soil rather than the crop. In the model outlined, 30 tonnes of composted garden/park waste is applied in three out of seven years, corresponding to an average input of 93 kg total N per hectare per year.

As the nitrogen is released over a number of years, it requires a lot of management and craftsmanship to retain it in the cultivation soil and avoid leaching.

Weeds are controlled in early spring and with catch crops

The management requirement is handled by only opening a single window of black soil in the autumn if and when there is a particular need to control root weeds. In this example, it is after harvesting potatoes. The rest of the time, the soil is covered with either a crop, cover crop or green manure, which also contributes greatly to weed control through competition.

Ideally, control of root weeds, especially chickweed, is most effective in early spring when there is still night frost and low humidity. The opportunity to keep weed pressure at an acceptable level should be taken before resorting to black soil in autumn.

Next steps: economy and climate

The Innovation Centre for Organic Farming has conducted a survey among organic consumers that shows that farmers cannot immediately expect a premium price for crops grown without any animal input. The contribution margin in a purely plant-based crop rotation is therefore challenged by the fact that two out of seven years in the rotation are clover grass, which has no income from crop sales. The next step in describing organic crop production without input from livestock production will be to calculate the economics of this and several different intermediate scenarios.

It is also necessary to focus on the effects of a higher proportion of organically bound nitrogen in the fertilisation plan. This can affect nitrous oxide emissions from the field. The Innovation Centre for Organic Farming has several ongoing trials measuring emissions from different organic farming systems.