Farming Without Fertiliser — Is It Possible?

Over the coming period, we at WUR want to contribute to these discussions about the dilemmas and major challenges with regard to sustainable agriculture and food supply. In this way, we want to explain the current themes as well as why these were chosen, what the possible solutions may be, and how they are being worked on. The first conversation is with Pieter de Wolf, Farm of the Future project leader and agronomist by trade who has worked on nutrient cycles and mixed systems throughout his career. We will discuss fertiliser, specifically artificial nitrogen fertiliser.

Fertiliser comes in many forms, such as compost, animal manure, artificial fertiliser, and human excrement, and it is required for the growth of plants (including grass). Plants mainly need nutrients to grow, and they are removed when the product is harvested. In terms of quantities, the macro-nutrients nitrogen, phosphate, and potash are the most important, but meso-nutrients and micro-nutrients (such as magnesium, manganese, boron) are also important for the growth of plants. For a long time, fertiliser was one of the scarcest production resources and was therefore the primary limitation on the growth of food production. The invention of artificial nitrogen fertilisers by way of the Haber-Bosch process in 1910 lifted this limitation for nitrogen. However, this does not apply to phosphate, which is obtained through mining. The amount of phosphate on Earth is finite.

Why do we use artificial fertiliser in the Netherlands? Isn’t there a lot of animal manure?

In fact, the Netherlands has a nitrogen deficiency, even though we have a lot of animal manure. If you add up the fertilising needs of all the crops, then you need more nitrogen than the amount available from animal manure. The use of nitrogen from animal manure has been limited to 170 kg N/ha by EU regulations. Many crops need more than 170 kg N, and are permitted by law to receive this. This is then supplemented with artificial nitrogen fertiliser. This is required for the current levels of production, and it is allowed.

The system is there for a reason: particularly in arable farming and field production of vegetables (with annual crops), the environmental deficit would increase significantly if all the nitrogen used was in the form of animal manure. Nitrogen from animal manure is slowly released through the mineralisation of organic matter, a process that also continues after harvesting crops. Fertilisation is more efficient and has lower losses by using less animal manure and supplementing this with artificial fertiliser that can be immediately absorbed. This is different for grass, which is a perennial crop that continues to absorb nitrogen for longer during the season.

It is the combination of organic and mineral nitrogen that contributes to efficient fertilisation with minimal losses. That mineral nitrogen is usually applied in the form of artificial fertiliser, although types of animal manure that contain relatively high amounts of mineral nitrogen are also available. The use of artificial fertiliser has a practical reason as well: animal manure must be worked into the soil (because of ammonia emissions), but this would lead to damage to the crops during the cultivation. Artificial fertiliser (in granular or liquid form) does not need to be worked in and can therefore be applied to crops during the cultivation without damaging them. As mentioned before, the combination of animal manure and artificial fertiliser can satisfy the crop requirement even if it is over 170 kg.

How does this work in organic farming, where artificial fertiliser is not used?

The use of artificial fertiliser is indeed not allowed in organic farming, and the European limit of 170 kg N/ha from animal manure still applies. This means that organic farming cannot add as much nitrogen.

This more flexible crop rotation is possible as a result of the higher financial gains from organically cultivated crops. Leguminous plants are often used to bind nitrogen as well, which means that there is more nitrogen available for the next crops. That nitrogen is not counted on the balance as it is calculated based on the supply of manure, and as a result, the calculated nitrogen efficiency seems better. If an organic farm optimises its choice of crop, crop order, fertilisation, and use of green fertilisers/catch crops, then the nitrogen efficiency does not need to be lower than on a regular farm.

There is a lot to do about nitrogen, but what about the usage and loss of phosphate?

The story is different for phosphate (P). We use relatively little artificial phosphate in the Netherlands: about 3% of the Dutch P imports consist of P fertiliser. For the most part, it can be explained by the other part of the P import: about 60% consists of P in raw materials for animal feed (in addition to import as feed). In so far this is not exported, this will become available in the Netherlands through animal manure. See the report "Mest en metropolen" (manure and cities).

In practice, Dutch farmers mainly complement the phosphate requirement of their crops with animal manure. If there is less animal manure, then arable farmers will use more artificial phosphate as alternatives (compost, human excrement) are not yet sufficiently available for agriculture. Greenhouse horticulture mainly uses artificial phosphate (cultivated on substrate or water). Early crops may sometimes need artificial phosphate as well, because phosphate is not available in the soil at low temperatures.

A lot of artificial fertiliser is used to supply the Netherlands with animal feed. Raw materials for animal feed, such as grains, rapeseed or soybean meal, come from regions inside and outside Europe, where cultivators also use fertilisers in line with the crop requirements. In these regions (large-scale arable farming areas), there are not many livestock farmers and animal manure is not available. As the Netherlands imports large amounts of animal feed, we have so much animal manure available that some people think that we could do without artificial fertiliser.

Is it possible to produce sufficient food without the use of artificial fertiliser?

It is important to make the distinction between phosphate and nitrogen. Unlike nitrogen, phosphate is stable, non-volatile, and difficult to dissolve. Phosphate losses are minimal, while nitrogen leaks away everywhere, in gases (ammonia, nitrous oxide) and in dissolved form (nitrate). In principle, you could close the cycle for phosphate if you managed to recover all phosphate that leaves the agricultural cycle through animal and plant products. The biggest challenge is to organise the return stream from the consumer/city, as that is where most of it is lost. As long as this is not the case, you need a supplement and artificial phosphate must be added, in the Netherlands or abroad, if you import phosphate through animal feed.

For nitrogen the losses are much greater, even if you do whatever you can to limit them, and you also need an external supplement if you want to keep the production at the same level. This can be supplemented in two ways: by using nitrogen-fixing crops, such as clover, alfalfa, and legumes, which can fix nitrogen from the air through a symbiosis with rhizobium bacteria. This form of nitrogen supply requires acreage, at the expense of other crops. If you want to fully compensate for nitrogen removal through nitrogen-fixing crops, you will need an estimated 20-25% of your crop plan for leguminous plants. The second option is to use artificial nitrogen fertiliser. This costs energy instead of acreage. That is ultimately the question: if you don't want to use artificial nitrogen fertiliser, you have to free up a large part of the acreage for the cultivation of nitrogen-fixing crops. In theory, all nitrogen added from artificial fertilisers and animal manure can be replaced, but that results in a phosphate deficiency, because these crops do need P. This phosphate must therefore be supplemented from an external source — either from human excrement or from artificial fertiliser — if you want to replace the supply of animal manure with leguminous plants.

Recently there was an article about a “vegan farmer” in Flevoland, who only used vegetable compost. Is that possible?

It is possible for a few companies: an individual company can work in this way due to nitrogen fixation with its own crops and supply using clippings from nature reserves. You are often talking about large areas of natural land (dozens of hectares) that are needed for one company. The amount of clippings from Dutch nature reserves is only sufficient for a few companies. This also applies to the amount of green and GFT compost: there is nowhere near enough compost available to meet the needs of land-based agriculture, from the standpoints of both nutrients and organic matter.

The main reason that it cannot be done everywhere is that an intensive, highly productive agricultural system (high output) also has a high need (high input). This applies to nutrients and to organic matter. The accumulation of organic matter is relatively low in intensive crops, which leave little crop residue and require intensive tillage, such as potatoes, onions, and flower bulbs.

Companies with an intensive farm plan need a relatively high external supply of organic matter in order to achieve a positive organic matter balance. Currently, cattle manure is actually a very favourable type of fertiliser for arable farmers: the ratio between nitrogen and phosphate matches the crop requirements well and it contains both organic and mineral nitrogen as well as a relatively large amount of organic matter. The biggest disadvantage of cattle manure lies with the dairy farmer: the emissions in the barn (ammonia) are mainly caused by the combination of faeces and urine.

Shifts often tend toward a higher scale level. You can ban artificial fertiliser as a farm, region, or even as a country, while at the same time externally supplying the minerals via animal feed. This kind of artificial-fertiliser-free company or artificial-fertiliser-free region can only exist if suppliers use artificial fertiliser. The other option, which also occurs, is the use of grass or other streams obtained from nature reserves. It is often a conscious policy to dispose of grass clippings in those settings.

Can legumes play an even greater role than they do now in the supply of nitrogen?

Legumes can indeed play a greater role in the supply of nitrogen. For example, combining leguminous plants with other crops in mixed crops would work. The best known is grass-clover, but there is also increasing interest in mixtures of grain and field beans or peas. Furthermore, leguminous green fertilisers are an option, often as part of a mixture.

At the Farm of the Future we are also looking at how we can make optimal use of leguminous plants in the farm plan. The greatest risk of too many legumes is in the accumulation of soil fungi and nematodes, which not only cause damage to the legumes themselves, but often to other crops as well. There is also increasing interest in leguminous plants in the cultivation plans abroad as well, which now often consist of grains and rapeseed.

The trouble is that these crops often yield less than the other crops, that other machines are needed and that there are no good sales opportunities yet.

What other developments are there in this area?

There are all kinds of plans to produce artificial fertiliser substitutes from animal manure. These substitutes then have the status of “artificial fertiliser” and do not need to be incorporated, may be applied above 170 kg/ha, and are available in liquid or granular form. I am sceptical about that: I understand that this will solve the problem of livestock manure in livestock farming, but the big question is how much energy (and chemistry) it takes to separate mineral nitrogen from organic matter, to concentrate it, or even to produce it entirely in dry, granular form. That could be close to or higher than the production of artificial nitrogen fertiliser via the conventional Haber-Bosch process.

Fertiliser is traditionally a livestock farming theme. You can see this again in the contours of the new fertiliser policy: livestock farming and the sale of fertiliser are the focal points, not use of the fertiliser itself. This is also striking in the movement around manure processing: these initiatives are primarily intended to add value to animal manure. I still think that a classic fallacy is being asserted here: that livestock farmers have to pay money to get rid of their manure is not so much a problem with the quality of the manure, but with the quantity. There's simply too much. My suspicion is that all manure processing factories will decrease in value if livestock farming (and therefore manure production) is reduced. Then we can simply sell unprocessed manure to arable farmers.

Furthermore, dairy farms are becoming increasingly land based, so that all the manure produced can be used to fertilise their own land. For arable areas such as Zeeland, Flevoland, the peat region, and the northern clay region, this means that the manure supply is drying up. How do you get your nutrients?

It is also important to focus on the recovery of nutrients from human waste flows. Phosphate (in the form of struvite) is now being recovered on a limited scale, and developments are also under way for nitrogen. For organic matter, the potential decrease in the external supply via animal manure may pose a challenge. At the same time: organic matter is digested plant material. In other words, plants make organic matter and animals just convert it. For arable farmers this means that they have to organise the organic matter accumulation more on their own farms, such as by including more dormant crops in the cropping plan, growing more green manures/catch crops, and leaving crop residues on the land.

This immediately undermines the basis of a sustainable cultivation system, specifically healthy, fertile soil.

From an agricultural point of view, you could argue that we also need to extensify cultivation systems a bit. Slightly more dormant crops and slightly fewer harvested crops lead to a lower nutrient requirement, a more positive organic matter balance, and better soil structure. At the same time, this is very expensive from a financial point of view, and not realistic in view of the rising land prices in the Netherlands. That is, unless the land market is restructured and compensation is paid for sustainability. But that's a topic for another time.