The problem of nitrogen assimilation in organic farming

Organic fertilisers contain different organic compounds which are broken down by the microflora into nitrate in various stages with the production of polypeptides, amino acids, amides, ammonia, nitrite and nitrate. The polypeptides have a more or less long life span and are the main source of the nitrogen reserve in the soil.

Organic fertilisers, which should not be confused with organic amendments, have little effect on soil humification. Their main objective is to provide nutrients to crops, whereas organic amendments aim to improve the biological properties of the soil (microbial biodiversity, CAC, humic reserve, etc.). Some organic fertilisers sold in garden centres contain a balanced proportion of various organic elements to both maintain the soil and feed the plants. Whatever their classification, all organic fertilisers have the disadvantage of being difficult to manage optimally during the growing season. Why is this?

Origin of nitrogen losses in organic farming.

Nitrogen is the major element that is subject to the highest losses during the input assimilation process (1). Phosphorus and potassium are less subject to losses when the soil is rich in CAC. Phosphate and potassium ions are fixed by CAC, while nitrate is not. In addition, phosphorus salts tend to block with other substances such as iron when their soluble form is in excess, which creates a notorious problem in agriculture, as phosphorus is then unavailable. One can end up with a phosphorus deficiency even though the soil is rich in this element blocked in the soil.

The main driver of the increase in atmospheric nitrous oxide comes from intensive agriculture with indiscriminate use of mineral or organic nitrogen fertilisers. Emissions from synthetic fertilisers dominate the releases in China, India and the USA, while emissions from the application of manure as fertiliser dominate the releases in Africa and South America. Europe is the only region in the world that has managed to reduce nitrous oxide emissions over the last two decades (3), probably due to new, more economical methods of applying mineral fertilisers in conventional agriculture. This study shows the weaknesses of an agriculture built only on organic inputs, as shown by other observations in France where there are areas of abundant spreading of organic fertilisers which produce harmful effects on the environment. This has long been the case in Brittany, where slurry and manure from intensive pig farms are used, causing the proliferation of green algae on the coast (4), even though significant progress has been made since 1970 to reduce effluents.

In both organic and conventional farming, the risks of releasing nitrous oxides into the atmosphere (a greenhouse gas 300 times more potent than carbon dioxide) and of nitrate leaching should not be underestimated. For this reason, the specifications for organic farming limit the use of organic fertilisers from the farm (compost, manure, green manure) or manufactured by specialised companies, to 170 kg of organic nitrogen per ha/year (imposed by European regulations for all forms of cultivation) (a) (2).

Why nitrogen management is more difficult in organic farming!

Precise nitrogen management is a well-known problem for organic farmers, which, unlike their conventional counterparts, is more difficult to organise. In integrated farming, it is much easier to manage nitrogen requirements and reduce losses, as a precise application of industrial fertiliser containing only this element is allowed.

Because nitrates are highly soluble in water and cannot be fixed by CACs, they are easily washed away when not taken up by plants and eventually end up in the water table. The higher the nitrogen content of an organic fertiliser, the more nitrate losses are possible. It should also be taken into account that the mineralisation of organic fertilisers depends on factors that are often uncontrollable (temperature, humidity, etc.). A nitrogen reserve can therefore develop faster than expected, exceeding the needs of the plants. The reverse is also true, for example, following an unexpected drought. Watering may not be sufficient if the temperature is too low. During a growing season, this type of problem is easily encountered, as evidenced by a laboratory analysis of nitrates in the soil.

Too much nitrogen fertilisation also has an impact on the pest communities of the crops. For example, the parasitism rate of diamondback moths is lower when the soil is less fertilized (5).

Nitrates can also be reduced by bacteria to atmospheric nitrogen (6), thus reducing soil fertility. In principle, nitrogen losses should be lower when the soil is well supplied with organic matter and clay, which are necessary to form CACs that have the property of fixing ammonium ions. However, a soil well supplied with organic matter is also rich in bacteria that rapidly convert ammonium ions into nitrates. For this reason, the addition of CAC to the soil is not a sufficient solution to limit nitrogen loss in organic farming. CAC slows down the loss of nitrogen, but cannot prevent it in the case of excess nitrogen inputs.

In periods of rapid crop growth, if nitrogen reserves are not sufficient and no external nitrogen supply is decided, in the medium term the soil will have a deficit of assimilable nitrogen, resulting in lower crop yields, loss of quality and the occurrence of deficiency diseases. In organic farming, this situation is common, for example, in wheat, where low nitrogen fertilisation results in a reduction in crop density and disease awareness.

Some vegetable crops such as lettuce have an intense need for nitrogen during a short period of their growing cycle. It is very difficult in organic farming to meet this intensive need for nitrogen from organic bottom dressing.
Nitrogen deficiency can of course be reduced by nervous organic fertilisers (such as blood powder) which contain mainly nitrogen, but their cost, while still acceptable to a home gardener, is no longer acceptable to a market gardener.

Nitrogen management: organic fertilizers for home gardeners.

Amateur gardeners can find organic fertilisers rich in guano, blood, bone or feather meal, granulated hair, castor oil cakes, etc. in garden centres. These organic fertilisers provide a nitrogen reserve that is largely exhausted 6 months after spreading, ranging from 93% for guano to 66% for meat bone meal and castor oil cake (7). Of course, these organic fertilisers also present a risk of environmental pollution which is not negligible for the same reasons as mentioned above for field crops.

Some remarks on nitrogen management on crop and livestock farms.

For crop and livestock farms, managing the amount of nitrogen needed is less of a problem and much less costly if the farmer has sufficient livestock and grassland. In a grassland, atmospheric nitrogen is fixed by bacteria and proteobacteria such as Azospirillum that colonise the rhizosphere of the grasses. The other elements (phosphorus, potassium, trace elements, etc.) found in manure come from the decomposition of the parent rock and the recycling of crop residues. For those who do not have enough livestock, one solution is to recover ammonia-rich liquids from industrial methanisation processes or installed on farms. These organic fertilizers must meet environmental safety standards.

Crop and livestock farms with grassland are often presented as the future solution for sustainable agriculture. However, in France, organic farming is increasingly developing without a livestock component. In 2011, 66% of organic farms had no livestock (8), which means that they have to use organic fertilisers from conventional agriculture. On the other hand, the potassium and phosphorus reserves in cultivated soils are mainly derived from mineral fertilisers before the conversion of farms to organic farming. These reserves will be depleted if losses are not compensated for by balanced organic inputs in sufficient quantities, by growing legumes or by importing industrial fertilisers.

Finally, the mechanisms for compensating for mineral losses on crop and livestock farms are more or less long depending on the nature of the soil, the climate and the season, with losses at each stage. For this reason, extensive permanent grasslands in conventional agriculture receive mineral fertilisers to increase fodder yields in order to obtain an average export of 200 kg of nitrogen per hectare per year (9) (grasslands sown with grasses and legumes also receive fertiliser at the end of winter (10)).

Without this fertiliser, these grasslands, which are often areas with low agronomic potential, would be much less profitable. It must be taken into account that a meadow must be maintained, which represents a cost in terms of time and financial means for the farmer (b). Finally, not all organic farmers run a mixed farm, or have enough animals and grassland to produce all their manure needs. They then call on other non-organic farmers to supplement their manure needs; manure that is therefore not organic; a practice that is very common in organic farming.

The difficulty of compensating for nitrogen losses with farmyard manures.

When the organic farmer has enough manure, he is faced with another problem related to the nature of manure itself, which is known to be very uneven in nitrogen, potash and phosphate, with the ratio of these elements varying according to the source (see table below) and after composting. By forcing the amount of manure to be increased in order to have more of the missing elements, the farmer often ends up with an excessive supply of the other elements, creating an imbalance in the fertilisation. There is a solution to this problem: growing legumes, but this has consequences for the profitability of the farm.

Legume crops are the only economically viable source of nitrogen that does not produce excess potash and phosphates. Enough legumes are needed in the crop rotation to compensate for nitrogen production. The cultivation of these legumes leads to a lack of profitability for exportable crops. While the organic farmer cannot produce a usable crop from his field mobilised by the legume crop, in conventional agriculture where nitrogen losses are corrected by chemical fertiliser, the same area is used to produce an exportable and saleable crop. In the end, there is a significant loss of yield in organic farming.

Of course, the amateur gardener is faced with the same difficulties if he decides for ideological reasons not to use synthetic fertilisers, except that in relation to the surface area he cultivates, the solutions available in garden centres, such as dried blood, are more within his means. An amateur gardener can afford to be a fervent supporter of all-natural methods by investing in expensive solutions that are still within his reach, but which a professional gardener will not be able to follow for financial reasons. What is possible on a small scale is not necessarily possible on a large scale; a situation often encountered in organic farming that is never mentioned in TV gardening programmes that focus on organic farming.

Benchmark values for the composition of some manures, slurries and droppings :

Source: extract from a table in Web agri - integrating the fertilising values of manures accessible here.

a) On crops, in vulnerable and non-vulnerable zones :
- The average amount of organic nitrogen applied per year and per hectare of arable land is a maximum of 115 kg
- The total annual nitrogen input (organic + mineral) per hectare is a maximum of 250 kg
- The average amount of organic nitrogen from the farm may not exceed 170 kg per ha/year

b) Mossing (removal of moss), chipping (spreading of dung), liming (addition of limestone), sowing (spreading of mole mounds), overseeding (elimination of bare areas), mowing of refusals (non-consumed plants) ... are the most frequent maintenance operations. Without good maintenance, grasslands end up deteriorating, characterised by a transformation of the flora, the appearance of diseases affecting forage grasses with yield losses, excessive growth of undesirable plants (thistles, nettles, buttercups, etc.) or low productivity plugging the gaps caused by predators, etc.