Fertilization

Food production would not be sustainable without earthworms. Indeed, earthworms are major soil engineers. For instance earthworms transform organic residues into plant nutrients, and facilitate groundwater flow by tunnelling earth. Agronomists Bertrand et al. review the ecological services of earthworms, focussing on the effect of tillage, fertilisation and pesticide usage.

Conventional mineral fertilisers are costly, they have a low efficiency because only a minor part of the fertiliser reaches cultivated plants, and they often pollute water, e.g. nitrate and phosphate pollution. Scientists Grassi et al. show that seagrass Posidonia oceanica can be used directly or after composting to fertilise organic crops.

Natural biochars are alternative fertilisers that could solve three major world issues : the increasing cost of mineral fertilisers, water pollution by excessive use of mineral fertilisers, and the global warming due to carbon transfer from soils to atmospheric CO₂. Biochars and carbon-rich solids formed by pyrolysis of biomass such as crop residues. Biochar can be used for soil fertilization, carbon sequestration and improvement of soil structure. Sigua et al. reported that addition of sorghum biochars increased total biomass of winter wheat by about 31% over the control plants. Their findings suggest that pyrolytic transformation of sorghum residues into sorghum biochars is a better strategy for both environmental and crop productivity improvement.

Biochar is a black solid created by pyrolysis of biomass such as crop residues and other agricultural waste. Research on Terra Preta soils in Amazonia suggest that biochar could improve soil properties and crop yield. However, we know little on the performance of mixture of biochar and fertilizers under European conditions. Glaser et al. performed field experiments in Northern Germany. They found that biochar addition to mineral fertilizers, biogas digestate or compost increase maize yield from 20 to 42%. Their findings demonstrate that biochar-fertiliser combinations have a better performance than pure fertilizers, in terms of yield and plant nutrition.

Manures and slurries contain recyclable crop nutrients such as nitrates (NO₃^–). However, field application of an excess of manures and slurries often leads to air pollution by ammonia (NH₃) and nitrous oxide (N₂O), a greenhouse gas. Therefore there is a need to optimise spreading techniques to decrease undesired pollution. Agronomists Langevin et al. designed a model that simulates the effects of spreading techniques on NH₃ and N₂O emissions. They found that taking into account soil compaction should decrease environmental impacts.

Agriculture is actually facing three major issues. The first issue is to produce enough food for the ever growing world population. The second issue is the increasing cost and use of mineral fertilisers. The third issue is the huge amounts of organic waste produced by human activities. Insects could solve those issues. Indeed insects can be used as food and feed. Also insect larvae often feed on decaying organic matter in nature. A such process can thus be used to transform organic waste, such as compost, into food and feed. Moreover, the compost residue can be used as plant fertiliser. Lalander et al. used black soldier fly to transform compost into biomass. They observed a high waste conversion and a reduction of microbial diseases.

Broadcast application of cattle slurry to grassland soils is a natural means of fertilisation. However this practice is actually not optimised and leads to nitrogen loss to the atmosphere and groundwater. Novel application techniques such as injection into the soil are therefore tested. Kayser et al. found that the type of application technique has no effect on nitrate leaching. Whereas the amount of applied slurry has an effect on nitrate leaching.

A catch crop is a crop planted between two regular crops grown in successive seasons or between two rows of crops in the same season. Catch crops are used in particular to reduce soil erosion and fertiliser leaching that occur when the soil surface is not planted. For instance nitrogen catch crops feed on nitrates and thus recycle soil nitrogen and decrease water pollution by nitrates. Catch crops are therefore a way to reduce the use of costly fertilisers. Tuulos et al. show that winter turnip rape, an oilseed crucifer, is an effective nitrogen catch crop adapted to the Nordic climate.

Rice is a major food worlwide. The cost of rice fertilisation using mined fertilisers such as nitrogen (N), phosphorus (P), and potassium (K) amounts to 30-35% of the cropping cost, and is still increasing. This issue could be solved by replacing mined fertiliser by biofertilisers that contain microbes helping the plant to grow, so-called ‘plant growth-promoting microorganisms’. The report by Rose et al. indeed show that inoculant biofertilizers can replace up to 52% of mined nitrogen fertiliser without loss of yield.

Today half of the world population would not be alive without the 68 million tons of nitrogen (N) fertiliser applied to agricultural soils yearly. However, synthetic N has become too much of a good thing because most applied N is not used by crop plants and escapes to pollute groundwater as nitrates and atmosphere as nitrogen oxides. An alternative and cheap solution is to return plant residues to the soil, thus producing ‘natural’ N by plant residue decomposition. Chen et al. reviewed the effects of crop residues on soil N behaviour to better understand the mechanisms of transformation, and to predict farming practices that optimise natural fertilisation.