Fertilization

Scientists have found that plants grow better with the help of some soil bacteria, named plant growth-promoting bacteria or rhizobacteria; from the Greek ‘rhiza’ meaning roots. Such bacteria form symbiotic associations with plant roots and enhance nitrogen and mineral fixation by plants. Rhizobacteria may thus be spread onto crop soils to enhance yields, but field experiments are so far rarely successful. Agronomists Berger et al. demonstrate that Kosakonia radicincitans promotes radish growth both in pots and field conditions.

Legumes are plant species that can fix atmospheric nitrogen, and thus legumes do not need costly and polluting fertilisers. Such an advantage is provided by special bacteria that live in small, round homes, named nodules, which are built by the plant in the roots. Basically, plants provide the home and food for bacteria, and, in return the bacteria transform atmospheric nitrogen into plant-edible nitrogen. Plant physiologists Voisin et al. study hypernodulating legumes, that are plants that have more homes to host bacteria, to design new species needing less fertilisers.

Classical fertilisers are expensive and are polluting waters when applied in excess. Therefore scientists are actually seeking sustainable alternatives such as biofertilisers. Megali et al. show that research reports on ‘effective microorganisms’, a commercial mixture of bacteria and yeast, conclude on an overall positive effect of this biofertiliser on crop yields. Nevertheless, effective microorganisms also induces higher vulnerability to insect attack in cornfields.

Adding the right amount of nitrogen fertilisers to crop soils is not an obvious task. Too much fertiliser will induce nitrate pollution and extra costs for farmers. Too little will decrease crop yields. Therefore agronomists have developped the nitrogen dilution curve, a simple math tool that helps farmers to calculate the best fertiliser amount to be applied. Reyes et al. show that the nitrogen dilution curve can be also used to manage grasslands.

Organic wastes from agriculture and food industry are increasingly digested then recycled as cheap soil fertilisers. However, waste digestion and spreading is a potential source of greenhouse gases. The agronomist K. Möller reviews the consequences of waste anaerobic digestion on soil fertility and nitrogen compound emissions.

Mineral fertilisers containing nitrates and ammonium are almost always applied in excess in crop fields. As a consequence soil waters and rivers are often contaminated by fertilisers, which induce pollution issues such as undrinkable water and unexpected algal blooms. Benckiser et al. review ecological techniques such as nitrogen fixation that recycle nitrogen within the soil, thus decreasing water pollution by nitrates.

Nitrogen (N) is a major element that plant need for growth. Legumes are nitrogen-rich plants that are named ‘green fertilisers’ because legume residues enrich the soil in nitrogen after plant death. This fertilising effect is actually assessed by measuring shoot N concentrations, which does not take into account root contribution. Agronomists Carranca et al. indeed found that roots contain  7 – 11% of total legume N. The power of legumes as fertilisers for the next crop is thus underestimated.

Crop yields depend on many factors such as fertiliser amount, rainfall and temperature. In order to better understand which factors rule crop yields, soil scientists Wang et al. analysed the Chinese Statistical Yearbooks including data of the major grain-producing regions of China from 1993 to 2013. They found that current fertilising strategies should be improved by taking into account regional climate conditions.

Classical fertilisers have a low efficiency becasue only a minor part of the fertiliser reaches plant roots. As a consequence a major part of classical fertilisers escapes rapidly toward groundwater, thus polluting drinking water with nitrates. Research has therefore invented controlled-released fertilizers to feed the plant slowly in the long run. Controlled-released fertilizers include polymeric material – a kind of plastic – to slow down fertiliser feeding. In other words controlled-released fertilizers are comparable to pasta that provide energy slowly in the long run, whereas classical fertilisers are similar to sweets that provide high energy fast. Agronomists Wang et al. have evidenced improved nitrogen uptake and reduced nitrogen loss using a polymer-coated urea fertiliser to grow flooded rice in southeast China.

Nitrogen (N) fertilisation is essential for crop and food production. However actual N fertilisation is not often very efficient and can induce pollution, e.g. by nitrates. There is therefore a need for indicators to compare farming systems. Scientists Godinot et al. developed a new indicator named ‘relative N efficiency’, which is specifically well suited to compare farming systems with different proportions of animals and crops.