Integrated soil fertility management seeks to improve crop yields, while preserving sustainable, long-term soil fertility through the judicious use of fertilisers, recycled organic resources, responsive crop varieties, and improved agronomic practices.
Together, these measures minimise nutrient losses and improve the nutrient-use efficiency of crops.
Integrated soil fertility management includes knowing how to adapt these practices to local conditions to maximise efficiency of the applied nutrients and improve crop productivity.
All inputs need to be managed following sound agronomic principles. Nature-based solutions are defined as actions to protect, sustainably manage and restore natural or modified ecosystems, and address societal challenges, simultaneously providing benefits for human well-being and biodiversity.
Nature-based solutions mimic natural processes that rely on ecosystems functioning to ensure food and livelihood security, healthier diets and more inclusive rural economies.
They can improve soil fertility and soil-nutrient content through different approaches, including the capture of atmospheric nitrogen through biological nitrogen fixation (BNF). Currently, symbiotic plants such as legumes are utilised, but at least 13 genera belonging to the prokaryote group are known to fix nitrogen.
BNF is one of the main ways farmers can avoid the over-use of synthetic nitrogen fertilisers. Indeed, more than 60% of the fixed nitrogen on the planet results from BNF.
Optimising BNF in agriculture is therefore urgently needed to help meet the demand of the world’s growing population for food.
Nature-based solutions also include the solubilisation of precipitated forms of phosphorus to increase its availability for plants. This is done by harnessing soil biodiversity and using micro-organisms to recover soil and water contaminated through the excessive use of fertiliser or carrying hazardous elements.
There is a wide range of technological-oriented solutions for better management of nutrients and fertilisation, including the use of sensors, variable rate applicators, nitrification inhibitors and modelling. The advantages range from a better diagnosis of deficiencies with sensors, a more efficient application in terms of dose, time, and application methodology, to alternative sources of nitrogen that reduce losses and increase efficiency of use.
The modelling tools contribute to a better diagnosis of the behaviour of the nutrients added through fertilising, and reduce the risks of contamination and eutrophication (which
occurs when the environment becomes enriched with nutrients, increasing the plant and algae growth in estuaries and coastal waters).
Sensor technology is useful in soil monitoring. Proximal soil sensing (where the sensor is close to, or in direct contact with, the soil) is a multidisciplinary approach that involves instrumentation, data science, geo-statistics, and predictive modelling.
The integration of these disciplines has allowed successful sensor application for the diagnosis of soil fertility attributes. Advances in sensors compatible with on-line measurement systems and portable sensor systems complement each other well and have proven useful.
Spectroscopy is the study of the interaction between matter and electromagnetic radiation.
Visible and near-infrared spectroscopy (Vis-NIRS) and mid-infrared spectroscopy (MIRS) have been used successfully to determine total soil nitrogen.
Modelling and management of spectral data are important steps for the development of techniques such as variable rate nitrogen fertilisation. That is, the application of the correct rate of nitrogen fertiliser in the right place at the right time using advanced precision-agriculture technologies.
Vis-NIRS and MIRS techniques have increased sensor cost-effectiveness and accuracy not only in the case of soil nitrogen, but also in the determination of different aspects of soil analysis directly affecting soil fertility, such as particle sizes, aggregation, surface roughness, and water content.
Optical sensors have proven to be useful tools in improving nitrogen-use efficiency and applying the appropriate dose of nitrogen and phosphorus fertiliser in different regions of the world, thereby helping to achieve the maximum expected yield.
A drawback of these strategies is the costs. However, technological advances have made possible the development of smaller, cheaper and portable equipment, and have increased the profits of farmers and significantly reduced emissions of, especially, nitrous oxide, the most important greenhouse gas after methane and carbon dioxide.
Even so, the adoption of this technology in subsistence farming would be complex due to cost, training, technical support and the lack of studies on the performance of the sensors in ‘hillside agriculture’.
That said, ’technified agriculture’ (farming using these types of advances) has been shown to generate savings for producers and reduce the addition of nitrogen without affecting yields.
Enhanced-efficiency fertilisers have the potential to reduce nitrous oxide emissions and improve crop productivity.
A meta-analysis of 43 studies on different continents found that nitrification inhibitors, double inhibitors (urease plus nitrification inhibitors) and controlled-release nitrogen fertilisers consistently reduced nitrous oxide emissions compared with conventional nitrogen fertilisers.
Bio-stimulants can be used to improve soil fertility and include humic and fulvic acids (both formed when plants break down, and found in the humus), amino acids and peptide mixtures.
Other nitrogen molecules considered bio-stimulants include betaines, polyamines and non-protein amino acids. These are diverse in the plant world, but their beneficial effects on crops are poorly characterised.
Promoting plant growth
Algae and plant extracts, chitosans (a sugar that comes from the outer skeleton of shellfish) and other bio-polymers (bio-degradible natural polymers produced by the cells of living organisms) are also used as bio-stimulants.
In addition, there are inorganic compounds considered beneficial elements, because they promote plant growth and may be essential for some species.
Advantages of bio-stimulant application include the improvement of efficiency in the absorption and assimilation of nutrients, and tolerance to biotic or abiotic stresses.
Bio-stimulants could complement and, in some cases, actually replace chemical products and improve plant metabolism and biochemical activities.
A variety of microbes can perform as plant bio-stimulants, improving the efficiency of nutrition, crop quality, plant growth, and tolerance to abiotic stress.
More research needed
However, bio-stimulants are relatively new products. Their regulation is not yet completely clear and non-existent in some cases, which may lead to the marketing of low-quality products. We need regulation and quality assessment.
More research is required to fully understand the mechanisms and functioning of bio-stimulants in crops and soils.
In addition, price is a factor that could hinder the research and adoption of bio-stimulants, as some of these can be more expensive than certain types of fertilisers.
The combination of the use of beneficial and environmentally friendly micro-organisms for agricultural production such as phosphorus solubilisers and nitrogen fixers, together with inorganic fertilisers, is an increasingly important area of research aimed at developing microbial formulations that enhance the benefits of mineral inputs and reduce negative effects on the environment.
Since most agricultural systems are nitrogen- and phosphorus-limited, this approach is likely to be of global interest. Evidence shows even a single polymicrobial inoculation can have positive effects on agricultural productivity.
The views expressed in our weekly opinion piece do not necessarily reflect those of Farmer’s Weekly.
This is an excerpt from a report titled ‘Soils for nutrition: state of the art’, released by the Food and Agriculture Organization of the United Nations in July 2022.