Agriculture must, literally, return to its roots by rediscovering the importance of healthy soil, drawing on natural sources of plant nutrition, and using mineral fertilizer wisely
Soil is fundamental to crop production. Without soil, no food could be produced on a large scale, nor would livestock be fed. Because it is finite and fragile, soil is a precious resource that requires special care from its users. Many of today’s soil and crop management systems are unsustainable. At one extreme, overuse of fertilizer has led, in the European Union, to nitrogen (N) deposition that threatens the sustainability of an estimated 70 percent of nature1. At the other extreme, in most parts of sub-Saharan Africa, the under-use of fertilizer means that soil nutrients exported with crops are not being replenished, leading to soil degradation and declining yields.
How did the current situation arise? The main driver was the quadrupling of world population over the past 100 years, which demanded a fundamental change in soil and crop management in order to produce more food. That was achieved thanks partly to the development and massive use of mineral fertilizers, especially of nitrogen, since N availability is the most important determinant of yield in all major crops2-5.
Before the discovery of mineral N fertilizers, it took centuries to build up nitrogen stocks in the soil6. By contrast, the explosion in food production in Asia during the Green Revolution was due largely to the intensive use of mineral fertilization, along with improved germplasm and irrigation. World production of mineral fertilizers increased almost 350 percent between 1961 and 2002, from 33 million tonnes to 146 million tonnes7. Over the past 40 years, mineral fertilizers accounted for an estimated 40 percent of the increase in food production8.
The contribution of fertilizers to food production has also carried significant costs to the environment. Today, Asia and Europe have the world’s highest rates of mineral fertilizer use per hectare. They also face the greatest problems of environmental pollution resulting from excessive fertilizer use, including soil and water acidification, contamination of surface and groundwater resources, and increased emissions of potent greenhouse gases. The N-uptake efficiency in China is only about 26-28 percent for rice, wheat and maize and less than 20 percent for vegetable crops9. The remainder is simply lost to the environment.
The impact of mineral fertilizers on the environment is a question of management – for example, how much is applied compared to the amount exported with crops, or the method and timing of applications. In other words, it is the efficiency of fertilizer use, especially of N and phosphorus (P), which determines if this aspect of soil management is a boon for crops, or a negative for the environment.
The challenge, therefore, is to abandon current unsustainable practices and move to land husbandry that can provide a sound foundation for sustainable crop production intensification. Far-reaching changes in soil management are called for in many countries. The new approaches advocated here build on work undertaken by both FAO10-12 and many other institutions13-20, and focus on the management of soil health.
Principles of soil health management
Soil health has been defined as: “the capacity of soil to function as a living system. Healthy soils maintain a diverse community of soil organisms that help to control plant disease, insect and weed pests, form beneficial symbiotic associations with plant roots, recycle essential plant nutrients, improve soil structure with positive repercussions for soil water and nutrient holding capacity, and ultimately improve crop production”21. To that definition, an ecosystem perspective can be added: A healthy soil does not pollute the environment; rather, it contributes to mitigating climate change by maintaining or increasing its carbon content.
Soil contains one of the Earth’s most diverse assemblages of living organisms, intimately linked via a complex food web. It can be either sick or healthy, depending on how it is managed. Two crucial characteristics of a healthy soil are the rich diversity of its biota and the high content of non-living soil organic matter. If the organic matter is increased or maintained at a satisfactory level for productive crop growth, it can be reasonably assumed that a soil is healthy. Healthy soil is resilient to outbreaks of soil-borne pests. For example, the parasitic weed, Striga, is far less of a problem in healthy soils22. Even the damage caused by pests not found in the soil, such as maize stem borers, is reduced in fertile soils23.
The diversity of soil biota is greater in the tropics than in temperate zones24. Because the rate of agricultural intensification in the future will generally be greater in the tropics, agro-ecosystems there are under particular threat of soil degradation. Any losses of biodiversity and, ultimately, ecosystem functioning, will affect subsistence farmers in the tropics more than in other regions, because they rely to a larger extent on these processes and their services.
Functional interactions of soil biota with organic and inorganic components, air and water determine a soil’s potential to store and release nutrients and water to plants, and to promote and sustain plant growth. Large reserves of stored nutrients are, in themselves, no guarantee of high soil fertility or high crop production. As plants take up most of their nutrients in a water soluble form, nutrient transformation and cycling – through processes that may be biological, chemical or physical in nature – are essential. The nutrients need to be transported to plant roots through free-flowing water. Soil structure is, therefore, another key component of a healthy soil because it determines a soil’s water-holding capacity and rooting depth. The rooting depth may be restricted by physical constraints, such as a high water table, bedrock or other impenetrable layers, as well as by chemical problems such as soil acidity, salinity, sodality or toxic substances.
A shortage of any one of the 15 nutrients required for plant growth can limit crop yield. To achieve the higher productivity needed to meet current and future food demand, it is imperative to ensure their availability in soils and to apply a balanced amount of nutrients from organic sources and from mineral fertilizers, if required. The timely provision of micronutrients in “fortified” fertilizers is a potential source of enhanced crop nutrition where deficiencies occur.
Nitrogen can also be added to soil by integrating N-fixing legumes and trees into cropping systems (see also Chapter 2, Farming systems). Because they have deep roots, trees and some soil-improving legumes have the capacity to pump up from the subsoil nutrients that would otherwise never reach crops. Crop nutrition can be enhanced by other biological associations – for example, between crop roots and soil mycorrhizae, which help cassava to capture phosphorus in depleted soils. Where these ecosystem processes fail to supply sufficient nutrients for high yields, intensive production will depend on the judicious and efficient application of mineral fertilizers.
A combination of ecosystem processes and wise use of mineral fertilizers forms the basis of a sustainable soil health management system that has the capacity to produce higher yields while using fewer external inputs.
The way forward
The following actions are required to improve current land husbandry practices and provide a sound basis for the successful adoption of sustainable crop production intensification. Responsibility for implementation rests with national partners, assisted by FAO and other international agencies.
- Establish national regulations for sound land husbandry. A supportive policy framework should aim at encouraging farmers to adopt sustainable farming systems based on healthy soils. Leadership is required to establish and monitor best practices, with the active participation of smallholder farmers and their communities. Governments must be prepared to regulate farming practices that cause soil degradation or pose serious threats to the environment.
- Monitor soil health. Policymakers and national institutions responsible for the environment are demanding methods and tools to verify the impact of farming practices. While monitoring soil health is a very challenging task, efforts are under way to implement it at global25, regional and national scales26. Monitoring the impact of agricultural production has advanced in developed countries, but is just beginning in many developing countries. FAO and its partners have developed a list of methods and tools for undertaking assessments and monitoring tasks27. Core land quality indicators requiring immediate and longer term development should be distinguished28. Priority indicators are soil organic matter content, nutrient balance, yield gap, land use intensity and diversity, and land cover. Indicators that still need to be developed are soil quality, land degradation and agrobiodiversity.
- Build capacity. Soil health management is knowledge-intensive and its wide adoption will require capacity building through training programmes for extension workers and farmers. The skills of researchers will also need to be upgraded at both national and international levels, in order to provide the enhanced knowledge necessary to support soil management under SCPI. Policymakers should explore new approaches, such as support groups for adaptive research cooperation29, which provide technical support and on-the-job training for national research institutions and translate research results into practical guidelines for small farmers. National capacity to undertake on-farm research must also be strengthened, and focused on addressing spatial and temporal variability through, for example, better use of ecosystems modelling.
- Disseminate information and communicate benefits. Any largescale implementation of soil health management requires that supporting information is made widely available, particularly through channels familiar to farmers and extension workers. Given the very high priority attached to soil health in SCPI, media outlets should include not only national newspapers and radio programmes, but also modern information and communication technologies, such as cellular phones and the Internet, which can be much more effective in reaching younger farmers.
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