Introduction to sustainable agriculture

Apart from air and water, food represents an important basic need for human survival and therefore its production and access is of vital importance. In contemporary societies, most food is produced by crops and livestock in agricultural systems. Developments in fertilizer and pesticides production as well as breeding have increased the agricultural output and efficiency substantially since 1940. The increased efficiency led to an increase in farm size in many high-income countries [1], but globally still around 70% of the food is produced by small farms [2]. Another development in agriculture related to the increased efficiency is a loss in product diversity. More than 40% of calories consumed worldwide come from only three different plant species – wheat, rice and maize [3]. The yields per hectare for these three crops have roughly doubled from 1940-2000, attributed to improved crop varieties and agricultural practices, such as increased use of fertilizers, pesticides and irrigation [4]. While these increases are astonishing, the practices to achieve them have also caused negative effects: pollution of soil, air and water through the use of fertilizers and pesticides, soil compaction and erosion of soil and loss of biodiversity, just to mention the main ones. Fortunately though, more and more people are getting aware of these effects and hence, the interest in sustainable forms of agriculture is increasing.

Permaculture and agroecology

My motivation to get involved with agriculture came through the contact with permaculture. The term permaculture was created by Bill Mollison and David Holmgren in 1978 from the conjunction of the expression permanent agriculture, meaning the practice of agriculture in a way that allows to go on potentially forever; in other words, farming in sustainable ways. However, the idea of sustainability is not only about the production of food, but also includes human wellbeing and society. The three basic ethics of permaculture are [5]:

• Care for the Earth
• Care for the people
• Fair share (setting limits to population and consumption and redistribution of surplus)

Permaculture is sometimes also described as a way of designing systems where cycles are closed. This means that waste products do not accumulate, but are rather purposefully used in another process, such that nutrients are not lost but recycled. Another concept termed – agroecology – shares some parallels with permaculture. In agroecology, the idea is to use ecological principles in agroecosystems. In both – permaculture and agroecology – methods that can contribute to the goal of a more sustainable agriculture are discussed and applied . In the following paragraphs, I will describe a series of such approaches and how they can help to avoid negative consequences of conventional agriculture practices.

Sustainable agriculture

Reducing Erosion

Erosion is the loss of soil and a great risk for future agricultural production. Factors that accelerate soil erosion include:

• soil compaction (for example caused by heavy machinery compressing the soil below it, displacing the air out of the pores between soil grains, which can result in a reduced ability of the soil to hold water),
• bare soil (soil not covered by plant cover or mulch)
• reduced soil life (for example reduction of earthworms which introduce pores in soil helping water to infiltrate).

In sustainable practices the method of no-till is encouraged, meaning to sow seeds directly into a covered field without plowing it first. One goal of this practice is to keep the soil covered at all time. No-till farming reduces the soil compaction as the heavy machines need to pass fewer times over the field. In addition, skipping the plowing step promotes soil life, since the fragile ecosystem of the soil does suffer from the heavy disturbance of being turned upside down.

Another benefit of this method is the increased capability of holding water and slowing down its flow and the impact of rain, which contribute to erosion. There are also some challenges with no-till farming though, for example it can be more challenging to maintain crop health, although other sustainable methods could be also used, as you will see in the next section. I will give more details about no-till farming in a future post.

Crop health

In conventional agriculture the use of chemical pesticides is common practice. Herbicides suppress the growth of unwanted plants such as weeds. Fungicides and insecticides help to protect crops from diseases and yield losses. However, these products can be harmful to Nature including ourselves, as they are associated to cancer and chronic diseases [6]  Therefore pesticide  use should be reduced or avoided altogether. A simple sustainable practice to improve crop health is crop rotation, or the growing of different crops in a temporal sequence in a given field. This method can be effective against soil-borne pathogens, because different plant species have varying susceptibilities towards certain pathogens. In monocultural fields where year after year wheat is grown, certain fungal spores can readily cause diseases. If however, other crops (e.g. soybean, alfalfa) are planted after a wheat harvest, such that the next wheat crop is only planted a couple of years later in the same field, the risk for this next wheat crops is reduced.

Another sustainable strategy to protect crops is the application of biological pest control, ranging from the introduction of beneficial insects to the application of microbial preparations and manipulation of the ecosystem in order to benefit from natural control mechanisms. The famous bacteria Bacillus thuringiensis (Bt) for example is effectively used to protect crops from insect damage [7].
A further sustainable approach to improve crop health is focusing on the concurrent planting of different crops in the same field, called intercropping, which can reduce weed pressure [8]. A common combination for intercropping comprises maize and bean. Through the different nutrient requirements of these plants and the increased soil cover, competing weed plants have less nutrient and light resources. Intercropping can have even more benefits, as we will see below in the section about fertilization.

Fertilization

While a large part of agricultural products consists of carbon, oxygen and hydrogen, available in the air and water, many nutrients (nitrogen, phosphorous, potassium and many more) stem from the soil. Thus, the constant export of nutrients from the soil through harvest needs to be replenished in order to avoid depletion. In conventional agriculture, a lot of the applied fertilizer is either chemically produced (in the case of nitrogen) or mined (phosphorous and potassium). The production of nitrogen fertilizers through the Haber-Bosch reaction requires a high amount of energy (around 1% of the global energy!) [9]. And the easily accessible resources for potassium and phosphorous are limited and concentrated in certain areas on the Earth, thus requiring transport over long distances.

In sustainable practices, one of the goals is to rely more on local resources for fertilization. The use of animal manure has a long tradition and is often combined with mineral fertilizers. In mixed farming systems with crops and animals, the use of manure and slurries are a good way for recycling a “waste” product and use it as fertilizers. In farms specialized in livestock with feedlots, where huge numbers of animals are kept, large amounts of manure accumulate. In this case, the animal waste represents rather a problem than a valuable resource. In some countries, regulations are in place to limit number of animals per farm and to encourage mixed farming systems, where manure can be recycled easily [10].

Apart from using animal manure and slurries as fertilizer, another sustainable practice is the incorporation of legume plants. Legumes have the ability to fix nitrogen from the air in a symbiosis with Rhizobia bacteria and thus do not rely solely on nitrogen resources from the soil. Therefore legumes are often used in crop rotations, providing a substantial input of nitrogen for subsequent crops [11]. In intercropping legumes are also a valuable plant to include, because fixed nitrogen can also be made available to plants growing concomitantly [12]. Together with benefits regarding crop health, intercropping represents a promising approach for sustainable agriculture. However, it also introduces new challenges in farming technique and variety selection.

While the use of manure and legumes can contribute to more sustainable farming practices, it does not solve the whole aspect of nutrient export through the harvest. Beyond macronutrients, plants also require micronutrients like iron and zinc for example. The concentration of nutrients in plant products is also associated to the health benefit for us as consumers. Therefore the food we eat is a rich combination of many nutrients, which ultimately end up in the sewage treatment plant and our water bodies. In order to close this cycle it would thus be necessary to use nutrients in sewage as fertilizers. And in Sweden for example there are already policies proposed to recover nutrients from human excreta (focusing on phosphorous in urine) [13]. However, it is important to highlight that the use of sewage also poses challenges, because it also contains toxic compounds such as heavy metals and organic pollutants, and therefore, more research is needed to find suitablenew ways to recover the nutrients to be used as fertilizer [14].

This was a first overview of some aspects of sustainable agriculture. In future posts, I will focus on different aspects in more detail including:

• Intercropping
• Fertilizers
• Crop health
• Diversity

Thanks for reading until the end! I hope you have enjoyed it and learned something new. I would be happy if you shared this article, given you found it interesting.

Be conscious 🙂

Christoph

References