Harnessing nature’s resources for sustainable farming
The year 2015 has been declared as the "Year of Soils" by the United Nations Food and Agriculture Organization. This couldn’t be more timely, considering the current drought conditions facing South Africa. While it is quite distressing, it is not the first time that we are faced with drought-related challenges, which provides some assurance. Charles Darwin once said: “It is not the strongest or the most intelligent who will survive, but those who can best manage change”.
It appears that adaptability is the key objective in this instance – that is to produce as much quality food as possible with as little water as possible. However, under drought conditions it is not only water use efficiency that has to be considered; one should rather have a holistic and sustainable approach to mitigate associated drought risks. These risks can be measured and managed with more sustainable technologies and practices.
A key consideration of sustainable farming under stressed environmental conditions is soil management. Soil under current intensive use and environmental stresses must be regarded as a delicate and living organism that has to be nurtured and protected to ensure long term productivity. It is said that a healthy and productive soil is a resilient soil, producing healthy plants with optimum vigour and are less prone to diseases. Conversely, poor management of soil results in greater inputs of water, nutrients, pesticides and energy to maintain yields, which becomes environmentally and economically unviable to maintain.
Understanding soil health impacts, in relation to drought and other environmental stresses, is possible through measurement tools for soil biological, physical and chemical attributes. Soil health indicators are a composite set of the aforementioned attributes which relate to functional soil processes and can be used to evaluate soil health status, as affected by management and climate change drivers (Allen et al, 2011). This basic assessment of soil health substantiates the age old, but relevant management proverb of “you can’t manage what you don’t measure”. Measuring these soil health indicators provides guidance to make strategic decisions as to where investment should be channelled to ensure that on farm risk is best managed.
SOIL BIOLOGY AND ROOT HEALTH
Soil biology is a critical part of soil management within sustainable agriculture and can provide key information about root health status. Root health under these trying conditions is of utmost importance. The root is responsible for anchorage and support of the plant, absorption and conduction, movement, aeration and even reproduction. Having highlighted these fundamental functions, it is clear that root health needs to be well understood and managed, especially under stress. Root health depends on a healthy community of soil microbes that decompose organic matter and contributes to the biological recycling of chemical nutrients.
The interaction between the two functional groups, plant roots and soil microbes, forms the basis of all ecosystems and has major consequences on its functioning (Nautiyal, 2012).
Valuing this close interaction with microbes, plant roots secrete 30% to 60% of the carbon captured as plant photosynthates to support mutualistic relationships with soil fungi and bacteria (Johnson et al, 2015).
SOIL MICROBES, AN ALLY IN TIMES OF PLANT STRESS
Crop productivity greatly depends upon the amount of available nutrients in the soil, which is governed by transformation of soil biomass (Nautiyal, 2012). The vast majority of plants in nature is thought to be associated with some kind of fungi in the soil, both mycorrhizal (reside on the roots and extend to rhizosphere) and endophytic (reside within plant tissue, roots, stems and/ leaves). Fungal species are responsible for the adaptation of plants to environmental stresses.
Some of the benefits conferred by mutualistic symbiosis of fungi on plants include tolerance to drought, metals, disease and temperature. The mechanisms involved in abiotic (non-living such as drought, hail, etc.) and biotic (living factors such as diseases, rodents, etc.) stress tolerance include: (1) rapid activation of host stress response systems after symbiotic plants are exposed to stress; and (2) synthesis of anti-stress biochemicals, such as alkaloids, by fungi (Rodriguez et al, 2004).
Scientific observations have shown that fungal symbiosis on plants activates defence systems more quickly than non-symbiotic plants affected by environmental stress. Redman et al, (2002) showed that while the maximum growth temperature for panic grass was 40°C, with fungal symbiosis, the plants could withstand temperatures of up to 70°C. In addition, Rodriguez et al (2004) showed that symbiotic plants, in the case of tomato and pepper respectively, survived desiccation for 24 and 48 hours longer than non-symbiotic plants.
Plant growth promoting rhizobacteria are bacteria that can enhance plant growth by a wide variety of mechanisms. Some of their functions are to fix nitrogen, solubilise phosphates and release siderophores to chelate iron and other metals. These bacteria also produce the enzyme ACC deaminase to facilitate plant growth and development by regulating ethylene levels, inducing salt tolerance and reducing drought stress in plants. Rhizobacteria are also able to produce phytohormones like indole acetic acid, cytokinins and gibberellins and these bacteria can therefore be classified as biofertilizers or phytostimulators, which are all responsible for plant growth, yield and nutrient uptake (Nautiyal, 2012). Therefore, harnessing the potential of nature’s resources is a lucrative option for sustainable farming.
A STRESSED PLANT IS A SUSCEPTIBLE PLANT
Drought stress promotes diseases through different mechanisms; it may alter the plants physiology, making it more susceptible. Severe stresses or deficiencies of nutrients cause injuries to the root cells making it weaker and easier to invade by soil pathogens. It may also reduce the ability of the plant to induce its defensive mechanisms which under normal conditions outcompete the pathogens.
Plant parasitic nematodes are among these pathogens and become especially opportunistic on plant roots under high temperatures and lack of water. The weakened plant will have to contend with these pathogens for the uptake of water and nutrients, with the added challenge of facing specialised defence mechanisms of the parasites, like effector genes. Two harmful plant parasitic nematodes are the rootknot nematode (Meloidogyne incognita) and the soybean cyst nematode (Heterodera glycines) which both possess effector genes which may be involved in suppression of host defenses (Hamamouch et al, 2012).
In a scenario like this, it comes down to the survival of the fittest, whilst pathogens have evolved to become more and more robust; the soil has become less and less resilient. This is due to continued unsustainable management practices. These include but are not limited to; mono-cropping which leads to a lack of biodiversity and diseases, irresponsible agro-chemical use and depletion of soil organic matter and biology.
SUSTAINABLE FARMING PRACTICES
Biodiversity and a healthy soil are pivotal to sustainable approaches to make farming more drought and stress resistant. Adopting practices that build a healthy soil is important to help plants cope with drought stress. Practices such as amending soil with organic matter should be considered to improve moisture retention, which will permit better root growth and establishment.
Organic matter and biology in the soil drives a number of soil functions. Increasing organic matter leads to better soil structure which results in improved water holding capacity, decreased risk of erosion and compaction, and an overall enhancement of soil fertility.
Protecting soil also includes using cover crops and intercropping to improve biodiversity. Also, cover crops can have stabilising effects on the soil by holding soil and nutrients in place. Cover crops in orchards and vineyards can buffer the system against pest infestations by increasing beneficial arthropod populations and can therefore reduce the need for chemical inputs (Sundar, 2006). In addition, practicing good crop rotation suppresses weeds and insects and, very importantly, helps to break the spread and carryover of plant pathogens which can be host specific. Considering legume crops as part of the rotation cycle where possible can also be beneficial in creating an organic rich soil with good biodiversity.
These are some strategies to improve overall soil qualities to ensure that water made available by nature is not lost and nutrients become more accessible to the plant. Preventive strategies which are adopted early can reduce inputs and help establish a sustainable production system. The key to adaptation under the current circumstances is to aim to have a knowledge intensive farming approach, which will result in being more economically and ecologically stable.
- Hamamouch, N., Li, C., Hewezi, T., Baum, T.J., Mitchum, M.G., Hussey, R.S., Vodkin, L.O., Davis, E.L. 2012. The interaction of the novel Hg30C02 cyst nematode effector protein with a plant b-1,3-endoglucanase may suppress host defence to promote parasitism. Journal of Experimental Botany.
- Allen, D.E., Singh, B.P and Dalal, R.C. 2011. Soil health indicators under climate change: A review of current knowledge. Soil Health and Climate Change, Soil Biology. 29: 25:45.
- Nautiyal, C.S. 2012. Microbes for soil sustainability and crop productivity. Environews ISEB India. 18: 3
- Johnson, D., Ellington, J and Eaton, W. 2015. Development of soil microbial communities for promoting sustainability in agriculture and global carbon fix. Peer J Preprints.
- Redman, R.S., Sheehan, K.B., Stout, R.G., Rodriguez, R.J. and Henson, J.M. 2002. Thermotolerance conferred to plant host and fungal endophyte during mutualistic symbiosis. Science. 298: 1581.
- Rodriguez, R. J., Redman, R. S., and Henson, J.M. 2004. The role of fungal symbioses in the adaptation of plants to high stress environments. Mitigation and Adaptation Strategies for global change. 9: 261-272.
- Sundar, I. 2006. Environmental and sustainable development. A.P.H Publishing cooperation. Darya Ganj. New Delhi.
By Venessa Moodley