Walter Jehne[*]

The nutrition of agricultural soils
Sustaining productive farming systems

Roundtable discussion of this paper

Life on earth over the past 3.8 billion years has substantially relied on photosynthesis, the conversion of sunlight, CO₂ and water into carbohydrates - initially by blue green algae and then plants to sustain itself.

Whereas sunlight, CO₂ and water are generally available, photosynthesis is often limited by the availability of some of the 16 major, minor and trace nutrients essential for plant growth.

Apart from nitrogen, which is sourced from the atmosphere, mineral nutrients have to be obtained from rocks or their weathered derivative, soils. The mineral nutrients must be present but also available for uptake by the plant roots; either via the absorption of soil water or through their selective uptake by symbiotic fungal associations called mycorrhizas.

Soils differ greatly in their level and composition of nutrients and in how those nutrients are held in either rocks, on soil surfaces, on organic matter or as chemical ions in soil solution.

Available nutrients - How nutrients are held fundamentally affects their solubility and hence their availability for uptake by plants and associated symbiotic associates. While soils may contain high levels of nutrients in total, such nutrients may not be soluble or available to the plant, significantly limiting the bio-productivity or fertility of that soil.

Consequently the factors that govern the availability of essential plant nutrients are often more important in governing the fertility of a particular soil than the level of those nutrients. Similarly the effectiveness of nutrient additions via fertilizers may also depend on how much remains available and for how long rather than on the levels that are added. For example many soils with high iron and aluminum oxide levels can occlude over 98% of the soluble phosphate added as fertilizers making it unavailable for uptake and the intended increased plant growth.

Soil microbes - The presence and activity of specific micro-organisms can often become the most important factor in enhancing and sustaining the fertility and productivity of soils. Indeed microbial associations govern most of the nutrition, and hence existence and bio-productivity of most of the world’s natural vegetation, particularly for perennial systems growing on more weathered, lower nutrient status soils.

While the importance of these microbial processes in the availability of nutrients to plants has been known for decades, their significance has not been widely appreciated in agriculture. As such the nutrition and productivity of agricultural plants has often been based on the speedy and opportunistic harvesting of natural or added nutrients as these are made available by periodic burning, soil disturbance or nutrient inputs.

As the availability and affordability of oil based fertilizer inputs decline, food securities will increasingly need to be based on sustainable perennial farming and soil nutrient systems. The understanding and management of the key microbial processes governing the fixation, solubility, availability, uptake and recycling of essential plant nutrients will become fundamentally important to sustaining agricultural productivities and food securities in this new global environment.

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The nutrition of agricultural soils

Whereas humans have evolved over the past million years as hunters and gatherers utilizing foods produced from such natural perennial bio-systems and nutrient processes, agriculture over the past 10,000 years but particularly industrial farming systems over the past 100 years have been based on fundamentally different nutrient strategies that have different impacts.

Rather than managing and harvesting food from broadacre perennial systems, intensive agriculture has concentrated and survived in particular unique locations where there were either highly fertile nutrient rich soils or where natural processes resulted in the regular addition and replenishment of essential nutrients. As a result most of the world’s agriculture has been, and still is concentrated on:

• The high nutrient soils formed as a result of recent glaciations, as for example in Europe, Russia and North America,

• On soils with previous or intermittent volcanic nutrient additions as in the case of Java, Darling Downs, Japan or,

• On river sediments such as in Egypt, Iraq, India, China in which nutrients are periodically replenished via the deposition of fresh nutrient rich sediments.

Such soil and nutritional resources have been fundamental to each of the world’s civilizations. Where natural processes have sustained soil fertilities such as in the Nile or where soils were managed to sustain and enhance nutrient cycling, as in China, civilizations have survived. However in most instances former civilizations, such as Babylon, Lebanon, Crete, Greece, Rome, Carthage, the Maya, Easter Island and others, have each failed directly because of their inability to sustain their original fertile soils and with that their essential food requirements.

Modern agriculture - Over the past 150 years ‘scientific agriculture’ has made it possible to grow crops even on soils of lower natural fertility via the addition of nutrients in fertilizers. This has enabled global populations to increase from some one billion to the present 6.5 billions over that period. However, the productivity of these crops and the viability of their dependent societies is now often highly dependent on the continued input of such fertilizers and their impacts on the soil.

The long term dependence on fertilizer additions raises sustainability challenges including:

• The finite limits and cost of accessing mineral deposits for conversion into fertilizer.
• The energy cost involved in the mining, transport, processing, distribution, and application of these fertilizers relative to the energy returns from the crops produced. As the oil used in these fertilizer processes becomes more expensive the returns from many current agricultural systems may no longer be economically viable.
• The serious degradation in the organic matter status and structure of many farmed soils as a result of inappropriate fertilizer use. This degradation, by decreasing water infiltration, availability, root proliferation and increasing toxic and saline effects can limit plant growth resulting in decreased fertilizer effectiveness as well as increased requirements to sustain former yields.
• The often rapid fixation of much of the added fertilizer nutrients onto soil surfaces so that they are unavailable for plant uptake and growth, or the leaching of more soluble fertilizer nutrients from soils into streams to create pollution risks.
• Nutrient deficiencies and toxicities as a result of nutrient in-balances due to inappropriate fertilizer additions. Healthy soils and plants generally require a balanced availability of all essential macro and micro nutrients with the overall growth response often being governed by the availability of the most limiting nutrient which may be affected by excesses of other added nutrients.
• The effect of inappropriate fertilizer additions on both the nutritional balance and value of the resultant crops and foods as well as the effect of fertilizers in altering the susceptibility of some crops to disease and stress factors.

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Processes for sustaining highly productive but low nutrient input farming

Although the above reinforce our dependency on and potential consequences of our common high nutrient input agricultural systems, as the availability and affordability of oil declines we will need to develop more sustainable and productive agro-ecosystems that are less dependent on repeated expensive high nutrient inputs and the degradation of natural soil capital.

Although such systems have existed and supported sustainable communities for millennia, often in remote tribal cultures and via shifting cultivation systems such as in Papua New Guinea and the Amazon, such traditional systems could not support the current global population. However an understanding of the nutrient dynamics in such agricultural systems may be critical in restoring the nutrition and bio-productivity of future low input farming systems.

Studies of the nutrient dynamics in such forests and in their periodic clearing, burning and use for shifting cultivation cropping confirms that natural bio-systems, far from being dependent on fertile high nutrient soils, have evolved means of sustainably supporting highly bio-productive and diverse systems even on extremely low nutrient soils. Rather than depending on high nutrient levels or inputs, as in our current farming systems, these natural productive bio-systems have evolved extremely efficient nutrient cycling systems. This enables adequate essential nutrients to be made available optimally where and when required for plant growth, despite the quantity of nutrients within that system often being extremely low.

As a result some of the world’s most bio-productive natural ecosystems occur on some of the world’s lowest nutrient status soils; such as the subtropical rainforests growing on silicaeous sand dunes at Cooloola and Fraser Island in Australia or the rainforests growing on low nutrient oxisols and latesols throughout the Amazon.

Microbial recycling - The very rapid and efficient cycling of the very limited quantities of essential nutrients is made possible through a range of soil micro-organisms, specifically mycorrhizal fungi. These fungi are directly involved in the bio-degradation of litter, the solubilization and efficient uptake of key nutrients from organic and mineral surfaces and their translocation to the actively growing plant tissues. Studies with radioactive labeled nutrients confirm that such microbial symbioses can recycle nutrients from fresh litter back into the plant within minutes, rather than the weeks or even years required for nutrients to recycle via chemical processes.

As it is possible to enhance and manage these microbial recycling systems, the potential exists to develop similar highly efficient nutrient cycling strategies in highly productive food and biomass producing agro-ecosystems even on low nutrient soils without high fertilizer inputs.

Provided that only 'stored solar energy' as biomass or sugars is used and no nutrients are quarried and exported from the site it should be possible for such bio-systems to be sustained at high levels of productivity and profitability. Similarly, provided any nutrients that are removed in the foods produced are returned to the site as recycled wastes, it should be possible to enhance and sustain the bio-productivity of such systems for many future food and energy crops.  

Research has confirmed that it is practical and feasible to select, introduce and enhance the activity of highly efficient microbial symbioses to optimize the nutrition and sustained productivity of different agro-ecosystems. Field and glasshouse inoculation studies confirm that such symbioses can greatly enhance the nutrition and growth of crop plants in unfertilized soils similar to those with optimal fertilizer additions. Although establishing such optimal symbioses and nutrient cycling systems involves more than just 'adding microbes', the fact that selected symbioses can be as effective as fertilizers in stimulating crop growth - while also being free, natural and sustainable - reinforces their significance for low input food systems.

Major opportunities exist to design highly productive low input agro-ecosystems based on these natural nutritional cycles and meet our food security imperative in a post cheap oil economy.

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