Soil sustainability

By Dr J Floor Anthoni (2000)
Agricultural land, the land that feeds the world, is in serious difficulty and it is not at all certain that it will be able to produce enough food for the world's population, in perpetuity. Confusion reigns about what sustainability is, but in this chapter we'll define some basic rules that will help people to understand it and to better manage their land. Does the green revolution deliver as promised? What is soil sustainability? How can we manage agricultural systems sustainably? How does the soil cycle work?

"Man is like every other species in being able to reproduce beyond the carrying capacity of any finite habitat. Man is like no other species in that he is capable of thinking about this fact and discovering its consequences." - William R Catton, Jr

please note that this document consists of two parts, part1 (this page) and part2

signs of fatigue

The world's agriculture is showing signs of fatigue. Modern cropping, even with minimal tilling, may not be sustainable.

disrupting the water cycle

By changing the landscape, humans are also invisibly disrupting the water cycle, leading to unexpected droughts and loss of water.

defining sustainability

Our means of food production should not only be maximal when the world's population peaks but more than adequate for thousands of years to come. An enormous confusion reigns about what sustainability entails, but in this chapter we'll boldly define the natural laws that rule it.

sustainable systems

Not all of today's agriculture is in difficulty. This chapter reviews the agricultural systems that have been a failure and those that work sustainably.

energy efficiency

The purpose of food is to feed humans, so that they can spend the energy to live. Food production should be energy-efficient, so that more food energy is obtained than work expended to produce it.


In order to earn its keep, farming must be economic. Income must exceed expenditure. Many improvements have been made as a result of new knowledge in the 'green revolution' and these improvements will continue. But the soil does not need to be compromised.


Often the question of ownership is an important factor in how the land is managed. This chapter looks at a number of issues.

permaculture and organics

The unsustainability of various farming practices has not been left unnoticed and alternative ways are being tried. This chapter discusses two of these.

what can we do?

Soil sustainability is obviously an issue that affects society. So what can society do about it?
go to soil contents page <==>  go to part2
-- home -- all about soil -- environment -- issues -- Revised 20010525,20020110,20051122,20070718,


Signs of fatigue and natural limits

Signs of fatigueSigns of fatigue of the world's agricultural soils, comes in many varieties but here are two that illustrate how the situation is deteriorating. The left graph comes from a trial in the tropical soils of the Philippines where two identical plots were farmed in a modern way, on the one hand by doublecropping (two harvests in one year) and on the other hand by applying fertiliser, and undoubtedly also using chemical pest control. One of the two plots received no fertiliser. It is an important study because it ran over 25 years. The result shows that the use of fertiliser doubles the yield as expected (see also the graph in previous image), but that the soil fatigues. After 30 years, yield would have dropped by half. 

The unfertilised plot (green) behaves in much the same way. Apparently, the very act of cropping degrades the soil beyond practical use, over a period as little as 50 years. A civilisation based on this kind of agriculture would vanish in a mere 100 years! The right-hand graph shows how cereal production is not keeping up with population growth. Many statistics, ranging from fishing to mining, show similar trends. And the world's population is still to double, so how will we ever be able to reach the year 10,000 sustainably?

Although the data on erosion varies from place to place and author to author, the following statistics may serve as a guideline: The natural soil renewal rate is about 1 ton/ha/yr in natural ecosystems, equivalent to 0.05-0.1mm per year.
In about 200 years of farming, the USA lost 80mm; New Zealand lost 30mm in 100 years. Soil loss today in the USA is about 18 t/ha but in China amounts to 40t/ha. Under the most careful and conservative cropping methods, soil loss still exceeds three times its natural formation.

In the 20 years from 1970 (Year of the first Earth Day) to 1990, deserts expanded by some 120 Mha, claiming more land than is currently planted in crops in China (1990), and the world's farmers lost 480 Gt of topsoil, roughly equivalent to India's entire cropland.

The entire surface of the land is about 13000 Mha; of which only 8800 Mha is productive (forest or farmed), the remainder being ice, desert or mountain top.
Each year 6-7 Mha is lost to erosion; 1.5 Mha is lost to waterlogging, salinisation and alkalinisation;
Each year 17 Mha of forest is cleared. At this rate all tropical rainforest (now 800 Mha) will be lost before 2040.

So the question of sustainability and how to achieve it, is not trivial. The table below may give an indication of how much of the world's Net Primary Production is used by humans. NPP is the photosynthetic production of plants, less what they use for their own life processes. Note that NPP is an energy flow, not a reservoir.

NPP account
NPP consumed Pg/yr NPP dominated Pg/yr NPP lost Pg/yr
consumed by humans
consumed by domestic animals
wood used by humans
converted pastures
tree plantations
occupied lands
fires, forage, wood 
land clearing
deforestation for crops
human occupation
Total (4%) 5.2 Total (31%) 41 Total (13%) 16.9
Total Net Primary Production (NPP) of the whole world estimated at 132 Pg/yr. Column 1 gives actual consumption. Column 2 gives the productivity of agricultural systems, and column 3 annual losses to nobody's benefit, but deforestation in column 3 produces land for column 2. 
Total NPP sequestered by humans is 58 Pg/yr or 39% of total NPP.
Sources: J S Olson et al, Carbon in live vegetation of major world ecosystems. 1983. Oak Ridge Nat Lab.
Vitousek P M et al: Human appropriation of the products of photosynthesis. 1986. Bioscience 36: 368-373.

All three columns give rather frightening consequences:

Please note that the terrestrial NPP of 132Pg carbon/year quoted above, differs from most sources which estimate world NPP at 50 Pg carbon per year. The difficulty in calculating NPP, is that it is a turnover factor. Satellite images can look at the green colour of chlorophyll and make an estimate of the standing crop in green leaves, but not NPP. In soil54.gif, the standing crop is estimated at 664Pg carbon, but that includes all the woody stems, which do not take part in primary production. The figures in the above table will undoubtedly be sharpened, but their message will probably remain.

A few more horror stories of soil degradation world-wide. (Source: World Watch institute, 1996)

  • China: Erosion affects more than a third of China's territory, some 367 Mha. In Guangxi province, more than a fifth of irrigation systems are destroyed or completely silted up by eroded soils. Salination has lowered crop yields on 7 Mha. Nearly 7 Mha are polluted by industrial wastes.
  • Russia: Eroded area increases by 0.4- 0.5 Mha/yr, and now affects 60% of Russia's arable land (not unlike the USA). Water erosion has created some 400,000 gullies, covering 0.5 Mha (not unlike the USA).
  • Iran: nearly all (94%) of Iran's agricultural land is estimated to be degraded, the bulk of it to a moderate or strong degree. Salination affects some 16 Mha of farmland, and has forced at least 8 Mha from production.
  • Pakistan: Gullies occupy some 60% of the 1.8Mha Pothwar Plateau. More than 16% of agricultural land suffers from salinisation. In all, more than 61% of agricultural land is degraded.
  • India: Degradation affects 25% of India's agricultural land. Erosion associated with shifting cultivation has denuded approximately 2.7 Mha of land east of Bihar. At least 2 Mha of salinised land have been abandoned.
  • Haiti: 32% of the land is suitable for farming, but 61% is farmed. Severe erosion eliminated 6,000 ha of cropland in the mid-1980s.
  • Australia: More than 4.5 Mha of drylands - 10% of all cropland - and more than 8% of irrigated area are affected by salting. The area affected by dryland salting doubled in size between 1975 and 1989.
  • The list goes on and on, illustrating how farming has become something of a nightmare. Everywhere people are struggling to understand what soil is, how it works and how to manage it sustainably. It also shows how high the pressure is to bring unsuitable land under cultivation and to overexploit suitable land.

    Another nightmare is that we appear to be running out of phosphate rock and sulphur, with only 80 years to go for phosphorus and 20 years for sulphur (see the world production and reserves table). These two minerals are necessary for nitrogen to work. Although nitrogen fertiliser can be made from air in unlimited amounts, P and S are scarce in concentrated form. Phosphorus is found in low concentrations in soils all over the world, but should farming rely on that alone, overall agricultural productivity would be limited considerably. Sulphur rains down from the atmosphere, but only in small amounts.

    A new vision
    It is comforting to know that everywhere in the world, farmers and scientists are becoming aware of the need for a new philosophy for farming. The underlying thoughts are eloquently expressed by Frederick Kirschemann, director of the Leopold Centre for Sustainable Agriculture at the Iowa State University (USA), in his speech Questions we aren't asking in agriculture: beginning the journey toward a new vision. Kirscheman observes that the vision for agriculture changed four times and that it is overdue for another change:
    • feeding the village: (12,000 BC-16th century) Tribes (American Indians, e.g.) were concerned with feeding the village in a way that introduced least risk (shifting farming).
    • building a nation: (17th century) Early pioneers brought visions of taming the land, clearing the forest and creating a vision of pastures and crops.
    • creating a free and democratic society: (18th, 19th century, Thomas Jefferson) a republic consisting of small independent farm landholders, free to speak their minds and to vote their conscience.
    • developing a lifestyle: (19th-21st century) a highly productive economic system that freed people from hard work to pursue lives of pleasure and leisure (and to have large governments and armies).
    The productivity-driven farming system has produced a vast range of almost insurmountable problems within a world limited in resources and ecosystem services (recycling). But a system with the following characteristics cannot be bad for the future:
    • if all waste in our farming systems became food for something else in the system.
    • if the biodiversity in all our farming systems was increased, rather than reduced.
    • if all of the energy used in our farming systems was current.
    Such a system is not possible with the current philosophy of top-down control of the environment, but needs bottom-up adaptive management with feed-back from how the environment responds. It requires biological diversity which conflicts with the present system of farms producing for a market, controlled by only a few food industries. Instead, consumers and farmers should market the farm within its limitations due to sustainability.

    In a following subchapter we'll define the rules for sustainability, based on our step-by-step understanding of soil, but first a look at what is perhaps the foremost cause of climate change.


    Disrupting the water cycle

    The change from forest to city is a massive one, for all to see, but invisibly, the water cycle is changed too, resulting in local climate change and loss of water. From sediment cores, drilled in temperate lakes in Europe, the period since 1850 saw a twenty fold increase in the loss of nutrients from the land. (G Diggerfeldt, 1972) Simultaneously, the landscape changed from forest to agriculture, and with it the rainfall in the lake's water catchment area.
    The following table shows hydrological and thermal properties of a number of ecosystems.
    ecosystem run-off seepage evaporation dissolved
    matter flow
    temperature rising
    climax forest very low low high very low low 60 m
    meadows low high low high medium 100 m
    crop field medium high low high medium 200 m
    urban area very high nil very low high high 1000 m
    water, wetland high nil medium very low low 60 m
    sand, gravel low high nil high high ?

    When forests are converted to crop fields, run-off (eluviation) and seepage (illuviation) increase, whereas evaporation decreases. At the same time, fertility is disappearing by a substantially increased dissolved matter (nutrients) flow. The temperature increases and evaporated moisture rises much higher. Because in a forest the evaporated moisture rises to only 60m, staying cool in the process, it is more likely to precipitate nearby. Once moisture rises to 200m, it will be transported much further and rain down somewhere else, where the temperature is cooler, such as the oceans, or the mountain tops.
    This explains the change in precipitation accompanying the clearfelling of forests. Around the Mediterranean Sea, after forests were cleared to produce ships and gunpowder, the climate became dry. The Anastasi Indians of North America lost their agriculture once their forests were felled, and rains dried up. Such examples are found all over the world.

    Urban areas are now becoming so large that they have started to influence the water cycle. With its higher temperature, urban air rises to considerable heights (over 1000m), and like huge chimneys, drawing neighbouring cooler air from crop lands, and exporting it to the oceans. In all, the water cycle may have been affected so much that global temperatures could rise just because of this. More importantly, precious water is lost from crop lands and populated areas. Note that forestry and padi culture disrupt the water cycle least.

    disrupting the water recyclingAnother effect of deforestation with subsequent farming, cropping and urbanisation is that of disrupting the recycling of water. In the pre-human situation, coastal rains would be soaked up by the forest, only to be re-evaporated, forming new clouds. These then rained down further inland, and so on. The rainwater was recycled many times before flowing back to sea.
    But today, the situation has changed. Rains now immediately drain back to sea, and the little amount of re-evaporated water forms clouds which are more reluctant to release their rains because the rising air has lifted them higher. As a result, less water falls inland and droughts become permanent. Further inland, deserts spread.
    This hardly visible deterioration of the water cycle, is most noticeable above large continents. In China, it has given rise to prolonged droughts, rapid desertification and the resettlement of people.

    Human-induced droughts also reduce the amount of water seeping down to recharge underground aquifers, both by there being less water, and surface water draining off more rapidly. As farmers require to pump more water from them, in order to compensate for the changed conditions, the aquifers are being depleted rapidly. As a result, many areas in the world have suddenly become unsuitable for farming.

    ALL rain comes from the sea
    A sobering thought
    As a sobering thought, Dr Wilhelm Ripl reminds us that ultimately, all processes depend on just two factors, sunlight and rainfall (which also depends on sunlight). Sunlight arrives in daily impulses, modulated by a seasonal component. This energy makes plants grow, and brings warmth to power the water cycle. Water is the universal 'processor' since all life processes depend on it. An exact quantity of water is used in photosynthesis to produce biomatter. It can work only if that water is transpired into vapour, for which sunlight is needed. Water is also needed to dissolve the nutrients of life, and to transport these to where they are needed, inside and outside living organisms. Water shapes the catchment area by natural erosion of the soil. Soil is formed under influence of water, plant roots and soil organisms. Temperature and moisture of the soil decide which kind of soil will form. The soil and local climate then select which plants will grow, and so on.
    When humans change the water cycle in a catchment area, they change its moisture, temperature and even the amount of sunlight reaching the surface. This must eventually result in profound changes, even though the process may take hundreds of years. Ultimately, such changes may cause the total demise of the world we once knew.
    Without even considering the burning of fossil fuels, deforestation, agriculture and urbanisation are creating a warmer, drier world, inviting the water vapour from oceans to condensate onto the cooler surfaces of (coastal) mountains and ice caps, thus ushering in a new ice age.
    Source: Wilhelm Ripl: Management of water cycle and energy flow for ecosystem control. 1994.

    As one can see, the preservation of the water cycle, which is perhaps no longer to achieve, is important for sustainability. But there is much more to it.


    Defining sustainability - sustainability is forever

    From the many sources on soil, erosion, agriculture and ecology, an overarching philosophy appears to be lacking. In this chapter we will boldly define a number of natural laws that govern sustainability, and that will enable farmers, horticulturists and home gardeners to do what is right. Note that the laws or principles outlined here have never been published before and they have not been proved by scientific method. However, they result from sound ecological consequence. I have not been able to find any data refuting this logic, but welcome scientists to prove me wrong, or to provide additional information. (e-mail Floor Anthoni)

    Let's first review the previous chapters to paint the essence of the life cycle of soil:

    Natural erosion wears soil down and washes it into the continental margins. Tectonic plates push it underneath continents, where it is homogenated and melts. Magma cauldrons rise up through the continental crust, bringing rocks of various type to the surface. Rocks weather slowly to produce soil of varying qualities. In the top soil, where the soil organisms live, the nutrients relevant to life are retained; all others are lost by leaching. Top soils all over the world have very similar compositions, which are often very different from their bed rocks. Nutrients are carefully cycled between the plants above and the soil organisms beneath the surface. On average, their tissue compositions are equal. As a rule, the soil contains much more living biomass than the plants above it (active leaves). Nutrients are cycled many times before they are lost. Erosion and leaching slowly diminish the top soil but in natural ecosystems, the rate of loss equals that of formation. The climate determines many of the soil's processes, its fertility and its properties.
    A number of basic and important natural laws can now be formulated:
    Sustainability accepts neither soil loss nor soil degradation. Sustainability is forever.

    The quality of a soil is like the life of an organism or a company. It cannot sustain continuous losses. Farming for sustainability cannot accept any form of soil loss or degradation. Like running a business, it must be run for the profit of the soil. If the soil does not improve, it will decay. Farming for sustainability implies farming for improved soil quality. Everything else, like economic and social benefits, should take second place.

    Sustainability includes the down-slope environment, the wetlands, rivers, lakes and the ocean. 

    Sustainability cannot be practised with a myopic view on a particular cropland. The influence of agricultural practices such as irrigation, groundwater pumping, fertilising, spraying should be considered all the way down the slope. If the sea's ecosystems cannot sustain the amount of fertiliser used and erosion caused, then farm practices should be altered to meet sustainability requirements in the sea. If the pumping of groundwater affects nearby wetlands, then the operation is not sustainable.

    The fertility of soil is entirely stored in its soil organisms and the vegetation above.

    Soil organisms store the fertility of the soil in their bodies. Their varied make-up (bacteria, fungi, worms, arthropods) matches the above-ground vegetation closely. Soil biota and plant communities develop in close synergy, the one matching the other. If this were not the case, nutrients would be lost unnecessarily rapidly. Plant vegetation can convert to soil biota and the other way around. It is a closed loop with low losses. From this basic law, a number of others derive:

    The more soil biota and the more live plant tissues, the more fertile is the soil.

    Since the fertility of the soil is stored in living organisms, the more of these, the more fertile the soil and the more rapidly nutrients can be sequestered by plants. Logically, in order to have a large standing crop of soil biota, a large amount of food is needed by way of plant roots and leaf litter. But ironically, the amount of oxygen should be limited to prevent soil organisms from 'burning' themselves up. This happens violently when ploughing the soil, as illustrated in the table below. The difference between no-till farming and tilling is just too large to go unnoticed. The figures show what was lost in 19 days after ploughing the crop residue in, but in the period following, more organic carbon was lost from the soil (134%) than the amount of crop residue ploughed in. It may explain why under-ploughing of nitrogenous crops has proved to be so ineffective.

    Tillage-induced 'flush' of organic matter
    type of tilling organic matter
    lost  in 19 days
    moldboard plough
    moldboard plough + disc harrow (2x)
    disc harrow
    chisel plough
    Source: D C Reicosky in J Glanz: Saving our soil, 1995

    The moldboard plough is a deep plough that turns the soil one or two spades deep. It does what a spade would do. The disc harrow has freely rotating discs that break the clods into finer crumbs. It does what a rake would do. A chisel plough consists of many narrow chisels, intended to rip the soil without turning it. It is a kind of rough rake. No-till cropping does not use ploughs or harrows but leaves the soil structure intact. Seeds may be drilled in place and weeds may be cut mechanically or sprayed. According to above table, it could be five times friendlier to soil than ploughing.

    Nutrients not stored in the bodies of living organisms, will be lost.

    What organisms can't use, is excreted in soluble form and becomes susceptible to transport by water. These nutrients, when not taken up by plants, are eluviated upward and out of the soil or illuviated downward into the deeper soil layers, where they can get lost in the groundwater aquifers or react with newly formed soil to form impermeable 'pans' of iron oxides, podsols, gypsum and the like. Loss of useless minerals (like salt, NaCl) this way, is part of the natural topsoil forming process. But the consequence is that tillage-induced 'flush' as described above, will lead to permanent loss of soil fertility. Other practices that reduce soil biota, such as burning, waterlogging, compaction, also result in loss of fertility.

    The nutrient in shortest supply determines the fertility of the soil.

    This aspect has been discussed in the chapter on soil fertility as part of the needs of plants (Liebig's law). But it also applies to the soil, although in a different way. If the soil organisms are considered a single organism, Liebig's law applies, but it has profound consequences. Plants are restricted by the nutrient in shortest supply, and they simply stop growing. Nothing is lost. But when soil organisms stop growing, all remaining free nutrients are eventually lost. So, a soil short of nitrogen but rich in all other essential nutrients, will lose all of these if nitrogen is not supplied. The whole soil will then start to match its composition to the nutrient in shortest supply and become poorer than it would otherwise have been. Hence the importance of 'topping up' with artificial fertilisers containing the elements in shortest supply, in order to gradually improve soil quality. This cannot be done by recycling from the same soil. Some artificial fertiliser is thus absolutely necessary for sustainable farming, and for organic farming!

    Mineral clay (without organisms) has the ability to bind cations (Cation Exchange Capacity, see soil classification/properties) and these form an intermediate storage. Although clay's binding capacity is far less than that of living organisms, it does play an important role in the formation of fertile soil underneath the soil biosphere (B and C horizons) where none or very few soil organisms are found.

    Note that permanent drought is very impoverishing to soil. Temporary droughts are less impoverishing because during the drought there is no water to leach nutrients away. Also, the soil organisms have turned into dried organic matter, still binding the nutrients inside their dried bodies. Since most have excellent drought-survival strategies (like making spores), and most are so small that they can regenerate very quickly, a good soil can recover its original fertility quickly after a temporary drought.

    Average composition of elements in plants must match those in the soil biota. Plant variety improves fertility.

    In order that nutrients cycle between plants and soil organisms, their average element compositions must be nearly equal. But the soil organisms are not plants, and plants have far-ranging concentrations of elements. This is why natural ecosystems (forests, prairies) have such an enormous variety of plants.  When a forest is cleared and planted with a monoculture, it is most likely that plants and soil biota are no longer matched. Following Liebig's law, the new 'ecosystem' will leach unused nutrients away and the soil becomes less fertile. This problem could be alleviated by planting rotations of different crops. Fallowing (allowing whatever wishes to grow), may also be of considerable benefit, as well as allowing weeds or ground cover to grow with the crops.

    Soil scientists make much ado about the Cation Exchange Capacity (CEC) of soils. It is the soil's capacity to bind soluble positively charged ions like Na+, K+, Ca++ and others. In the table one can see actual values of the CEC in mole/kg soil. The soils from the top down, contain progressively more clay, except for the last one which contains clay and a high concentration of live biota. As one can see, clay-rich soils may have CECs over ten times that of sandy soils, but 'organic' soils have ten times higher CEC still. The difference in natural fertility between a sandy soil and an organic soil is of the order of hundred times! Organic soils have very high Anion Exchange Capacity too (not measured). They store the valuable nutrients inside living matter, from where it can be released again. 

    To relate CEC to amount of fertiliser, assume one square metre to contain 300 kg soil (20cm deep at density 1.5). One ha contains 3 million kg of it. A CEC of 1.0 gives 3 million moles/ha. A mole weighs between 23 (Na) and 40 (Ca), say 30 g on average, equals 90 Mg/ha, or 90 t/ha. Since this is the active part of fertiliser, multiply by 3 or 4 again to equal roughly 300-400 t/ha for a CEC=1.0! 

    Kind of soil CEC
    sandy soils 0.02-0.04 fast very low
    sandy loam 0.02-0.17 fast very low
    loam 0.07-0.16 fast-moderate low
    silt loam 0.09-0.3 moderate moderate
    clay loam 0.04-0.6 low high
    organic soil 0.5-3.0 very low very high
    source: University of Hawaii, 
    College of Tropical Agriculture and Humand Resources

    Various soil topics: 

    Some further thoughts about nutrient circulation
    The ideas expressed here, follow from the observation that nature has evolved its ecosystems over a very long time period, while gradually improving them. The thousands of organism species contributing to the functioning of such ecosystems, have evolved to give them the variety and resilience found today. In the process, nature has found ways to retain its most precious assets, the nutrients, in the most efficient way. When a tree drops a leaf, its nutrients must be retained to grow another leaf tomorrow. This is what soil organisms do.

    Suppose there were two competing ecosystems in the same area, one efficient at circulating and retaining its nutrients; the other not. Then the first would accumulate a larger asset, more biomass and health. It would outcompete the less efficient one. It follows then, that soil organisms have evolved over time to retain and circulate nutrients as efficiently as possible. 

    These nutrients are stored in the bodies of the living soil organisms, the animals, fungi and bacteria. The soil organisms will be able to do so only if their combined nutrient composition equals that of the living plant life above. It is not necessary that the soil contains an equal quantity of nutrients as the forest above, but only the capacity to recirculate the dying plant matter. Note in this respect, that trees over their very long time spans of hundreds to thousands of years, are able to amass a wealth of nutrients inside their dead woody tissues, even though wood contains fewer nutrients than leaves.

    Soil creatures, not being able to produce their own energy (like plants do), utilise the energy in woody tissues and the carbohydrates in leaves and fruits. They find energy easily from the dead bodies of animals, and from their wastes. It follows then that animals, whether grazers or carnivores, are not allowed to be 100% energy efficient in the digestion of their food. Their wastes must retain enough energy to power the soil's decomposition process. By contrast, the soil's organisms are 100% energy-efficient, burning the last atom of carbon in the process. If they were not, the soils would eventually accumulate rich stores of carbon, which is not the usual case (except in bogs and tundra). 

    In 2005 we discovered that the second law of thermodynamics applies to decomposition, which means that the energy stored in biomolecules is insufficient to decompose them fully. A slush of short organic molecules remains, which can be decomposed only when extra fuel is added, like sugars. Only plants can make these. Thus complete decomposition is not possible without living plants. For more read the section on the Dark Decay Assay on this website. It is an important discovery made by us.

    Sustainable systems

    On a scale from very artificial to very natural, the following agricultural systems can be distinguished:

    go to soil contents page <==>  go to part2