Signs of fatigue and natural limits Signs 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.
 

All three columns give rather frightening consequences:

• For 5.2 Pg/yr in direct consumption (col 1), humans are prepared to lose 16.9 Pg/yr permanently (in col 3). It could destroy all ecosystems in a very short period of time, the very reason why this section on soil was written. The seriousness of the situation is partly hidden by increasing agricultural yields, while not showing the amount of degradation of our seas.
• Net efficiency of human agricultural systems appears to be 5.2/41 = 13%.
• Of the 132 Pg/yr world production, humans claim 58 (=41+16.9) for themselves. The rest is for the ten billion other species.
• Of the remaining 80-90 Pg/yr, humans claim by deforestation a large share of 10 Pg/yr.
• How much should we leave alone as natural ecosystems for the millions of other species? There isn't much left!

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.

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.
 
 

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.

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.

Another 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.

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.

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.

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 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.
 

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:
 

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.
 

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.
 

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.
 

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.
 

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.

Sustainable systems

• hydroponics: there is no soil. The plants' roots stand in a flowing solution of nutrients and water. A glasshouse envelops the plants to regulate temperature and moisture. Carbon dioxide can be added to the atmosphere. Spraying for pest control can be done with windows closed, requiring minimal amounts. Productivity is high and optimal. Since no soil is used, it is essentially sustainable, but it requires much energy. Used solutions are often flushed out to sea.
• soil as substrate: in many cases of cropping, the soil is treated as just a substrate for the roots to hold on to. The soil's nutrient concentrations are maintained entirely or mostly artificially. If the soil is separated from the natural water table, as is the case in some traditional glasshouse horticulture, this system could be sustainable. But out in the open air and subjected to ploughing and tilling and rain, this system is not sustainable.
• seasonal monocultures in open air: these are the modern farms, controlled by artificial fertilisers, chemical pest control while lacking adequate cycling of nutrients. These systems cannot be sustainable, although no-till practices make a big improvement.
• mixed  farming: this is the traditional system of farming with crop and pasture rotations, confined meat animals and recycling of wastes, with some addition of artificial fertiliser and moisture control (irrigation). These systems have existed for over a thousand years and can be sustainable.
• padi culture: in the tropics, where enough water falls in the wet monsoon, the land is terraced into stacked ponds. In these shallow ponds, rice is cultivated, alternated with other crops in the dry monsoon. It is an agricultural system that has proved its worth for over a thousand years. Padis produce their own nitrogen supplies and as a result, the soil has remained rich and fertile. The terraced hillsides reduce soil erosion to the extent of permanent sustainability. Padis harvest their own water supplies. During the wet season, when waterlogged, padi soils lack oxygen and convert carbondioxide to methane, a powerful global warming gas.
• perennial monocultures: plantations of rubber, coffee, tea but also grasslands on flat to easy country, could be sustainable. Various improvements could be made to these systems, taking the sustainability rules above into consideration. (see also trees for grassland). Forests for paper and timber appear to be the most sustainable of all agricultural systems. Although the trees are harvested by clear-felling, the soil is immediately replanted, causing a minimum of erosion.
• perennial multicultures: these are the natural ecosystems that have sustained themselves for millions of years.