Soil fertility 2

by Dr J Floor Anthoni (2000)
Farmers have known for a very long time that certain substances, when added to the soil, improve production. These are now called fertilisers. For reasons of cost and ease of use, chemical fertilisers have replaced natural ones. Although plants can't distinguish the difference, artificial fertilisers can easily be over-used, resulting in damage to the soil, rivers and ocean. Learn to know how to produce more, while damaging the soil less.


This document consists of part1 and part2 (this page)

plant needs
What do plants need? Liebig's law states that the need in shortest supply will be the main factor limiting growth. Often overlooked needs are light and warmth.
watering A plant's most important need is water. In most places on Earth, water is a problem. There is either too much of it or too little. Water is needed by soil organisms too, so a farmer's most urgent task is to manage the supply of water.
nutrients Nutrients are found in the rocks. Once weathered into soil, these become available to plants. This supply is not enough, the reason why all terrestrial ecosystems recycle their nutrients with minimal losses. Agricultural soil should recycle its nutrients too, but there are insurmountable problems.
fertilising Is fertilising necessary and how is it done? How can the fertility of the soil be enhanced and maintained?
trees for grassland Bringing variety in a monoculture can bring additional fertility. Here the case for trees in grassland is studied. 
salt Because plants do not need salt the way animals and humans do, salt is easily lost from our soils, particularly through modern farming. Produce does not only taste weak, it also contains fewer salts. Salt deficiency in society may arrise from other causes too.


Essential elements for life in the universe, solar system, earth, sea, plants, animals.
NPKnowledge: Iowa State University publications about fertilising and soil management.
more about soil: go to soil index <==> go to part1
-- home -- soil -- environment -- issues -- Revised: 20010527,20051122,20070718,

abundance of elementsWhereas H, O and C are copiously available in water and carbon dioxide, the macro nutrients N, P and S are not. In this barchart one can see the relative abundances of the elements for life. Note that the scale extends over 6 decades from 1 to one million ppm.
The brown bars represent the elemental concentration in the planet's crust and the green bars those in plants. Not taking notice of H, O, C, Si, Al, Fe which are the abundant elements contributing to soil hard matter, water and carbon dioxide, the remaining ones are more important to the chemistry of plants and animals. Of these the four macro nutrients are N, Ca, K, Mg, P, S. Of these six, the ratios vary between 1/3 rd of a division (50%) to nearly 2 divisions (1%). Only nitrogen is less abundant in the crust than in plant matter. 

But scientific data is not unanimous on this issue. Below follows a table as published by W Larcher, 1980 :

element stored
in soil ppm
land plants
dry matter ppm
land plants
average ppm
N nitrogen
P phosphorus
S sulphur
K potassium
Ca lime
Mg magnesium
Fe iron
Mn manganes
Zn zinc
Cu copper
Mo molybdenum
B boron
Cl chlorine
Ni nickel
Se selenium
Co cobalt
I iodine

The table shows soil concentrations and those found in plants and a concentration factor that shows how much plants have to concentrate the element. As one can see, rather large differences between plants exist, and nitrogen is the element in shortest supply. A word of caution on the use of the 'dry matter' weight, which is obtained by driving all water out. Plants and trees have two very different kinds of tissue: the dead woody stems and the living matter consisting of hair roots, bark and leaves. Their 'ash weight', obtained by driving all carbon and oxygen out, differs tenfold in favour of living tissues. Ash contains the elements in the table above.

Depending on which fertiliser formula to use, plants need about eight times more nitrogen than phosphorus and sulphur. In recent history, too much phosphorus was applied to soils because it was believed that it 'disappeared' in the soil's processes. Ratios of N:P = 2:1 were common but nowadays, soils are replenished in ratios N:P = 6:1 (and N:P:K = 100:18:22 is now a world-wide average), which is more in line with what has been harvested from the soil. The soil's ability to 'fix' (bind) phosphorus compounds is problematic in the sense that it is no longer readily available to plants, but a blessing since it won't leach away easily. Alkaline soils release phosphorus reluctantly but soils rich in soil organisms, make this mineral freely available.

yield, fertiliser and pollutionThese graphs show a number of important facts relating to the use of artificial fertiliser. The world is now using an amount of fertiliser corresponding to 90 million ton of nitrogen per year, equivalent to 90 x 200 = 18 trillion ton (Gt) of vegetable matter or 9 Gt carbon per year. To bring this into perspective, the primary production of the entire world ecosystems is 160 Gt carbon per year, 100 on land and 60 in the sea (Woodwell & Pecan in Carbon and the Biosphere). The total amount of N applied since 1950 is about 2.5Gt (the area under the curve = 0.5 x 50 years x 100 Mt); the mass of the atmosphere is 5,200,000 Gt and the total amount of nitrogen there 4,000,000 Gt, which appears to make human production pale in insignificance (note that one gigaton Gt equals one petagram Pg).

However, it is estimated (Vaclav Smil, 1997) that around 175 Mt nitrogen flow into the world's croplands every year, and about half this total becomes incorporated into plants. Synthetic fertiliser provides about 40% of all the nitrogen taken up by these crops. Because they furnish - directly as crops and indirectly as animal foods - about 75% of all nitrogen in consumed proteins (the rest comes from fish and from meat and dairy foodstuff produced by grazing), about one third of the protein in humanity's diet depends on synthetic nitrogen fertiliser. Children born today, may grow up with 50% of their bodies' protein made from artificially fixed nitrogen.
Note that most fish is caught in coastal seas where they grew on a supply of plankton originating from the fertiliser in the run-off from the land, and thus 50% artificial fertiliser. Dairy grassland is now heavily fertilised with 'artificial' nitrogen, although most hill country grasslands obtain theirs naturally from clover. It is obvious that much of it goes to waste and that there are ecological limits.

Note also that the elements in artificial fertiliser are in no way distinguishable from those in natural fertiliser. Mankind cannot make new elements. A nitrate anion fixed from nitrogen and oxygen by humans, is exactly the same as one fixed by a bacterium or a thunderstorm.

Fertility of Argentine Pampas croplandIn the right-hand diagram above, one sees the tremendous progress made in the yield of all crops, of which the cereal grains, shown here, are a good example. In the course of fifty years, the use of fertiliser grew ten-fold, in order to double productivity, but fertility expressed as productivity per tonne of fertiliser dropped world-wide, as showh in the diagram on right, for Argentine Pampas cropland. The grain yield expressed as tonnes production per ha, grew twofold, following the world average, but the fertility of the area expressed as production for the amount of fertiliser used, dropped 10 to 20-fold! Such farmland is probably close to the state of ecological collapse, with erosion accelerating.
(Source: Hall, A J et al "Field crop systems of the Pampas" in Pearson C J (Ed), Ecosystems of the world: Field crop ecosystems. Elsevier, Amsterdam. 1992).

It must be noted that those gains were not made by artificial fertiliser alone, but also from irrigation, multiple harvests and improved crop varieties. In the top diagram, the amount of nitrate in rivers is shown. Note in this respect that 50ppm is the health limit for drinking water and a marine aquarium is in serious trouble with this concentration. In  Holland, the nitrate concentrations in groundwater average 134ppm and 243ppm underneath dairy farms! Pollution control measures in Europe have flattened the Rhine pollution, but elsewhere nitrate concentrations are on the rise. It is interesting to note that the pollution concentration in rivers is less in high rainfall areas (due to water dilution) and more when the catchment area has much crop land (the source of it).

Note at this point that a number of crops do not react well to nitrogen:

Rhizobium root nodules
Some plants have root nodules infested with nitrogen-fixing bacteria. Receiving oxygen and food from the plant, these bacteria provide the plant with nitrogen.
Azolla ferns
Azolla ferns (Azolla pinnata) grow in rice paddies. They can fix nitrogen from the air and make it available to rice plants.

Genetic engineers may ultimately succeed in splicing the Rhizobium bacterium into the root cells of our most important food plants, endowing them with their own nitrogen factories, but this is no easy task.

Effects of nitrogen fertiliserThe graph here shows how the soil and plants react to nitrogenous fertiliser. It is a conceptual diagram, in the sense that it shows the principles of what is happening, rather than exact data, because each plant species reacts differently. It is an important graph, because it also represents a typical ecosystem response to an artificial addition of a natural component. Horizontally the amount of fertiliser applied, vertically its relative effects. If you wish to tag some figures here, saturation corresponds to 50 kg/ha; sufficiency to 20-30 kg/ha. The coloured bands in the background correspond to the qualitative conditions of being limited, sufficient, saturated and excessive. To bring this into perspective, many places in the world, experience fertiliser-laden rain (acid rain) to the tune of 50-100kg/ha, originating from industrial processes. This unsolicited input has a major influence on forests, dunes, marshes, and croplands.

As fertiliser is applied, its effect is immediately visible in an increase of net primary productivity (NPP at around 10 kg dry matter for 1 kg N), which decreases sharply after saturation is exceeded (perhaps 16 t/ha vegetation with 8 t/ha harvest maximum) and nitrogen gradually becomes poisonous. Ironically, the amount of green foliage increases with further N application, but at a much slower rate, due to reduced NPP. Nitrogen mineralisation represents the amount that is accommodated in the soil and foliage. It drops sharply after exceeding sufficiency. Excessive nitrogen now leaches away with rain water and as the soil rejects excessive nitrogen, bacteria convert more of it into gas (NH3, N2 and N2O). Nitric oxides (N2O, laughing gas) are very powerful greenhouse gases, about 280 times more potent than carbon dioxide. Their contribution to global warming already amounts to 6%. As the world attempts to squeeze more production out of the dwindling area of cropland, nitric oxide's contribution to global warming will increase sharply.

Estimated global losses of nitrogen amount to 10kg/ha on flat land and 50kg/ha on sloping land (2-4 degrees) in windy areas. Nitrogen loss in the form of ammonia (NH3, a potent greenhouse gas) escaping into the air, is about 25 kg per head of cattle per year. Phosphorus losses would be ten times less, because phosphorus compounds are less soluble and much less of it is applied.

Nitrogen compounds in the biosphere
  nitrogen gas ammonia urea amino acids proteins
Formula N2 NH3 NH2.CO.NH2 n(CH2).m(NH2).COOH -
Nitrogen share by weight 100% 82% 47% 8-27% ~16%
Biosphere abundance (Gt) 10,000* 10 0.01 10 1
(*) does not agree with the 4 million Gt at the beginning of this chapter. Treat these values as indications of their magnitude.
Source: V Smil, SciAm July 1997

Reference: V Smil: Global population and the nitrogen cycle. SciAm July 1997.

Potassium (K) is important for photosynthesis and in the formation of amino acids and protein from ammonium ions. Potassium deficiency shows as premature death of leaves, and an increased sensitivity to stresses. Potassium, like phosphorus may become fixated in the soil, and thus unavailable to plants, however, this element is much easier leached from soils than phosphorus. Optimum pH for availability is 6-8. The three-layer smectite clays contain potassium in their structure, but the two-layer kaolinitic clays do not, and are usually deficient in potassium.

Calcium (Ca) deficiency in plants causes stunting of roots and leaves. Plants need large quantities of calcium. Lucerne contains nearly 3%. Lime is added to soils to regulate soil acidity, and over-liming (alkaline soil with pH above 7) could reduce the availability of nitrogen, phosphorus and potassium.

Sulphur (S) is not required in large quantities by growing plants, but is nevertheless an important nutrient. Its cycle in nature is similar to that of nitrogen. Small amounts of sulphur are obtained from sulphur dioxide (SO2) in the atmosphere, produced by volcanoes and the burning of fossil fuel. Sulphur is recycled from the oceans in the form of dimethyl sulphide (CH3.S.CH3)

From the previous chapters we have seen that farming for the very long term is a delicate balancing act. Fertilisers can help the soil, but they can also cause damage. The natural ecosystems of the world have never needed additional fertilisers, so why do humans need it now?

In very primitive societies, the production of food was motivated by hunger. The search for food stopped when the tummy was full. Overharvesting was unknown. As societies became more sophisticated, the reasons for producing food changed. Agricultural societies started to produce food for others. Today's farmers do it for money. They are able to do so because of world trade, a monetary system, transport, subsidies and means of preservation. Let's be honest: food is not produced because someone somewhere else on the planet is hungry. It is not produced 'to feed the world'. The free market system just happens to distribute it to those who can afford to pay, making it seem so.

Everywhere in the world and over many thousands of years, farming has been a hit-and-miss affair. Land was cleared and farmed. If it failed, the land reverted back to scrub and forest or was lost altogether, leaving the bones of the land, the naked rock, behind. Today's farming is very much the same, but in the meantime we hope to have learnt from some of our mistakes.

There are three principal reasons for applying fertiliser:

The table below shows some of the most popular fertilisers in use:
Commonly used fertilisers
type of fertiliser formula active element comments
Ammonium bicarbonate NH4.HCO3 N 18% Highly volatile. Needs sealed bags. Best applied underground. Up to 50% lost after application. Cheap to make.
Ammonium nitrate NH4.NO3 N 35% Very potent fertiliser made by the Haber-Bosch industrial process. Dissolves easily into water and is taken up quickly by plants. Leaches out quickly too.
Urea NH2.CO.NH2 N 47% Slow-release fertiliser that needs to be converted by soil bacteria before ammonia is available to the plant. Urea pellets are easy to pack, handle, store and apply.
Rock phosphate Ca3(PO4)2 P 19%
Ca 38%
Adds both phosphorus and calcium. Mined from guano (sea bird excretions) deposits.
Super phosphate CaSO4 53%
CaP2O5 34%
S 12%
P 11%
Ca 24%
Made from rock phosphate or bones, ground to a powder, then mixed with sulphuric acid. Popular in New Zealand and Australia.
Super phosphate Ca(H2PO4)2 P 26% 
Ca 17%
The most common form of phosphate fertiliser, highly soluble
Triple super CaHPO4 P 23% Ca 29% Widely used, slightly soluble in water.
Lime, calcite CaCO3 Ca 40% Adds calcium to the soil and combats acidity. 
Potassium oxide K2O K 83% Adds potassium
Other fertilisers used: liquid ammonia (NH4.OH), ammonium sulphate ((NH4)2.SO4), muriate of potash (KCl), potassium carbonate (K2CO3), lime (CaO, Ca(OH)2, CaCO3, dolomite (CaMg(CO3)2), calcium bicarbonate (Ca(HCO3)2),

Soil tests give soil concentrations in ppm (parts per million). Two different values are obtained for phosphorus, depending on the type of test: Bray or Olsen. Olsen figures are generally 30% lower than those from Bray tests. Soil is considered optimal with P concentrations of 10-20 ppm; deficient with 0-5 ppm. Typical application rates are 50 kg/ha (P2O5) for high intensity farming.
Potassium is extracted from soil with ammonium acetate, giving an optimal soil at 90-130 ppm and deficient soil at 0-50 ppm. Typical application rates are 40 kg/ha (K2O) for high intensity farming. Note that 1000 ppm = 1 kg/ton.
If fertiliser is withheld, highly productive dairy pasture degrades by 5% and hill country by 10-15% per year, to level off at around 30-40% less yield.

In traditional farming systems, there was enough recycling of animal and human wastes to keep up with losses from harvesting. The cropland was cycled between cropping and grazing. Confined meat animals were fed the residues from crops and the feedlot crops that were grown specially for them. Their wastes were recycled onto the land, just in time before the new crop needed its nutrients. Crop diseases were combated by suitable rotations, rather than by chemical means. It is a way of farming that has earned its existence for well over a thousand years, and that is finding much support today by those advocating permaculture.

But modern cropping and meat production have been allowed to proceed independently, the one being located far away from the other, making transport costs too high to justify recycling of wastes. It is a freemarket idea that has no respect for sustainability, but it can be changed by relocating chicken and pork farmers to where their wastes can be recycled.

We have seen from the preceding chapters already, that the soil's natural fertility is contained in the soil organisms and hardly anywhere else. This will be treated more firmly in the chapter on sustainability. In deciding how to treat the soil, be it by ploughing, fertilising or pest control, the sustainability-conscious farmer must first of all think about the soil biota. What do they need? How do they wish to be treated? What to do to get more of them? In this respect, artificial fertiliser is not the same as recycled wastes. Artificial fertiliser contains only the nutrients for plants. Their wastes and roots then feed the soil. By contrast, animal wastes feed the soil and the soil makes the nutrients available for the crops. It is an important difference.

There is also a huge difference between perennial crops such as tea, coffee, rubber, pasture, and seasonal crops such as beans, wheat, potatoes. Perennial crops do not require ploughing, a continuous disruption of the soil, which is very detrimental to soil biota. The longevity of these crops allows the soil to adjust to the new plant community above it and to retain those nutrients that are most needed. Perennial crops do not have sudden needs for fertiliser like seasonal crops do.

Most human crops differ remarkably from natural ecosystems and their communities in their complexity. Human crops are almost all monocultures, whereas natural communities have the highest possible variety for the given locale of climate and soil type. A high variety of plants means that their average composition of nutrients better matches that of an optimal soil community. In other words, plant variety increases soil fertility. This is explained in more detail in the chapter on sustainability. Although no proof exists (yet), it is to be expected that meadows with a variety of grasses and other plants, maintain a more fertile soil than those with a monoculture of one species of grass.

Minimising risks
Being intimately involved with his farm, knowing its history and having observed how it reacts to varying circumstances and trials, the farmer is the person most suited to judge environmental risks. Here are some general practices to reduce risk to the environment:

Using natural rock as fertiliser
Proponents of permaculture and organic farming use pulverised natural 'hard rock' as a means of natural fertiliser. It is spread over the land. Soil organisms then cover it and bring it into the moist and acid environment that promotes weathering at a rate exceeding that of natural weathering of 1 ton/ha.  Of course, at the C horizon, the base rock weathers at this rate, bringing new nutrients. Often the hard rock is a metamorphosed sedimentary rock like greywacke or an igneous rock like basalt. The chemical constitution of these two differ, but on average one could expect the following yield in nutrients (see table of abundance of elements):
Estimated composition of rock fertiliser
element rock [1]
rock [1]
soil [2]
N - nitrogen
P - phosphorus
K - potash
S - sulphur
Ca - lime, calcium
Mg - magnesium
[1] average concentration of the Earth's crust, for lack of details on greywacke, granite and basalt.
[2] see table above

As one can see, it is a very inefficient way of increasing fertility, when only 50 kg of 1000 kg (5%) rock produces useful fertiliser, compared to about 40-50% in artificial fertilisers, and it does not add nitrogen. The table also illustrates the limits posed to natural soil productivity when produce is taken off the land and not recycled. It roughly corresponds to 10kg fertiliser per ha (N:P:K=8:1:1, excluding N) for forest soils weathering at the rate of 1t/ha, and perhaps 40kg/ha for agricultural land which weathers faster.
Reader please note: these figures are estimates and I'd be interested to obtain actual measurements and data from field experiments. E-mail Floor Anthoni.

Trees for grassland
trees for soilThis diagram illustrates an ecological idea for extending the fertility of grassland soils while at the same time providing better protection against erosion. The idea is to plant suitable trees in moderately hilly grassland. Their roots reach much deeper down than those of grass, so that they are able to draw nutrients from deeper down, thereby also assisting the weathering process. The leaf and branch fall feeds the soil organisms who return the nutrients to the A horizon. The following benefits are obtained:
The idea is particularly useful for sloping terrain that is unsuitable for cropping and difficult to fertilise. The trees themselves need to satisfy the following requirements: Some poplar trees may be suitable, but here is certainly a challenge for scientists and the farming community.

A question of salt
Two of the most important elements in the human body are sodium and chlorine, known as salt (NaCl, sodiumchloride).  Chlorides play an essential role in the neutrality and pressure of extracellular fluids and in the acid-base balance of the body. Hydrochloric acid is produced in the stomach for the digestion of food. It is also lost in sweat, urine and faeces (92%). The body's supply of chlorine can deplete rapidly through excessive perspiration or loss of acid in the body. It is found in animal products, including dairy products, but little in vegetables.

Sodium is an element that functions with chloride and bicarbonate to maintain the balance of positive and negative ions in body fluids and tissues. Sodium has the property of holding water in body tissues. Excess sodium may result in edema or water retention. Too little of it disturbs the tissue-water and acid-base balance, necessary for good nutritional status. The hormone aldosterone controls the balance of sodium and water in the body. Symptoms of sodium deficiency may include feelings of weakness, apathy, nausea, cramps. Sodium is found in all animal foods and table salt. (See also periodic table/nutrient deficiency)

There is no doubt that salt is an important nutrient for humans, yet a number of recent developments could render society suffering from salt-related deficiencies. What follows in this subchapter, are my own observations, not (yet) verified by scientific method. However, they are important enough to raise awareness. Note, that although we will talk only about salt, it includes a host of other minerals that are not essential to plants but only (or mainly) to animals (boron, magnesium, fluorine, iodine, iron, chromium, manganese, molybdenum, selenium, silicon, vanadium and so on). I am using the word salt as also including the salt balance. Many people ingest an excess of salt from commercial foods like snacks, fast food, bread, cereals, etc., but this is pure table salt, lacking the balance of essential minerals.

Salt loss in soils
Plants do not need salt for their functioning (see also abundance table/soil&plants), but they include salt in their body tissues when absorbing water through their roots. The process by which plants absorb water, is called osmosis. Water is drawn from a weak concentration (the soil), through a 'semi-permeable' membrane, to a higher concentration of salts (the plant). Plants always maintain their body fluids more concentrated than the soil. They do so by evaporating pure water through their leaves. If the soil is dry, plants need higher concentrations than when the soil is moist, reason why desert plants are saltier. When the soil is salty, plants also need higher concentrations, like mangrove trees standing in seawater.

But salt is highly soluble in water, and is lost rapidly from the soil. Plants do not mind this, but the soil organisms do. They need salt just like humans do, and they store it jealously inside their body tissues, cycling it between them and the plants above ground.  As modern farming becomes more reliant on artificial fertilisers, rather than the soil's natural fertility, the soil organisms are lost, and with it the pool of underground salt. As a result, modern produce has become tasteless  and 'watery', while providing less salt.

A number of beliefs are doing the rounds in society, limiting the amounts or balance of salt in our bodies:

Sea salt, rock salt and table salt
The best replenishment of body salts comes from salt, rich in other minerals, such as natural sea salt. But the salt available in shops is table salt, a very refined form of sea salt. Table salt has lost all other minerals, but iodine is added at the end to prevent the iodine-deficiency disease goitre (the swelling of one's thyroid glands in the neck). This fact alone should have warned scientists of the possibility of other, as yet unrecognised, salt-related deficiencies. In any case, pure sea salt gives perhaps the best replenishment of our mineral needs. (See detailed composition of sea water)

The problem is that nearly all salt products on the market have deliberately been mislabelled as 'sea salt' or 'rock salt', their contents being ordinary refined table salt, but with bigger crystals. Pure sea salt is recognisable by the following qualities:

Rock salt is the salt found underground as compressed haline rock. It originates from an inland sea or lake having dried up, then covered by sediments. In principle, rock salt has the same composition as today's sea salt, but geological processes such as leaching, may have changed it. In order to be safe, use sea salt.

more about soil: go to soil index <==> go to part1