Soil fertility

the factors that add to soil productivity

by Dr J Floor Anthoni (2000)
Farmers have known for a very long time that certain substances (such as dung and ash), 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 and environment less. What do plants need, how are nutrients formed and maintained and what can we do to increase the natural fertility of the soil?

This document consists of part1 (this page) and part2.

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.


NPKnowledge: Iowa State University publications about fertilising and soil management.
more about soil <==> go to part2
-- home -- soil -- environment -- issues -- Revised: 20010527,20051122,20070718,

Plant needs
In the chapter on geology, we've seen that the base rocks from which soil is weathered, ends up quite different in composition from place to place, but in practice all fertile soils on earth follow the rather constant chemical composition of plants, which is similar to animals. This can be understood from the way plants, animals and soil form an ecosystem, cycling the available nutrients many times before they get lost. In the process, unnecessary concentrations of elements (like salt and chlorine) do get lost, resulting in concentrations of available soil elements, closely matching plants' needs, everywhere on Earth. Although their ratios of elements are similar, soils may vary considerably as to their densities, and thus fertility.

Underground, the soil nutrients are not kept in solution but inside the bodies of the living organisms (and some are adsorbed onto clay platelets). No wonder then that the amount of life in good soils is 2 to 10 times more than that above ground. The nutrients become available when some organisms die, which happens frequently because they grow fast. But it does not happen in sudden boosts, as is needed for a monoculture that has been planted all at the same time, like a potato crop. In this respect, natural, productive soil appears to need more fertiliser than it actually does. Modern farming, driven by economic constraints, is forced to use artificial fertilisers, often to the detriment of the soil's natural fertility.

The ecologist Edward S Deevey Jr discovered that living matter consists mainly of the six elements hydrogen (H), oxygen (O), carbon (C), nitrogen (N), phosphorus (P) and sulphur (S), in the ratio H:O:C:N:P:S = 2960:1480:1480:16:1.8:1, which is an average for all living organisms on Earth. Of these, the woody plants far outnumber all others, so the formula is biased towards these. The ratio H:O:C:N:P:S = 1600:800:800:9:1:5 is often used for land plants, and 212:106:106:16:1:2 for marine plants and soil humus. From an ecological perspective, it would not be surprising if scientists discover that these ratios for terrestrial life (green matter in plants + animals) are the same as for soil biota (bacteria + fungi + animals). By comparison, the most common component of  plants are the carbohydrates (sugars, starches and woody substances), represented by H:O:C = 12:6:6 atoms, or as masses 1:6:8.
With C, O and N having similar atomic masses (12, 16, 14), as a rule of thumb, each unit of nitrogen belongs to 200 units of life (dried) and 100 units of carbon.

What every plant needs for growth is:

The soil biota have similar requirements, but since they do not photosynthesise, they need neither light nor carbon dioxide. The requirements above are often called 'limiting factors' because each could limit the plant's growth. More accurately, they should be called 'life-determining factors'.

Liebig's Law
The scientist Liebig discovered that all of the above needs need to be satisfied, and that the one in shortest supply will be the main cause of limiting growth. Thus in winter, when it freezes, plants do not need either carbon dioxide or water or nutrients. What they need is warmth first.

Sunlight and warmth
Sunlight and warmth go together, since the only input of energy comes from the sun (see oceanography/radiation). Seasonal cycles affect particularly the temperate areas. But it can be influenced considerably. A glasshouse for instance, traps heat radiation by trapping visible light but preventing infrared radiation from escaping. In cold climates, glasshouses are often heated by burning fossil fuel. Cropland can be sheltered from cold winds, by means of shelter belts. Heat from sunlight can be trapped by stands of vegetation. Evaporation from soil causes enormous loss of warmth, but it can be minimised by mulching or planting a soil-covering crop.

The amount of sunlight in summer may be too much, causing the soil to dry out. Sheltering trees can be planted that lose their leaves in winter. Crops can be spaced properly to prevent them shading each other out.

Carbon dioxide
Carbon dioxide is rather scarce in our atmosphere, where it is found as one molecule in every 30,000. All plants on the planet compete for this resource, since all places on earth connect to the same atmospheric pool of carbon dioxide. The most successful plants, living in warm tropical areas scavenge it more successfully than plants living in cool areas with less light.
Only recently did nature evolve a plant, capable of converting carbon dioxide more efficiently than any other plant, while also using less water. Their photosynthetic conversion requires four biochemical steps, rather than the usual three, a process that saves it both energy and water. These plants, called C4 plants, include the bamboo-like grasses, and the agricultural crops sugarcane, maize and sorghum. They are about twice as efficient in converting sunlight and need four times less water. C3 plants have maximum sunlight conversion efficiency of 15% and C4 grasses up to 24%. In practice, due to leaf shading, these figures are five times lower. Photosynthesis in C3 plants converts 0.1-0.4 g CO2 with 1 kg water, whereas C4 plants convert 0.4-0.8 gram.

Succulent plants are active at night, taking up CO2 with their stomata (leaf pores) wide open, when other plants close theirs to minimise respiration. During the night, CO2 is absorbed and converted into chemical storage as oxaloacetic acid and then as malate. During the day, these compounds are converted and normal C3 photosynthesis takes place, with the plant's leaf pores closed to prevent unnecessary evaporation. This special form of CO2 fixation is called Crassulean Acid Metabolism (CAM). CAM plants are succulents, agaves, lilies, bromeliads, orchids, cacti, euphorbia, geraniums and many more. They use a minimum of water. (For more details and differences between C3, C4 and CAM plants, see the table below)

As can be expected, the C3 plants, which are limited in their CO2 uptake, react more vigorously to CO2 increases than the C4 plants. They also still outnumber the C4 plants, which are limited by temperature.

In externally heated glasshouses, carbondioxide from burnt fossil fuel for heating, is often piped into the glasshouse to enhance growth.

Water and nutrients will be discussed in their own subchapters below. See also the periodic table of elements for essential nutrient needs and symptoms of deficiency in plants, animals and humans.

Differences between C3, C4 and CAM plants
characteristic C3  C4 CAM
leaf structure laminar mesophyll, parynchymatic bundle sheaths mesophyll arranged radially around chlorenchymatic bundle sheaths laminar mesophyll, large vacuole
chloroplasts granal mesophyll granal, bundle-sheath cells granal or agranal. granal
chlorophyll a/b ratio ~ 3:1 ~ 4:1 < 3:1
CO2-compensation concentration at optimal temperature 30-70ppm <10 ppm in light: 0-200 ppm
in dark: <5 ppm
primary CO2 acceptor RuBP PEP in light: RuBP
in dark: PEP
first product of photosynthesis C3 acids (PGA) C4 acids (malate, asparate) in light: PGA
in dark: malate
carbon-isotope ratio in photosynthates -2 to -4 % -1 to -2 % -1 to -3.5 %
photosynthesis depression by O2 yes no yes
CO2 release in light yes no no
net photosynthetic capacity slight to high high to very high in light: slight
in dark: medium
light-saturation of photosynthesis at intermediate intensities no saturation at highest intensities at intermediate to high intensities
redistribution of assimilation products slow rapid variable
dry-matter production medium high low
From W Larcher: Physiological plant ecology, 1980. After Black 1973, Laetsch 1974, Tieszen 1975, etc.

Water is by far the most restrictive of a plant's needs. In spite of the massive size of the water cycle, which causes rain and snowfall, water is in short supply in most areas of the world, at least during one or more seasons. Water is not only necessary for a plant's survival but also for its soil biota, on which it ultimately depends. Likewise, the success of farming, depends mainly on how to keep the underground 'circus' alive, and with it, the above ground vegetation.

Plants need water, a large amount of it when growing. The table below gives an indication of how much water is transpired to produce one kg of dry matter.

Average transpiration ratios for various plant types
Water amounts in kg per kg dry matter (transpiration ratio).
C3 plant type water C4 plant type water
Potatoes and beets
Sunflowers (young)
Sunflowers (flowering)
Tropical foliage trees & crop plants
Temperate foliage trees
Oil palms
Maize/sorghum in field experiments
Maize in growth chamber

CAM plants



Source: W Larcher: Physiological plant ecology. 1980. Springer Verlag.

A hectare of highly productive grain produces 8 ton of grain and some 10 ton dry matter, requiring some 10 million litres of water during the season (4 months) for photosynthesis alone, or 100,000 litres per day, or 1000mm of rain!
It is common sense therefore, to irrigate crops for higher productivity, and also to increase the cropping area. Particularly as an insurance against the vagaries of weather and climate, farmers all over the world are tapping whatever water resources they can find. The most common of these are ground water and artificial lakes.
Ground water and aquifers
Although soil and rock are compressed by tremendous forces, there are nonetheless gaps and cracks that have been interconnected by flowing water. One would have expected that water, being three times lighter than rock, is pushed up as sediments and rocks are pushed down by their own weight, so that free water cannot exist at depth. However, as can be observed in limestone Karst systems, water can exist deep down to 300 m and perhaps even deeper. What's more, these underground aquifers are interconnected as if it were a single underground lake, accessible by all who live above it. 

Pumping groundwater aquifers is so attractive because the water does not need to be transported. But aquifers replenish slowly. The deeper they are, the longer it takes. Saudi Arabia is estimated to have some 2000 cubic km of 10,000 - 30,000 year old water stored in aquifers to 300m deep.

Ogallala Aquifer USAThe Ogallala aquifer in the USA spans eight states, covering some 452,000 square km, and estimated to hold 3700 cubic km of water, a volume equal to the annual flow of more than 200 Colorado rivers, an underground 'lake' of 120m deep. Today, the Ogallala alone, waters 20% of US irrigated land, depleting it by 12 cukm/yr. In several decades of pumping, the 3700 cukm reservoir has been shrunk by 325 cukm, facing extinction 300 years from now. It is the typical tale of all ground water reservoirs in the world.

Bangladesh is sinking into the sea because its groundwater has been pumped so extensively. In other places the lower water table is drying out valuable wetland areas. One may think that it is a stupid idea to pump water from underneath the plant's roots in order to put it on top of the land, where much of it evaporates. Yet this is exactly what has been happening all over the world. Since the groundwater is used by all but owned by none, it follows the 'tragic of the commons' (why should I limit my use, when the other guy is not?), unless rigorously managed by governments.

Groundwater is formed from water penetrating the soil and sinking to deeper levels. As it is pumped, the water table drops, encouraging water to flow more freely and thereby carrying substances that should not be there. The table below gives an idea of the kinds of threats to groundwater systems and how these affect humans. Note that the effects on the environment are not mentioned.

Chemical threats to groundwater
threat source effects where
pesticides runoff from farms, backyards, golf courses, landfills organochlorides linked to reproductive and endocrine damage in wildlife; organophosphates and carbamates linked to liver and nervous system damage and cancers. USA, eastern Europe, China, India
nitrates fertiliser runoff; manure from livestock operations; septic systems. restricts amount of oxygen reaching brain, which can cause death in infants (blue baby syndrome). Mid-Atlantic USA, north China plain, western Europe, northern India.
underground petroleum storage tanks benzene and other petrochemicals can cause cancer, even at low exposure USA, United Kingdom, parts of former Soviet Union.
chlorinated solvents metals and plastics degreasing; fabric cleaning; electronics and aircraft manufacturing. linked to reproductive disorders and some cancers. western USA, industrial zones in East Asia.
arsenic naturally occurring nervous system and liver damage; skin cancers Bangladesh, eastern India, Nepal, Taiwan.
fluoride naturally occurring dental problems; crippling spinal and bone damage. northern China, northwestern India.
source: World Watch Institute: Vital Signs 2000.
Irrigation from artificial lakes
About 6000 years ago the Sumerians invented irrigation by diverting water from the Euphrates river to their crop lands. It improved yield and living conditions considerably. Today, wherever feasible, rivers are dammed for hydro electricity and irrigation. The high water pressure makes it possible to transport high volumes of water through a system of reticulated pipes. When carefully managed, it allows farmers to extend their cropping season and to increase productivity. One would think that irrigation is just another form of rainfall, but it is not.

The water collecting in a reservoir is the run-off from rain falling on the upper-catchment area. In its journey to the lake, it has dissolved valuable nutrients but also the not so valuable salts that have been discarded by living soil. If this water were applied to soils that experience a good soaking several times per year, the salts would be washed further down the slopes, eventually ending in the sea. But so often it is the irrigated land's main source of water. As water evaporates from the soil, it leaves the salts behind, resulting in gradual salinisation which degrades the land. Much irrigated cropland has been lost this way. As stated before, it is difficult (or risky) to bring dry land into production. Irrigation from lakes can help in some climate situations, mainly to reduce the risk of drought. Hydro lakes do reduce the flow of the river, resulting in less flooding downstream and thus less soil fertility replenishment. The Aswan Dam in Egypt has caused such problems.

The table below shows how much world agriculture depends on irrigation of its crops. Not surprisingly, the driest countries rely on it the most, and it is in these places that irrigation brings its problems. In the table below, padi culture has been included as irrigated land, but this is a sustainable form of water harvesting. The growth of irrigated cropland first kept pace with world population growth, but is now falling behind, mainly because the most suitable land has been used. About 20% of irrigated land is damaged by salinisation.

Irrigated area in the top 20 countries and the world
country irrigated 
% of
country irrigated 
% of 
United States
North Africa
Saudi Arabia



Source: UN FAO 1996 Production Yearbook.; various other sources.

Water harvesting
Having a water lake above each farm sounds like a good idea. The stored water can reach lower farmland through the water table or by being piped there. Small lakes or ponds are used in this way to provide for drinking water for grazing stock, but the larger lakes are too much of an engineering challenge. 
One sound ecological way is to leave a stand of forest above each farm, crowning the hill tops. Forests can soak up large quantities of water and release these slowly down-slope. Hill tops are difficult to farm because of their low water tables, but they are relatively flat, offering access to tractors, a reason why many have been denuded. But in Japan, steep hillsides and hill tops have been left alone, clad in their native forests.
Water saving
Water can be saved by reducing evaporation direct from the soil. Water evaporates faster in high temperatures and wind. So if wind speed can be reduced at soil level (and above it) while the soil can be kept cool, much water loss can be avoided. Covering the soil with mulch and erecting wind break hedges is one solution. In Spain and around the Mediterranean Sea, where the climate is too dry in summer, farmers till the soil under their olive trees to prevent weeds drawing water and mulch the soil with tilled, dry soil. However, this method leaves the soil wide open to erosion when sudden rains appear.

Irrigation through open and unpaved channels, and applying it to the land through surface furrows, may lose 50% of the water into the soil where it is not needed and through evaporation. Applying water to crops by means of drip irrigation, although more expensive, can reach up to 95% efficiency in water use. Water savings have been achieved by replacing high pressure sprinklers that make fine droplets, with low pressure sprinklers making large droplets.
In many places in the world, fresh water is now a commodity that can be traded in the freemarket. With the aim of encouraging farmers to conserve water, it has also opened the way to feudal land ownership and water rights being bought by industries and cities, who are in a better position to offer higher bids.

Water horror stories

It is evident that practically everywhere on Earth, the amount of irrigation water is seriously overdrawn. Prospects for increasing agricultural yield are therefore not optimistic. It is not only land that needs water in large volumes, but also industries and people. As the world's population grows and becomes urbanised (where else are jobs found?), water may shift to where it is valued more, the industries and cities. In 25 years, India will add some 340 million people to its cities, more than the current population of the USA and Canada combined. Saving water is not just an agricultural problem, but should be achieved in cities as well.
world fresh water resourcesThis diagram shows how the world's fresh water resources are heading for a climax. Net fresh water falling on the land is about 40,000 cubic km/yr. Most of this runs off in floods and won't penetrate the soil. Some of the flood water is caught in dams (green area), which increases both the base flow and the amount of accessible water. Back in 1950, human consumption was only a fraction of accessible water, but by 1950 it became 50% and by the end of the millennium it stood at 80%. Only rapid building of dams can prevent total human demand from catching up with the amount of accessible water, but this can no longer be achieved. As a result, there will be a world-wide shortage of drinking and industrial water after the year 2020.

more about soil <==> go to part2