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:

• sunlight: to obtain the energy for photosynthesis.
• carbon dioxide: necessary input for photosynthesis. The atmosphere cycles this effectively.
• oxygen: when plants rest at night, they need oxygen, while producing carbon dioxide. In slow-growth areas such as the Boreal forests, respiration during the long winters is almost equal to photosynthesis during the short summers.
• warmth: to be able to perform the biochemical processes of life. Plants have adapted to a wide range of temperature, but the warmth of the tropics promotes highest productivity.
• water: the biochemical process of photosynthesis requires much water. Water or the lack of it, causes problems in most geographic areas.
• macronutrients: the main nutrients N,P,K,S,Ca,Mg and micronutrients, the trace elements.

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.

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.
 

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.

The 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.
 

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.
 

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

• United States: The High Plains Aquifer System (Ogallala) underlies 20% of all US irrigated land and contains some 3700 cukm. Net depletion in 30 years amounts to 325 cukm. More than 65% of this depletion has occurred in the Texas High Plains, where irrigated area dropped by 26% between 1979 and 1989. Current depletion is estimated at 12 cukm/yr.
• United States, California: Groundwater overdraft averages 1.6 cukm/yr, amounting to 15% of the state's annual net groundwater use. Two thirds of the depletion occurs in the Central Valley, the country's (and to some extent the world's) fruit and vegetable basket.
• United States, Southwest: Overpumping in Arizona alone totals more than 1.2 cukm/yr. East of Phoenix, water tables have dropped more than 120m. Projections for Albuquerque show that, if groundwater withdrawals continue at current rates, water tables will drop an additional 20m by 2020.
• Mexico City and Valley of Mexico: Pumping exceeds natural recharge by 50-80%, which has led to falling water tables, aquifer compaction, land subsidence, and damage to surface structures.
• Arabian Peninsula: Groundwater use is nearly three times greater than recharge. Saudi Arabia depends on nonrenewable groundwater for roughly 75% of its water, which includes irrigating 2-4 Mt/yr wheat. At this depletion rate, groundwater reserves would last only about 50 years.
• North Africa: Net depletion of groundwater in Libya totals nearly 3.8 cukm/yr. For the whole of North Africa, current depletion is estimated at 10 cukm/yr.
• Israel and Gaza: Pumping from the coastal plain aquifer bordering the Mediterranean Sea exceeds recharge by some 60%. Salt water has invaded the aquifer.
• Spain: One-fifth of total groundwater use, or 1 cukm/yr, is unsustainable.
• India: Water tables in the Punjab, India's bread basket, are falling 0.2m annually across two-thirds of the state. In Gularat, groundwater levels declined in 90% of observation wells monitored during the 1980s. Large drops have also occurred in Tamil Nadu.
• North china: The water table beneath portions of Beijing has dropped 37m over the last 4 decades. Overdraft is widespread in the north China plain, an important grain-producing region.
• Southeast Asia: Significant overdraft has occurred in and around Bangkok, Manila and Jakarta. Overpumping has caused land to subside beneath Bangkok at a rate of 5-10 cm/yr for the past two decades.

This 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.