Factbox 1.
Elements That Plants Need

By Professor Nils Nykvist

In addition to light, carbon dioxide, and water, which are necessary for photosynthesis, and oxygen for respiration, plants also need for their growth the elements nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulphur (S), which are required in relatively large amounts and therefore are called macronutrients. Other elements, such as iron (Fe), manganese (Mn), copper (Cu), zinc (Zn), boron (B), and molybdenum (Mo), are needed in smaller amounts and are therefore called micronutrients.

The elements are taken up from the soil by the plants as ions dissolved in water. Fertilizers are, from a chemical point of view, salts consisting of positive and negative ions.
Other elements that are dissolved in the soil water are also taken up by the plants, even if they are not directly necessary. However, some of these may be necessary for humans and animals. One of these elements is sodium, which is found only in small quantities in plants and therefore must be added as sodium chloride to food.

In New Zealand a lack of the element cobalt (Co) in plants had a significant impact on the country’s economy. In certain areas with volcanic soils in the North Island, sheep farming failed, because the sheep died of an unknown disease. The land was therefore of less value, and people started instead to plant trees in those areas.

Later it was found that the cause of the disease was lack of cobalt, which could easily be fixed by enriching the feed with a cobalt salt or by fertilizing the land with superphosphate containing small amounts of cobalt. However, before this solution was found, forestry had grown so much that it is now one of the most profitable livelihoods in the country.

All plant nutrients are originally from the bedrock, with the exception of nitrogen coming from the air, where it is available as free nitrogen (N2). The bond between the atoms of nitrogen in the N2 molecule is very strong, and free nitrogen is bound to other elements only with great difficulty.

However, at high temperature and at high pressure, nitrogen gas combines with oxygen gas, resulting in various forms of nitrogen oxides. This occurs also at lightning discharges and in combustion engines. The nitrogen oxides form with water nitrate ions, which with rainfall reach the ground, where they can be taken up by plants.

The amount of nitrogen oxides in the air has increased in recent decades and may be a contributing factor to an increase of forest growth in some countries.

Many nitrogen compounds in the soil have been formed by some organisms able to fix free nitrogen from the air. Among these so-called nitrogen-fixing organisms are many different species of free-living bacteria in the soil, which in the presence of organic matter may form nitrogen compounds from free nitrogen.

Of greater significance are the so-called cyanobacteria that have the ability to form their own organic matter through photosynthesis and therefore sometimes are called blue-green algae.
By their ability to both photosynthesize and fix nitrogen, they can colonize such inhospitable environs as the Antarctic region, deserts, and hot springs. The cyanobacteria are very important for the cultivation of paddy rice in the tropics, where they can bind up to about 75 kilograms of nitrogen per hectare per year on unfertilized rice fields in Southeast Asia.

In recent years, their contribution to the algal bloom in the Baltic Sea has been widely debated in the media. As the cyanobacteria are nitrogen fixing, a great supply of phosphorus is probably the main reason for the abundance of these organisms.

The best-known nitrogen-fixing organisms are the bacteria that live in symbiosis with legumes, on whose roots they form characteristic nodules. They are of great importance in agriculture, because legumes are mostly a considerable part of the farmers’ crop rotation.

Although nitrogen fixation by bacteria on legumes does not give yields as high as those of chemical fertilizers containing nitrogen, it is environmentally preferable, because the nitrogen compounds do not leach out from the soil as easily as do chemical agents.

Nitrogen-fixing plants also include many trees, such as species of acacia that are planted on a large scale in many tropical countries (Fig. 30). The fixation of nitrogen is improved by the application of other plant nutrients, particularly phosphorus.
Figure 30. Nitrogen-fixing roots of Acacia mangium from a forest nursery in Sabah in northern Borneo, Malaysia.

Nitrogen compounds in soil and water may be lost by so-called denitrification when nitrate in oxygen-poor soils is converted to free nitrogen or nitrous oxide (N2O, laughing gas), which is a greenhouse gas.

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