Nitrogen is taken up by plants as the nitrate and, to a lesser extent, the ammonium ion. Nitrates and ammonium ions are utilized in the plant to form protein. Plants use large quantities of nitrogen; it is associated with vegetative growth. Consequently large dressings of nitrogen are given to leafy crops, whereas fruit, flower or root crops require limited nitrogen balanced by other nutrients to prevent undesirable characteristics occurring.
Although plants live in an atmosphere largely made up of nitrogen they cannot utilize gaseous nitrogen. They are able to take up soluble nitrogen from the soil water as nitrates and ammonium ions. Both are derived from proteins by a chain of bacterial reactions as shown in Figure 21.2.
Ammonifying bacteria convert the proteins they attack to ammonia. Ammonia from the breakdown of protein in organic matter or from inorganic nitrogen fertilizers is converted to nitrates by nitrifying bacteria. This is accomplished in two stages. Ammonia is first converted to nitrites by Nitrosomonas spp. Nitrites are toxic to plants in small quantities, but they are normally converted to nitrates by Nitrobacter spp. before they reach harmful levels. Ammonifying and nitrifying bacteria thrive in aerobic conditions. Where there is no oxygen, anaerobic organisms dominate. Many anaerobic bacteria utilize nitrates and in doing so convert them to gaseous nitrogen. This denitrification represents an important loss of nitrate from the soil, which is at its most serious in well-fertilized, warm and waterlogged land.
Although plants cannot utilize gaseous nitrogen, it can be converted to plant nutrients by some micro-organisms. Azotobactor are free-living bacteria that obtain their nitrogen requirements from the air. As they die and decompose, the nitrogen trapped as protein is converted to ammonia and then to nitrates by other soil bacteria. Rhizobia spp. which live in root nodules on some legumes (see Figure 21.3) also trap nitrogen to the benefit of the host plant. Finally, nitrogen gas can be converted to ammonia industrially in the Haber process, which is the basis of the artificial nitrogen fertilizer industry.
Excess nitrogen produces soft, lush growth making the plant vulnerable to pest attack and more likely to be damaged by cold. Very large
Nitrogen is needed by plants to form proteins and is associated with leafy growth.
Denitrification/ -r /
Nitrate \ Nitrite I I
Nitrogen fertilizers v Y
1 Animals I
Figure 21.2 Nitrogen cycle. The recycling of the element nitrogen by organisms is illustrated. Note the importance of nitrates that can be taken up and used by plants to manufacture protein. Micro-organisms also have this ability but animals require nitrogen supplies in protein form. Gaseous nitrogen only becomes available to organisms after being captured by nitrogen-fixing organisms or via nitrogen fertilizers manufactured by man. In aerobic soil conditions, bacteria convert ammonia to nitrates (nitrification) whereas in anaerobic conditions nitrates are reduced to nitrogen gases (denitrification)
Figure 21.3 Rhizobium nodules on legume
Figure 21.3 Rhizobium nodules on legume quantities of nitrogen are undesirable since they can harm the plant by producing high salt concentrations at the roots (see conductivity) and are lost by leaching. Large quantities are usually applied as a split dressing, e.g. some in base dressing and the rest in one or more top dressings.
Nitrates are mobile in the soil, which makes them vulnerable to leaching. In the British Isles it is assumed that all nitrates are removed by the winter rains so that virtually none are present until the soils warm up and nitrification begins or artificial nitrogen is applied (see nitrogen cycle). Nitrates leached through the root zone may find their way into the groundwater that is the basis of the water supply in some areas. Nitrification also leads to the loss of bases; for every 1 kg N in the ammonia form that is oxidized to nitrate and leached, up to 7 kg of calcium carbonate or its equivalent is lost. Nitrogen is also lost from the root zone by denitrification, especially in warm, waterlogged soil conditions. When in contact with calcareous material, ammonium fertilizers are readily converted to ammonia gas which is lost to the soil unless it dissolves in surrounding water. For this reason urea or ammonia-based fertilizers should not be applied to such soils as a top dressing or used in contact with lime. Nitrogen fertilizers used in horticulture and their nutrient content are given in Table 21.2.
Ammonium nitrate is now commonly used in horticulture. In pure form it rapidly absorbs moisture to become wet; on drying it 'cakes' and can be a fire risk. Pure ammonium nitrate can be safely handled in polythene sacks and as prills. Ammonium sulphate has a highly acid reaction in the growing medium. Urea has a very high nitrogen content and in contact with water it quickly releases ammonia. Its use as a solid fertilizer is limited, but it is utilized in liquid fertilizer or foliar sprays. The addition of a sulphur coating to urea not only creates a controlled release action, but also a fertilizer with an acid reaction. Other manufactured organic fertilizers include urea formaldehydes (nitroform, ureaform, etc.) which release nitrogen as they are decomposed by microorganisms, isobutylidene urea (IBDU) which is slightly soluble in water and releases urea and crotonylidene (CDU, e.g. Crotodur). The latter breaks down very slowly and evenly, which makes it ideal for applying to turf.
Natural organic sources of nitrogen, including dried blood, hoof and horn and shoddy, amongst others, are generally considered to provide slow release nitrogen, but in warm greenhouse conditions decomposition is quite rapid.
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