O level Notes: Agriculture - Soil fertility

Nitrogen is of greatest importance and there is need to describe how it becomes available and removed from the soil through the nitrogen cycle. 

The nitrogen cycle

The nitrogen cycle refers to the changes in forms and location of nitrogen element within the plants, animals, soil and atmosphere. 

Atmospheric nitrogen

This nitrogen will be in gaseous form. It is the most abundant gas and it is unavailable for plant use until it is fixed into the soil where it changes form to nitrates. Nitrogen will be fixed from the atmosphere through bacterial activities on root nodules of legumes. When these nitrogen fixing bacteria are feeding, they are able to convert atmospheric nitrogen to ammonium compounds and these will be converted to nitrates which can be available for use by plants. Nitrogen can also be added into the soil by lightning effect. The oxidation process that occurs to the nitrogen will change it into nitrates which are soluble and can then fall into the soil together with rain. 

Nitrogen in the soil

Besides bacterial fixation and lightning effect, nitrogen can be fixed into the soil through the decomposing animal and plant matter. The dying animals and plants are decomposed due to the action of fungi and bacteria. The decomposing matter will further be broken down through ammonification process to form ammonium compounds. Ammonium compounds are acted upon by the nitrifying bacteria (nitrosomonus and nitrosococcus) to form nitrites. Both ammonium compounds and nitrites are not available for crop absorption. Nitrifying bacteria will further break them down to nitrates. The nitrates are soluble and ready for plant use. Plants can then assimilate them and use them to build up their tissues during growth and development. Nitrogen can also be added into the soil through the use of nitrogen containing fertilisers during farming. Most compound fertilizers such as C, D, L, S and straight fertilizers such as urea and ammonium nitrate can be used to supply nitrogen into the soil. Nitrogen can return into the atmosphere through the action of denitrification bacteria. It will change nitrates to nitrogen gas and that will eventually escapes back into the atmosphere. Nitrogen can also escape back into the atmosphere by volatilisation. This is the vaporisation of nitrogen as gas due to effect of direct heat. 

Here is what we discussed in this topic

• Nitrogen cycle refers to the changes in forms and location of nitrogen elements within the plants, animals, soil and atmosphere.
• Nitrogen is available in the atmosphere as nitrogen gas.
• Nitrogen can be fixed into the soil through nitrogen fixing bacteria, lightning and decomposing animal and plant matter.
• The nitrogen fixing bacteria are found mainly on root nodules of legume crops and as they feed they convert atmospheric nitrogen into ammonium that will then be changed into nitrates through the nitrification bacteria action.
• The lightning strikes oxidise the nitrogen gas into nitrates which are fixed into the soil during rain.
• The matter of dying animals and plants is decomposed by the action of fungi and bacteria.
• Decomposing material is then converted to ammonium compounds through ammonification process.
• Ammonium compounds are changed to nitrites and then to nitrates by nitrifying bacteria.
• Nitrates are available for assimilation by the plants.
• Plants will then use nitrogen to build up their tissues.
• Animals will get nitrogen containing substance through feeding on plant matter.
• Some nitrogen  is  lost  back  into  the  atmosphere  by  de-nitrification  and volatilization process.

Definitions

• Ammonium compounds-  these  are  compounds  made  up  of  nitrogen  and hydrogen molecules from ammonification of decomposing animal and plant matter.
• Nitrites- are insoluble compounds made up of nitrogen and oxygen atoms and cannot be absorbed by the plants.
• Nitrates- these are soluble compounds made up of nitrogen and oxygen atoms and can be absorbed by the plants.
• Oxidation- it is a process which involves the adding up of oxygen to other molecules to form compounds.
• Nitrification – this is a process which breaks up compounds of ammonia to nitrites and nitrates.
• Denitrification – the breaking down of nitrates into nitrogen gas which will later escape into the atmosphere.
• Ammonification – the process by which decomposing animal and plant matter is broken down into ammonium compounds.