The first thing to know / remember is what happens when a plant photosynthesises which is as follows:
Sunlight hits the leaf. Within the leaf is chlorophyll which is green in colour and which acts as the catalyst for the following reaction. The plant absorbs water (usually via its roots) and carbon dioxide (usually from the air) and, using the energy from the sun, converts these two things into carbohydrate plus oxygen plus water.
Carbon dioxide + water + energy = carbohydrate + oxygen + water
6CO2 + 12H2O + energy (sunlight) = C6H12O6 + 6O2 + 6 H2O
It's as simple as this. A plant takes carbon dioxide out of the atmosphere and converts it into a simple sugar (a carbohydrate) which is stored within the plant to be used either as a future energy source or the carbon is used to create the 'backbone' of the plant (starches, cellulose etc). Whilst locking up the carbon, the plant emits life-giving oxygen (plants are the source of all the oxygen in the atmosphere!)
Note also that the sun's light energy is converted into chemical energy in the process. Hence, all our fossil fuels and all our biofuels are simply sunlight energy that has been captured by plants and turned into chemical energy which is then 'stored' in the plant.
When the plant grows, it pulls nutrients up from the soil as needed. For example, nitrogen is used to form proteins within a plant; potassium is used to regulate water flow and in the formation of cellulose.
Typically, the amount of carbon in a plant will be around 45% (with a range of 35%-50%). It is the nitrogen which varies and gives the C:N ratio of the plant. For example, a plant made up of 45% carbon and 5% nitrogen will have a C:N ratio of 9:1. A plant with 45% carbon and only 0.5% nitrogen will have a C:N ratio of 90:1
When a plant dies or is trampled by an animal, it will, if in moist conditions and in contact witht the soil, begin to be broken down by the different organisms in the soil. Effectively, the smaller 'bugs' in the soil eat the plant material. They need carbon and nitrogen in a specific ratio in the material they consume to enable them to grow properly.
If there is an excess of nitrogen in the plant material, they will release into the soil all that N which they don't need. However, if there is an excess of carbon in proportion to nitrogen (and 20:1 seems to be the critical level) then the bacteria will need to get some additional nitrogen from the soil to build the proteins in their bodies. This 'steals' the nitrogen from the growing plant, and is why high carbon material (eg straw, woodchip), when added to the soil, can result in a nitrogen deficiency in the following crop.
To give an idea of the C:N ratios of different materials:
So, if FYM is added to a soil, it will typically provide more N than the bacteria need, so some N will be available for the following plant growth.
- Grasses are typically 20:1 to 25:1
- Clover is 15:1 to 22:1
- FYM is 14:1 to 20:1
- Wheat/Barley/Oat stubble is 90:1 to 160:1
- Lucerne hay is 12:1 to 15:1
Straw, on the other hand, will not provide enough N (in relation to the C) and hence the bacteria will have to use up soil N in order to fully break down the stubble. Therefore unless either additional N is added or (in a healthy soil) there is sufficient N available, the growing plants will show signs of nitrogen deficiency.
One final point. I mentioned that N might be available in a 'healthy' soil. What happens is that the bacteria that have eaten the plant material themselves get eaten by larger fungi and protozoa who in turn get eaten by nematodes, insects and earthworms. Each of these, as they get higher up the food chain, tend to have a higher C:N ratio in their bodies. This means that they don't need all the N (ie protein) from the smaller organism they've eaten and hence they excrete the excess into the soil where it becomes available to the plant root.