2. Lack of sunlight - Lack of sun light will lower photosynthesis activity, thus cause an imbalance of the symbiotic activity of Nitrogen Fixing Bacteria that resides at the roots nodules. The bacteria continue to consume "food" stored at the roots that is supposedly supplied from the photosynthesis process. This cause a depletion in carbon component in the plant. If left unattended the plant condition may deteriorate.
The above diagram illustrate the complete nitrogen cycle. The plant cannot take in nitrogen from the atmosphere directly. It relies on bacteria to convert them (through multiple stages) into a form that can be absorb into the plant. As illustrated above, the end cycle is Nitrates (NO3-) that is assimilated into the plant roots.
NH3 + CO2 + 1.5 O2 + Nitrosomonas → NO2- + H2O + H+
NO2- + CO2 + 0.5 O2 + Nitrobacter → NO3-
Nitrifying bacteria converts ammonia to nitrite (NO2-) and subsequently converts nitrite to nitrate (NO3-) which shall be assimilated by the plant as nutrient. To accomplish this task, the bacteria needs CO2 and O2 from the atmospheric air. This explains the importantance of having good soil aeration to ensure higher nitrogen production for the plant roots.
This is why I like to water plants in the morning and also in the evening, always twice daily. Obviously apart from the main purpose for moisting the soil, the other important purpose is to flushed out the "burned" air so that more fresh CO2 and O2 replaces the pores in the soil.
The presence of earthworms in soil helps keep the soil aerated and its worm casting contains Nitrate (NO2-) and bacteria too, thus increase the nitrifying acitivities in the soil.
Another important information is that these bacteria also involved in the formation of soil aggregation. They produce organic compounds called polysaccharides (complex carbohydrates ; Cn (H2O)n-1 where n is usually large number between 200 and 2500), that binds soil particles together in aggregates. These aggregates can vary in size, and they do not necessary fit together thus creating spaces and pores within and between the soil. These spaces and pores are essential for storing air and water, microbes and nutrients in the soil.
I hope by now you will appreciate these lowly creatures. Though they may seemed insignificant, they largely contributes to our food chain.
In case you wish to understand a little bit more, please refer to the useful links below that explains what Bacterias do in the soil. I have extracted this piece of information from the US Department of Agriculture and posted it in this blog.