Recently I have felt some pressure to justify my decision to become what I call an 'organic gardener' which is to avoid the use of chemical fertilisers and pesticides in the garden. Well here goes!
Why avoid artificial fertilisers?
The use of artificial fertilisers can and does cause marine pollution, with nitrate run-off building up in rivers, affecting the flora and fauna there and flowing out to create 'dead zones' in the sea where fish and other marine life cannot access enough oxygen and die, due to algal blooms.
Artificial fertilisers are made from fossil fuels such as oil, which is a finite resource; the extraction and use of which causes global warming, which is humankind's most pressing environmental issue. If you are in any doubt about the reality of man-made climate-change, a quick check of the business interests of the sources of information which present it as 'doubtful' will probably resolve the issue. A useful New Scientist guide to popular misconceptions about climate change is here: (http://www.newscientist.com/article/dn11462-climate-change-a-guide-for-the-perplexed.html#.U7q-NfldWSo)
This is not to mention the human suffering in the form of wars which have been fought over the control of this resource under various pretexts.
The production of chemical fertiliser itself can cause pollution (150 million tons of toxic spoil a year) from mining phosphate rock which affects both the land where it is produced, the water supplies and the people involved in the mining and those who live in the area.
Phosphorous is running out. The Global Phosphorus Research Initiative run by Dana Cordell of Linkoping University in Sweden, says we could hit “peak phosphorus” production by around 2030. (See http://e360.yale.edu/feature/phosphate_a_critical_resource_misused_and_now_running_out/2423/)
The high temperatures which need to be maintained to keep the chemical reaction going in the production of conventional fertilisers also require large amounts of energy, which will usually be taken from non-renewable sources.
If the soil is treated well to avoid erosion, and adding organic matter will help do this, there should be no need to use fertiliser at all, unless you are growing plants which need extra food such as tomatoes, soft fruit, squashes, leafy greens, roses and bedding plants. Feeding plants 'as a matter of course' regardless of whether they need it, cannot be good practice, just as for watering or potting on. I see good gardening as being based on careful observation, not rigidly following instructions on a label, where no doubt, doses may be exaggerated to increase consumption. If your plant looks sickly, it could be for a variety of reasons...just see Jeremy's post 'Why did my plant die!'
Chemical fertiliser can be easy to over-apply, which can lead to weak growth, scorching and pest infestations which can also lead to diseases. Admittedly this isn't as much of a problem with the slow-release versions.
There are sustainable alternatives, such as seaweed, and animal and even pelleted human waste. (See http://www.theguardian.com/environment/2013/nov/06/nutrient-recovery-fertiliser-human-excrement-slough)
Why avoid artificial pesticides?
They can lead to the pollution of groundwater, which can affect the flora and fauna of the local area, some of which are also beneficial to the gardener. The present government's U-turn on the issue of the poisoning of bees with neonicotinoids which is was prepared to do even in the face of scientific opposition from the EU shows how easy it is to remain unaware of ecological issues to do with pesticides until campaigners make a big push, which is no doubt just how the companies selling them would like it to be. For more information, here's an article: http://www.theguardian.com/environment/2014/jul/04/syngneta-banned-pesticide-bees)
With all of these pesticides, it pays to do one's own research and not rely on the idea that 'if it's in the shops it must be safe'.
I think taking care of the land and local ecology for future generations is ultimately more important than my temporary and mainly aesthetic use of the land my garden encloses. I will, for example choose plants which aren't as attractive to slugs, rather than making a great effort to defend them! There are so many different plants to try growing, as can be seen in this site's Plant Finder..
There are sustainable, effective alternatives such as biological control (for greenhouses and conservatories), the use of fatty acids for aphids, beer traps and even organic slug pellets made from ferrous phosphate, which can be used as a last resort, rather than the conventional ones made from metaldehyde, which affect birds and hedgehogs.