For some products there is very little difference between organic and non-organic because there's almost no GMO, pesticides or artificial fertilizers used to grow them. For other products there is a huge difference, with non-organic types having a huge amount of toxins hidden within them.
Lists such as the "dirty dozen" and "clean fifteen" that rate pesticide loads on different fresh produce provide a good starting point. However pesticides are only one of the problems. We also need to consider genetic modification, chemical fertilizers, preservatives, hormones, antibiotics, detergents, heavy metals in the soil and other food additives.
We've looked at the research on a range of food types and made a summary of what you should focus on buying organic and what you should be less worried about, broken down into red, yellow and green zones. While in reality there will be variation by country and brand there is a lot of consistency in modern farming practices and this will give you a good start point.
Fruit and Vegetables
The major factor for fresh produce is the amount of pesticide residue. Based on this we recommend anything in the red zone below is bought organic. However for the green zone unless it is almost the same price I generally wouldn't bother getting organic. The yellow zone is where you have more discretion based on price and how concerned you are about that particular provider.
Where you can't get your fresh produce organic another mitigation is to thoroughly wash them before you use it. You can actually remove 80-90% of the pesticides just by doing this. The recommended process is to soak for 15 minutes in one of the following:
Saltwater solution (10% ratio)
White Vinegar solution (10% ratio) - note this can add a vinegar taste
Baking soda solution (1tsp in 2C of water)
Some key tips for washing the fruit:
Only wash them before you use them (washing them and leaving them to sit will make them go bad faster).
Only use cold water.
Don't soak berries or they'll go soggy. Just rinse them.
Dried fruit
These are generally worth buying organic as the non-organic ones have high amounts of preservative added. They also often come from fruit that has heavy use of pesticides such as grapes and apricots.
Nuts and grains
Any foods based on wheat, soy and corn best to buy organic due to the GMO, glyphosate and associated links to Cancer, Liver/Kidney Damage & Severe Hormonal Disruption. A lot of the nuts are also heavily treated when not organic: cashews - endosulfan, pistachios - phosmat, macadamia - atrazine.
Meat
Fish - Saltwater fish will generally have been caught naturally and just kept on ice until sale - particularly if caught locally. However, be wary of farmed fish where they are often fed a lot of antibiotics and, in the case of salmon, chemicals to make the meat a deeper pink. There are also some countries where they will treat the fish with ammonia and/or formaldehyde to kill any bugs and preserve it for longer. Due to this I'm always wary of any seafood imported from a country with lower food hygiene standards.
Chicken - Non-organic chickens are often heavily loaded with hormones and antibiotics to maximize the size of the chicken and the amount of meat that they get. They then use peracetic acid and highly chlorinated water to process and sterilize the meat before injecting salted water directly into it. Due to the huge range of chemicals used in growing and processing I'd definitely recommend only buying organic chicken.
Red Meat - While not as bad as chicken, non-organic sheep and cattle are also fed non-organic GMO food and may be treated with antibiotics. In particular, US and Canadian cattle are grown with a lot of hormones that are actually illegal in Europe and other countries.
While you can pay for organically certified meat another option can be to buy from any small local farmers or butchers who can tell you specifically how the animals are farmed. Key things to check for are that they're grass fed, that the grass is not chemically fertilized and that the animals aren't treated with hormones or antibiotics unless actually sick.
Another great option to get read meat is to go hunting and butcher it yourself. You can generally be confident that this will be all natural and toxin free. However, whether this works for you will generally be based on the wildlife and laws around hunting where you live.
Processed foods
These are often the worst as not only will they be based on non-organic foods but will then often have preservatives, salt and other artificial components added in. If you read the packet and find a list of chemicals or ingredients that are just represented by numbers then I would definitely look for an organic alternative.
Summary
Overall organic food can be hard to source in some areas and is nearly always more expensive. Using the lists here should help you to minimize your toxin load while rationalizing what you invest in buying organic.
Keep in mind that the advice here is general so if buying specific brands frequently it can be worth doing specific research on what practices thy use in their farming and processing. Food providers are generally pretty open about what processes they use - if they're reluctant to tell you then that's probably an indication that it's not something you want to be eating.