The first and best reason to consider using organic fertilizers instead of chemicals is that there is almost zero chance that you will burn (overfertilization causing injury or death) the plants with them. When fed organic substances, plants will only take in the nutrients they need, leaving the rest in the soil. Additionally, organic fertilizers are broken down slowly in the soil by microorganisms, which ensures a steady supply of nutrients to your plants; also, lots of soil microorganisms are good for the soil and consequently, your plants as well.

Chemical fertilizers, on the other hand, are in a highly soluble form and are generally of a much higher concentration than organic fertilizers. Upon applying them to the soil, they are quickly taken up by the roots. Because they are so concentrated, this rapid action will cause the plant to take in toxic levels of nutrients if the fertilizer is overapplied, leading to injury and even death if the levels are high enough. Additionally, chemical fertilizers leave salts behind in the soil. If the plant is not flushed periodically (every 1-2 months), these salts will build up to levels that are dangerous to the plants. (As a related note, if the soil is not flushed just prior to harvest, the taste of the smoke will be adversely affected.) Finally, chemical fertilizers have an adverse effect on soil microorganisms, including earthworms.
Beyond the issues of soil chemistry and nutrient uptake, there is little question that using organic substances are better for the environment, even when growing indoors. Organic fertilizers - blood and bone meal, fish emulsion, manure, worm castings - are renewable. Petroleum, which chemical fertilizers are synthesized from, is not.

For the outdoor grower, choice of fertilizer has an even more profound effect. Successful outdoor growing is closely linked with the health of the soil. Chemical fertilizers, as mentioned, have an adverse effect on soil life, which decreases the biodiversity and overall health of the soil. Chemicals are also far more soluble than organics, and are often washed away with rain or a too-heavy watering. Not only does this not help your plant, it also causes a potential pollution problem - for instance, toxic algae blooms in lakes and ponds are often linked with fertilizer run-off from lawns.

Organics are not without their drawbacks, however, especially to the indoor grower. Some organic fertilizers, fish emulsion in particular, have an odor that may offend delicate noses. (However, any security measures involving air filtering or ionization should keep the smell to a minimum.) Also, because organics encourage soil life, there are sometimes more problems with insects, particularly fungus gnats. Finally, organics require a greater investment of time and effort: chemical fertilizers' main advantage is their ease of use.

In the event that you face insect problems, there are a number of organic controls at your disposal. The easiest homemade, all-purpose bug killer is about a teaspoon of soap (I prefer Dr. Bronners Eucalyptus or Peppermint, which are generally eco-friendly and may have additional insecticidal effects) in a spray bottle of water. Spray any bugs you see, the plants (including the undersides of the leaves!) and the soil surface thoroughly with this solution. The soap coats the outsides of the bugs' bodies, which suffocates them.

Another, stronger option is Tobacco tea. The nicotine in tobacco is one of the more potent poisons known, and will generally wipe out even mites, which are usually quite difficult to kill. It's worth remembering that nicotine is poisonous, so keep that in mind. (Unless you also smoke cigarettes, in which case, go crazy.) Here is a recipe:

Buy a package of Chewing Tobacco and put the whole package into 1 gallon of warm water. Let it stand in a warm place overnight 12 hours minimum. Filter the solution using a coffee filter and put it into a clean spray device. You can add 4 tablespoons of dish soap to this and spray the foliage down. Make sure you get the mites. Spray the tops and bottoms of the leaves. Once you have done this sparay the floor and walls in the grow area. Bear in mind that the spray may brown the leaf tips and visible pistils. This is a very powerful contact insecticide.

If you feel like going the store-bought route, a product called Safer Insecticidal Soap has been used with good results. As a final resort, you can use insecticides made from pyrethins, which are synthesized from certain varities of Chrysanthemums. Although they are reportedly non-toxic to humans and animals, they are a potent toxin and probably shouldn't be used anytime near harvest.

With any insecticide, multiple treatments over a 1- to 3-week period will probably be necessary to kill the bugs, plus any new ones that hatch after your first applications.

Finally, perhaps the best route is to go outside and catch some ladybugs (or order them from a nursery or garden supply). Ladybugs are vicious insect killers, but won't touch your plants.
Blood Meal: 13 - 0 - 0

Blood meal has one of the highest concentrations of Nitrogen of any organic fertilizer, and is consequently a popular choice for the vegetative growth period. In its dry and slow-acting form, it can be mixed in with the soil at a rate of 1 to 2 tablespoons per gallon of soil mix. However, many growers prefer to use it as a soluble fertilizer as it acts very quickly without much danger of burning - much like the action of a chemical fertilizer, but without the risks.
To make blood meal tea, soak 1 tablespoon of blood meal in a gallon of water for 5 to 7 days. The longer you wait, the higher concentration of N the tea will have. Shake well, then strain out the solids and water your plants with the tea.
Bone Meal: 1 - 11 - 0

Bone meal is high in Phosphorus, and is most suitable for the flowering period. However, as it is a slow-release fertilizer, it is fine to add to the soil earlier in the grow period. (Perhaps the best course of action is to add it to the mix you perform your final transplant into.) One caution about bone meal, especially in Europe, is that many growers will not use it for fear of spreading Mad Cow Disease. Although this has not been proven, it is wise to bear this in mind.
Fish Emulsion: 5 - 1 - 1

Fish emulsion is a liquid solution made from decomposed fish and sometimes other ingredients. It is an exceedingly gentle fertilizer and is thought by many growers to be the best "first fert" to use on young plants. Its NPK ratio is also ideal for vegetative growth. It is usually mixed with water at a rate of 1 to 3 tablespoons per gallon.
Worm Castings: 0.5 - 0.5 - 0.3

Also known as worm compost or good ol' worm shit, this may be the single best all-purpose fertilizer. Although the nutrient levels are relatively low, worm castings somehow have amazing effects on plant vigor, and anyone who has used them can testify to their effectiveness. They are very gentle on plants, making them ideal for seedlings, and also contain micronutrients. Worm castings can be used as part of the soil mix (no more than 15% total volume is suggested) or can be made into tea (1 part WC to 5 parts water) and applied as watering solution or as a foliar fertilizer.
Kelp Meal: 1 - 0.5 - 2.5

Kelp meal provides over 60 trace elements, plus growth promoting hormones and enzymes. As such, it is often used to ensure the plant is properly supplied with micronutrients. Can be used as part of the soil mix (1-2 tablespoons/gallon) or brewed into tea at the same rate.

great post.. i just want to mention i remember reading in the anarchist cookbook a long time ago that same tobacco recipe used for killing people. just be sure not to drink that shit on a dare u might end up in the emergency room

pk i have a 12-0-0 bloodmeal fertilizer.....so to make the tea a simple soke a teaspoon of it in water for 5 to 7 days...then i water my plants with it like i would normally water them with the regular water?

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I have never grown marijuana or intend to. Anything I post is knowledge gained from reading, not personal experience. Any pictures I post were aquired on the web or are fakes.

there are organic nutrients (like GH Flora Nova) that will burn your plants.
And not to be pedantic but if we use common usage of the word: organic (chemistry of compounds containing carbon chiefly from biological origin) you would be hard-pressed to find a non-organic nutrient.

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major was talking about tobacco, I want to let everyone know about an issue I face because I roll my own smokes. States all have different regulatory on what chemicals are in your tobacco and I have found that one of the new chemicals they have put in the tobacco I smoke, (buglar, top,gambler,no.2 and n0.7 will kill my leaves on my plant if I have the residue from rolling my own smokes on my fingers and I touch the leaves. This is something you all might want to concider if your think you can just use any old tobacco laying around for pest control.