Do I need fertilisers outdoors and what are some suggested soil conditioners?
If your soil is good (rich in minerals good drainage ect)  and its not a permanent location, you might not even need to fertilise at all however it’s suggested you add a little during the flowering cycle to give it a boost when forming it’s heads. The suggested nutrients to look for to assist here are rose flowering nutrients it is a common misconception that cannabis is very similar to tomatoes, Rose nutrients however are best as the requirements for the best rose formation and head formation are close to identical.
If However your growing in the same location year after year or if your soil is poor to begin with, you will need to add amendments. Some organic amendments will improve the texture and drainage as well as supply nutrients, and they have the advantage of not building up into a toxic condition. Below are some common organic additives commonly used to condition soil.

Nitrogen
Blood meal: Blood meal is a by product of the meat-packing industry and has a nitrogen content of 12-14%. The nitrogen is available in a short time, but it does little for the mechanical properties of the soil.

Cottonseed meal: A by-product resulting from the extraction of oil from cottonseed. Commercial cottonseed meal has nitrogen content of 6-7%. Generally it is used as a partial source of nitrogen in mixed fertilisers. The nitrogen is readily available. 
Urea: Urea is a white crystalline compound containing 46% readily available nitrogen.
Fish emulsion :This is prepared from non-edible fish and waste from fisheries. It has about 8% nitrogen.

Phosphate
Bone meal: There are two kinds of bone meal  raw and steamed. Steamed bone meal has less nitrogen than raw, but more phosphoric acid. This material releases its nutrients slowly so can be used without fear of injuring the crop.
Rock phosphate: Mineral occuring in deposits throughout the world. Its effectiveness is dependant on its degree of fineness, and the reaction of the soil.
Slag: Finely ground by-product of steel manufacture. Its free lime content makes it of special value in the reclamation of acid soils.

Potasium
Wood ashes: Ash from wood is a great natural source of Potasium   
Seaweed/Kelp: Seaweed or kelp is a great way to add  Potassium and a few other minerals to your soil . It can generally be found in most of your organic all round additives

Other
Compost :A great source of many organic minerals and a great way to re-use those scraps that would normally just go into the bin.
Fulvic Acid : Fulvic acid improves a plants all around general health and greatly assist in boosting nutrient/mineral uptake .
Lime: Lime is a good way to stop your soil from going to acidity, limestone chips are a good long term solution for both draining and promoting bacterial activity.
Guano:The composted or fossilized excrement of a roosting vertebrate ( eg bat).
Castings: Composted or fossilized Invertebrate excrement (eg worms )
Gypsum: Gypsum assists in the uptake of potassium and sulfur and the conditions soil.