Horse chestnuts will make a great washing detergent, if you use them as follows:
collect as many fresh fallen horse chestnuts in autumn as you can
clean roughly, so that they can be stored dry and clean
store in cool place, make shure they will not mould, but stay fresh, basket is a good idea
take 4-6 chestnuts depending on size and amount of clothes, cut them each in 4 pieces
let them soak in ca. 200 ml cold water, several hours or overnight, liquid will get a whitish colour
cast off, using only the liquid as detergent in washing machine or handwash, chestnuts may go to compost
For me the result was really great, no funny smell, all clean, very good for machine low temperature color wash, with high temperature white stuff you may want to add some washing soda, depending on water hardness and stains. If you want some nice fragrance, you may add a drop of natural essence.
Further research welcome:
did not try it for pure wool yet
did not try it at really hot temperature (cause i do not use it anyway)
If you want to make shure to have enough detergent into the next season, you could alternatively peel the chestnuts, grind them in an old (electric) coffee grinder (not too fine) and fill them in jars with lid; this way you prevent running out, because if the chestnuts get too dry over time, they become really hard to cut. This one i did not try myself yet:
experiment with ground horse chestnuts kept in jars about amount and how long it will keep