My planting medium now contains 76% clay, 18% sand and larger particles and 6 % organic material. I want to mix in sand and organic material so I end up with a medium of 40% clay, 35% sand (larger inorganic particles) and 25% organic material, such as compost. All these items are by volume.
I’d like a formula I could use since I have other planting beds with different mixtures.
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You can’t do it with simple % alone because you can’t add in %s. You need quantifiable #s. So take your original percenages and multiply them by the total amount of soil you have to get a weight in grams (use metric its easier). Your constant here is Clay because your not adding any of that. For clay to go from 76% to 40% it requires a 90% increase in the total weight of the medium.
Start like this. you have 100kgs of dirt. To dillute the clay down to 40% you will need to add 90 kgs of sand and organics.
Now that you have your total weight just divide it out and subtract what was originally there
Organics to be added =190*25% - 100*6% = (47-6) = 39kgs of Organics
Sand to be added = 190*35% - 100*18%= (66.5 -18) = 48.5kgs
SO! For every 100 kgs of original mix you had you need to add 39kgs of Organics and 48.5 kgs of Sand
Remember the trick is figure out what is constant (in this case clay) find how much you need to dillute the constant by (in this case 90%) to give you a new total weight. Then find out how much wieght of each variable you’d like in the mixture ( in this case 47kgs and 66.5kgs) and subtract what was already present (6kgs and 18kgs). This will give you your answer.
I hope you can get i figured out
O i forgot to add to figure out the dillution rate (90% in this ex.)
simply divide the current mix by the mix you’d like (76%/40%=1.9 or 90% increase)
