I'm a tree farmer near Edmonton, Alberta. My entire operation is in pots ranging from 100 ml to 40 liters
I just got a new load of compost from City of Edmonton.
I performed the following test:
Packed a pot solidly with compost to a level similar to what I use transplanting.
Applied 1 inch of water, waited for it to drain, and added about another inch. (1 liter of water total for a 3 liter pot.)
Let it sit over night.
Applied half a liter of water.
Collected the first leachate out of the bottom of the pot.
This according to one paper published out of U. Florida gives a consistent approximation of the saturated paste method, with a lot less work.
Measured the electrical conductivity. 10.8 mS/cm . My water is moderately high, at about .7 mS/cm
Made up a batch of my standard soil mix which is compost, field soil, peat moss in roughly even proportions. 5.8 mS/cm
Tested the soil in an old potted plant. 1.2 mS/cm
I'm using an Oakton PCS tester. New batteries. Temperatures have been about 7 C, but I'm assuming that the meter corrects for this. (If it doesn't then the readings are going to be even higher.)
My understanding is that 1.5 mS/cm will start showing problems with a lot of plants, and anything over 4 is a death sentence for anything but a halophyte.
Can I wash this compost? Spread it out in a 2 foot thick pile, and set a sprinkler to put several feet of water through it?
I am asking this question also on National Gardening Assoc. forum and on UBC botany forum.