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.

  • Wouldn't the pH be more important than conductivity ( implying some salt ) ? I would expect low conductivity water would leech out salts. Is that about temperature zone 1 ? Oct 5 '19 at 19:46
  • MilliSiemens, not MilliSieverts. I use PPM. Rinsing should work, but you'll have to turn the compost at least once during the process in order to avoid dry 'hot spots'. Compost tends to be lumpy, and water will avoid the clods. Oct 6 '19 at 16:23
  • I started taking pH yesterday. Old compost 6.3, new compost 5.3. This is good, as my soil is about 8. Oct 7 '19 at 12:24
  • ppm makes an assumption about the average weight of the ions. May not be a good assumption. Oct 7 '19 at 12:25

Normally for high EC we would tend to suspect very fine clay particles in the compost being tested, but from the description it sounds as though you have used a method (all liquid) that eliminates soil particle size, so that may not be it. I think the supplier has placed an unfair burden on you and is up for some explanation of what they put in their compost. Other users may have blindly used the material and are suddenly faced with unexplained deaths in their gardens. The EC measurement does not tell us what mixture of salts is present, and simply leaching may pull out the wrong component for a serious grower. I think a call to the supplier is in order, followed by action on their part to explain and make good.

  • In the for what it's worth, I've left the pots in place, and have been running a half liter of water a day, and collecting the initial drip. EC has been dropping about 15% a day. Fine clay isn't unreasonable, the water from the new compost is somewhat turbid. And in my field soil there is enough clay and silt, that when I pour water through it, the EC goes LOWER. Oct 7 '19 at 12:28

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