Mystery of the disappearing soil

[Ozark Lady]

Who likes a mystery?

Okay for me this is a mystery, hopefully, some of you more seasoned gardeners and wise sages have some clues or even answers for me.

I live on the south side of a hill. There is a fairly steep drop, but the garden is the "flattest" of the slope. It drops about 4-6" in a three foot wide width of a raised bed.

We double dug the beds down to a depth of about 3'. We removed the larger rocks, and then we invented a sift for the rest of the soil, using 1" chicken wire. Which took out several 5 gallon buckets of rocks per 3x6 bed.

When the soil was returned to the bed it was layered with purchased bags of soil. The soil was built back up to level with the garden tiles (4" treated timbers). After each rain, the soil was back down to a 'sunken garden' status.

Okay, air pockets were cleared and filled with dirt. The compost in the bags of soil likely continued to break down... I explained all this to myself, and added more bags of soil to the beds. Then I added manure and mulch, planted and enjoyed a season.

By spring, I removed all the mulch, and I had sunken beds again. I went to the gravel quarry and got a pick up load of topsoil, which I just placed on top of the beds... topsoil goes on top right? I then, added aged manure, planted it and mulched. enjoyed a season again. But again, midseason, I had sunken beds! In beds that were not all season, I added more soil, the others I just kept building the mulch up to keep all well covered.

About the third year, the rocks were too bad to continue, so the beds had to be double dug and rocks removed again. Not quite as large of rocks and not quite as many of them as when dealing with virgin soil. More soil was added, when refilling the beds. Again, soil had to be topped up at midseason, and was a sunken garden by fall.

This has gone on for nearly 20 years. I look all around the beds, no signs of the soil. No deep compost, so shouldn't be that it just broke down. In most years no double digging, so shouldn't be air pockets. I even tried no till, thinking all that digging was making smaller soil particles and causing the beds to sink. Still mid season sunken bed. It is not blowing away, the beds are only uncovered for 3-4 days and then it has plastic over it to kill pests with solar power. The plastic steams nicely, so soil can't be escaping then.

There are about 1-2 earthworms per foot... but still a lot of grubs, are there.
Lizards, snakes, toads and treefrogs are plentiful, so it is a healthy area.

The beds are covered with chicken wire cages shaped like hoop houses to keep the chickens, ducks, and geese out of the plants. I want the poultry in the paths to eat the slugs and other bugs. I had a tick on me yesterday, February 22 with an ice storm! I know likely off of my goats. But, no time of the year is safe from bugs.

Rocks continue to work to the surface and soil continues to disappear.

Any ideas or suggestions as to where it is going?

[rainbowgardener]

Interesting puzzle... Couple more questions. That is a pretty steep slope. Is your raised bed framed/ boxed in or just piled? If framed, is the box built up on the down hill side so the bed is flat, or does the bed slope? Did you fill the box all the way or leave some inches of box room above the soil?

The back half of my property is a steep hillside. I have put retaining walls and paths on it. I know even though it is planted and mulched, the soil tends to keep sliding down the hill. It ends up filling in all the space behind the retaining walls and then washing over the edge.

So I'm thinking some of your topsoil may be ending up at the bottom of your hill... it's why bottom lands make such good farms.

[Ozark Lady]

There are 4" treated timbers around the edges. Rocks found in the beds have been placed on the outside of the timbers, to help keep down weeds.

On the downhill side, about 4' away from the southern most bed, is a blackberry/raspberry patch, that is a lot lower than the beds, just naturally occurring. And just beyond that maybe 20' is a fence, this fence has been there 20 years, and has a build up at the base of it... leaves etc. But dirt should get trapped there, if it is wandering off with rainwater. I know, clay soil has pretty small particles and could mix well with water and get through a lot of small holes, but shouldn't it at least discolor the blockage? Shouldn't there me marks showing where the water is running? Like small ditches? Are they disguised by the naturally occurring rocky terrain?

The raised bed is brought up to level by: the lower edge of the bed has two timbers, the upper edge only one. They fitted together to avoid losing the soil with the long sides extending past the short sides. The long sides are the ones where the soil would be sneaking out. The ends are not a different elevation. The drop is north to south and not also east to west.

I ran the beds across the slope, because there is just too much difference to go for 6 feet... originally the beds were 18' long, but too difficult, so they were re-done to 6 feet lengths.

Should I cement the lower edges, just in case the clay is sneaking out in a rainwater suspension? What would be small enough to filter the water and let it pass but hold my clay?

Can clay seep through the logs themselves while suspended in water?

I appreciate the brain storming here. This mystery has baffled me for years.
You would think that in time, the issue would resolve itself or I would find the answer wouldn't ya?

I added sand to a couple beds for plants that like sand... the sand always ends up on top, no matter how well I mix it in. And the clay settles under it... smaller particles, I get it... but those beds also sink... I don't get it.
The clay in this bed, after the sand rises is below the grade of the existing garden... and still it sinks...

Underground cave? Are the rocks rising and the soil dropping into the place the rock had? Do the rocks ever get done rising?

How can I test various ideas to see what is going on?

[tomf]

Stay up at night and watch the beds, you may have trolls steling your dirt.

I know soil will compact but this is a bit much.

[Ozark Lady]

I think you may be right Tom.

[imafan26]

Your foundation for your bed may not be deep enough to keep the hill from sliding under it. Gravity always wins!

It may be small amounts, or large amounts every time it rains, you won't see because it spreads out as it flushes away. The corners are usually the places that leak the most.

My yard slopes toward the street and I am always sweeping my soil back into my garden.

[rainbowgardener]

You mentioned caves. That made me think -- whether or not there are caves, I think much of your area the bedrock is limestone, which is very porous. It's how the stalagtites in caves form, the ground water seeping through the limestone and then dripping down. So you could have rainwater + clay particles that seeps down into your bedrock.

[Dillbert]

a key question would be:
after "topping off" - when the soil sinks, does it sink to the same level ?

you can't count on organic matter to fill up an area. as it decomposes it's volume decreases way big time, something like 600 to 1.

to prevent sifting out - try spun bond stuff - I.e. floating row covers - as a liner
but I don't think that's the problem.

I suspect you're just seeing the sink due to the organic matter collapsing -

[rainbowgardener]

Unless OL was planting in pure compost, it seems like too much sinkage. Topsoil doesn't shrink that much. I have raised beds sitting on my concrete patio and they shrink very little. What I add in mulch makes up for what they lose and the bed stays full. When I go out in spring (when it finally gets here! ) and turn the winter mulch under, the bed will not have sunk.

[Ozark Lady]

Down the hill, way down the hill, there is outcroppings of limestone, just laying on the surface. Or where erosion has washed all the soil off of them.

We were double digging the beds, and had this long metal thing, which we used to put under things, rocks, roots and lift them. It is heavy metal bar and about 6 feet long, I don't know what it is, we call it the digging bar.
Anyhow, hubby was jabbing at something with it, and I was 20-30 feet away and could feel all the vibrations. This is only where the garden is, setting fence posts we often use that digging bar to make the hole, and there is no vibration within 6 feet of the digging. We had fun jabbing and seeing where we could and could not feel the vibrations in the garden. What would that be? We decided likely a deep slab of rock, or even the top of a cave. Not experts by far, only guessing. We dug 3 feet down and found only fist-sized rocks and clay soil.

When septic was dug, we looked over the soil profile, and it was about 2 inches of topsoil, and then virgin red clay and rocks. This was outside the garden area and not where vibrations were felt.

Compost was not added except the first year, below the ground level or the timbers. When the beds shrink, they shrink below ground level of the un-dug paths. Compost, manure and mulch have been added to the top. Which I remove the top to warm the bed in spring, so it is not counted in the shrinkage. And anything that broke down small would not be removable.

If the soil and water are percolating down into bedrock, I would see nothing and it would lower the beds. And each time it is after rains, the spring rains, and the fall rains... It does pretty much lower equally, there are a few holes that are round and about 2", I assume those are animal, snake, or something dug out. And the round holes are not in every bed.

The bedrock sounds likely, and if that is the case, only moving the garden would solve that problem.

[tomf]

What about digging down to the bed rock and putting landscape fabric down?

It is called a digging bar, I use one.