Assuming there is no big sink hole under your garden :) then it is because all organic matter continues to decompose over time and turns to humus which has very little volume. It has gradually been doing that for 34 years but just wasn't as noticeable. That is why new stuff has to be added at least one a year and preferably more often. This fall would be the ideal time to build it back up with the addition of some fresh soil and as much good compost as you can get your hands on and improve the soil quality and fertility for next season all at the same time.

Dave

You need SOIL....good old ordinary soil, with amendments of organic matter every once in awhile. Truthfully, the mineral, gritty components of a real soil are more important to gardening than all of the compost in the world.

Many, many years ago, in one of my soils science classes we learned (and I have never forgotten because it's so true) that the pore spaces, the voids in a soil or potting mix are extremely important, ultimately the most important aspect.

Organic particles may start out as coarsely textured but will decompose into microscopic fluids in a short time. The pore spaces provided by sand, silt, and clay create the infrastructure of solids and voids required for a strong root system, for nutrient uptake, and gas exchange. Root cells give off CO2 and take up O2...all of the soil biota gives off CO2.....gas exchange between soil and air atmosphere is crucial. That can only happen in a reasonably porous soil or medium.

I find it odd that the bed is sinking if you never actually removed anything from it. Did you? That is, if the original soil is all there, decomposing organic matter that you add to it isn't ever going to make the bed lower than ground level. That organic matter might start out making the bed a lot higher than ground level, such that it eventually sinks back down, but it shouldn't end up lower than ground level. Is your bed near the water table, such that water motion deep underneath is removing soil?

Good points, all. Catherine, it does sound as though a lot of the organic matter has (as you want it to do) broken down. Our dozer guy covered a tree stump and informed us that in a few years we would want to add some loam to the area because a low spot would follow. That's not a problem, we just add more soil and the depressed area is better than a suckering tree stump. I say this just to point out how much bark mulch and woody material can compress from it's original volume. I created a garden at my parent's place in very sandy soil by tilling in bark mulch and wood chips and some composted manure. It added some good organic matter that improved the structure and let some moisture hold; otherwise everything would have dried out in a flash. I'm sure that has all collapsed by now (I intentionally mounded it) but that garden had a mix of small shrubs and perennials.

Rhizo, so interesting about soil infrastructure. I know that here, a bit of unscreened loam over construction gravel fill, we have ugly plantain where we tend to walk (path) because the soil gets compressed and compacted. Good grass cannot cope with that, but plantain, no problem ;)

At my parent's place, I swear there was hardly any clay (in the very thin loam overlaying the sandy soil). I've often wondered how to get some clay into the soil -- loam, or can one just ad some new kitty litter clay. Can one make or 'fake' "loam"?

Just out of curiosity, what goes on with the soil outside the garden? Do you put anything on that? Is it weeds, grass, or mulched in anyway? For example, I put wood chips on the paths around my beds.

After 34 years, relative perspective may be skewed... it may not be the garden soil that is sinking, it may be the soil outside the garden is being added to, but nothing is ever harvested from it.

Nope. If you didn't take any soil out of the bed, and then added organic matter to the bed, the bed is NEVER going to end up lower than it was originally. Organic matter won't compact the original soil. The point that the soil around the bed may be rising is an interesting proposition. The OP should tell us whether soil was originally removed from the bed 34 years ago, and replaced with organic matter. If that's what happened, then slow decopmosition and compacting of the organic material is a simple explanation.

Thanks everyone. I sure wish we'd kept all the soil that was removed a long time ago to add on to the house. We do have a huge property, and an endless supply of more soil.....but it might leave some big holes around. No, we never removed any soil when we moved here. I'm not sure if the original owner of the property did.

I agree.....the concept that maybe the yard around the garden has grown more is interesting. Yes, it's all grass that we've cut (and not collected) for those 34 years........so that's a possibility.

Maybe its feeling lower this year is because of all the incredible rain we've had. Everything has had its feet wet for about 2 months now and nothing's growing well. This has all encouraged me to buy a couple more stock tanks (Behlen's food-grade stock tanks) and set them up on cement blocks in the garden. I'm already using about 5 of them.....mostly in the back yard.

Even if it never rains this much again, at least I won't have to lean over out there much any more with all the raised stock tanks.

Thanks so much for all your input.

If you add organic matter to a garden the soil level will only raise over time. It can raise slowly in case of sand, as OM fills the pores, and faster in clay. It can not sink. Concur that decomposing stumps or logs may be a reason, but if not, it may be due to geological reasons. From Chicago to Cleveland there are numerous limestone areas, including my own, where things like this can happen.

"It can not sink."

Not so, Grasshopper. The minerals taken up by plants come from actual minerals. There is also organic material in the soil before the gardening ever starts. Who knows how much these affect subsidence?

My father's Eisenhower-era garden in the Northeast was a foot below the grass by the time we left the place after eighteen years. I know. I had to mow the lawn AND weed the dirt. He saved and composted EVERYTHING, never chemically fertilized anything, had manure delivered and spread to overwinter every Fall, and grew a lush garden designed to keep a zillion canning jars full. It still shrank (shrunk?). There was definitely more coming out of there than there was going in.

It's not an unheard of thing and doesn't imply a future sinkhole. What was the problem? My guess is probably not nearly enough manure, but I dunno, really...

this can be computed. Say I produce a head of cabbage (3lbs) per square foot, at 0.5% ash (whole foods catalog number), and the (mineral) soil is typically 150 lbs per cubic foot. It will take about 1000 crops to lower the soil by one inch. Soil subsistence seldom creates sinkholes. Much more often it will create shallow depressions, as is this case. A possible other hypothesis is soil compaction through human traffic, or more abundant watering having leached the mother rock a bit. But they seem like small effects.

3 cabbages per year from a square foot of garden is certainly a pretty optimistic number. So the calculation is an interesting one, and I suspect it proves the theory isn't relevant. The idea of mineral leaching is a good one. The difference between a garden bed and the soil next to it is deep cultivation, and that deep cultivation makes the soil far more permeable. So irrigation of a garden bed can, in principle, carry away a lot more dissolved minerals than an uncultivated bed, in which the water just soaks in and eventually evaporates there. In principle, what this ought to do is to very slowly drop the bed level, and very slowly increase the level of the surrounding soil.

Same issue separate question. And my first post.

The corner of my parking lot, a 10'x10' landscaped area, might be sinking a bit, but it has a lot of plants already planted and have likely established their roots. What would be the best route to build it up? Do I need to remove all the plants first? My neighbor has a few hundred pounds of sand and wood chips he wants me to use. I don't know if those are the right materials now after reading this thread.