Concrete is lime. Other than being a lump of stuff, like stones ans rocks, there would be no harmful effects from those clumps of concrete. Adding organic matter by planting cover crops or other means may be a good idea but would not be needed to offset any harmful effects of the concrete.

Can you dig out a couple of inches of the soil under the dump spot to remove most of the mix?

It's not toxic - it's just going to make that spot more alkaline for a while.

" it's just going to make that spot more alkaline for a while." Maybe, maybe not. Depends on a number of factors. The only way to know if the concrete has raised the soils pH is with a soil test.

Limestone is alkaline in nature, so its addition will raise the PH of the soil.

Its addition might raise the pH but there are lots of variables. It depends on how much mix was dumped (and does the OP mean concrete mix or cement?) whether the mix stayed dry or got wet and if it got wet did it set. I don't think a small amount of cement on a large volume of soil and removed quickly would make much difference. More info about the situation would help.

Cement, aka limestone, will raise the PH. It might raise it a teensy bit (most likely) or it might raise it substantially (less likely), but it *will* raise it. This minor change in PH in a small area will probably make very little difference in the grand scheme of things. Some plants like alkaline soils and others prefer a more acidic environment but most plants adapt. A lilac for instance would probably happily soak up as much limestone as you can give it, while an azalea would prefer acidic soil.

If it is fresh mixed up cement it will burn the roots kill the grass or whatever is planted near it. EPA says it is toxic even dry because if it rains or snows then it becomes wet and it will burn and kill out that area if not properly cleaned up and (IF) a concrete Truck dumped it or washed their truck off in any spot then whatever is planted near it that the roots will and can come in contact with will more than likely die. Cement burns and blisters the skin if it comes in contact and it will blister and burn killing vegetation in and around it and should be dig out until soil looks free of any wet or dry cement. I know I had the EPA come out because someone reported the cement truck cleaned his truck near their property to mine but still a distance in my property not going onto the property of that neighbors and then the contractor come out to that same area to rinse their wheelbarrows and shovels rinse it out saturated the ground using water to rinse and epa fined the contractor & cement company and now they have to pay loads of money to clean it up and restore my neighbors property even though their property hasn't come in contact with the saturated soil because it was my property because the epa said it could yet kill their weeds and grass through extension of the roots growing under the soil on my yard. Even though many states oh it wont hurt it it's lime well think again the EPA Laws states different that it is toxic

Dana, Thank you for sharing your experience. I was definitely concerned about the matter, and of those who responded, only you showed the same concern.

The guys mixed and dumped powdered concrete all over this property and it is causing me major problems even though I hauled in topsoil and plant virtually everything with good soil around it. i am ending up completely replacing the soil in some areas it's so bad. I hate that people seem to be minimizing this or saying it isn't a problem. It is a problem, it absolutely does make the soil very alkaline, you can find more on the internet. I'm an avid gardener and at wit's end with this at my place.