Re: why are plants growing better in old motor oil?

There is a common misconception in the public press that motor oil is somehow harmful or toxic. This probably arises from the damage done by oil spills. However, in the case of oil spills, the vast amount of damage arises from the fact that oil - and certainly crude oil - is sticky, messy stuff. Birds get coated in oil and can no longer fly. They also can't swim because the oil changes both their buoyancy and their interaction with water - oil is a "surface active agent" or "surfactant". Ditto for other forms of aquatic life.

That is, they don't tend to die from the toxicity but the physical aspects of exposure to oil. That said, of course drinking too much is very harmful but the same could be said for pretty much anything, including water. However, motor oil isn't particularly toxic.

So, with regards to the plants that are growing in your yard and the motor oil soaked dirt, the motor oil could physically interact with the plant and clog up root pores and such, but it also might not. In your garden, that appears to be the case. Further, motor oil is quite edible to a wide variety of microbial organism and as such, you may be producing a soil with a rich bioshpere. It depends a lot on the ratio of motor oil to soil, but if you are talking about the stuff that has been leaking out of your engine over time, there is enough to provide nutrients without killing the biota.

On top of all this, you said that the weeds are actually doing better in that part of the yard. Another misconception about plants is that they need carbon from the soil. They don't. But they do need macronutrients such as potassium, phosphorus, and nitrogen which is why we add fertilizer to our lawns to make them grow better. They also need a whole variety of micronutrients including iron, sulphur, and copper. These are the sorts of elements that could be found in old motor oil or where a car has been stored for years. Soil enrichment from microparticles worn off of an engine in which the oil was used would enhance the growth of plants in the area. And weeds, being weeds, would take full advantage of this.

Finally, in your message, you said that you got "used dirt to fill in your driveway." It is quite likely that there were a lot of seeds for weeds and other invasive species in that dirt which means that the weeds had a head start on things.

So, my best guess as to what is going on is that you got soil with a lot of seeds in it, which was rich in micro-nutrients, and with motor oil that has little or no effect on plant growth. The result is weeds growning like crazy in one area of your garden.

Hope this helps.