I believe that organic farming is much more likely to cause soil degradation and environmental harm than conventional farming for several different reasons:
- organic farming's main tool to combat weeds and regrowth is tillage, which causes immense amounts of damage to the soil directly and allowing the soil to blow away or wash away. Conventional farmers know how damaging tillage is and are increasingly moving away from it as pesticide alternatives become available. It was tillage that turned the fertile crescent of Mesopotamia into the deserts of Iraq.
- Organic farmers still use pesticides and herbicides, but they are limited to those that are certified organic. Organic pesticides and herbicides aren't necessarily any safer for human consumption or for the environment, they're just "organic". For example, some organic farmers literally salt their soil. When the only thing you have in your toolbox is a hammer, that's what you use, even when the situation calls for a scalpel.
- Conventional farmers care far more about the value of their land than organic farmers do. Farmers don't have retirement funds, they've just got land on which they've spent the last 40 years paying off the mortgage. A conventional farmer with healthy soil can sell his land for a lot more than one with unhealthy soil. An farmer with land that's "certified organic" doesn't care, it's that "certified organic" stamp that he's getting paid for.
- Organic herbicides and pesticides are much less efficient than conventional ones. So they have to use much larger quantities. Which means more runoff. For instance, they sometimes spread massive amounts of manure on their fields, cause massive algal blooms in the waterways and destroying them.
- Conventional farmers are constrained by economics. They're selling a commodity, where price == marginal cost. So the only way to make a profit is to have lower costs per unit of output than your competitors. Their major costs are diesel, herbicides & pesticides -- minimizing those is both good for the farmer's pocketbook and the environment. An organic farmer's price is not set at the marginal cost, so it's often better for them to increase inputs to increase yields, even when it's not as efficient.
P.S. I gave 5 arguments. They are not equal. The first one (tillage) is orders of magnitude more significant than the other 4. Tillage destroys the soil, and is far more "unnatural" than any sort of herbicide or pesticide organic or conventional.