🚀 go-pugleaf

Most plant nutrients bind quite tightly to soil particles due to their 
charge. (Nutrients are available in soil as polar ions.) It would take 
quite a long time for these nutrients to leach out of soil once they're 
there. In fact, as the most efficient way to remove said ions from the 
soil is to have plant roots extract them, the nutrients may even stay in 
the soil LONGER after logging then before.

The exception to this is nitrogen, which is available to plants in the 
form of an ion with the same charge as your average soil particle. Hence 
it is repelled by the soil, and tends to leach out with soil water 
movement. Trees will add minute amounts of nitrogen to soils through 
dropping dead leaves, and larger amounts by attracting animals that urinate, 
defecate and decompose on the soil. But it's a matter of degree only: 
trees don't provide that much nitrogen unless they fix it (take it from 
the atmosphere and change it chemically into a usable form, with the help 
of microorganisms.

Your soil scientist may have been thinking solely of these inorganic 
nutrients in answering your question, and he answered it accurately. Soil 
_Structure_, on the other hand, is at least as important in forest 
ecosystems as soil nutrient level, if not more so. A soil's structure 
depends in large part on the amount of organic matter deposited, which 
trees do in abundance. Cut the trees and the soil structure suffers, as 
microorganisms eat the organic matter with nothing new to replace it.

Chris