All of these questions have already been answered in this thread, so I'll give you the abbreviated versions....my thinking is this~if it's worth it to you to learn about these things, it's worth it to read the whole dang thread. It's no different than reading a book on the same subject...sure, you have to sift through all the nonessential info but it's really worth it. I'll tell you why...my answers aren't the only answers.
I don't have my birds in a penned run, so my answers regarding that aren't from experiencing chickens in a run all their life. We used to do that some when I was young and it wasn't ideal but we didn't get sick birds. The run was huge and we didn't put too many birds in it, it had good drainage overall and it had good airflow. Now I don't use a run....I create situations when raising chickens where I don't have to confine birds to a stationary run.
Overstocking the soil is having too many birds on the same soil for too long...you will see a barren, slick run that gets slimy in wet weather, gets packed down so that it cannot absorb fluid, gets too much nitrogen on the same soil, is dry and dusty in hot weather with no areas of moist, cool earth. This creates unbalanced life in the soil where the beneficial microbes cannot thrive but the bad ones will have an overgrowth. In nature, soil has many filters and ways of cleansing itself. If you remove the grass, the topsoil(the loose, absorbent layer at the top that is lost with soil erosion by water, wind and foot traffic), the natural sieve that is normal soil...all these things help the soil stay healthy and balanced...when removed, you have poor conditions for drainage, for cleanliness, for health.
Air flow..this isn't only essential to the coop...a run, though fully open to air, can be in a situation that doesn't allow for good cross breezes or sunlight. Both of these things also help with drainage or wicking of moisture from a run.
Rain isn't the problem...not even if you live in a rainy area. Rain is cleansing the soils on a regular basis if the soils are healthy. The soil is the key to balance in the area of your livestock...all their waste go there, they are constantly in contact with it, they eat it, they dust in it, they live on it. Guard the soil integrity and you invest in your animal's health...that goes for dogs and cats, as well as livestock.
You know when I see the start of trouble for many people on this forum? When they ask a question like this one~"How many birds can I put in a ___ X ____ pen, coop, run, yard?" No one can answer that question effectively. You know why? Because one has to find that out for themselves and you find out by starting with a low number and then adjusting upwards, not the other way around. By the time you find out you have too many birds in one space, it is often too late and the damage is done....then you have to do the long process of healing the land or getting rid of the number of birds to fit your coop size, etc.
When someone asks that question, I can almost guarantee they are going to start out with the maximum amount of birds that someone says will fit in that space. Everyone wants everything
right now...no one wants to wait and do it right. No one wants to earn their own knowledge..they want someone to tell them how to do it and then they want to do it right then, immediately, because they just
want to.
Then you hear a lot of people sending out urgent posts with lots of exclamation points at the end of the title....I don't even bother to click on those. They merely irritate me to even see the title~this is someone who rushed into something and now wants to rush everyone into giving them a solution to a problem they have created...and they want that solution RIGHT NOW.
Deep litter? We have written tons about that here and there is a whole thread devoted to it on the forum, so I won't go there tonight. I've already rambled on too long....maybe tomorrow.