I have a 10 thousand sq.ft. free range area fenced in for my chickens. The flock consists of 8 hens and one rooster. I have had them 4-5 years and want to keep them. But I am building a second hen house and want to run some new chickens over the same ground. How can I be sure my existing flock is disease free and the ground is not harboring any threats to the incoming chickens?