(taxpayers rented out 40 acres of farmland for over 10-year period with cash rent received that last five years of rental arrangement; land then placed in CRP; taxpayers maintained the property during period of ownership including repairing and mowing waterways, maintaining fences collecting and negotiating rent and paying all bills associated with the property; on sale, taxpayer claimed capital gains deduction; deduction denied because taxpayer failed to present evidence indicating participation in farming activities; no self-employment tax paid on CRP income).