In this chapter, the following terms have the meanings indicated. Terms Defined. “Agricultural operation” means a business or activity that: Tills, crops, keeps, pastures, or produces an agricultural product, including livestock, poultry, plants, trees, sod, food, feed, or fiber by in-ground, out-of-ground, container, or other culture; Has a gross annual income of $2,500 or more, or has eight or more animal units; and Includes all land owned or managed for agricultural production by the taxpayer filing a tax credit claim. “Base year” means the year before the agricultural operation converted to a phosphorus based nutrient management plan. “Commercial fertilizer” means any substance containing a recognized plant nutrient used for its plant nutrient content, and designed for use or claimed to have value in promoting plant growth, except unmanipulated animal and vegetable manure, marl, lime, wood ashes, and gypsum. “Department” means Maryland Department of Agriculture. “Nutrient management plan” or “plan” means a plan prepared in accordance with Department requirements to manage the amount, placement, timing, and application of animal manure, fertilizer, biosolids, or other plant nutrients in order to: Minimize nutrient loss or runoff; and Maintain the productivity of soil when growing agricultural products.