This is storage for manure, called the 'manure pit'. Farmers know the amount of manure their pigs produce, and build the size of their manure storages accordingly.
The size of the manure storage is built according to the number of pigs that live in the barn. This allows the farmer to store the manure until the time is right to spread it on the land. The best time to spread manure on the land differs from farm to farm depending on the soil and land type, time of the year and the type of crops they plan to grow.
Manure contains three major nutrients: nitrogen, phosphorous, and potassium. It also adds valuable organic matter to the soil. Farmer John fertilizes his fields once a year using pig manure as a natural material added to the soil to supply plants with the essential nutrients that help them grow. This is similar to adding manure to your garden at home to help your plants grow better.
Farmers apply manure as part of a nutrient management plan on their land once or twice a year, when conditions are right. They match the nutrients in manure and fertilizer to what is required by the crops and soil in an environmentally friendly way.