Different types of farms may have distinctly different nutrient flow patterns. Farms that exclusively grow grain or vegetables have a relatively high annual nutrient export (figure 7.2a). Nutrients usually enter the farm as either commercial fertilizers or various amendments and leave the farm as plant products. Some cycling of nutrients occurs as crop residues are returned to the soil and decompose. A large nutrient outflow is common, however, because a large portion of the crop is usually exported off the farm. For example, an acre of tomatoes or onions usually contains over 100 pounds of nitrogen, 20 pounds of phosphorus, and 100 pounds of potassium. For agronomic crops, the annual export of nutrients is about 100 pounds of nitrogen, 6 pounds of phosphorus, and 50 pounds of potassium per acre for corn grain and about 150 pounds of nitrogen, 20 pounds of phosphorus, and 130 pounds of potassium per acre for grass hay.
It should be fairly easy to balance inflows and outflows on crop farms, at least theoretically. In practice, under good management, nutrients are depleted a bit by crop growth and removal until soil test levels fall too low, and then they’re raised again with fertilizers or manures (see chapter 21).
A grass-fed beef operation that uses little to no imported feed should also be able to easily balance imports and exports because few nutrients leave the farm (as animals) and few nutrients are brought on to the farm (figure 7.2b). Most of the nutrients on this type of operation complete a true cycle on the farm—they are taken up from the soil by plants, which are eaten by the animals, and most of the nutrients are then returned to the soil as manure and urine. The same type of flows will occur on all integrated crop and livestock farms that produce all of their own feed.
A contrasting situation occurs on dairy farms if all of the forage is produced on the farm but grains and minerals are purchased (figure 7.2c). Many dairy farms in the northeast U.S. do not have the land base to grow all the needed feed and tend to emphasize growing forage crops. In this situation, there are more sources of nutrients coming onto the farm—with concentrates (commonly mixtures containing corn grain and soy) and minerals usually comprising a larger source of nutrient inputs than fertilizers. In a study of forty-seven New York state dairy farms an average 76% of N came onto the farms as feeds and 23% as fertilizers. The percentages were pretty much the same for P (73% as feeds and 26% as fertilizers). Most of the nutrients consumed by animals end up in the manure—from 60% to over 90% of the nitrogen, phosphorus, and potassium. Compared with crop farms, where a high percent of the crop grown is sold, fewer nutrients flow from dairy farms per acre. Under this situation, nutrients will accumulate on the farm and may eventually cause environmental harm from excess nitrogen or phosphorus. This same problem exists for any animal farm that imports a high percentage of its feed. To put it another way, these farms have an inadequate land base to produce all their feed and therefore also have an inadequate land base on which to apply their manure at environmentally safe rates. Animal operations that import all feeds and have a limited land base to use the manure have the greatest potential to accumulate high amounts of nutrients. Contract growers of chickens are an example of this situation.
Two different nutrient flows occur when manure on livestock farms is applied to the fields used for growing the feeds. The nutrients in manure that came from farm-grown feed sources are completing a true cycle. The nutrients in manure that originally entered the farm as purchased feeds and mineral supplements are not participating in a true cycle. These nutrients are completing a flow that might have started in a far-away farm, mine, or fertilizer factory and are now just being transported from the barn to the field.
If there is enough cropland to grow most of the grain and forage needs, low amounts of imported nutrients and export per acre will result. Relatively low amounts of nutrients exported per acre as animal products make it easier to rely on nutrient cycling on a mixed livestock/crop farm that produces most of its feed than on a farm growing only crops.