Pathways and Movements of Nutrients

The concept of ‘cycle’ was previously used when we discussed the movement of nutrients from soil to plant to animal and back to soil, as well as the global cycles of carbon and nitrogen (refer to chapter 2).

There are farmers who reduce their reliance on nutrient supplements and attempt to depend more on the natural cycles of soil nutrients—contrasting with the use of bought commercial fertilizers—to supply plants with fertility. But is it truly feasible to rely indefinitely on the natural recycling of all nutrients to satisfy a crop’s requirements? Let’s first understand what a nutrient cycle is and how it differs from other methods that nutrients use to move from one location to another.

When nutrients transition from one location to another, that is considered a flow. There can be various types of nutrient flows. When you purchase fertilizers or animal feeds, nutrients are ‘flowing’ onto the farm. When you sell products like sweet corn, apples, alfalfa hay, meat, or milk, nutrients are ‘flowing’ off the farm. Flows that involve products entering or leaving the farm gate are managed intentionally, regardless of whether you are thinking about nutrients. Other flows are unplanned—for instance, when nitrate is lost from the soil by leaching to groundwater or when runoff waters carry nutrients along with eroded topsoil to a nearby stream.

When crops are harvested and taken to the barn to feed animals, that is a nutrient flow, as is the return of animal manure to the land. These two flows together form a true cycle, because nutrients return to the fields from which they originated. In forests and natural grasslands, the cycling of nutrients is very efficient. In the early stages of agriculture, when almost all people lived near their fields, nutrient cycling was also efficient (refer to figure 7.1a). However, in many types of agriculture, especially modern, ‘industrial-style’ farming, there is little real cycling of nutrients, because there is no easy way to return nutrients shipped off the farm. In addition, nutrients in crop residues don’t cycle very efficiently when the soil is without living plants for long periods, and nutrient runoff and leaching losses are much larger than from natural systems.

The first significant disruption in the cycling of nutrients occurred as cities developed and nutrients began to routinely travel with farm products to feed the growing urban populations. It is rare for nutrients to travel many miles away from cities and return to the soils on which the crops and animals were originally raised (refer to figure 7.1b,c). Thus, nutrients have accumulated in urban sewage and polluted waterways around the world. Even with the construction of many new sewage treatment plants in the 1970s and 1980s, effluent containing nutrients still flow into waterways, and sewage sludges are not always handled in an environmentally sound manner.

The trend toward farm specialization, mostly driven by economic forces, has resulted in the second disruption in nutrient cycling by separating animals from the land that grows their feed. With specialized animal facilities (refer to figure 7.1c), nutrients accumulate in manure while crop farmers purchase large quantities of fertilizers to keep their fields from becoming nutrient deficient.

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 (refer to 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 significant