Building Soils for Better Crops, Third Edition

Implications of Nutrient Flow Patterns


Long-distance transportation of nutrients is central to the way the modern food system functions. On average, the food we eat has traveled about 1,300 miles from field to processor to distributor to consumer. Exporting wheat from the U.S. Pacific Northwest to China involves an even longer distance, as does the import of apples from New Zealand to New York. The nutrients in concentrated commercial fertilizers also travel large distances from the mine or factory to distributors to the field. The specialization of the corn and soybean farms of the Midwest and the hog and chicken mega farms centralized in a few regions, such as Arkansas, the East Coast’s Delmarva Peninsula, and North Carolina, has created a unique situation. The long-distance flows of nutrients from crop farms to animal farms require the purchase of fertilizers on the crop farms; meanwhile, the animal farms are overloaded with nutrients.

Of course, the very purpose of agriculture in the modern world—the growing of food and fiber and the use of the products by people living away from the farm—results in a loss of nutrients from the soil, even under the best possible management. In addition, leaching losses of nutrients, such as calcium, magnesium, and potassium, are accelerated by natural acidification, as well as by acidification caused by the use of fertilizers. Soil minerals—especially in the “young” soils of glaciated regions and in arid regions not subject to much leaching—may supply lots of phosphorus, potassium, calcium, and magnesium and many other nutrients. A soil with plentiful active organic matter also may supply nutrients for a long time. Eventually, however, nutrients will need to be applied to a continually cropped soil. Nitrogen is the only nutrient you can “produce” on the farm—legumes and their bacteria working together can remove nitrogen from the atmosphere and change it into forms that plants can use. However, sooner or later you will need to apply some phosphorus or potassium, even to the richest soils. If the farm is in a mixed crop/livestock system that exports only animal products, it may take a long time to deplete a rich soil, because so few nutrients per acre are exported with those products. For crop farms, especially in humid regions, the depletion occurs more rapidly, because more nutrients are exported per acre each year.

The issue eventually becomes not whether nutrients will be imported onto the farm, but rather, what source of nutrients you should use. Will the nutrients brought onto the farm be commercial fertilizers; traditional amendments (limestone); biologically fixed nitrogen; imported feeds or minerals for livestock; organic materials such as manures, composts, and sludges; or some combination of sources?

Three Different Flow Patterns

There are three main nutrient flow patterns, each one with implications for the long-term functioning of the farm and the environment: Imports of nutrients may be less than exports, imports may be greater than exports, or imports may equal exports.

Imports are less than exports.

For farms “living off capital” and drawing down the supplies of nutrients from minerals and organic matter, nutrient concentrations continually decline. This can continue for a while, just like a person can live off savings in a bank account until the money runs out. At some point, the availability of one or more nutrients becomes so low that crop yields decrease. If this condition is not remedied, the farm becomes less and less able to produce food, and its economic condition will decline. This is clearly not a desirable situation for either the farm or the country. Unfortunately, the low productivity of much of Africa’s agricultural lands is partially caused by this type of nutrient flow pattern, as increasing population pressure elevated land-use intensity, and fertilizer prices are too high for poor farmers. In previous times under the system of shifting cultivation, agricultural fields would have been allowed to return to forest for 20 or more years, during which time there would have been a replenishment of nutrients in the topsoil. One of the greatest challenges of our era is to increase the fertility of the soils of Africa, both by using fertilizers and by building up healthier soils.

Imports are larger than exports.

Animal farms with inadequate land bases to produce all needed feed pose a different type of problem (figure 7.2c). As animal numbers increase relative to the available cropland and pasture, larger purchases of feeds (containing nutrients) are necessary. As this occurs, there is less land available—relative to the nutrient loads—to spread manure. Ultimately, the operation exceeds the capacity of the land to assimilate all the nutrients, and pollution of ground and surface waters occurs. For example, in a study of New York dairy farms, as animal density increased from around 1/4 of an animal unit (1 AU = one 1,000-pound animal, or a number of animals that together weigh 1,000 pounds) per acre to over 1 AU per acre, the amount of N and P remaining on farms increased greatly. When there was 1/4 AU per acre, imports and exports were pretty much in balance. But at 1 AU per acre, around 150 pounds of N and 20 pounds of P remain on the farm per acre each year. The nutrient flow pattern on farms with high animal densities—with large imports, mainly as feeds, greatly exceeding exports—is not environmentally acceptable, although under current conditions it may be more economical than a more balanced pattern. In addition, some farmers, mainly organic ones, try to build up their soil organic matter and nitrogen supply by annual applications of manure or compost. This also causes an unacceptable buildup of nutrients in soils. In a survey from 2002 through 2004 of thirty-four organic farms from seven states in the Northeast, encompassing 203 fields, it was found that approximately a third of the soils had below-optimal levels of nutrients. However, about half of the fields were found to have excessive levels of P. Other ways need to be found to add organic matter through on-farm practices such as intensive use of cover crops and rotations with perennial forages.

Imports and exports are close to balanced.

From the environmental perspective and for the sake of long-term soil health, fertility should be raised to—and then maintained at—optimal levels. The best way to keep desirable levels once they are reached is to roughly balance inflows and outflows. Soil tests can be very helpful in fine-tuning a fertility program and making sure that levels are not building up too high or being drawn down too low (see chapter 21). This can be a challenge and may not be economically possible for all farms. This is easier to do on a mixed crop-livestock farm than on either a crop farm or a livestock farm that depends significantly on imported feeds. As discussed above, because such a high percentage of the nutrients in feeds are excreted, animal products end up exporting relatively low amounts of nutrients off the farm. So if all the feeds are farm grown, adding an animal enterprise to a crop farm will tend to lower the nutrient exports.

In order to help balance nutrient imports and exports, routine soil tests should become a part of every farm’s practices, because they will indicate whether nutrients are being depleted or accumulating to higher levels than needed.

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