. . . an economical use of fertilizers requires that they merely supplement the natural supply in the soil,and that the latter should furnish the larger part of the soil material used by the crop.
—T.L. LYON AND E.O. FIPPIN, 1909
Both nitrogen and phosphorus are needed by plants in large amounts, and both can cause environmental harm when present in excess. They are discussed together in this chapter because we don’t want to do a good job of managing one and, at the same time, do a poor job with the other. Nitrogen losses are a serious economic concern for farmers; if not managed properly, a large fraction (as much as half in some cases) of applied N fertilizer can be lost instead of used by crops. Environmental concerns with N include the leaching of soil nitrate to groundwater; excess N in runoff; and losses of nitrous oxide, a potent greenhouse gas. For P, the main concerns are losses to freshwater bodies.
High-nitrate groundwater is a health hazard to infants and young animals because it decreases the blood’s ability to transport oxygen. In addition, nitrate stimulates the growth of algae and aquatic plants just as it stimulates the growth of agricultural plants. The growth of plants in many brackish estuaries and saltwater environments is believed to be limited by a lack of N. So, when nitrate leaches through soil, or runs off the surface and is discharged into streams, eventually reaching water bodies like the Gulf of Mexico or the Chesapeake Bay, undesirable microorganisms flourish. In addition, the algal blooms that result from excess N and P cloud water, blocking sunlight to important underwater grasses that are home to numerous species of young fish, crabs, and other bottom dwellers. The greatest concern, however, is the dieback of the algae and other aquatic plants. These plants settle on the bottom of the affected estuaries, and their decomposition consumes dissolved oxygen in the water. The result is an extended area of very low oxygen concentrations in which fish and other aquatic animals cannot live. This is a serious concern in many estuaries around the world.
|Table 19.1: Comparing Soil N and P|
|Nitrogen becomes available from decomposing soil organic matter.||Phosphorus becomes available from decomposing soil organic matter and minerals.|
|N is mostly available to plants as nitrate (NO3 )—a form that is very mobile in soils||P is available mainly as dissolved phosphate in soil water—but little is present in solution even in fertile soils, and it is not mobile.|
|Nitrate can be easily lost in large quantities by leaching to groundwater or by conversion to gases (N2, N2O).||P is mainly lost from soils by runoff and erosion. However, liquid manure application on well-structured soils and those with tile drainage has resulted in P loss to drainage water.|
|Nitrogen can be added to soils by biological N fixation (legumes).||No equivalent reaction can add new P to soil, although many bacteria and some fungi help make P more available to plants.|
Denitrification is a microbial process that occurs primarily in surface layers when soils are saturated with water. Soil bacteria convert nitrate to both nitrous oxide (N2O) and N2. While N2 (two atoms of nitrogen bonded together) is the most abundant gas in the atmosphere and not of environmental concern, each molecule of N2O gas—largely generated by denitrification, with some contribution from nitrification—has approximately 300 times more global warming impact than a molecule of carbon dioxide.
Phosphorus losses from farms are generally small in relation to the amounts present in soils. However, small quantities of P loss have great impacts on water quality because P is the nutrient that appears to limit the growth of freshwater aquatic weeds and algae. Phosphorus damages the environment when excess amounts are added to a lake from human activities (agriculture, rural home septic tanks, or urban sewage and street runoff). This increases algae growth (eutrophication), making fishing, swimming, and boating unpleasant or difficult. When excess aquatic organisms die, decomposition removes oxygen from water and leads to fish kills.
All farms should work to have the best N and P management possible—for economic as well as environmental reasons. This is especially important near bodies of water that are susceptible to accelerated weed or algae growth. However, don’t forget that nutrients from farms in the Midwest are contributing to problems in the Gulf of Mexico—over 1,000 miles away.
There are major differences between the way N and P behave in soils (figure 19.1, table 19.1). Both N and P can, of course, be supplied in applied fertilizers. But aside from legumes that can produce their own N because of the bacteria living in root nodules, crop plants get their N from decomposing organic matter. On the other hand, plants get their P from both organic matter and soil minerals. Nitrate, the primary form in which plants absorb nitrogen from the soil, is very mobile in soils, while P movement in soils is very limited.
Most unintentional N loss from soils occurs when nitrate leaches or is converted into gases by the process of denitrification, or when surface ammonium is volatilized. Large amounts of nitrate may leach from sandy soils, while denitrification is generally more significant in heavy loams and clays. On the other hand, most unintended P loss from soils is carried away in runoff or sediments eroded from fields, construction sites, and other exposed soil (see figure 19.1 for a comparison between relative pathways for N and P losses). Phosphorus leaching is a concern in fields that are artificially drained. With many years of excessive manure or compost application, soils saturated with P (often sands with low P sorption capacity) can start leaking P with the percolating water and discharge it through drain lines or ditches. Also, liquid manure can move through preferential flow paths (wormholes, root holes, cracks, etc., especially in clay soils) directly to subsurface drain lines and contaminate water in ditches, which is then discharged into streams and lakes (see also chapter 17).
PROBLEMS USING EXCESS N FERTILIZER
There are quite a few reasons you should not apply more N than needed by crops. N fertilizers are now quite expensive, and many farmers are being more judicious than when N was relatively cheap. However, there are other problems associated with using more N than needed: (1) ground and surface water become polluted with nitrates; (2) more N2O (a potent greenhouse gas and source of ozone depletion) is produced during denitrification in soil; (3) a lot of energy is consumed in producing N, so wasting N is the same as wasting energy; (4) using higher N than needed is associated with acceleration of decomposition and loss of soil organic matter; and (5) very high rates of N are frequently associated with high levels of insect damage.
Except when coming from highly manured fields, P losses—mainly as dissolved P in the runoff waters—from healthy grasslands are usually quite low, because both runoff water and sediment loss are very low. Biological N fixation carried on in the roots of legumes and by some free-living bacteria actually adds new N to soil, but there is no equivalent reaction for P or any other nutrient.
Improving N and P management can help reduce reliance on commercial fertilizers. A more ecologically based system—with good rotations, reduced tillage, and more active organic matter—should provide a large proportion of crop N and P needs. Better soil structure and attention to use of appropriate cover crops can lessen loss of N and P by reducing leaching, denitrification, and/or runoff. Reducing the loss of these nutrients is an economic benefit to the farm and, at the same time, an environmental benefit to society. The greater N availability may be thought of as a fringe benefit of a farm with an ecologically based cropping system.
In addition, the manufacture, transportation, and application of N fertilizers are very energy intensive. Of all the energy used to produce corn (including the manufacture and operation of field equipment), the manufacture and application of N fertilizer represents close to 30%. Although energy was relatively inexpensive for many years, its cost has fluctuated greatly in recent years, as has the cost of fertilizers, and is expected to be relatively high for the foreseeable future. So relying more on biological fixation of N and efficient cycling in soils reduces depletion of a nonrenewable resource and may save you money as well. Although P fertilizers are less energy consuming to produce, a reduction in their use helps preserve this nonrenewable resource—the world’s P mines are expected to run out in the next fifty to one hundred years.