19.1: Introduction

... 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 prioritize the management of one nutrient and neglect the other; it’s important to consider balanced nutrition. And when applying manures and composts, which contain N and P (as well as other nutrients, of course), there is no alternative to taking both into consideration. Nitrogen losses are an 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 through runoff and leaching into tile drains.

Figure 19.1. Different pathways for nitrogen and phosphorus losses from soils (relative amounts indicated by width of arrows). Based on an unpublished diagram by D. Beegle, Penn State University.

High-nitrate groundwater is a health hazard to infants and young animals because it decreases the blood’s ability to transport oxygen. There is accumulating evidence that high-nitrate drinking water might have adverse health effects on adults as well. In addition, as surface waters become enriched with nutrients (the process is called eutrophication) there is an increase in aquatic plant growth. 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, undesirable microorganisms flourish when nitrate leaches through soil or runs off the surface and is discharged into streams, eventually reaching water bodies like the Gulf of Mexico, the Chesapeake Bay, Puget Sound or the Great Lakes, and increasingly many others around the world. 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, and despite government efforts to curtail the flow of nutrients, most of these dead zones appear to be growing rather than shrinking (the Gulf of Mexico’s dead zone still averages three times larger than the goal set by the U.S. Environmental Protection Agency).

Nitrogen can also be lost from soil by denitrification, a microbial process that occurs primarily when soils are saturated with water. It is especially problematic in soils with poor structure due to compaction or other causes, frequently a result of excessive tillage. 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 climate change impact than a molecule of carbon dioxide. According to the U.S. Environmental Protection Agency, N₂O accounts for 55% of the agricultural greenhouse gas emissions and 5% of the total emissions of all economic sectors combined, which is equivalent to twice the impact from the aviation industry.

Phosphorus losses from farms are generally small in relation to the amounts present in soils. However, small quantities of P loss have great effects on water quality because P is the nutrient that frequently limits the growth of freshwater aquatic weeds, algae and cyanobacteria (also called “blue green 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 eutrophication increases algae growth, which makes fishing, swimming and boating unpleasant or difficult. When excess aquatic organisms die, decomposition removes oxygen from water and leads to fish kills. This is a large concern in the freshwater lakes near the authors’ homes in Vermont and New York where dairy farming is prevalent, and in recent years it has created a very extensive low oxygen (hypoxia) zone in the western part of Lake Erie.

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 Upper 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, there is no biological process that can add P to soils, and plants get their P from soil minerals as well as from decomposing organic matter. 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 N loss from soils occurs when nitrate leaches, is converted into gases by the process of denitrification, or is volatilized from surface ammonium. When water exceeds plant needs, large amounts of nitrate may leach from sandy soils, while denitrification is generally more significant in heavy loams and clays. On the other hand, P is lost from soils in lesser quantities when it is carried away in runoff or in 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). But generally lower P losses are associated with higher impact per unit of nutrient on water quality, so the overall environmental concerns with both N and P are therefore significant. Except for highly manured fields, P losses in runoff and erosion from healthy grasslands is usually quite low because both runoff water and sediment loss are very low. 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 can 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). Cover crops help lessen nutrient loss by preferentially filling many of the large continuous pores with roots, causing more water to flow through the main matrix of the soil and allowing for better nutrient retention.

Problems Using Excess N Fertilizer

There are quite a few reasons you should not apply more N than is needed by crops. N fertilizers are costly, and many farmers are judicious with application rates. However, there are other problems associated with using more N than needed: 1) groundwater 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 accelerated decomposition and loss of soil organic matter; and 5) very high rates of N are frequently associated with high levels of insect damage. For many farmers, the challenge is knowing the correct N fertilizer rate for their crop in the particular growing season. With this uncertainty and with the risk of yield losses from insufficient fertilizer applications, they tend to apply more than needed in many years. Good N management tools can help address this concern.

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 the use of appropriate cover crops can lessen N and P loss 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 and P 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%. In the late 2010s energy (and N fertilizer) costs decreased from their record high levels, but it still makes sense for both environmental and economic reasons to use N fertilizers wisely. 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 will run out at some time in the future.

Table 19.1 Comparing Soil N and P

Nitrogen

Phosphorus

Nitrogen becomes available from decomposing soil organic matter, commonly supplying about one third or more of crop uptake. N is mostly available to plants as nitrate (NO3^–), a form that is very mobile in soils. Some ammonium (NH4^+) and small nitrogen-containing organic molecules such as amino acids are also taken up by plants. Nitrate can be easily lost in large quantities by leaching to groundwater or by conversion to gases (N2, N2O). Nitrogen can be added to soils by biological N fixation (legumes). Cover crops can store nitrogen that would otherwise be lost by leaching and denitrification, providing the N to the following crop.

Phosphorus becomes available from decomposing soil organic matter and minerals. P is relatively immobile and is only available to plants in small concentrations as dissolved phosphorus in the soil solution, mainly as H2PO4^– and HPO4^–2. P is mainly lost from soils through runoff and erosion. However, excessive fertilizer P or manure application on well-structured soils and on those with tile drainage has resulted in P loss to drainage water. No equivalent reaction can add new P to soil, although many bacteria and some fungi (especially mycorrhizae) help plants take up more P. Cover crops can mobilize P from soil and store it in their tissue, providing extra P to the following crop.