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Competency Area 6: Nutrient Management and Planning

PO 46. Describe environmental effects from nutrient loss by:

  1. Erosion
  2. Runoff
  3. Volatilization
  4. Denitrification
  5. Leaching

Erosion is the loss of soil particles, often due to wind or water.  Ammonium N (NH4+) is a cation, and thus is readily adsorbed onto the soil CEC.  This holds the ammonium N in a readily-available form which is not susceptible to leaching or denitrification.  However, since most of the CEC is in the clay fraction of the soil, and since this is the fraction that is most susceptible to detachment and erosion, there can be a significant loss of available N when erosion occurs.  Similarly, P is held mainly in insoluble forms in the soil, thus making erosion the predominant mechanism for P loss.  When the eroded soil enters water, the dilution effect will solubize some of the P, which can lead to eutrophication.

Runoff represents a loss of nutrients dissolved in water that drains off a field.  P is commonly lost through runoff.  The capacity for a soil to fix P is very large; however, it is not infinite.  It is possible to add enough P to use up most of the available Fe and Al (which bind it to soil particles), at least in the surface soil.  When this happens, the soil is said to be saturated with P.  Thus, additional P is not fixed, and it remains soluble and can be lost with runoff.  This becomes very important when large amounts of P are added to the soil surface.  The surface layer of soil can become saturated very quickly, and then when there is runoff over the surface, it interacts with this saturated surface soil layer and picks up significant amounts of soluble P and transports it off the field.  If this runoff goes into a water body, the P is immediately available to cause eutrophication.  In very sandy soils with little natural Fe and Al, there is little capacity to hold the P, and thus it becomes saturated more quickly.  In these soils, soluble P loss is an even greater concern, and even P leaching can be significant.

Volatilization is the loss of nutrients in the gas form.  Urea nitrogen (found in urea fertilizer, UAN solution fertilizer, and manure) is unique in that the available ammonium N (NH4+) from urea can be rapidly converted to ammonia (NH3), which is a gas.  If this reaction occurs on the soil surface, the ammonia is lost into the atmosphere.  Losses of over 1/3 of the N from urea can occur within one week of surface application of a urea-containing material.  Tillage or ½ inch of rain, which incorporates the urea, will minimize this loss.  The sooner tillage or rain occurs after application, the smaller the losses will be; thus immediate incorporation or timing application just before rain are important to reducing this loss.  Other common N sources, such as ammonium sulfate and ammonium nitrate, are not susceptible to volatilization loss.  Also, urease inhibitors have been developed as an additive to urea to effectively reduce this loss when incorporation is not possible.  Volatilization can represent a significant loss to the crop and the ammonia that goes into the atmosphere represents a significant potential pollution problem.

Denitrification is the loss of gaseous forms of N due to anaerobic conditions, like when soils become saturated with water.  This is especially a problem in poorly-drained soils.  Denitrification occurs when all organisms in a saturated soil begin to run out of oxygen.  Some of these organisms have developed the ability to extract the oxygens from nitrate (NO3-) to survive.  In the process, the N from the nitrate is released as either dinitrogen (N2) or nitrous oxide (N2O), both of which are gases which are unavailable to plants.  Nitrous oxides are greenhouse gases with serious environmental implications.

Leaching is similar to runoff in that nutrients in the soil solution leave with water.  Most N sources are rapidly converted to nitrate N (NO3-) by bacteria in the soil.  This is an important process because nitrate is the most common form of N taken up by plants.  However, since nitrate is an anion (has a negative charge), it is not held by the soil CEC.  Therefore, if water percolates through the soil, it can easily carry significant amounts of nitrate with it.  If the nitrate is leached below the rooting zone of the crop, it is no longer available, and if it leaches to the groundwater, it represents a pollution problem with public health implications.  This is especially a problem on well-drained soils.

Nitrogen is a very dynamic element in the soil.  It is constantly changing forms and is very mobile.  As soon as N is applied to the soils, it begins to change and move.  Unfortunately, while some of these changes result in greater availability of the N to the plant, many of these forms can be lost from the system.  These losses not only represent a loss from potential uptake by the crop, but the N that is lost can end up creating environmental problems.  One of the most important management factors to minimize these losses is to time the application of N as close to the time of crop uptake as possible, thus reducing the time that the N is exposed to losses.

Management to minimize P loss is a compromise.  Incorporating the P positions it so that surface runoff and erosion will not have access to the added P, thus reducing the potential for loss.  Also, the mixing that occurs spreads the P out, so it contacts more soil and the soil does not saturate as rapidly.  However, tillage to incorporate P usually increases the potential for erosion, which is the major loss mechanism for P.  The ideal is to place the P below the soil surface in a way that minimizes soil and residue disturbance.  Direct injection of manure or fertilizer P can be very effective at reducing P loss.  Finally, timing can help.  Most P sources are highly soluble when first applied.  If a runoff or erosion event occurs immediately following application, loss can be very high.  However, with time the P reacts with the soil and becomes less soluble.  Thus, timing P applications when it is less likely that there will be significant runoff or erosion events can reduce losses.  Finally, soil properties which control water movement can play a very important role.  If a soil is compacted or crusted, or has a low amount of residue cover, then more water will runoff, increasing the potential for P loss.  Managing soil drainage can have a similar impact.