PO 46: Describe environmental effects from nutrient loss by:

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.