Phosphorus application can result in increased risk of P delivery to surface waters. Risk of P runoff loss, especially for dissolved and bio-available fractions, is greatest immediately following P application and declines with time. The risk appears to be similar for application of fertilizer P and organic P for a given rate of application. The potential for P runoff can be greater with manure application because manure is often applied to meet crop N needs, and the amount of P applied generally far exceeds crop need or the amount of P removed in crop harvest (Figure 3). The result is that STP increases with continued manure application, resulting in increased risk of P runoff loss. There are three ways in which producers can manage P application to reduce runoff risk: rate of P application; time of P application; and method of P application.
Discussion Question: Why is the risk of runoff loss, especially for dissolved and bio-available fractions, greatest immediately following P application, and then decreases with time?
Answer: Soil soon becomes absorbed to compounds and soil colloids making it less easily carried in runoff. Also, the added organic material of manure may stimulate soil aggregation thereby increasing rate of water infiltration and reducing runoff, and making the soil less susceptible to erosion.
Rate of P Application. Runoff P concentration can be expected to be greater with higher rates of, and more frequent, P application. In a runoff study conducted in eastern Nebraska, total P loss was approximately 80% more when the P application rate was doubled by applying compost with high P concentrations, compared to compost with lower P concentration (Fig. 4), even though the actual amount of compost applied was similar for both cases. Risk of P loss in runoff can be expected to increase as the rate of P application increases.
Does more frequent application of P reduce the chance of P loss from runoff? This is an important question for farmers who need to dedicate time to P application on their fields and also need to manage the timing of those applications. There is no evidence that applying the same amount of P in infrequent applications at higher rates (e.g., four times as much once in four years) results in more risk of P runoff loss than do annual applications with proportionally lower rates of application. If infrequent application allows better timing of application, and otherwise more careful application, risk of P loss may be less with infrequent applications at higher P rates sufficient to meet crop needs for several years. Therefore, risk of P loss can be reduced with careful application when the probability of runoff events is low.
Time of P Application. The main consideration in assessing the effect of time of application on risk of P runoff is the probability of a runoff event, and the likely magnitude of such an event, occurring shortly after P application. As discussed in Lesson I of this series, the risk of P runoff decreases with time after application as P becomes adsorbed, and soil aggregation improves due to manure application. This decrease in risk with time is illustrated by results from western Nebraska (Fig. 5) where P loss was 5 times as great where beef feedlot manure was applied one day before as compared to one year before. The effect of time of occurrence of the runoff event after application is further illustrated by the results of a study with swine liquid manure in Iowa. Phosphorus concentration in runoff was much higher following surface application, as compared to the injection of swine manure, if the runoff event occurred one day after application ( Fig. 6). If the runoff event occurred 10 days after application, P concentration in runoff was much reduced for the surface application and was similar for the two application methods.
In a study which took place in eastern Nebraska, runoff events occurred most frequently in May. March runoff events, which were due to snow melt, were less frequent than for May, but of much greater magnitude. No runoff events occurred from September to February, and events were infrequent in July and August. In this study, P loss was about 15% more with winter application, as compared to spring (late April) application.
Increased risk with winter application onto frozen ground can be substantial where snowmelt events often result in considerable runoff. The main point is that the effect of time of application on risk of P runoff depends on the probability of a runoff event, and the likely magnitude of such an event, occurring shortly after P application.
Method of P Application. Issues of P application methods are concerned with the advantages of injection or incorporation of P, as compared to surface application without incorporation. Research results on the effect of incorporation of surface applied manure or fertilizer have been inconsistent. Generally, however, dissolved P in runoff, and maybe other P fractions as well, is more of a concern with surface application, if the runoff event occurs shortly after application. This is illustrated in Figure 6, where P concentration in runoff was much higher with surface application than with incorporation, if the runoff event occurred shortly after application. P loss was similar with both application methods when the runoff event occurred 10 days after application. In eastern Nebraska, under natural rainfall conditions, P losses following annual manure applications for three years were similar for incorporation and surface application.
The effect of incorporation or injection of manure or fertilizer P on the potential for P runoff also depends on the erosion risk associated with the incorporation or injection operation. Erosion risk might not be changed if the land is typically tilled or if injection can be done without much disturbance of ground cover. In short, risk of P runoff can be decreased by incorporation or injection of both fertilizer or manure P, provided this can be done without increased erosion risks. However, on more erodable land, surface application of P may be the option of least risk, as incorporation or injection might result in increased erosion.
Discussion Question: In addition to the potential for increased or decreased P loss with incorporation, what else needs to be considered when choosing between surface application and incorporation?
Answer: Ammonium-N loss, odor and attraction of flies may be worse if the manure is not incorporated.