Overview and Objectives


The importance of managing agricultural phosphorus (P) to protect water quality was addressed in Lesson I of this series [Manure Phosphorus (P) and Surface Water Protection I:  Basic Concepts of Soil and Water P].  Factors contributing to P loss from the land to surface waters include site and management factors (source factors) and transport factors (Table 1).  An interaction of source and transport factors is needed to have runoff P loss. If one set of factors contributes minimally to risk, then the overall risk is not likely to be great, even if the other set of factors offers greater risk potential.  This raises the concept of 'critical source area' (Fig. 1), the area where risk of P loss is greatest because both source and transport factors are high.

Figure 1.  The 'critical source area' concept for loss of agricultural P to surface waters.  (From Sharpley and Sheffield, Livestock and Poultry  Environmental Stewardship Curriculum)

This lesson describes how source factors, including soil characteristics and management practices, affect P delivery to surface waters; and also discusses how crop producers can control these factors through their management practices.

Table 1:

Site and management factors  


Transport factors

Soil P levels


P application practices, including time, rate and method of application

Erosion from rainfall, snowmelt and irrigation events

Field management practices, such as tillage practices and use of cover crops

Surface and subsurface drainage


Percolation and underground movement of P to seepage areas


Distance from P source to concentrated water flow or water body


Atmospheric deposition


Upon completing this lesson, a student should be able to:

  1. Identify source factors, and analyze their importance, for P delivery to surface waters.
  2. Describe the relationship of soil test P and total soil P with P in runoff.
  3. Identify P application factors, and analyze their importance, for P delivery to surface waters.
  4. Discuss the reactions of manure P with soil following manure application.
  5. Discuss deep plowing as a means of reducing P delivery from cropland.
  6. Name and discuss the role of three management practices that can reduce the potential impact of source factors contributing to P loss  from a field.

Discussion Question: What are the major source factors in your state?

Answer: Erosion and runoff are the most common. Drainage can be important where practiced. Distance to water is always a consideration. Wind erosion is less often considered but can be important.

Copyright 2005

Development of this lesson was supported, in part, by a University of Nebraska Cooperative Extension grant. A contribution of the University of Nebraska Cooperative Extension, Journal Series 1044. Any opinions, findings, conclusions or recommendations expressed in this publication are those of the author(s).