Methods to determine phosphorus loss from farm fields

Movement of phosphorus (P) from farm fields to surface waters can elevate P in water systems above critical levels for aquatic plant growth and thus enhance nutrient enrichment and seasonal deficient oxygen, a process called eutrophication. Phosphorus commonly controls vegetative production in freshwater bodies, and hence the potential for eutrophication. The sourcing of P from production fields (including P from manure and fertilizer) is now one focus area considered as being an important contributor of total P entering surface waters, and hence significantly contributing to water quality concerns.

Background. In April 1999, the Iowa Natural Resources Conservation Service (NRCS) issued an Interim Conservation Practice Standard, Nutrient Management Code (590). This standard is the guidance used by NRCS staff and the private sector when providing technical assistance to producers requesting assistance on nutrient management. Under some situations the technical guidance in this standard may be required if the producer is voluntarily participating in cost share programs that address water quality concerns. The NRCS in each state is required to revise their state Nutrient Management standard (590) in accordance with guidance provided by national policy and in the national 590 standard. For P, the national standard provided states with three options for guidance on application of P. In other words, there is a choice of three methods states can use to assess the risk of P loss from farm fields, and thus determine the potential management changes needed to modify P application. This is a field-specific assessment of the potential for P transport from the field. These options are 1) soil test, 2) soil P threshold level, and 3) P Index rating. The state NRCS has until April 2001 to implement one of these methods in the Iowa 590 standard.

Soil test. This assessment method is very similar to an agronomic interpretation of P need. The soil is tested using routine soil test P methods for crop production, and test results are interpreted using tables developed for crop response (Table 1). At soil tests less than optimal, P is applied based on crop need (or at a nitrogen [N] need for the crop). At some intermediate (optimal to high) level, P is applied based on the crop removal. Eventually, P application is withheld at even higher soil tests (Table 1, Excessive). The theory behind this risk assessment method comes from the knowledge that as soil test P increases, dissolved P in runoff increases.

Table 1. Risk assessment option using soil test P method.
Soil Test P Level	P Application
Low	N based
Medium	N based
High	P based (e.g.,1.5 times crop removal)
Very high	P based (e.g., crop removal)
Excessive	P based (e.g., no application)

This environmental P interpretation does parallel agronomic use interpretation, like that currently recommended in Iowa State University publication PM 1688, General Guide for Crop Nutrient Recommendations in Iowa (for an example, see Table 2 for corn P interpretation and recommendations).

Table 2. P recommendations for corn grain production.
	P Soil Test (ppm)
Soil Test Category	Very Low	Low	Optimum	High	Very High
Bray P₁ and Mehlich-3 P:
Low subsoil P	0–8	9–15	16–20	21–30	+31
High subsoil P	0–5	6–10	11–15	16–20	+21
Olsen P:
Low subsoil P	0–5	6–10	11–14	15–20	+21
High subsoil P	0–3	4–7	8–11	12–15	+16
	lb P₂O₅ to apply (lb/acre)*
	100	75	50	0	0
*The recommended amounts of P₂O₅ for the optimum soil test category are based on nutrient removal for the reported yield. The amount shown in the table for the optimum soil test category is for 140 lb of corn grain per acre. Although P₂O₅ is not recommended at the high soil test category, a small amount equivalent to that contained in 100 pounds of a common complete NPK grade, applied as a starter fertilizer banded to the side and below the seed row, may be advantageous under conditions of limited soil drainage, cool soil conditions, or crop residues on the soil surface. None is recommended for the very high soil test category.

There are significant advantages and problems in using the soil test approach to modifying P applications for water quality purposes. Advantages include 1) uses soil tests and sampling methods that are familiar to farmers and advisers; 2) follows agronomic guidelines for crop P need; and 3) applies simple decision process and easy regulatory control. From the standpoint of optimal agronomic and economic P resource use and protection of soil and water resources, the soil test P risk assessment method makes a lot of sense. Disadvantages include 1) research-based correlation between soil test level and P reaching surface waters is limited; 2) management practices (recent P application, rate, method, source, timing, and tillage) can override the effect of soil test level on P losses; 3) beyond edge of field management can affect P losses (distance to surface water, connectivity between the field and water body, grassed waterways, and buffers); and 4) soil P tests do not predict soil erosion (P leaves fields in conjunction with soil particles). There is also the issue of where and how to collect soil samples for best prediction of P loss.

Phosphorus threshold. This assessment method is very similar to the soil test method. Instead of interpreting soil tests as given in Table 1, and relating to crop need, an environmental soil P threshold level is determined (Table 3). This environmental soil P threshold could be determined from a routine soil P test, an environmental soil P test, P saturation of the soil, or some other soil test. Advantages and disadvantages are similar to those described for the soil test method. The largest disadvantage is that no threshold value has been correlated to a critical P loss concentration from farm fields (mainly due to the linear increase found in dissolved P loss with increasing soil P level).

Table 3. Risk assessment option using soil P threshold value (TH).
Soil Test P Level	P Application
<3/4 TH	N based
³3/4 TH and <1 1/2 TH	P based (e.g., crop removal
³1 1/2 TH and <2 TH	P based (e.g., crop removal)
³2 TH	P based (e.g., no application)

Phosphorus Index (PI). The PI is an integrated approach to estimating the risk of P loss from farm fields and movement to surface waters. Instead of looking at just one test, it integrates the many field-specific factors that influence P loss and potential movement to surface waters: erosion, sediment delivery, relative field location in the watershed, buffer strips, soil conservation practices, soil test P, precipitation, runoff, tile flow, and P application (fertilizer or manure) method, timing, and rate.

The PI has several advantages over other risk assessment methods: 1) estimates erosion and sediment losses because total P is an important aspect of P supply to surface waters; 2) accounts for beyond field edge effects on P reaching surface waters; 3) includes P applications; and 4) adjusts for P management strategies and soil conservation practices. The PI also could include some characteristics of the other methods, for example, an environmental P threshold. As for any of the P loss assessment methods, the predicted risk of P delivery to surface waters indicated by a PI should be field tested with representative situations (calibrated against measured P delivery) and interpreted for surface water quality impacts.

The PI is more complex and difficult to determine, but is a more reasonable and effective approach to assessing risk of P loss from fields and delivery to surface waters than soil test or threshold methods. Because of the integrated system, the PI is useful for understanding the important factor or factors causing a high P loss risk, and can help identify management practices to lower that risk. And that is the goal, to reduce risks of P loss, help water quality, and provide producers options for P management.

The Iowa approach. The Iowa NRCS, through work and discussion of the State Technical Committee, has decided from the three possible methods to develop a PI for use in the Iowa 590 nutrient management standard. Other midwestern states are also taking this approach. A PI is currently under development in Iowa by a team of NRCS employees, Iowa State University Extension specialists, and Iowa State University and United States Department of Agriculture (USDA) soil scientists. Once recommended by the USDA State Technical Committee and adopted by NRCS, an electronic version of the Iowa Phosphorus Index and user’s guide will be available on the Web at http://www.ia.nrcs.usda.go

Winter 2000