Quality Water for Idaho Current Information Series No. 963

Water is the lifeblood of Idaho. More than 22 million gallons of water are used in the state each day. More than 97 percent of this water irrigates 4.1 million acres of farmland. Eighty percent of this water comes from surface sources (rivers and reservoirs); the other 20 percent is groundwater. Currently, the quality of water used in Idaho is very good compared with water in other areas of the United States and the world.

Because water is so vital to Idahoans, agricultural best management practices (BMPs) to protect water from phosphorus pollution are becoming more important. Phosphorus is a common water pollutant in Idaho's lakes and rivers. Phosphorus originates from many sources, including agriculture.

Phosphorus is essential to all forms of terrestrial life. It is widely distributed over the surface of the earth in biologically available forms, cycling within plants, animals, soil, and water in the phosphorus cycle. A simplified phosphorus cycle is shown in figure 1. In commercial agriculture, fertilizer is the major phosphorus addition to this cycle.

Fig. 1. Phosphorus cycle in an agricultural setting. Note that chemical fertilizers are the primary phosphorus input into the system.

Many human activities contribute phosphorus to surface waters. Agricultural land enriched with phosphorus by fertilization or manure can contribute substantial amounts of phosphorus to surface waters as the result of runoff and/or erosional processes. Activities associated with modern agriculture often significantly increase soil erosion and water runoff from land and transport sediment into surface waters.

Surface water pollution with phosphorus is controllable -- by reducing soil erosion and keeping soil out of creeks, streams, rivers, and lakes.

Specific BMPs for phosphorus fertilizer and manure management that should be employed to protect surface water quality in many areas of Idaho include:

1. Soil erosion control
2. Fertilizer recommendations based on research and soil sampling
3. Correct phosphorus fertilizer placement
4. Variable fertilizer management
5. Efficient manure management
6. Barnyard and/or feedlot runoff control
7. Conservation tillage and reside management
8. Buffer (filter) strips

Numerous BMPs for the control of runoff and soil erosion are available. These practices reduce contaminant transport to surface waters. Practices for runoff and soil erosion control include both management practices and physical structures.

Management practices designed to control runoff and soil erosion are:

• Permanent vegetative cover -- establishment and maintenance of perennial vegetative cover to protect soil and water resources on land retired from agricultural production
• Conservation cropping sequence (rotation) -- a sequence of crops to provide organic residue for erosion reduction
• Conservation tillage and residue management -- tillage practices that leave residues from the previous crop on the soil surface
• Contour farming -- tillage, planting, and cultivation on sloping land performed on the contour of the landscape perpendicular to the slope
• Strip cropping -- farming operations with alternating strips of row crops, hay, or small grain
• Cover crops -- ground-hugging crops planted after row crop removal to prevent soil erosion
• Buffer (filter) strips -- strips or areas of close-growing vegetation (usually grass) for removing sediment, organic matter, and other pollutants from runoff and wastewater
• Mulching -- use of residue from an off-site source for erosion prevention

Structures designed to control runoff and soil erosion include:

• Diversions -- channeled ridges perpendicular to slopes
• Fences -- barriers that enclose or divide land areas and prohibit stock access to critical streambank areas
• Grade stabilization structures -- structures to stabilize slope gradients, control erosion, and prevent the formation of gullies
• Grass waterways -- graded, vegetated channels for water runoff
• Ponds/sediment basins -- sturctures to trap water and sediments
• Terraces -- earthen embankments of channels and ridges, perpendiclar to the slpe, designed to intercept and transport runoff at nonerosive velocities.

The University of Idaho has developed more than 30 fertilizer guides for Idaho crops. The data base used to develop these fertilizer guides is extensive and has been collected for more than 30 years. Fertilizer guides are based on years of field research and take into account the amount of residual phosphorus in the surface foot of the soil profile.

Soil sampling is a very important BMP that considers the amount of plant-available phosphorus already in the soil profile. Soil sampling should be done 3 to 4 weeks before planting a crop. The soil samples should be representative of the field. Normal sampling depth for phosphorus analysis is 12 inches.

Soil samples for phosphorus should be taken at least once during each crop rotation cycle. Maintain a record of soil test results on each field to evaluate long-term trends of nutrient levels.

Fig. 2 Methods of applying phosphorus fertilizers on agricultural fields. Broadcast applications (left) leave a portion of the fertilizer near the surface and thus are not recommended for the protection of surface water quality. Banding and pop-up placement of phosphorus (right) are BMPs.

Phosphorus should never be placed on the soil surface without incorporation. Banded applications below seeding depth, placement with the seed (pop-up), and/or broadcast-incorporated applications are superior to surface broadcast treatments because they minimize erosional losses of phosphorus. Banding of phosphorus below or with the seed is the best BMP (fig. 2) because no phosphorus is left on the soil surface for erosional loss. A broadcast-incorporated application leaves a portion of the phosphorus fertilizer on or near the soil.

To use a variable management strategy, follow these steps: (1) divide the field into different management units based on yield potential, (2) take separate soil samples from each management unit in the field, (3) use fertilizer guides to apply nitrogen based on yield potential for each management unit, and (4) apply phosphorus, potassium, and sulfur based on soil sampling and analysis from each management unit.

As with commercial phosphorus fertilizers, threats to surface water from manure will be minimized if applications are incorporated or injected beneath the soil surface. Manure should be tested for phosphorus content, and a soil test should be taken to help determine the amount to apply to meet crop demand. The amount of phosphorus in the manure can be used as a fertilizer phosphorus credit. The period of major concern for application of manure from a surface water quality standpoint is the winter and early spring months when manure is difficult to incorporate and runoff is most likely.

Site considerations include land slope and proximity to surface waters. When suitable sites for land application for manure are not available, the use of manure storage facilities is recommended. Manure management BMPs are not only environmentally sound but also are very often cost effective.

• Use soil erosion control practices to minimize runoff and soil loss.
• Test soil and apply phosphorus at recommended rates for crop production in Idaho.
• Credit phosphorus contributions from manure and other organic wastes.
• Band phosphorus below the soil surface or broadcast and incorporate it.
• Limit manure applications on untilled lands.
• Avoid manure applications to sloping, frozen, saturated, or eroding soils.
• Control runoff from barnyards and feedlots.
• Install buffer (filter) strips adjacent to surface waters receiving runoff from croplands.

This publication is one of a series on water quality issues produced by the University of Idaho Cooperative Extension System for the people of Idaho. The material is based upon work supported by the U.S. Department of Agriculture, Extension Service, under special project number 90-EWQUI-1-9216.

Issued in furtherance of cooperative extension work in agriculture and home economics, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, LeRoy D. Luft, Director of Cooperative Extension System, University of Idaho, Moscow, Idaho 83844. The University of Idaho provides equal opportunity in education and employment on the basis of race, color, religion, national origin, gender, age, disability, or status as a Vietnam-era veteran, as required by state and federal laws.

10,000, November 1992 Printed with special grant funds from USDA

Comments to author: karenl@uidaho.edu

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College of Agricultural and Life Sciences, University of Idaho.
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Revised: January 3, 2002

URL: http://www.uidaho.edu/wq/wqpubs/cis963.html