Purpose of HUA
The Idaho Snake-Payette Rivers Hydrologic Unit Area (HUA) is one of 74 projects funded nationally by USDA. These 5-year projects have the purpose of accelerating the transfer of best management practice (BMP) technology necessary to protect both ground and surface waters while still maintaining farm profitability. Program efforts focus on irrigation, nutrient, and pesticide management for groundwater protection, and on structural practice implementation for surface water protection. The HUA projects offer agriculture the opportunity to demonstrate that education coupled with a voluntary BMP implementation program can protect and even enhance existing water quality.
The Snake-Payette Rivers HUA comprises over 840,000 acres in Canyon, Gem, Payette, and Washington counties in southwestern Idaho. Within the project area are 3,400 farms covering more than 500,000 acres. Virtually all of the productive farmland is irrigated. Agriculture within the HUA is very diverse as over 50 different high value crops are grown.
Nitrate-N is the most common pollutant detected in aquifers in the Snake-Payette Rivers HUA and across the USA. The public is demanding that agriculture judiciously use fertilizers and minimize potential environmental contamination -- such as leaching losses of NO3-N into groundwater. The groundwater resource beneath much of the HUA is particularly vulnerable to NO3-N contamination due to its shallow depth and the intensive use of irrigation and agrichemicals required by agriculture above it. The Environmental Protection Agency (EPA) drinking water standard for nitrate-N of 10 parts per million (ppm) is exceeded by between 5 to 8 percent of wells in the HUA. This compares unfavorably to the USA on the whole where the drinking water standard is exceeded 2.4 percent of the time.
This survey of current grower nitrogen management practices was a necessary first step for the development of both education and implementation plans focusing on the improvement of N management in the HUA. Specific survey objectives included:
|Crop||Acres in HUA||Fields surveyed|
Basis for Fertilizer Application
A major goal of the survey was to determine a farmer's basis for applying N to a field. Soil and/or plant tissue samples are used as a basis for applying N on 55 percent of the acreage surveyed. Nitrogen recommendations are based on soil samples only on 33 percent of the acreage, while both soil and tissue sampling are used on 19 percent of the acreage.
The selected crops had a major impact on the likelihood of using soil and/or plant diagnosis as a means of determining the N fertilizer application rate. Virtually all of the potato, onion, and sugarbeet acreage utilized soil or plant tissue diagnosis for N management. Conversely, only 25 percent of the cereal acreage (wheat and barley) was soil tested. The likelihood of testing was related to the economic value of the crop.
Expense did not appear to be the primary reason for a lack of soil testing on 45 percent of the surveyed acreage. On the contrary, in these situations 88 percent of the growers felt that soil sampling was not necessary. This is contradicted by the fact that higher yields were reported for growers using soil testing for five of the six crops evaluated. Expense and the practicality of using soil sampling do not appear to be major sampling obstacles for the farmers interviewed.
Nitrogen Application Rates
The amount of N applied to farmland in the HUA is crop dependent. Average N application rates on onion, mint, potatoes, and sugarbeets are 297, 248, 204, and 187 lb/acre, respectively. Nitrogen application rates on cereals averaged 131 lb/acre. Nitrogen applications on legume crops (beans, alfalfa, and clover) were less than 100 lb/acre.
The average rate of N applied to cropland in the HUA was 108 lb/acre in 1991. The average ranged from 45 lb N/acre for legumes to 240 lb N/acre for shallow rooted high value crops (onions, potatoes, and mint). Deep rooted perennials (orchards and hops) and deep rooted annual crops (sugarbeets, small grains, and corn) were intermediate with application rates of 135 to 175 lb N/acre.
Approximately 32 percent of cropland in the HUA received less than 50 lb N/acre. This reflects the relatively large acreage of alfalfa. Eighteen percent of the cropland received more than 200 lb N/acre.
Split Applications of Nitrogen
Nitrogen fertilizer is most likely to be applied as split applications on potatoes, mint, and onions. The fact that these crops are shallow rooted and that existing water management often creates leaching conditions for nitrates has resulted in an average of 3 to 5 applications of N on these crops. Conversely, split applications of N on corn, sugarbeets, and grains are less common.
Irrigation Effect on N Use
The two major types of irrigation systems, furrow and sprinkler, had an effect on the quantity of N fertilizer applied to some crops. The greatest observed differences were in mint where growers applied an average of 120 lb more N per acre with furrow irrigation than with sprinklers. Nitrogen applications under furrow irrigation were 43 and 29 lb per acre greater than with sprinklers in sugarbeet and small grain production, respectively. Only with potatoes did N use appear to be similar under both types of irrigation systems.
Approximately 55 percent of the acreage in the HUA utilizes soil and/or plant tissue testing and analysis as a tool for nutrient management. As a group, fields which were soil tested received both lower rates of N and utilized the BMPs of split fertilizer applications more than fields where the soil was not tested.
Growers were more likely to use soil and/or tissue testing on the higher value crops (potatoes, onions, and sugarbeets). Onions received the highest N application rates.
The results of this survey will allow the targeted development of education and implementation programs to best meet the goals of the Snake-Payette Rivers HUA.
This brochure, WQ-17, was prepared by T. D. Steiber and R. L.
Mahler. Mahler is the University of Idaho extension water quality coordinator,
located in the Soil Science Division, University of Idaho, Moscow, Idaho
83844-2339. Stieber is a water quality extension agent. Project office located at
1630 Third Ave. S. #3, Payette, ID 83661. Telephone: (208) 642-6128.
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All contents copyright © 1997-2003. College of Agricultural and Life Sciences, University of Idaho. All rights reserved. Revised: January 3, 2003