Sustainable Potato Cropping Systems for Irrigated Agriculture in the Pacific Northwest

Introduction: The Pacific Northwest (PNW) is an important area for potato (Solanum tuberosum) production which accounts for about 117 million metric tons of potato production annually (55% of the total U.S. production) on about 256,000 ha (i.e., 45% of the total U.S. acreage under potato). The crop value in the PNW is estimated at 1.3 billion dollars. Idaho and Washington rank number 1 and 2, respectively, in the country for potato production. The Columbia Basin in eastern Washington provides ideal conditions for high per acre yields of up to 80 Mg ha-1. However, there are some environmental concerns with regard to soil and water quality. Increasing public concern about environmental quality and the sustainability of agroecosystems has emphasized the need to develop and implement management strategies that maintain and protect soil and water resources. Both of these issues are related to maintaining the quality of soil resources through improved soil management. Groundwater protection from agri-chemical and nutrient contamination is a major environmental issue in some parts of the PNW under intensive agricultural production. Irrigation is essential for high yields and net returns for a myriad of crops in the PNW, particularly for potato. Potatoes are typically grown on soils low in organic matter content that are highly susceptible to agri-chemical leaching under poor irrigation scheduling, and wind erosion after harvest. Improved irrigation and nutrient management practices are important to minimize leaching losses. Groundwater is the major source of drinking water in the PNW. Groundwater protection from nitrate (NO3-N) contamination is an important public concern and a major environmental issue in this region and other parts of the nation. Overall, in Washington, 23% of the 574 wells sampled contained NO3-N concentrations in excess of the EPA's maximum contaminant level (EPA-MCL, i.e. 10 mg/L).

Currently most of the potato production is under pivot irrigation. Drip irrigation offers a significant potential for improved water and pest management in potato cropping systems but is not being utilized on a commercial scale in the PNW. Drip irrigation is considerably more efficient and offers the opportunity to inject synthetic pesticides and/or biological pesticides (bio-pesticides) into the root zone which could lead to reduced application rates as this placement may be more efficacious than broadcast applications. Combined with biological control measures like the use of cover crops, more extensive rotations and bio-pesticides, drip irrigation offers real opportunities for improved yield and quality with lower inputs of water and pesticides. The effects of drip irrigation as compared to the sprinkler irrigation on yield and tuber quality of potato are not fully investigated. There is also a compelling need to monitor the water transport in the soil and to use soil water monitoring as a tool to schedule irrigation in an effort to minimize excess water application and nutrient losses.

Research Areas: My collaborative research contributes to the following research areas:

1. Develop nutrient and irrigation management recommendations for irrigated potato rotation system to optimize production and crop quality, and minimize potential negative environmental impacts. This experiment will be conducted on a commercial scale pivot irrigation system in the Columbia Basin production region. Potato cultivar 'Ranger Russet' will be used in this study. Different rates of pre-plant N application (0 to 168 kg N ha-1), and rates (112 to 336 kg N ha-1) and frequencies (2 to 10) of in-season N application will be evaluated to investigate their effects on tuber yield, grade, and specific gravity. Porous cup suction lysimeters will be used for collection of leachate below the root-zone for evaluation of water leaching and quality of leachate in terms of nitrate concentrations. Petiole sampling will be taken at weekly interval 5 weeks after emergence to evaluate the nutritional status of the plants. Capacitance probes that work on the principles of dielectric constant will be used to measure the soil moisture content. Plant samples will be taken at various times during the growing season to estimate the total dry matter (DM) and partitioning of DM and nutrients into various parts of the plant. Soil samples will be taken at two weeks interval at 30 cm increment down to 120 cm depth to analyze the concentrations of extractable NO3-N and NH4-N. Nitrogen uptake by the plants and tubers, residual soil N at different depth soil horizons (0 to 120 cm by 30 cm increment), and nitrate-nitrogen leached below 120 cm depth will be evaluated.

2. Evaluation of N management for sprinkler vs. drip irrigated potatoes. This experiment will be conducted using 'Ranger Russet' cultivar to evaluate sprinkler and drip irrigation effects on tuber yield and quality. The N treatments will include: (i) conventional, i.e. 112 kg ha-1 N applied at planting (soil residual N at planting and dry granular form N applied during land preparation) plus 224 kg ha-1 N applied in season as fertigation using UAN32, applied in five weekly applications, 3 weeks after the seedling emergence; and (ii) programmed N, i.e. N application based on the crop requirement: Pre-plant N application as in treatment (1), plus: in season application as needed to keep the petiole N concentration in the optimal range. Petiole samples will be taken on a weekly basis starting 3 weeks after emergence. The petiole nutrient analysis will be conducted following standard procedures. If the petiole NO3 concentrations show below the optimal concentraton, N application will be made as fertigation. Capacitance probes will be installed under both irrigation treatments at 15, 30, 60, 90 and 120 cm depths. These sensors provide water content data at these depths. Since these measurements are done on a continual basis, both within and below the root zone of potato, the water mass balance can be calculated to determine the water leaching below the root zone. Using the estimate of water leaching and the concentration of NO3-N in the soil solution (from SL), the quantity of N leached below the root zone in various treatments can be calculated.

3. Evaluate the soil and plant growth parameters of a potato growth model, and validate the model for major potato cultivars and different management practices. This research will be carried out with major cooperation from ARS and university scientists, i.e, the ARS-Alternate Cropping Systems Laboratory, Beltsville, MD, WSU-Center for Precision Agricultural Studies, and WSU-Biosystems Engineering Department, Pullman, WA. The overall objective of this cooperative research project is to evaluate and improve an existing crop model designed to simulate potato growth and development (SIMPOTATO). SIMPOTATO was developed for a wide range of applications at the farm and regional levels to provide guidelines on irrigation and fertilizer management and predicting yield response to irrigation and fertilization regimes, estimating movement of water and nitrogen below the root zone in response to various levels of applied water and nitrogen, estimating economic benefit of supplemental applied water and nitrogen, estimating distribution of nitrogen in the soil during the growing season and after crop maturity. Development of model-based decision aid tools for potato production will be investigated using real time weather data. The WSU Center for Precision Agricultural Systems manages a public network of 60 weather stations installed in southeastern Washington (PAWS). PAWS provide growers and crop consultants with a variety of weather data. Using the PAWS system, development of web-based decision aid tools delivering model outputs in conjunction with weather data will be evaluted. This project will focus on a first phase of delivering basic information such as potato phenological development and will prepare the basis for the development of more sophisticated tools, designed to aid water and nitrogen management decisions.

4. Long term evaluatin of changes in soil fertility and quality indices as influenced by cover crops and tillage effects in a potato rotation system. Soil properties including soil pH, organic matter content, soil moisture content, KCl extractable NO3-N and NH4-N, Mehlich 3 extractable P, K, Ca, Mg and micronutrients including Cu, Zn, Fe, and Mn will be evaluated to monitor the long term effects of different cover crops and tillage treatments in a potato rotation system under center pivot irrigation.

Major Collaborators:

ARS: Hal Collins, - Vegetable and Forage Crops Research Unit, Prosser, WA

Rick Boydston - Vegetable and Forage Crops Research Unit, Prosser, WA

V.R. Reddy - Alternate Cropping Systems Laboratory, Beltsville, MD

Dennis Timlin - Alternate Cropping Systems Laboratory, Beltsville, MD

Non-ARS: Claudio Stockle - Washington State University

Fran Pierce - Washington State University

Clyde Fraisse - Washington State University

Research Support:

William Boge, Biological Science Laboratory Technician