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|Background||Nitrogen fertilization under furrow irrigation|
|Efficiency of fertilizer placement||Response to Deficit Irrigation|
|Nitrogen fertilization under sprinkler irrigation|
Prior to the establishment of a state "groundwater management area" and a federal "hydrological unit area" in northeastern Malheur County, positive changes in irrigation and fertilizer use efficiency were already taking place. Driving motivations for increases in efficiency occurred in the late 1980's associated with economic incentives to improve potato quality. During the 1984 and 1985 seasons, potato tuber quality was inadequate to meet the needs of potato processors, Ore-Ida Foods Inc. and J. R. Simplot Co. In particular a condition called "dark-ends" in fried tuber slices resulted from tubers grown on stressed potato plants. The stresses aggravating the occurrence of "dark-ends" were poorly defined. Growers lost contracted acres.
Intensive research at the Malheur Experiment Station from 1986 through 1989 discovered how to grow quality tubers. Researchers included Lynn Jensen, Zoe Ann Holmes, Tim Stieber, Eric Eldredge, Monty Saunders and myself. Funding at that time was provided by the Oregon Potato Commission, the Malheur Potato Growers, and USDA grants.
The Malheur Experiment Station research determined the soil water requirements for potato production and how to carefully monitor soil water status using soil moisture sensors. Growers modified their irrigation and other growing practices so as to minimize water stress on the potato plants during tuber development. Lynn Jensen and the station also proved that "dark-ends" were less prevalent under sprinkler irrigation than under furrow irrigation but that drip irrigation was of less benefit. Simultaneously growers' field scale potato production confirmed the reduction in "dark-ends" by using sprinklers. Growers purchased and leased sprinkler irrigation systems. Varieties were identified that expressed fewer negative characteristics when subjected to stress. One of these varieties, Shepody, increased in popularity with growers and processors. Growers regained contracted acres at favorable prices.
The research trials on "dark-ends" at the Malheur Experiment Station produced high quality tubers with far lower inputs of N fertilizer (100 to 180 lb N/ac) than growers had been using at that time (300 to 400 lb N/ac). By 1991 average grower N fertilizer applications to potatoes had shrunk to 225 lb N/ac. The lower fertilizer N rates on potatoes led to reductions in net nitrate loading to the environment. These nitrate loading decreases had little opportunity to be reflected in groundwater nitrates by 1990 and 1991 at the onset of the "groundwater management area."
Starting in 1992, new trials were initiated to examine a) the nitrogen rates and timing needed for potato production under furrow irrigation and under precision sprinkler irrigation, b) the possible contribution to plant available nitrogen from the mineralization of soil organic matter, and c) the response of potatoes to deficit irrigation. Nitrogen fertilizers were only applied during the growing season when potatoes had the greatest opportunity to take up nitrogen. Researchers on these new topics included Erik Feibert, Monty Saunders and myself and research has been supported by the Oregon Potato Commission, ODEQ, ODA, and EPA 319 funds.
In three years of trials, Shepody and Russet Burbank varieties had similar responses to N rates and N timing. Potato yields from these trials were consistently above county averages. Optimum yield responses were obtained using 0 to 120 lb N/ac depending on the year and experimental site. Rates of N over 120 lb N/ac were never economical. In 1993 and 1994, with wheat as the previous crop, 120 lb N/ac was the optimum N rate, over all varieties. In 1992, with alfalfa as the previous crop, there was no positive potato tuber yield or grade response to N fertilization.
There was no advantage in any of the three years in split applications of the N fertilizer compared to a single application before tuber set. The lack of response to split N applications was probably caused by the careful control of irrigation duration, based of crop evapotranspiration.
The limited N fertilizer responses realized in these trials were possible because of the careful irrigation management where little or no nitrate leaching occurred, the placement of N fertilizer (Figure 3), the silt loam soils that facilitated moisture retention, and high rates of N mineralization. Careful irrigation management for potatoes in Malheur County can be achieved by daily monitoring of the soil water potential then irrigating before the soil dries to -60 kPa and replacing no more than the accumulated crop evapotranspiration, Etc. Growers may run irrigation sets for more time than necessary to replace soil water.
In 1993 and 1994, the unfertilized potatoes had the highest tuber specific gravity. During each of the three years, the experimental processing varieties COO83008-1 and AO82611-7 had high total marketable yields, high US Number One yields, and high US Number One >10 oz tuber yields.
Oregon and Idaho N fertilizer guides for potatoes suggest that larger amounts of fertilizer N are necessary than found in these trials. Since the differences between the guides and potato responses to N fertilizer were large in these trials, the guides should be reexamined. Greater fertilizer N use efficiency may be possible.
The seven varieties used did not respond in the same way. With progressive amounts of deficit irrigation, several varieties lost grade, several varieties lost yield, and several varieties lost both yield and grade. All highly productive varieties tested lost economic return with reduced irrigation.
2. Careful irrigation management for potatoes grown on silt loam soils in Malheur County can be achieved by irrigating before the soil dries to -60 kPa and replacing no more than the accumulated Etc.
3. Positive potato yield and quality responses to N fertilizer were less in these trials than would have been expected by the fertilizer guides. All fields used had crop histories with little N fertilizer for several years before planting potatoes. Consistent with N fertilization history, soil residual nitrates at planting were low at planting in every field used. All trials were conducted with careful irrigation management which favored efficient use of residual and fertilizer nitrogen. The careful irrigation management also reduced nitrate leaching losses. Nitrogen fertilizer applications shanked into the hill at planting or shortly after planting (Figure 3) may be inherently more efficient of N fertilizer than preplant fall and spring broadcast applications.
4. The substantial amounts of nitrate-N and ammonium-N were contributed from organic matter mineralization in these trials. The large magnitude of the mineralized nitrogen indicates the importance of taking these natural sources into account when estimating crop N fertilizer needs.
5. Growers are reexamining nitrogen fertilization practices so that money is not wasted on unneeded N fertilizer. Where money can be saved on nitrogen fertilizer, we can expect that less nitrate will be lost to deep percolation.
Four hill trials are then conducted at Powell Butte and Hermiston.
Oregon Preliminary Yield Trial is the third test for the potatoes that made it through the first two trials. The potatoes are examined for appearance, yield, market grade, fry color, and specific gravity. The preferred potato is one with high specific gravity, the higher the specific gravity the less oil the potato will soak up during frying. The best potatoes will move on to the fourth stage, The Oregon Statewide Trial. Once again the potatoes must pass all the criteria of the preliminary trial before it can move into the fifth trial, The Western Regional Trial. The entries in this trial are grown throughout the northwest in Washington, Idaho and Oregon. The Malheur Experiment Station conducts an Oregon Preliminary Yield Trial.