Four potato (Solanum tuberosum L.) varieties were grown under four season-long sprinkler irrigation treatments in three successive years (1992-94) on silt loam soil in eastern Oregon. The check treatment was irrigated when soil water potential (SWP) at the 0.2-m depth reached -60 J·kg-1 and received at most the accumulated evapotranspiration (Etc) to avoid exceeding the water-holding capacity of the top 0.3 m of soil. The three deficit irrigation treatments were irrigated when SWP at the 0.2-m depth reached -80 J·kg-1 and had the following percent of the accumulated Etc applied at each irrigation: 1) 100%, 2) 70%, and 3) 70% during tuber bulking with 50% thereafter. Based on regression of applied water over 3 years, potatoes lost both total and U.S. No. 1 yields when irrigations were reduced. Based on regression on applied water, when irrigation was reduced gross revenues declined more than production costs, resulting in a reduction in profits. Leaching potential, as determined by the SWP treatments, was low for all treatments. The results of the study suggest that deficit irrigation of potatoes in the Treasure Valley of Oregon would not be a viable management tool, because the small financial benefits would not offset the high risks of reduced yields and profits from the reduced water applications.

Onion (Allium cepa L., `Great Scott') was grown on silt loam soils and submitted to four irrigation thresholds (-25, -50, -75, and -100 kPa) in 1992 and six irrigation thresholds (-12.5, -25, -37.5, -50, -75, and -100 kPa) in 1993 and 1994. Irrigation thresholds (soil water potential measured at 0.2-m depth) were used as criteria to initiate furrow irrigations. Onions were evaluated for yield and grade after 70 days of storage. In 1992 and 1994, total yield, marketable yield, and profit increased with increasing irrigation threshold. In 1993, total yield increased with increasing irrigation threshold, but marketable yield and profit were maximized by a calculated threshold of -27 kPa due to a substantial increase of decomposition during storage with increasing threshold.

Long-day onion (Allium cepa L. `Vision') was subjected to five soil water potential (SWP) treatments (–10, –20, –30, –50, and –70 kPa) using subsurface drip irrigation in 1997 and 1998. Onions were grown on 1.1-m beds with two double rows spaced 0.56 m apart and a drip tape buried 13 cm deep in the bed center. Soil water potential was maintained at the five levels by automated, high-frequency irrigations based on SWP measurements at 0.2-m depth. Onions were evaluated for yield and grade after 70 days of storage. In 1997, total and colossal (bulb diameter ≥102 mm) yield increased with increasing SWP, but marketable yield was highest at a calculated –21 kPa because of greater decomposition in storage in wetter treatments. In 1998 total, marketable, and colossal-grade onion yield increased with increasing SWP. Onion profits were highest with a calculated SWP of –17 kPa in 1997, and at the wettest level tested in 1998. Storage decomposition was not affected by SWP in 1998. Maintenance of SWP at –10 and –20 kPa required, respectively, 912 and 691 mm of water in 1997 and 935 and 589 mm of water in 1998. Onion crop evapotranspiration from emergence to the last irrigation totaled 681 mm in 1997 and 716 mm in 1998.

Onion (Allium cepa L.) production in the Treasure Valley of eastern Oregon and southwestern Idaho has been based on furrow irrigation with 318 kg·ha-1 N fertilizer and average yields of 70 Mg·ha-1, but these practices have been implicated in nitrate contamination of groundwater. Drip irrigation, introduced in the early 1990s, has several advantages, including reduced leaching losses. Since onion plant populations and N fertilizer rates can affect economic returns, studies were conducted in 1999, 2000, and 2001 to determine optimum plant populations and N fertilizer rates for subsurface drip-irrigated onion. Long-day onion (`Vision') was subjected to a combination of seven nitrogen fertilization rates (0 to 336 kg·ha-1 in 56-kg increments applied between late May and early July) and four plant populations (185, 250, 300, and 370 thousand plants/ha). Onion was grown on silt loam in two double rows spaced 0.56 m apart on 1.1 m beds with a drip tape buried 13 cm deep in the bed center. Soil water potential was maintained nearly constant at -20 kPa by automated irrigations based on soil water potential measurements at a 0.2-m depth. Onion bulbs were evaluated for yield and grade after 70 days of storage. Onion yield and grade were highly responsive to plant population. Onion marketable yield increased, and bulb diameter decreased with increasing plant population. Within the range of plant populations tested, gross returns were not always responsive to plant population. Returns were increased by the increase in marketable yield obtained with higher plant population, but higher plant population also reduced the production of the largest sized bulbs which had the highest value per weight. Onion yielded 95 Mg·ha-1 with no applied N fertilizer, averaged over plant populations and years. Onion yield and grade were not responsive to N fertilizer rate or interaction of N fertilizer rate with plant population. Preplant soil available N, N mineralization, and N in irrigation water all contributed N to the crop. Onion N uptake did not increase with increasing N fertilizer rate.

Crop over‐fertilization has economic and environmental consequences. Sugarbeet (Beta vulgaris L.) N fertilizer requirements could be lower than expected when planted after shallow rooted onion (Allium cepa L.). Sugarbeet was planted on an Owyhee silt loam (coarse‐silty, mixed, mesic Xerollic Durorthid) for 2 yr, where the previous onion crop had received 0, 60, 120, 240, and 480 kg N ha−1. Soil nitrate and ammonium were measured in 0.3‐m increments to 1.8 m deep after harvesting onion, and before and after growing sugarbeet. Nitrogen uptake by plant parts, and beet and sucrose yields, were measured. Averaged across years, sugarbeet recovered 336, 316, 338, 400, and 505 kg N ha−1 when N fertilizer of the previous onion crop was 0, 60, 120, 240, and 480 kg ha−1, respectively. The corresponding reduction in available inorganic N from the top 1.8 m of the soil during sugarbeet growth was 27, 82, 62, 120, and 152 kg ha−1. Nitrogen recovered by sugarbeet was largely supplied by sources other than preplant available N. Recovered sucrose yield was near maximum when the N rate on the previous onion crop was 240 kg ha−1, which resulted in preplant NO3–N levels of about 70 kg ha−1 in the top 0.6 m of the soil. Sucrose yield did not improve when petiole NO3–N in late June exceeded 6 g kg−1. In conclusion, sugarbeet may not require fertilizer N when grown after onion fertilized with about 240 kg N ha−1