A fundamental way to schedule irrigation is through the monitoring and management of soil water tension (SWT). Soil water tension is the force necessary for plant roots to extract water from the soil. With the invention of tensiometers, SWT measurements have been used to schedule irrigation. There are different types of field instruments used to measure SWT, either directly or indirectly. Precise irrigation scheduling by SWT criteria is a powerful method to optimize plant performance. Specific SWT criteria for irrigation scheduling have been developed to optimize the production and quality of vegetable crops, field crops, trees, shrubs, and nursery crops. This review discusses known SWT criteria for irrigation scheduling that vary from 2 to 800 kPa depending on the crop species, plant product to be optimized, environmental conditions, and irrigation system. By using the ideal SWT and adjusting irrigation duration and amount, it is possible to simultaneously achieve high productivity and meet environmental stewardship goals for water use and reduced leaching.

Quinoa (Chenopodium quinoa Willd.) yield potential needs to be further achieved by good management practices to meet the increasing global demand. Two years of orthogonal field experiments were undertaken to investigate the effects of irrigation onset criteria using soil matric potential (SMP) (−15, −25, and −55 kPa), nitrogen fertilizer rate (80, 160, and 240 kg ha⁻¹), and plant density (20, 30, and 40 plants m⁻²) on quinoa growth, seed yield, weight, and protein content. Initiating irrigations at an SMP of −15 to −25 kPa achieved significantly (p < 0.05) greater seed yield (37.2 g plant⁻¹), thousand kernel weight (2.25 g), and protein content (21.2%) than −55 kPa (25.2 g plant⁻¹, 2.08 g, and 19.8%, respectively). The 240 kg ha⁻¹ nitrogen rate had significantly (p < 0.05) greater thousand kernel weight (2.26 g) and protein content (21.3%) than 80 (2.07 g and 19.5%, respectively) and 160 kg ha⁻¹ (2.14 g and 20.7%, respectively). The yield under 20 plants m⁻² reached 39.5 g plant⁻¹, which was 13.5 g plant⁻¹ higher than 40 plants m⁻² (p < 0.05). The quinoa consumed most of the available nitrogen in the soil (410–860 kg ha⁻¹), indicating that quinoa should be part of a sound crop rotation program.

Stevia ( Stevia rebaudiana Bertoni) is a perennial herbaceous plant native to Paraguay, where it was used by the native Guarani peoples for centuries. Although steviol glycosides from stevia are powerful natural noncaloric sweeteners, stevia has been cultivated and commercially available only for the past 50 years. Cultural practices are still in development, and productivity potential in the United States is largely unknown. Currently commercial growers and processors worldwide are seeking to maximize the productivity of rebaudioside A, a steviol glycoside. The trials reported here examined the effects of location, harvest strategy, and cultivar on stevia dry leaf yield, steviol glycoside content, and steviol glycoside yield. Six or seven stevia cultivars were grown for ≈5 months at four western U.S. locations, with an irrigation criterion of 20 kPa. Stevia at every location was subjected to two harvest strategies: either one harvest at the end of the season or two harvests, one midseason and another at the end of the season. The main plots at each location were the stevia cultivars, and the split plots were the harvest strategies in a randomized complete block, split-plot design with four replicates. Dry leaf yield, leaf steviol glycoside content, and leaf steviol glycoside yield varied by cultivar, location, and cultivar by location, but not by harvest strategy or interactions of harvest strategy with location or cultivar. Dry leaf yield averaged 4.12 Mg·ha ⁻¹ with significant differences by cultivar and location. One of the steviol glycosides, rebaudioside A yield averaged 300 kg·ha ⁻¹ with significant differences by cultivar and by interactions of location with cultivar. Leaf productivity was greater at Ontario, OR, than at Hanford, CA, Indio, CA, or Yuma, AZ. Dry leaf yield greater than 7 Mg·ha ⁻¹ and rebaudioside A yields greater than 500 kg·ha ⁻¹ were observed at Ontario. Stevia perenniated at Hanford and Indio, providing the option of multiyear harvests from a single planting.

Onion ( Allium cepa ) varieties for commercial production in eastern Oregon and southwestern Idaho are evaluated annually in replicated trials conducted at the Malheur Experiment Station, Oregon State University, near Ontario, Oregon. Characteristics evaluated include bulb yield, market grade, and the frequency of single centers. After the emergence of iris yellow spot virus (IYSV) as a threat to commercial onion production in the early 2000s, onion varieties at the Malheur Experiment Station have been evaluated for virus symptoms since 2004. Varieties showed differences in the severity of IYSV symptoms each year. Symptom severity increased over the years from 2004 to 2006, and variety virus ratings showed a strong negative correlation of severity with yield in 2005 and 2006. Marketable yield after 3 months of storage averaged 781, 534, and 551 cwt/acre in 2004, 2005, and 2006, respectively. Averaging over varieties, yield of bulbs larger than 4 inches in diameter was 438 cwt/acre, 56 cwt/acre, and 76 cwt/acre, and the average virus severity ratings were 1.1, 1.3, and 2.7 in 2004, 2005, and 2006, respectively. A few varieties showed a combination of high yield, large bulb size, low incidence of virus symptoms, and a predominance of single-centered bulbs. With the prevalence of IYSV, variety tolerance to IYSV has become an important production factor in the Treasure Valley.