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Abstract

Procedures are considered which allow the researcher to determine the number of experimental units required to achieve stated experimental objectives. It is proposed that, prior to obtaining estimates of sample sizes, the researcher must obtain estimates of the important factors which influence the experimental variation. Experimental situations considered are those which range from problems in which only one factor influences the experimental variability to a complex example in which several factors must be considered.

Open Access
Authors: and

Abstract

Peach [Prunus persica (L.) Batsch cv. Harken] fruit were harvested at the firm-mature stage on 5 different dates. Fruit of uniform size were sampled randomly on each date from each of 4 quadrants per tree and evaluated for color, firmness, and soluble solids using the red and green sides of each fruit. A statistical model was used to identify and separate the sources of within- and among-tree variation in fruit quality indices. All quality indices were affected by harvest date. Fruit- and tree-related interactions accounted for a major part of the total random variation. Variance estimates were used to determine numbers of trees per treatment and fruit per tree needed to detect fruit quality differences of a desired magnitude. In general, 6 trees per treatment and 12 to 16 fruit per tree could be expected to detect meaningful differences in fruit quality in experiments involving 2 treatments.

Open Access

Large, precision weighing lysimeters are expensive but invaluable tools for measuring crop evapotranspiration and developing crop coefficients. Crop coefficients are used by both growers and researchers to estimate crop water use and accurately schedule irrigations. Two lysimeters of this type were installed in 2002 in central California to determine daily rates of crop and potential (grass) evapotranspiration and develop crop coefficients for better irrigation management of vegetable crops. From 2002 to 2006, the crop lysimeter was planted with broccoli, iceberg lettuce, bell pepper, and garlic. Basal crop coefficients, K cb, defined as the ratio of crop to potential evapotranspiration when the soil surface is dry but transpiration in unlimited by soil water conditions, increased as a linear or quadratic function of the percentage of ground covered by vegetation. At midseason, when groundcover was greater than 70% to 90%, K cb was ≈1.0 in broccoli, 0.95 in lettuce, and 1.1 in pepper, and K cb of each remained the same until harvest. Garlic K cb, in comparison, increased to 1.0 by the time the crop reached 80% ground cover, but with only 7% of additional coverage, K cb continued to increase to 1.3, until irrigation was stopped to dry the crop for harvest. Three weeks after irrigation was cutoff, garlic K cb declined rapidly to a value of 0.16 by harvest. Yields of each crop equaled or exceeded commercial averages for California with much less water in some cases than typically applied. The new crop coefficients will facilitate irrigation scheduling in the crops and help to achieve full yield potential without overirrigation.

Free access

A 3-year study was conducted in central California to compare the effects of furrow, microjet, surface drip, and sub surface drip irrigation on vegetative growth and early production of newly planted `Crimson Lady' peach [Prunus persica (L.) Batsch] trees. Furrow treatments were irrigated every 7, 14, or 21 days; microjet treatments were irrigated every 2-3, 7, or 14 days; and surface and subsurface drip (with one, two, or three buried laterals per row) treatments were irrigated when accumulated crop evapotranspiration reached 2.5 mm. The overall performance showed that trees irrigated by surface and subsurface drip were significantly larger, produced higher yields, and had higher water use efficiency than trees irrigated by microjets. In fact, more than twice as much water had to be applied to trees with microjets than to trees with drip systems in order to achieve the same amount of vegetative growth and yield. Yield and water use efficiency were also higher under surface and subsurface drip irrigation than under furrow irrigation, although tree size was similar among the treatments. Little difference was found between trees irrigated by surface and subsurface drip, except that trees irrigated with only one subsurface drip lateral were less vigorous, but not less productive, than trees irrigated by one surface drip lateral, or by two or three subsurface drip laterals. Within furrow and microjet treatments, irrigation frequency had little effect on tree development and performance with the exception that furrow irrigation every 3 weeks produced smaller trees than furrow irrigation every 1 or 2 weeks.

Free access

A 3-year study was done to determine the effects of furrow, microspray, surface drip, and subsurface drip irrigation on production and fruit quality in mature `Crimson Lady' peach [Prunus persica (L.) Batsch] trees. Furrow and microspray irrigations were scheduled weekly or biweekly, which is common practice in central California, while surface and subsurface drip irrigations were scheduled daily. Trees were maintained at similar water potentials following irrigation by adjusting water applications as needed. Tree size and fruit number were normalized among treatments by pruning and thinning each season. Surface and subsurface drip produced the largest fruit on average and the highest marketable yields among treatments. Drip benefits appeared most related to the ability to apply frequent irrigations. Whether water was applied above or below ground, daily irrigations by drip maintained higher soil water content within the root zone and prevented cycles of water stress found between less-frequent furrow and microspray irrigations. With furrow and microsprays, midday tree water potentials reached as low as –1.4 MPa between weekly irrigations and –1.8 MPa between biweekly irrigations, which likely accounted for smaller fruit and lower yields in these treatments. To reduce water stress, more frequent irrigation is probably impractical with furrow systems but is recommended when irrigating during peak water demands by microspray.

Free access