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Soil solution monitoring has been suggested as an appropriate procedure to optimize fertigation timing and application rate. Soil solution NO₃-N concentrations were measured for two growing seasons on a sandy loam soil when 5, 20 or 30 g N per season per tree were fertigated daily to apples as calcium nitrate from mid May-mid July. Soil solution NO₃-N concentrations at 30 cm depth changed rapidly in response to both the initiation and cessation of fertigation, with values ranging from 10-20 ppm, 60-100 ppm and 100-200 ppm for the low to high treatments respectively. The rapid response to NO,-fertilizers implied a potential to control closely the timing of N fertilizer applications. In another experiment, `Empire' apple trees were fertigated 5 times/week from May 31 to August 9 with 30 g N/tree applied either as ammonium sulphate or as calcium nitrate. With calcium nitrate as the N source, NO₃-N rapidly increased when fertigation was initiated and fell when fertigation ended. In contrast, with ammonium sulphate, NO₃-N was low for about 30 days after initiation of fertigation, then increased to 100 ppm and remained elevated for 40-50 days after fertigation ended. The potential control of N nutrition appeared to be less exact when fertigating NH₄-N.

A randomized, complete block, split-plot experimental design with six replicates was established and maintained annually for the first five fruiting seasons (1999 to 2003) in a high-density apple [Malus sylvestris (L) Mill var. domestica (Borkh.) Mansf] orchard on M.9 rootstock planted in Apr. 1998. Main plot treatments involved eight different nutrient regimes, each containing three tree subplots of each of five different cultivars (Ambrosia, Cameo, Fuji, Gala, and Silken). This report compares a +phosphorus (P) treatment, involving annual fertigation at bloom time of 20 g P/tree as ammonium polyphosphate (10N–15P–0K), to a −P treatment. Both treatments also received nitrogen, potassium, and boron nutrients through fertigation. Drip fertigation of P increased 2 M KCl-extractable P to 0.4-m depth within 0.5-m distance of the drippers. Leaf and fruit P concentrations were consistently increased by the +P treatment with few differences among cultivars. P-fertigated trees also had a 20% increase in cumulative yield overall cultivars during the first five fruiting seasons. Standard fruit quality measurements, including fruit size, soluble solids concentration, titratable acidity, and red coloration were unaffected by P application. However, reductions in incidence of water core at harvest, increased resistance to browning, and elevated antioxidant content of harvested fruit measured in some years imply a role for P in apple membrane stability. The cumulative results indicate that applications of 20 g P as ammonium polyphosphate annually at bloom would be advantageous for apples receiving adequate fertigated applications of nitrogen, potassium, and boron. Best apple performance was associated with leaf P concentrations above 2.2 mg·g⁻¹ dry weight and fruit P concentrations between 100 and 120 mg·kg⁻¹ dry weight.

Use of crop evapotranspiration (ETc), when a precise crop coefficient value (Kc) is used, provides a reliable tool (irrigation scheduling) for determination of water requirement. In this process, Kc should be modified by percentage of ground shade (GS) and tree canopy maturity (M). In an experiment in Idaho with ET-based irrigation scheduling, each tree with a full microjet sprinkler system received an average of 6461.7 L (994 mm), whereas each one with a full drip system used 3996 L (614.1 mm) of irrigation water. In general, deficit drip irrigation was shown to initially increase yield as a result of induction of stress and the production of a higher number of fruit spurs. However, production declined if the extreme water deficiency was repeatedly applied to the trees over several years. Using a microjet sprinkler system, a partial root zone drying regime reduced fruit size but slightly improved fruit color. Application of water at 65% full drip rate, applied on both sides of the tree row (DD), reduced fruit size. However, when the 65% of full drip rate was applied to only one of the alternating sides of the tree every other week (PRD), fruit size was larger than those with DD treatment.

Mature, fruiting ‘Ambrosia’/‘M.9’ apple [Malus ×sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.] trees were subjected over three growing seasons to a split-plot experimental design involving four irrigation main plot treatments and three subplot crop load treatments with six replicates. This semiarid production region is traditionally irrigated 01 May to 01 Oct. during which time an average of ≈ 15 cm of precipitation occurs. Irrigation treatments were applied through 2 × 4 L⋅h⁻¹ emitters per tree and included I1: daily application of 100% evapotranspiration (ET); or I2: 50% daily ET; or I3: 50% ET applied to one side; and I4: 50%, 25%, or 18% ET-application, applied every second day, 2007–09, respectively. Crop load treatments were imposed annually ≈4 to 5 weeks after full bloom to create low (2.5, 3, and 3.75 fruits/cm² trunk cross-sectional area (TCSA), medium (4.5, 6, and 7.5 fruits/cm² TCSA), and high crop loads (9, 12, and 15 fruits/cm² TCSA), 2007–09, respectively. Leaf and fruit nutrient concentration was affected more by crop load than by any deficit irrigation strategy. Increased crop load increased concentrations of leaf nitrogen (N), calcium (Ca), and fruit Ca in 2 of 3 years and consistently decreased concentrations of leaf and fruit phosphorus (P) and potassium (K) and, in 2 of 3 years, fruit boron (B). Reductions in seasonal water applications (as with I4) reduced leaf P in 2 of 3 years. But, when significant, (usually only 1 of 3 year) increased fruit Ca, magnesium (Mg), P, K, and B concentrations. Crop load also had a dominant effect on fruit nutrient removal rates expressed as kilograms per hectare. High crop load increased removal of all measured nutrients in most years. In contrast, imposition of deficit irrigation strategies often (2 of 3 years) reduced fruit P, Mg, and B removal rates but had little effect on N, Ca, and K. Cumulative evidence suggests that deficit irrigation applied to N, P, K, and B fertigated high density ‘Ambrosia’ apple orchards in combination with crop load reduction to maintain fruit size should usually not create additional nutrient problems. However, low fruit Ca concentrations may occur if the crop is very low. Fertigation of 20 g K/tree/year was insufficient for older trees because inadequate K occurred in all treatments by the third year.

Cristalina and Skeena sweet cherry cultivars (Prunus avium L.) on Gisela 6 (Prunus cerasus × Prunus canescens) rootstock were maintained for the first four growing seasons in a randomized, replicated split-split plot experimental design with two main plot irrigation frequency treatments, the two cultivars as subplots and three soil management subsubplot treatments. The same amount of irrigation water was applied through four drip emitters per tree at either high (I1, four times daily) or low frequency (I2, once every second day) beginning in the second year. Three different soil management treatments were established the year of planting and included: 1) NK fertigation with a herbicide strip (control), or additionally; 2) maintenance of a 10-cm thick bark mulch over the herbicide strip; and 3) annual fertigation of 20 g phosphorus (P) per tree per year immediately after bloom. I1 irrigation increased soil moisture (0- to 20-cm depth) throughout each growing season. The I1 irrigation resulted in higher leaf and fruit concentrations of the immobile nutrients P and potassium (K) and larger trunk cross-sectional area than I2 trees. I1 irrigation, in general, did not affect initial yield or fruit size. Fruit from I2 irrigation had higher soluble solids concentration (SSC), color, and total phenolic concentration at harvest in 2008 and lower titratable acidity (TA), firmness, and stem pull force suggesting an acceleration of fruit maturity. When compared with the control soil management treatment, P fertigation resulted in leaves and fruit with higher P concentrations, a higher 2008 crop yield, and a delay in 2008 crop maturity as indicated by lower harvest color and SSC and higher stem pull force. Mulch application, relative to control treatments, resulted in trees with higher vigor (but only with I1 irrigation) and leaf K concentration and had few effects on initial fruit yield or quality. There were important differences in cultivar responses to treatments. ‘Cristalina’ vigor was lower than ‘Skeena’ whose fruit had lower firmness and pedicel retention than ‘Cristalina’.

‘Cristalina’ and ‘Skeena’ sweet cherry cultivars (Prunus avium L.) on Gisela 6 (Prunus cerasus × Prunus canescens) rootstock planted in 2005 were maintained since 2006 in a randomly blocked split-split plot experimental design with six blocks of two irrigation frequency main plot treatments within which two cultivar subplots and three soil management sub-subplots were randomly applied. The focus of this study was the growth, yield, and fruit quality response of sweet cherry to water and soil management over three successive fruiting seasons, 2009–11, in a cold climate production area. The final 2 years of the study period were characterized by cool, wet springs resulting in low yield and yield efficiency across all treatments. Soil moisture content (0- to 20-cm depth) during the growing season was often higher in soils that received high-frequency irrigation (HFI) compared with low-frequency irrigation (LFI). HFI and LFI received the same amount of water, but water was applied four times daily in the HFI treatment but every other day in the LFI treatment. Consequently, larger trunk cross-sectional area (TCSA) and higher yield were found on HFI compared with LFI trees. Soil management strategies involving annual bloom time phosphorus (P) fertigation and wood waste mulching did not affect tree vigor and yield. Increased soluble solids concentration (SSC) occurred with LFI. Decreased SSC occurred with delayed harvest maturity in trees receiving P fertigation at bloom. The largest fruit size was correlated for both cultivars with low crop loads ranging from 100 to 200 g fruit/cm² TCSA. Overall cool, wet spring weather strongly affected annual yield and fruit quality, often overriding cultivar and soil and water management effects.

Direct application of fertilizers in irrigation water (fertigation) has been advocated as an efficient method of fertilizing fruit trees. However, more information is needed on the relationship between irrigation and N inputs in order to target fertigation to meet plant demands. Soil solution NO₃-N concentration was measured at three sites in response to the method of fertilizer application in which 25 g N/tree per year was either spring-broadcast with sprinkler irrigation or fertigated at 8 weekly intervals through drip irrigation; the amount of irrigation water in which 50 g N/tree per year was given in 63 daily fertigations with either 4 or 8 liters of water/day for two soil types and the concentration of fertigated N in which either 75 or 150 ppm NO₃-N was given in 63 daily fertigations. Soil solution NO₃-N concentration decreased rapidly for broadcast fertilizer with sprinkler irrigation and was lower than for weekly fertigation with drip irrigation. Doubling the amount of irrigation water effectively halved the soil solution NO₃-N concentration in both the silt loam and loamy sand soils, although concentrations were higher in the silt loam soil. Movement of applied N below the root zone was halted for the silt loam soil by mid-summer with the lower amount of irrigation water, but was only delayed in the loamy sand soil. Doubling the average concentration of N in the irrigation water resulted in a doubling of the concentration of NO₃-N in the root zone. A simple model was devised to predict the soil solution NO₃-N concentration based on N and water inputs and fitted to measured values for daily and weekly fertigation.

A simple flatbed-scanner-based image acquisition system was developed for the measurement of `Gala'/M9 (Malus ×domestica Borkh.) apple tree root growth in rhizoboxes with a transparent acrylic sheet on one side. A tree was planted in the center of each rhizobox, and a modified flatbed scanner was periodically used to directly capture high-resolution digital images of roots growing against the transparent wall. Total root length in the images was either measured manually, or by computer mouse tracing, or automatically with a computer image analysis system. Correlations were made among the different measurements. High quality root images were obtained with the adapted scanner system. Significant linear relationships were found between manual and computer traced root length measurements (r = 0.99), traced and automatic measurements (r = 0.76) and manual and automatic measurements (r = 0.75). Apple roots appeared on the transparent wall 34 days after transplanting, and grew rapidly thereafter, reaching a maximum on the transparent wall 59 days after transplanting. Our results showed that the use of a flatbed scanner for the acquisition of root images combined with computer analysis is a promising technique to speed data acquisition in root growth investigations.

Limiting irrigation to increase fruit quality as well as conserve limited water resources is of increasing importance. We examined the links of aboveground growth and physiology to root growth and distribution under cultural practices associated with restricted irrigation and mulching in an apple (‘Gala/M.9’; Malus ×domestica Borkh.) orchard in a semiarid climate. Trees were either mulched to maintain a 10-cm depth or left unmulched. After orchard establishment, half the trees (partially irrigated) received daily drip irrigation only sufficient to meet 50% of daily evapotranspiration for 45 days before fruit harvest using one emitter per tree. The other trees continued to receive 100% irrigation using two emitters. Over three growing seasons, fruit yield was strongly affected in 2 of 3 years by partial irrigation if trees were unmulched but not if mulched. Fruit size and other quality parameters were minimally affected by partial irrigation. Total fine root length in fully irrigated trees was nearly double that of partially irrigated trees. Our results suggest that increases in soil moisture associated with mulching enabled mulched apple trees to tolerate deficit irrigation with minimal consequences for production and quality of apples, but with an overall less extensive and smaller root system.

Growers plan most of their horticultural activities around certain shoot phenological stages, such as bloom, veraison, and harvest. Timing of root growth in relation to these stages of the shoot is of interest in fertilization scheduling and in understanding carbon allocation demands of the root system. With the recent use of minirhizotron root observation tubes, a much greater understanding of patterns of root growth has been made possible. In Fredonia, N.Y., 5 years of root investigation in `Concord' grape indicate considerable variability in timing of root flushes. Root flushes could occur any time between bloom and veraison, but were generally not observed after harvest. Wine grapes in the Napa Valley exhibited similar patterns. In apple, root flushes may occur at bloom, but often not after harvest. Consequently, we rarely observed the bimodal distribution of root flushes commonly depicted in textbooks for apple and grape. Our data suggest that general perceptions of the timing of root growth may be in error.