Two multiyear field studies were conducted to compare the phytoavailability and effectiveness of a variety of commercial foliar B fertilizer sprays applied at the pink flowering stage to 'Fuji'/EMLA.26 apple trees grown under irrigated semi-arid conditions. Treatments included products that differed by initial chemical form of B, physical state, and presence of additives of varying composition. Additional treatments were polymeric urea added to one B product and soil application of one B product. Boron application rates varied from 0.56 to 1.68 kg·ha–1·yr–1. All of the B sprays increased flower cluster B concentration in all years. The B sprays at the lower rate sometimes but not always increased leaf B concentration. Increasing the B rate substantially increased plant tissue B concentrations. In general, there was little substantive difference between the tested products/product mixtures on plant tissue B concentrations. Flower cluster B in the ground-applied B treatment was similar to the water control; however, leaf B concentration corresponded to the B spray treatments, indicating effective uptake of B from the soil during the early summer. Sodium polyborate-based products increased flower cluster Na concentration but not leaf Na concentration. The amount of Na contributed by Na polyborate-based products applied at commercial rates apparently was too small to be of horticultural concern. Fruit quality was excellent and was not affected by the experimental treatments in any year. Flower cluster and leaf B concentrations returned to near or at control levels in the season following the last spray application, validating the recommendation for annual B fertilizer applications to maintain adequate tree B status.
Boron (B) is an essential micronutrient that is often in inadequate supply in many deciduous tree fruit orchards and must therefore be added as fertilizer. It can also occur at phytotoxic levels because of over-fertilization, use of high-B irrigation water, or naturally in arid soils that are natively high in B. Tree B status is usually characterized by leaf analysis although other diagnostic criteria are being evaluated. Several tests are used to characterize soil B status. Symptoms of B deficiency include blossom blast, poor fruit set and development, shortened internodes, terminal bud death, and shoot dieback. To ameliorate deficiency, B fertilizer may be broadcast or sprayed over the soil surface or sprayed on tree canopies. In some regions, maintenance applications of B fertilizer are made to prevent development of B deficiency. Sodium borates or orthoboric acid are usually used. Fertilizer rates and timing vary with location and farming practices. Symptoms of B excess include reduced or no yield, impaired fruit quality, leaf marginal chlorosis and necrosis, defoliation, and shoot dieback. Boron toxicity is alleviated by leaching B-enriched soil to move B below the root zone.
Differential fertilizer application method (single dry, split dry, fertigated liquid), irrigation method (drip, microjet), and nutrient source (N vs. N+P in year 2+) were established in Spring 1992 in a newly planted Gala and Fuji apple orchard. In Spring 1993, the drip-fertigated Gala trees had 3 times and the drip-fertigated Fuji trees had 8 times more flower clusters per tree than the other treatments Fruiting was not allowed in 1993. Trunk cross-sectional area (TCSA) in Fall 1992 was not influenced by treatments. By Fall 1993, TCSA was still independent of treatment for the Fuji trees; however, the Gala trees fell into two size groups - (larger) microsprinkler-fertigated and split dry broadcast; and (smaller) drip fertigated and single-time spring dry broadcast. TCSA had increased 284% (Fuji) and 265% (Gala) since planting. None of the treatment effects were substantially influenced by fertigating with N+P vs N only. Leaf concentrations of most nutrients were consistently lower in 1993 than in 1992. Leaf Fe was higher in 1993 because the orchard was dustier. Leaf N was lower in the microsprinkler-fertigated trees than in all other treatments. Fertigation with N+P did not consistently produce higher leaf P than the N-only treatments. Leaf Mn varied with treatment: microsprinkler fertigated < drip fertigated, single dry < split dry. Treatment effects on all other elements were inconsistent (K, Ca, Mg, B, Cu) or absent (Zn, Fe).
Boron (B) deficiency symptoms often appear in the reproductive tissues of apple (Malus domestica Borkh.) and pear (Pyrus communis L.) without attendant vegetative symptoms. The primary symptoms are blossom blast, and internal and external cork in fruit. Leaf analysis is the principal diagnostic procedure for evaluating tree B status in Washington orchards. Because flower cluster and fruitlet B contents are influenced by soil moisture availability during the previous autumn, variability in fall precipitation limits the ability of leaf analysis to accurately predict tree B demands the following spring. In the 1960s, Woodbridge and Crandall of Washington State University reported that the B content of apple and pear buds was constant during the winter. The objective of the current study is to determine if winter bud analysis is an accurate predictor of flower and fruitlet B status. Temporal changes in bud, flower cluster, and fruit B are presented for two apple orchards and one pear orchard during the period 1988 to present, as well as the relationships between bud, flower cluster and fruit B concentrations.
Postbloom zinc (Zn) sprays are replacing dormant and postharvest sprays as the primary means for applying Zn in commercial apple (Malus ×domestica) orchards. We conducted a multiyear field study comparing the phytoavailability of Zn in 11 commercially available Zn spray products, plus reagent-grade Zn nitrate and a water-sprayed control, applied postbloom at identical Zn concentrations to `Golden Delicious' apple trees. Two sprays were applied per season (mid-May and mid-June), at per-spray rates of either 0.5 lb/acre in 2000 or 1.0 lb/acre in 2001 and 2002. No sprays were applied in 2003 in order to evaluate carry-over effects. The Zn sprays had no effect on fruit number, bitter pit or russeting, or on leaf green color. Zinc concentrations of detergent plus acid-washed leaves (a procedure used to remove surface residues of the Zn sprays) sampled in August and of unwashed winter buds sampled the following January were used as indices of tree Zn status. Leaf Zn concentration generally increased in the order: Zn phosphate < Zn oxide = Zn oxysulfate < chelated/organically complexed Zn ≤ Zn nitrate. There was little consistent difference among chelated and organically complexed Zn products. Leaf Zn concentration varied considerably between seasons, and was not related to Zn application rate. All of the Zn sprays increased leaf Zn concentrations to desirable levels. Because the inorganic Zn-based products typically are substantially less expensive per unit of Zn, it may be less costly and just as effective to use a higher rate of an inorganic Zn product as to use a lower rate of a more expensive chelated or organically complexed Zn product. On the other hand, use of low rates of highly phytoavailable Zn products minimizes release of the nutritionally essential but potentially ecohazardous metal into the environment. There was no detectable lasting effect of the three previous seasons of Zn sprays on leaf Zn in 2003. Similarly, there was no detectable effect in any year of the Zn spray treatments on bud Zn concentration the following winter. These results suggest that the amount of Zn supplied by the sprays at the tested rates was insufficient to promote substantial Zn accumulation within the trees, thereby validating the recommendation for annual application of Zn nutritional maintenance sprays.
Concerns about food safety prompted a case study of the arsenic and Pb contents of tree fruits grown on lead arsenate-contaminated soil. The arsenic concentration in apricot (Prunus armeniaca L.) and `Gala' apple (Malus domestica Borkh.) fruit was positively related to concentrated HCl-extractable soil arsenic. Fruit arsenic in both species did not exceed 70 μg·kg-1 fresh weight (fw). Fruit Pb was below the limits of detection of 20 μg·kg-1 fw for apricot and 24 μg·kg-1 fw for apple. All of these concentrations are substantially below levels associated with human health risk. `Riland' apricot trees did not show arsenic phytotoxicity at soil, fruit, and leaf arsenic concentrations associated with phytotoxicity symptoms in `Goldrich' apricots. The apple trees showed no visual symptoms of arsenic phytotoxicity.
Late dormant copper (Cu) sprays and mid-summer foliar Cu sprays are being promoted within the Washington apple industry as a means to enhance fruit typiness and red skin color, respectively. While there appears to be theoretical bases for these practices, they have not been tested for horticultural significance. Differential late dormant spray treatments of Cu hydroxide (the Cu source most commonly recommended by agricultural consultants) were imposed in two `Delicious' orchards. Flower cluster Cu was positively related to Cu rate, but the sprays had no effect on leaf Cu or on six fruit typiness variables. Differential mid-summer spray treatments of water, Cu sulfate, and Cu oxysulfate solutions were imposed in three `Delicious' orchards and one `Fuji' orchard. The Cu sprays increased leaf Cu, but had no effect on market color grade measured using a commercial color sorter. The results appear to reflect Cu physicochemistry and timing of application. These preliminary results call into question the utility of the Cu sprays for improving apple fruit quality characteristics when trees show no visual signs of Cu deficiency. They do suggest some alternative ways to manage Cu nutrition in deciduous tree fruit orchards.
Nitrification-induced subsoil acidification is a major problem encountered with the use of ammonium- or urea-containing fertilizer solutions for drip fertigation of tree fruit crops. We conducted a laboratory experiment to evaluate the soil acidification potential of the four fertilizer N solutions most frequently used for fertigation within the Washington tree fruit industry. Treatments were five orchard soils x four commercial N solutions (calcium nitrate, calcium-ammonium nitrate, ammonium nitrate, urea-ammoniun nitrate) x four N rates (0, 100, 200, 500 mg N/kg). Air-dry subsamples of each soil were inoculated with fresh soil known to exhibit nitrifying behavior amended with treatment solutions. Subsamples were maintained at simulated field capacity of –15 kPa. Soil pH was measured after 5 weeks incubation. The treatment solutions were reapplied and pH measured after another 5 weeks. The soil were then leached with distilled water and further incubated to determine if pH would increase as has been observed in the field. The fertilizer solutions acidified the soils in direct relation to their ammonium plus urea content. The calcium nitrate solution was acidifying because it contains ammonium nitrate as an impurity. We will present the pH “rebound” data.
Fruit trees grown in soils contaminated with lead arsenate (PbHAsO4) pesticide residues are subject to arsenic (As) phytotoxicity, a condition that may be exacerbated by use of phosphate fertilizers. A potted soil experiment was conducted to examine the influence of phosphate fertilizer on accumulation of As and lead (Pb) in apricot (Prunus armeniaca) seedlings grown in a lead arsenate-contaminated Burch loam coil. Treatments were fertilizer source (mono-ammonium phosphate [MAP], ammonium hydrogen sulfate [AHS]) and rate (0, 8.7, 17.4, and 26.1 -mmol/liter), and presence/absence of lead, arsenate contamination (231 -mg/kg coil). Plant biomass accumulation was reduced by lead arsenate presence and by high fertilizer rates, the latter due to soil salinization. Phytoaccumulation of As was enhanced by lead arsenate presence and by increasing MAP rate but was not influenced by AHS rate, salinity, or acidity of soil. Phytoaccumulation of Pb was enhanced by lead arsenate presence but was not influenced by fertilizer treatment.
Farmers often mix fertilizers, pesticides and other agricultural chemicals together in a spray tank to allow applications of multiple products in a single spray. Because polyborate-based B fertilizers may increase solution pH, adding B to tank-mixed sprays may impair the stability and efficacy of alkalinity-sensitive pesticides and growth regulators if an acidifier is not included. We conducted a laboratory experiment to determine the influence of 10 commercial B fertilizer sources in factorial combination with B concentrations ranging from 0 to 4 lb/100 gal (4.8 g·L-1) on solution pH values of distilled water and two natural waters. Two boric acid-based compounds produced acidic reactions relative to background water pH at all tested B concentrations. Their pH responses were influenced by initial water composition. Seven B products produced moderately to strongly alkaline reactions at all but the highest B concentration, regardless of the form of B (polyborate vs. boric acid) initially present in the formulated products. One polyborate product formulated with an acidifier showed intermediate pH behavior. The dependence of solution pH on B rate of the polyborate-containing products was identical in all three waters. The maximum pH values generated by all products occurred in the B concentration range <0.1 to 0.25 lb/100 gal (0.12 to 0.3 g·L-1). Solution pH values declined with increasing B concentration above this range. The pH responses qualitatively conformed to known aqueous chemical behavior of B and the product additives. The complexity of the interaction between initial water chemistry, B concentration, and B fertilizer product reinforces the need to measure the pH of B-amended spray water before adding pH-sensitive compounds.