Four apple (Malus domestica Borkh) cultivars (`Fuji', `Spartan', `Fiesta', and `Gala') on Malling 9 (M.9) rootstock were grown in the field with three N rates (5, 20, and 35 g N/tree per year), supplied as Ca(NO3)2, and fertigated daily for 9 weeks. In the second year, leaf SPAD readings (chlorophyll readings obtained with the Minolta-502 SPAD meter) increased over the growing season for all cultivars, and leaf N decreased. Leaf SPAD and leaf N measurements increased in response to N fertigation rate at all sampling times. `Gala' consistently had lower SPAD readings than the other cultivars, and, with the exception of the first sampling time, `Fuji' had higher and `Fiesta' lower leaf N concentrations than other cultivars. There were strong relationships between leaf N concentration and SPAD readings for all cultivars until mid-July (r2 = 0.44 to 0.89), but not later in the growing season. Differences in SPAD readings and leaf N concentration due to cultivar and over time were as great as those due to N treatments, indicating that in the future, determination of critical SPAD values for apple leaves must be standardized for cultivar and sampling time. SPAD readings could be used to assess the need for N early in the growing season in fertigated orchards where rapid changes in nutrition programs can be undertaken readily.

Environmentally sound management of N in apple orchards requires that N supply meets demand. In 1997, newly planted apple trees (Malus domestica Borkh. var. Golden Delicious on M.9 rootstock) received daily applications of N for six weeks as Ca(15NO3)(2) through a drip irrigation system at a concentration of 112 mg l(-1) at 2-8, 5-11 or 8-14 weeks after planting. Irrigation water was applied either to meet estimated evaporative demand or at a fixed rate. In 1997, trees were harvested at 5, 8, 11 and 14 weeks after planting; and in 1998 at 3 weeks after full bloom. The amount of fertilizer N recovered was similar in trees in both irrigation treatments, but efficiency of fertilizer use was greater for trees receiving demand-controlled irrigation than fixed-rate irrigation. This was attributed to lower N inputs, greater retention time in the root zone and less N leaching in the demand-controlled irrigation treatments compared with fixed-rate irrigation treatments. Less fertilizer N was recovered by trees receiving an early application of N than a later application of N and this was related to the timing of N supply with respect to tree demand. Demand for root-supplied N was low until 11 weeks after planting, because early shoot and root growth was supported by N remobilized from woody tissue, which involved 55% of the total tree N content at planting. Rapid development of roots > 1 mm in diameter occurred between 11 and 14 weeks after planting, after remobilization ended, and was greater for trees receiving an early application of N than for trees receiving a later application of N. Late-season tree N demand was supplied by native soil N, and uptake and background soil solution N concentrations were higher for trees receiving demand-supplied irrigation compared with fixed-rate irrigation. Total annual N uptake by roots was unaffected by treatments and averaged 6-8 g tree(-1). Nitrogen applications in 1997 affected growth and N partitioning in 1998. Trees receiving early applications of N had more flowers, spur leaves and bourse shoots than trees receiving later applications of N. Consequently, more N was remobilized into fruits in trees receiving early applications of N compared with fruits in trees receiving later applications of N. Demand for N in the young apple trees was low. Early season demand was met by remobilization from woody tissues and the timing of demand for root-supplied N probably depends on whether flowering occurs. Method of N delivery affected the efficiency of N use. We conclude that N demand can be met at soil solution N concentrations of around 20 mg l(-1).

Pulsed application of overtree irrigation for evaporative cooling of `Jonagold' apples (Malus domestica Borkh.) reduced visible solar injury by 15.8% (1991) and 9.4% (1992). Maximum fruit surface temperature was reduced by 8.1 °C on a day when the average surface mean of nonsprayed fruit rose to 45.6 °C. Air heated more slowly than the exposed fruit surface and was cooled only 1 to 2 °C by overtree irrigation. Cooling did not affect fruit size, firmness, or redness but reduced soluble solids concentration and increased titratable acidity. Storage breakdown was unaffected in the first year but was reduced by 6.0% in the second year.

A study was undertaken in order to quantify the effects of rootstock and irrigation level on the rooting pattern of fertigated apple trees. A profile trench observation method was used to map the number and location of root intersections at two planes relative to drip emitters for NP-fertigated McIntosh apple trees (Malus domestica Borkh). Four replicates of all combinations of three rootstocks (M.9, M.26 and M.7, planted in 1990) and two irrigation treatments [F1 (daily, watered to field capacity) and F2 (weekly, watered to 50% available soil water capacity, imposed in 1991)] were investigated after the 1994 growing season. Dwarfing rootstocks and F2-irrigation had lower top vigour and yield per tree. Cumulative yield efficiency was greater for trees on dwarfing rootstock but was unaffected by irrigation treatment. Root numbers per observation plane paralleled rootstock vigour, being lowest for M.9 and greatest for M.7. No effect of irrigation on root number was observed. Root distribution was affected by an interaction between rootstock and irrigation treatment with average root location moving closer to the surface and emitter for the dwarfing rootstocks M.9 and M.26 but not for M.7 under F1-irrigation. Key words: Vigour, yield, drip emitter, root number and distribution

. 2000. Drip-fertigation of apple trees affects root distribution and development of K deficiency. Can. J. Soil Sci. 80: 353-361. The locations of root intersections were mapped relative to micro-emitters in spring, 1996 for Gala apple (Malus domestica Borkh) on M.26 rootstock, which were established and grown for 8 yr under micro-irrigation with fertigation. Root distribution was mapped for trees that had been irrigated daily between May and October using drip or microjet systems and fertigated each year during May. Drip-irrigated trees had nearly half of their total root intersections within 30 cm depth and lateral distance from the emitter. Roots of microjet-irrigated trees were more widely distributed over a soil volume extending to 60 cm depth and 90 cm lateral distance from the trees. Leaf K concentrations had decreased to near deficient concentrations for drip-irrigated trees after 5 yr.Root distribution mapping and soil sampling were undertaken in spring 1997 for N-fertigated Gala on M.9 apples, planted in 1992 on the same soil, with or without annual K applications during 1992-1996. Fertigation of 15 g K tree -1 each year in July-August did not affect root distribution, which remained laterally and vertically restricted but prevented the decline of exchangeable soil K below 50 mg kg -1 , exchangeable K/Ca + Mg + K ratios below 0.04 and the appearance of K deficiency symptoms in the trees. There was no evidence of long-term soil-K enrichment after 5 yr of K fertigation. Nous avons cartographié au printemps 1996 l'emplacement par rapport aux distributeurs des racines sectionnées apparaissant à la face d'une tranchée d'observation. L'expérience était réalisée sur un verger expérimental de pommiers Gala (Malus domestica Borkh), formés sur porte-greffes M.26, cultivés pendant 8 ans en irrigation fertilisante goutte-à-goutte. Les arbres avaient été irrigués tous les jours de mai à octobre au moyen de goutteurs ou de micro-gicleurs et fertirrigués en mai chaque année. Chez les arbres irrigués au goutte-à-goutte, près de la moitié des coupes de racine se situaient dans les 30 cm en profondeur et latéralement par rapport au goutteur. Chez les arbres irrigués par micro-gicleurs, les racines étaient plus largement réparties, soit dans un volume de sol s'é-tendant à 60 cm en profondeur et à 90 cm latéralement à partir du tronc. Au bout de 5 ans, les concentrations foliaires de K des arbres irrigués au goutte-à-goutte étaient tombées près du seuil de carence. Au printemps 1997, les mêmes observations étaient répétées avec en plus des prélèvements de sol, sur des pommiers Gala formés sur porte-greffes M.9, plantés dans le même sol en 1992, avec ou sans fumure potassique annuelle dans les 5 premières années. L'apport par fertirrigation de 15 g K par arbre chaque année en juillet-août n'avait pas d'effet sur la répartition des racines dans le profil, lesquelles demeuraient confinées tant en profondeur qu'en largeur, mais il a néanmoins permis d'éviter la chute du K échangeable du sol en dessous de 50 mg kg -1 et celle ...