Tree Fruit & Grape News, July 1999

Comparing Fertigation and Broadcast-Application of N, P and K Fertilizers in Orchards - Part 1

G.H. Neilsen and D. Neilsen
Pacific Agri-Food Research Centre, Agriculture and Agri-Food Canada, Summerland, B.C., Canada V0H 1Z0 

This article was too long to include it entirely so we have divided it into Part 1 Nitrogen and Part 2 Phosphorus, Potassium and other. Part 2 will be published in the next issue.

Summary 

Broadcast or foliar fertilizer applications have been traditionally used to improve or sustain the nutrition of many irrigated, deciduous fruit tree orchards in western North America. Recent developments, including adoption of low-pressure, micro-irrigation systems and planting at higher densities (especially for apple), have increased interest in controlled application of fertilizers directly with irrigation (fertigation). This article summarizes recent research, emphasizing results from high-density apple orchards and comparing fertigation to traditional broadcast fertilize application with respect to mobility of N, P and K in the soil and response of fruit trees to application of these nutrients. It is adapted from a more detailed article to appear in Hort Technology in June 1999.

Virtually all fruit crops in Western North America are irrigated, and irrigation systems range from gravity flow (flood irrigation) to a variety of sprinkler, low-pressure-microsprinkler or drip-distribution systems. Low-volume irrigation systems are well suited to the high-density apple orchards that are increasingly important in British Columbia. For example, "new" apple cultivars, like Gala and Fuji are propagated on dwarfing rootstocks such as M.9 and M.26 and planted at tree densities which vary from approximately 240 trees/acre (600/ha) (some "traditional" central-leader plantings) to nearly 800 trees/acre (2,000/ha) (slender-spindle plantings) and even in excess of 2,400 trees/acre (6,000/ha) ("super spindle" plantings). At present, low volume irrigation systems are being installed in nearly all of the new, high- and very high-density orchards in British Columbia. Application of nutrients with irrigation (fertigation) is the most efficient method of nutrient delivery in this type of irrigation system. Fertigation offers increased flexibility in managing orchard nutrition programs because of the potential for more closely synchronizing nutrient application with plant demand. There is a need, however, to assess the effectiveness of any new fertilization strategy relative to traditional methods. In the irrigated fruit growing regions of western North America, the traditional method of meeting fruit crop nutrient requirements has involved a combination of soil and foliar applications. However, this review will focus on the major nutrients N, P and K, which, when required, have traditionally been applied via broadcast or banded application to the soil. The emphasis will be on comparing and contrasting fertigation and broadcast fertilizer application, as they affect the mobility of NPK in orchard soils and response of fruit trees to their application. This article focuses on high density apple orchards on dwarfing rootstocks with low volume irrigation systems.

Nitrogen Broadcast Application

Soil Mobility. Remarkably few measurements of change in soil N status have been made following broadcast application of N-fertilizer in irrigated fruit orchards. The mobility of fertilizer-N in response to irrigation has long been recognized for irrigated crops in both humid and arid regions, especially for those on sandy soil. Mobility of applied N fertilizer was observed in an orchard lysimeter study of nine years duration at the Summerland Research Station in which the amount of irrigation water applied via sprinkler varied from 37 to 100 per cent of the maximum for the area. The experimental soil was a sandy loam, containing 64 per cent sand in the surface horizon and grading to 90 per cent sand at depth. The amount of N drained from the 5 ft (1.5 m) deep base of the lysimeter and thus through the rootzone of 'Macspur McIntosh' apple on 'Malling- Merton 111' (MM.111) rootstock was generally high relative to the amount of applied fertilizer, especially in the first year of an increased irrigation rate.

Table 1. Average quantity of N leached below 5 ft (1.5 m) depth in an orchard lysimeter at Summerland Research Station planted to 'Macspur McIntosh' on 'Malling-Merton 111' rootstock with variable irrigation and 145 lb N/acre (162 kg/ha) applied annually.^z

Plant Response. In the irrigated fruit growing areas of the Pacific Northwest, practical experience and local research, primarily in high- pressure, sprinkler-irrigated orchards, has resulted in recommendations to apply N at rates ranging from 67-134 lb N/acre (75-150 kg/ha), usually as ammonium nitrate (34N-0P-0K) and more infrequently as urea (46N-0P-0K) or calcium nitrate (15.5N-0P-0K), often as a single application in fall or early spring. Excess N applications can have adverse effects on fruit quality by reducing red colour development while internal ethylene production, indicative of accelerating maturity, continue to increase. Actual grower application rates can exceed recommended values and usually exceed crop N removal rates, which have been estimated to be 20-30 lb N/acre (22-33 kg/ha) for a 40-60 ton/acre (45-67 t/ha) crop of Golden Delicious. Furthermore, N applications are usually made within herbicide strips to avoid N-uptake by strongly competitive orchard floor vegetation.

Figure 1a

Figure 1b

Figure 1c

Figure 1d

Fertigation 

Soil Mobility. The mobility of NO₃-N (the end product of all fertilizer N applications to soil), particularly when applied by fertigation, is well known. Porous ceramic cup soil solution samplers have been utilized to monitor changes in macro nutrient status (especially N) of the soil in response to fertigation regime. Collecting samples from fixed locations relative to irrigation emitters and at fixed times relative to the cessation of irrigation has indicated that the NO₃-N concentration in soil solution is very responsive to N-fertigation. For example, in fertigation management studies in British Columbia, soil solution NO₃-Nconcentrations increased to high values at 1 ft (30 cm) depth soon after the commencement of fertigation with calcium nitrate and declined to background values soon after cessation of fertigation (Fig. 1a-d). This pattern was similar for both a silt loam (Fig. 1a) and a loamy sand soil (Fig. 1b) with solution concentrations approximating the concentration of the fertigation solution and thus directly correlated with the rate of applied fertilizer. Soil solution concentrations were also directly influenced by the volume of applied irrigation water so that doubling irrigation rates from 1-2 gal/tree (4-8 L) per day almost halved soil solution NO₃-N concentrations for both silt loam (Fig. 1c) and loamy sand (Fig. 1d) soils. The form of N fertilizer applied also influences soil solution NO₃-N concentration. When applied as NH4-N there is a delay in the increase of NO₃-N at the start and a slowing of the decrease at the cessation of fertigation (Fig. 2). Consistent with the work of Klein and Spieler (1987b), only low concentrations of NH₄-N were measurable despite fertigating all N as NH₄-N. Control of soil solution NO₃-N over time is clearly more precise if NO₃-N forms are used and the manipulation of N to achieve high or low soil NO3-N concentrations at different times during the growing season is a practical possibility. A comparison of the seasonal variation of soil solution NO₃-N concentration when the same amount of N fertilizer [0.9 oz N/tree (25 g)] was applied, either as a single broadcast application (standard practice under sprinkler irrigation), or as multiple drip-fertigated doses, indicated a higher average soil solution NO₃-N concentration during the growing season for the fertigated treatment (Fig. 3).

Figure 2.

Figure 3.

Plant Response. Generally, research has indicated that fertigation permits the amount of N applied to be reduced relative to broadcast rates while maintaining a similar N status of fruit trees. In Michigan, similar yield and leaf N concentration of several fruit tree species was measured when the amount of trickle-applied N was reduced to 50 per cent of the normal recommended broadcast rates. Efficient use of fertigated N was also measured in England where the best balance between shoot growth, fruit bud production, fruit set and cumulative yield of 'Queen Cox'/M.9 apple in the first four growing seasons was achieved at only 0.7 oz (20 g) fertigated N split into 80 daily doses per tree [23 lb N/acre (26 kg/ha]). Fertigation rates can be much closer to the estimated N removal rates for the crop.

The consequences of excessive application of N are similar, whether fertigated or broadcast. Decreased fruit color and delayed harvest for Starking Delicious apple were observed when fertigated N applications exceeded 223 lb/acre (250 kg/ha) in Israel. Both NH₄-N and NO₃-N forms have been effectively used to fertigate fruit trees. Urea, ammonium nitrate and calcium nitrate have been found to be acceptable N sources for fertigation with use of calcium nitrate being advantageous when soil pH is low and Mn toxicity a potential problem.

As indicated above, it is possible to monitor variation in soil solution N-concentrations using suction lysimeters. Less information is available concerning an optimum soil solution N concentration for apple tree growth. For example, there were few effects of either irrigation or fertigation treatments on leaf N, tree growth and yield despite differences in soil solution NO₃-N concentrations observed in Fig. 1, implying the lowest soil solution N concentrations (50-75 ppm) were adequate for growth of young apple trees. Midsummer leaf N concentration of trees receiving NH₄-N rather than NO₃-N was significantly lower (2.27 vs. 2.33%) only in the year following initiation of differential N-forms when fertigating despite differences in soil solution NO₃-N concentrations during the growing season (Fig. 2). The different soil solution N-regimes illustrated in Fig. 3 resulted in leaf N concentration averaging 2.54 per cent over five years for fertigated trees and 2.40 per cent for trees receiving broadcast N fertilizer. Few measurable differences in growth and yield were observed, however, between the two treatments. In Israel, a 42 ppm NO₃-N concentration (Hoaglund solution) resulted in most efficient use of N by daily fertigated Golden Delicious apple grown in outdoor pots, even though plant N-uptake continued to increase to 154 ppm. With the capability of monitoring variation in soil solution N-concentrations using suction lysimeters, additional research is required to determine a minimum soil solution N concentration and duration capable of supporting adequate tree growth, and to determine the consequence to fruit production of maintaining optimum N-concentrations at different times during the growing season. 

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