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Ministry of Agriculture & Lands

Tree Fruit & Grape News, October 1999

Comparing Fertigation and Broadcast Application of N, P and K Fertilizers in Orchards - Part 2

By G.H Neilsen and D Neilsen,
Pacific Agriculture Research Centre, Summerland, BC

(Part 1 was printed in the July 1999 Issue)

Phosphorus Broadcast Application

SOIL MOBILITY: The mobility of broadcast P fertilizer under sprinkler-irrigation has been reported for several soils. In a Washington State study, it was concluded that irrigation rather than P fertilization rate [27-214 Ib/acre (30-240 kg/ha)] affected movement of P into the root zone but that downward movement was not large even for coarse textured soils (Table 2.)

Table 2. Average depth of penetration of 54 lb P/acre (60 kg/ha) applied as monoammonium phosphate or triple superphosphate in 3 different soils and subjected to sprinkler irrigation.z

Irrigation Applied [inches (cm)] Quincy sand Warden silt loam Calcareous Warden silt loam
 

[Inches (cm)]
1.6 (4) 1.9 (4.9) 2.3 (5.8) 1.2 (3.1)
6.3 (16) 2.7 (6.9) 2.6 (6.6) 1.3 (3.3)
Significance: * * *
* Significanty different at p=0.05 level of probability.
z Data adapted from Lauer (1988).

The effect of continuous irrigation was to move P somewhat deeper into the soil but the effect was small. Poor downward movement of broadcast P into the root zone of many orchard soils has long been recognized and it is known that P-mobility is reduced in finer-textured and other soils with a high P-sorption capacity.

PLANT RESPONSE: Provided there is sufficient movement of P into the main rooting zone in orchards, tree P status can be increased by broadcast applications of P. This is illustrated by increased leaf P concentration of apple and pear trees after applications of monoammonium phosphate fertilizer to the soil surface in a range of irrigated orchards in Washington State. Lack of growth response of fruit trees to broadcast fertilizer application is also common, however, initial growth of newly planted apple trees is often improved following application of high rates of P-fertilizer in the planting hole, especially on fumigated, replanted orchard sites.

Figure 4.

Figure 4b

Figure 4c

Fertigation

SOIL MOBILITY:  Increased mobility of P dissolved in irrigation water has been observed, especially for sandy soils but also for fine-textured clay loams. The improved mobility has been attributed to movement of P in mass flow with irrigation waters after saturation of reaction sites near the zone of P application. The extent of P mobility is illustrated from a fertigation study carried out at Summerland. Trees in an orchard on a loamy sand soil received annual applications of 1.4 oz N (40 g) and 0.6 oz P (17.5 g), as mixtures of ammonium nitrate (34N-0P-0K) and ammonium polyphosphate (0N-15P 0K), through a single drip emitter. Extractable soil P values were measured at 1 ft (30 cm) depth frequently during the growing season at various distances from the emitter and perpendicular to the tree row. Increases in extractable soil P were measurable as far as 1 ft (30 cm) from the emitter, regardless of whether the same amount of P was applied as a single dose, four weekly doses or 30 daily doses in May (Fig. 4). Changes were most apparent directly beneath the emitter, (0 distance), with elevated extractable P concentrations at 1 ft (30 cm) depth immediately after application. This contrasts with the behaviour of surface broadcast P fertilizer which failed to penetrate below 3 inches (7 cm) depth for sandy soil after 16 irrigation applications totalling 6.3 inches (16 cm) (Table 2). This is a clear indication of the mobility of fertigated P.

PLANT RESPONSE: Fertigated P can efficiently modify the P-status of fruit trees. Fertigating P increased leaf P concentration of apple in the first growing season (Table 3). P-fertigated trees also exhibited increased yield in the first fruiting season. This supports observations that improving first year P-nutrition will accelerate establishment and flowering of newly planted trees. Fertigation of P was less effective for increasing leaf P concentration beyond the first year although fruit P concentration was higher for fertigated trees in the second fruiting season. Fertigation of P may therefore have a role in augmenting fruit P concentration.

Table 3. Comparison of leaf and fruit P concentration and fruit yield for Summerland 'McIntosh' apple trees on 'Malling 26' rootstock receiving various rates of fertigated P and adequate N for the first three growing seasons.z

  Leaf P (% dry mass) Fruit P 
(ppm fresh mass)x		 Fruit Yield 
[lb (kg)/tree]
P ratey Yr. 1 Yr. 2 Yr. 3 Yr. 2 Yr. 3 Yr. 2 Yr. 3
P1 0.19 0.18 0.15 131 72 3.3 (1.5) 11.7 (5.3)
P2 0.23 0.17 0.15 119 84 5.3 (2.4) 10.8 (4.9)
P3 0.20 0.19 0.16 129 97 5.7 (2.6) 11.5 (5.2)
P contrasts
Linnear *** NS * NS **** *** NS
Quadratic NS NS * NS NS NS NS
z Data adapted from Neilsen et al. (1994)
y P1=0, P2=0.6 oz (17.5 g) and P3=1.2 oz (35 g) P/tree annually as ammonium polyphosphate (10N-15P-0K).
x 1 ppm = 1 mg/kg

Potassium Broadcast Application 

SOIL MOBILITY: Mobility of potassium broadcast directly beneath a drip emitter on a K-fixing soil was high directly below the emitter but restricted at lateral distances 12 inches (30 cm) or more from the emitter as observed in California. Concentrating the fertilizer in a band directly beneath the emitter resulted in unit area K-application rates more than 50-times usual surface broadcast rates. However, it is generally recognized that moderate to heavy surface applications of K-salts usually increases available K in the surface soil with adequate movement into the rooting zone of most orchard soils. The mobility of normal rates of surface applied K is highest in sandy soils, reduced for soils with high exchange capacity, and very limited for soils known to fix K.

PLANT RESPONSE: Broadcasting high rates of K directly beneath drip emitters (Table 4) increased leaf K concentration of prune trees throughout the growing season. The high requirement of heavily cropping fruit trees for K and their response to broadcast K-application when leaf K concentrations decline below 1% has long been recognized. Few instances of K-deficiency have been identified in the Pacific Northwest of North America. However, a single broadcast application of 200 Ib K/acre (224 kg/ha) as KC1 (0N-0P-50K) was sufficient to ameliorate K-deficiency (leaf K concentration <1%) for three years in a soil with low available K, averaging 39 ppm (mg/kg) throughout the surface 12 inches (30 cm) in the Similkameen Valley.

Table 4. Average NH4OAc-extractable K concentration in a Colusa clay loam soil at the end of a single growing season at various depths and distances from emitters through which 2.47 lbs (1.12 kg) K had been broadcast beneath the emitters (+) or fertigated (+) or not (-) over the irrigation season.z

 

NH40Ac - extractable K (ppm)x
 

Distance from emitter (ft (cm))
  0 1 (30)

2 (60)
  Broad-
casty		 Fertigated Broad-
cast		 Fertigated Broad-
cast		 Fertigated
Soil depth inches (cm) K K no K K K no K K K no K
0-6 (0-15) 2131 520 270 164 512 250 164 305 289
6-12 (15-30) 2714 414 141 137 395 137 141 180 98
12-18 (30-45) 3284 332 113 109 258 102 121 117 70
18-24 (45-60) 3288 230 74 78 407 70 78 94 94
24-30 (60-75) 1634 100 70 74 228 66 74 63 106
30-36 (75-90) 176 66 90 86 90 82 98 86 86
z Data adapted from Uriu et al. (1980)
y Single time application at start of irrigation
x 1 ppm = 1 mg/kg

Fertigation

SOIL MOBILITY: The mobility of fertigated-K is well documented and illustrated by changes in extractable soil K at selected depths and distances from an emitter through which K had been added (Table 4). At the end of a single growing season in a clay loam soil, expected to inhibit downward movement of fertilizer K, increases in NH4OAc extractable K could be discerned to depths of 2-2.5 ft (60-75 cm) and lateral distances as great as 2 ft (60 cm). Thus the distribution of soil K was similar in depth but greater in lateral extent than similar changes previously cited after broadcasting K directly beneath the emitter at rates which exceeded by more than 50 times normal unit area surface broadcast rates under flood irrigation. 

PLANT RESPONSE: As a result of the improved mobility of fertigated-K in soils, increased leaf K concentration is a well documented consequence of K-fertigation on a range of fruit crops. Amelioration of deficiency symptoms and occasionally increased yield can be consequences of fertigating K. However, the ready availability of fertigated-K may adversely affect plant uptake of other applications including Ca and especially Mg. The anticipated Ca-K antagonism in fruit, with adverse effects on quality and storage, has not been observed for apples when tree K status has been improved from deficiency conditions. Increased red coloration of fruit and increased titratable acidity have been observed but only when fertigated-K ameliorated K-deficiency and not when leaf K values were adequate.

Direct Comparison of Broadcast-Fertilizer Application and Fertigation 

Ideal information for assessing the relative performance of fertigated and broadcast-fertilized trees in irrigation regions would be side-by-side site comparisons with both treatments receiving the save fertilizer and irrigation regime and with experimental duration spanning a significant portion of an orchard's life cycle. Unfortunately, few such comparisons have been made in irrigated regions. In humid regions, as in the Netherlands, trials established at four locations in 1986 on both 'Jonagold' and 'Elstar' compared single time broadcast N [0.5 oz/tree (15 g)] to the same rate of N broadcast with daily trickle irrigation or fertigation for 3 months. Increases in first year shoot growth were observed from irrigation alone for both cultivars. Additional increases from fertigation for 'Jonagold' and for yield for both cultivars in the first fruiting year were measured. Yield differences from fertigation tended to decrease as trees matured. In New York, similar trials were designed to compare broadcast; broadcast and irrigation; and fertigated treatments. Broadcast fertilization at high rates of N, P, K, Mg, B and Zn was applied three times over a 9-week period while fertigated applications were made in 10 weekly applications to 'Redchief', 'Oregon Spur', 'Mutsu' and 'Empire' on the more vigorous MM.I06 rootstock. Similar growth and yield responses to irrigation were observed as in the Netherlands, whereas responses to fertigation were less dramatic with yield increases occurring early (years 2-4) but not later for fertigated trees. 

Short term comparisons in mature orchards between broadcast and fertigated fertilizer applications usually emphasize the ability to achieve similar yield, growth or N-uptake at lower N-rates when fertigating. Limited comparisons have been made of the effects of fertigation on fruit quality despite the ease of altering the timing of nutrient application via fertigation. Lack of knowledge of the seasonal and crop load variation in the nutritional requirements of fruit trees at different growth stages probably limits optimum use of the flexibility inherent in fertigation. Such information is not easily generated by comparisons of broadcast and fertigated nutrient applications due to the many combinations of fertigation scheduling available.

Root Distribution In Relationship to Fertigation

A range of plant, environmental and orchard management factors are known to affect the distribution of fruit tree roots. Concentration of fruit tree roots occurred around drip emitters not delivering nutrients and near emitters for newly fertigated mature trees. Roots were shallower and closer to the emitter for 'McIntosh' apple on M.26 and M.9 rootstock, relative to M.7 rootstock after daily NP-fertigation through drip emitters in a 4-year study on a sandy loam soil at Summerland. Nearly half the number of root intersections observed for 'Gala' apple on M.26 rootstock were measured within 1 ft (30 cm) depth and lateral distance from drip emitters after 5 years of annual NP-fertigation (Fig. 5). Roots were more widely distributed for similar trees fertigated through microjet emitters which were estimated to wet a 6 ft (2 m) wide strip in the loamy sand soil. The development of a more restricted root zone in orchard planting systems, subjected to frequent fertilizer and water additions offers a potential for improved control and manipulation of the main rooting zone. However, it will be necessary to maintain optimum soil conditions in this smaller root volume.

Figure 5.

 

Soil Acidification 

A problem which has arisen from continued fertigation of ammonium-containing N fertilizers into restricted soil volumes is soil acidification resulting from nitrification of the applied NH4-N. Declines of 2 pH units have been measured in fertigated orchards within a single year. This parallel declines in soil acidification concentrations and increases in solubility of toxic elements such as A1. The result may be reduced tree growth and induced nutrient deficiencies, such as for K, which have not previously been observed in that orchard. The sensitivity of orchard soils to fertigation-induced acidification varies with the buffer capacity of the soil and can be estimated by an acidification resistance index to assess soil susceptibility to acidification. Soils with little resistance to acidification should receive N fertigated in NO3-rather than NH4-forms.

Conclusion

Comparisons drawn from NPK-fertilizer experiments carried out under irrigated growing conditions, typical of western North America fruit orchards indicate that N is very mobile in the soil, regardless of whether it is broadcast or fertigated. An advantage to fertigation is increased flexibility in application with similar plant response possible at reduced N rates and a potential for multiple reduced rate applications timed to more closely coincide with plant N-demand. The timing of N application can be readily modified. The more controlled application of N through fertigation thus offers potential to reduce the leaching of excess N and contamination of groundwater, providing excess irrigation can also be avoided. A more sophisticated under- standing of the plant's N-demand will be required to take maximum advantage of the flexibility of fertigation. The mobility of P and K is much greater when fertigated than broadcast, increasing the potential to apply these nutrients rapidly when required. Caution will be needed to avoid luxury uptake of these nutrients. Fertigation through low-pressure irrigation systems, such as drip and microjet, concentrates root development in smaller soil volumes, increasing tree reliance on fertigated nutrients and requiring optimum soil quality in the root zone. An important aim must be to minimize rapid and excessive acidification which can result from the fertigation of NH4-N.