Nitrogen, phosphorus and potassium fertigation of apples

Trickle irrigation and fertigation have a significant impact on tree growth and yield in the first four years. Two key strategies in developing a fertigation program for apples are:

  1. Good water management and irrigation scheduling to keep nutrients in the root zone. Applying too much water can leach nutrients past the roots making them unavailable for uptake.
  2. Rate and timing of nitrogen (N), phosphorus (P) and potassium (K) applications to match crop demand and maximize uptake.

OMAF and MRA currently does not have any published guidelines for apple NPK fertigation. Research from British Columbia, New York and Ontario can provide some insight in planning a fertigation program for new and established apple orchards.

In drier climates, where considerable amount of fertigation research has taken place, irrigations are scheduled and fertilizers applied in each irrigation cycle. However, in Ontario this approach may not be the best. The total amount of irrigation water applied in our growing season is variable depending on how much it rains. This can make scheduling a fertigation event a challenge. A better approach for Ontario's climate, similar to practices in NY, would be to apply a single fertilizer application once a week in one irrigation cycle. If any additional water is needed that week, it is applied without any fertilizer.

The first step in planning a fertigation program is getting a soil test done. Information on taking a soil sample can be found at www.omafra.gov.on.ca/IPM/english/apples/soil-diagnostics/soil-testing.html. Application timing and rate guidelines based on current research for N, P and K are described in the table below.

Nitrogen (N)

Rate
  • OMAF and MRA Pub 360 Guide to Fruit Production provides nitrogen guidelines new and established plantings (www.omafra.gov.on.ca/english/crops/pub360/4nutrit.htm#fruct) Ontario research suggests that these N rates are adequate for fertigating a new planting (1-3 years).
  • Fertigated rates can be 50% less than broadcast surface applied N (NY, BC).
  • Calculating weekly nitrogen rate: divide the total annual nitrogen application rate by number of application weeks (example: 40 g/tree/10 weeks = 4 g N/tree/week)
  • N demand in young trees is quite low, 5-10 g N/tree/year
  • Large increase in N inputs may lead to only small changes in tree N uptake if tree demand is already met
Seasonal Timing
  • Apply for 10 weeks beginning around full bloom
  • Maximum soil N uptake begins around bloom and continues through shoot leaf canopy development
  • Before full bloom, spur leaf canopy development depends on remobilized N stored from the previous season, soil N uptake is minimal.
Irrigation timing
  • Leaching N and water below the rooting zone increases if irrigation is not scheduled to meet plant demand or if N and water are applied in large amounts over a short period of time.
  • Surface drip: nitrate-nitrogen is more uniformly distributed in the soil when applied during the middle half of the irrigation cycle rather than at the beginning or end of irrigation cycle
Comments
  • Use high grade quality water soluble fertilizers to avoid emitter plugging.
  • Urea is very mobile with water in the soil, similar to nitrate-nitrogen sources apply in the middle of the irrigation cycle to keep in rooting zone.
  • NY and BC research have reported a decrease in soil pH when ammonium-nitrogen based fertilizers are used for a long time. In slightly acidic to acid soils, pH less than 7.0, monitor soil pH regularly and alternate between ammonium-nitrogen and nitrate-nitrogen fertilizers. In well buffered soils, pH greater than 7.2, acidification is less likely to occur, monitor soil pH.

Phosphorus (P)

Rate
  • P fertigation is usually not considered necessary
  • Soil test P between 12-20 ppm is probably adequate.
  • NY research found no beneficial crop response to fertigated P.
  • BC research has shown a positive response to fertigated P in newly planted trees, younger orchards and a single post bloom application. This may reflect their growing and soil conditions.
Comments
  • Phosphate fertilizers are incompatible with magnesium or calcium fertilizers. When mixed together precipitates can occur plugging emitters.
  • Check irrigation water calcium (Ca), magnesium (Mg) and bicarbonate (HCO3) concentrations. To avoid phosphate precipitates, Ca and Mg combined levels should be less than 50 ppm and HCO3 less than 150 ppm.

Potassium (K)

Rate
  • Soil test potassium 120-150 ppm is probably adequate.
  • OMAF and MRA Pub 360 Guide to Fruit Production provides potassium guidelines new and established plantings (www.omafra.gov.on.ca/english/crops/pub360/4nutrit.htm#fruct)
  • Calculate weekly K2O rate by dividing total annual application rate by number of weeks to be applied (example: 90 g K2O/tree/15 weeks = 6 g K2O/tree/week)
Seasonal Timing
  • Apply over 15 weeks (NY)
  • K applied in first 8 weeks of season fruit size was greatest (NY)
  • Can be applied over the whole season
Comments
  • K movement through soil profile depends on soil texture. It could be a very small distance from the emitter as great as 40-60 cm down the soil profile with a horizontal diameter spread of 60 cm (New York).

Other factors affecting a fertigation program include soil texture, cultivar and root stock. Use annual leaf analysis along with a soil testing to evaluate the fertigation program at the end of the season. More information about apple nutrition, soil testing and leaf analysis can be found at www.omafra.gov.on.ca/english/crops/pub360/4nutrit.htm#fruct .


For more information:
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E-mail: ag.info.omafra@ontario.ca