Soil Acidification for Blueberries
David W. Lockwood
Univ. of TN/Univ. of GA
TN Fruit & Veg. Conf.,2012
Blueberries have unique nutritional
requirements
• Evolved in acidic soils where fertility is usually
quite low
• In such soils, the predominant form of
available nitrogen (ammonium) is usually
present at levels that would be toxic to many
plants
• Blueberries can directly ab sorb ammonium
(most plants absorb nitrate)
Blueberry Nutrition
• In comparison to other plants, blueberries
have very low nutritional requirements
• Most nutrient problems in blueberries can be
avoided by simply maintaining the soil pH
between 4.0 & 5.0
• A high soil organic matter content is desirable
for blueberries.
Nitrogen - blueberries
• Effect of organic matter:
– For each 1% of soil organic matter, about 15#
of N is released/acre/year
– Ammonium form is preferred over the nitrate
form
– Excess N results in
•
•
•
•
Reduced fruit size
Delayed ripening
Fruit softening
Reduced cold hardiness
Acidifying Soils
• ~ 1 yr. is required for supplemental S to oxidize &
reduce soil pH
• Oxidized sulfur is available as aluminum sulfate
and ferrous sulfate
– They are required in larger amounts (6 & 8-fold,
respectively) than elemental sulfur & they can be toxic
to blueberries (Al & Mn become very available when
pH is < 5.0)
• Soils high in organic matter rarely need
supplemental N
Amt. Sulfur (lb/A) required to lower
soil pH to 4.5
Current pH
5.0
5.5
6.0
Soil Type
Sand
Loam
175
530
350
1050
530
1540
Clay
800
1600
2310
Lowering Soil pH With Elemental
Sulfur
• Medium textured silt-loam soils –
– Apply 2# elemental sulfur / 1,000 ft² for each
0.1 unit change in pH
– When > 30# S/1,000 ft² required, use a split
application
• Apply 2/3 of required amount
• After 3 or more months, retest the soil
• Add the remainder if needed
Sulfur Application Rates (continued)
•Fine-textured (clay) soils –
Use 1/3 more sulfur
•Coarse-textured (sandy) soils –
Use 1/3 less sulfur
Applying Sulfur
• Elemental sulfur –
– Apply at least 6 months prior to planting
– Thoroughly incorporate in soil
• Direct contact with roots can injure or kill them
– May need to make split applications
• Injecting sulfuric aid or phosphoric acid through
the drip irrigation system
– Thoroughly mix acid with water in irrigation line
– Monitor water pH (4.5 – 5.5)
Soil Acidification Trials
• Goal: to evaluate the potential for using
injection of sulfuric acid in trickle irrigation to
maintain a desirable soil pH in the root zone of
the blueberry plants
• Treatments:
– Multiple sulfuric acid injection into irrigation
– Single surface application of elemental sulfur
Soil Acidification
• Initiated study 9/2009
– 1st acid injection
– Surface application of elemental sulfur
• 2010 – 5 acid injection treatments
• 2011 – follow up soil testing
Soil pH Readings
7
6
5
4
3
Acid
2
Sulfur
1
0
Acid Injection Details
• pH of acid solution at emitters: 3.2 – 3.7
• Gallons of solution used in treatment:
– 325 – 350
•
•
•
•
Treatment time: 2 hours
Flush time: 20 minutes
Gallons of water used in flush: 43 – 78
pH of water at emitters after flush: 6.1
Acid Injection: Conclusions
• Acid application is not stable
– Would be more feasible in areas where 100% of
water was from irrigation
• Elemental sulfur application is easier, cheaper,
safer and may only need to be done 1 time
• Initial pH adjustment with elemental sulfur
needs to be done well in advance of planting
(1 year or longer, depending on degree of
initial pH adjustment)
Pre-plant Blueberry Fertilization
• Soil test upper 8 in., 8 – 12 in.
– Soils with a natural pH over 6.0 or very high
calcium may not be suitable
– Very high phosphorus levels (>300#/A) may
reduce iron availability
Fertilizing Blueberries
• Young plants
– Objective: Maximize survival & growth
• Bearing plants
– Objective: Balance vegetative growth &
fruiting
Blueberry Fertilization
• Multiple applications
– Young plants: every 4 – 6 weeks (bud break to
early Aug.)
• Mature plants:
– 2 to 3 applications of N (30 # N/A/ application*)
• 1st at bud break
• Last after harvest
* For 12 ft. between row spacing
Acidifying Effect of Some Common
Fertilizers & Soil Amendments
Pure CaCO₃ needed to neutralize
Material
the acidity in 100# of material
Ammonium nitrate
60
Ammonium sulfate
110
32% liquid N
55
Urea
81
Sulfur-coated urea
118
Diammonium phosphate
70
Elemental sulfur
312
Aluminum sulfate
45
Fertigating Blueberries
• N rates:
– 1st yr. plants
1 to 1 ½ pounds/week
– 2nd yr
2–2½#
– 3rd yr
3 – 3¼ #
– 4th+ yr
4 – 5 #/week
– Reduce rates during fruit ripening
• Phosphorus & potassium
– ½ the rate of nitrogen
• During ripening, use equal amounts of N & K
• Do not use P if water iron levels are high
Iron Deficiency
• Appears on young
leaves first
• Occurs at a soil pH
above 5.3
• Shoot & leaf size
reduced
• Symptoms increased
in soils that are poorly
drained or compacted
Iron Deficiency - Correction
• Foliar applications of
iron chelate:
– Quick fix
– Temporary
• Lower soil pH
Maintaining Soil pH - postplant
• Soil test frequently
• Use ammonium sulfate as a nitrogen
source
• Use ferrous sulfate instead of aluminum
sulfate for further pH adjustment
– Injecting sulfuric acid into trickle system
Soil Acidification
• Sulfur reaction in the soil is slow
– Quick changes in soil pH should not be expected
– Since oxidation of sulfur is the result of microbial
activity, fall and winter sulfur applications well not
change soil pH (until the soil temperature rises)
– Soil pH adjustments with elemental sulfur should
be monitored with routine soil sampling
– Once soil pH has been acidified to the desired
level, it should remain low for a fairly long period
(>5 years), but soil monitoring is still desirable
In Order for Sulfur to Work:
• Sulfur must be mixed with the soil to provide
contact
• The soil must be moist
• The soil must be aerated (bacteria need O₂)
• The soil must be warm for rapid bacterial
growth
• Time is required for the reaction to go to
completion