Gypsum Rate Evaluation for Wild Lowbush Blueberry (Vaccinium angustifolium Ait.) Soils

Abstract

Four rates of gypsum (CaSO₄·2H₂O) were compared at two commercial wild lowbush blueberry fields (Addison and Belfast, ME) to determine the most suitable rate for increasing blueberry nutrient uptake under Maine soil conditions. Gypsum treatments (1,121, 2,242, 3,363, or 4,484 kg·ha⁻¹) were compared to diammonium phosphate (DAP; 448 kg·ha⁻¹) and a control. Gypsum and diammonium phosphate were applied pre-emergent in May 2009 to 0.9 m × 15 m plots arranged in a randomized complete block design with six blocks in each field. Composite leaf and soil samples were collected in July 2009 and analyzed for nutrient concentrations. Within each treatment plot, stems from four randomly placed 0.023 m² quadrats were collected in October 2009 for stem length, branching, and flower bud measurements. Soil Ca and S concentrations were increased by gypsum at both fields. Gypsum increased leaf N and P concentrations only at the field that was deficient in these nutrients (Addison). Diammonium phosphate increased leaf N and P concentrations compared to the controls at both fields. Gypsum at 2,242 kg·ha⁻¹ or higher corrected P deficiency; but only the 3,363 kg·ha⁻¹ and 4,484 kg·ha⁻¹ rates corrected leaf N deficiency. At Belfast, diammonium phosphate did not increase flower bud density or yield. At the deficient Addison field, a lower rate of gypsum (3,363 kg·ha⁻¹) than that recommended for Canadian soils (4,000 kg/ha) was effective in correcting leaf N and P deficiency. Flower bud density and yield were raised by diammonium phosphate but not by any of the gypsum treatments.

Keywords:

• fertility
• mineral nutrition
• calcium sulfate
• blueberry production
• organic

INTRODUCTION

Diammonium phosphate (DAP) has been used by Maine blueberry growers for wild lowbush blueberry production for more than a decade. However, environmental issues in blueberry production, i.e., runoff of dissolved reactive phosphorus (DRP) to nearby streams from heavy use of fertilizers, poses a threat to the nearby aquatic ecosystem. A survey of 76 Maine blueberry fields (Smagula, unpublished data) indicated that plants in a majority of the fields had adequate leaf N (>1.6%; Trevett, 1972) but inadequate leaf P concentrations (<0.125%; Trevett, 1972). In spite of having adequate levels of leaf N, some blueberry growers still fertilize their fields with DAP (18-46-0) resulting in an unnecessary addition of N to the soil, increasing the potential risk of surface runoff. Application of gypsum has been reported to effectively reduce the runoff of DRP and NH₄-N by 80 and 59%, respectively. This is due to the conversion of readily desorbable soil P into a less soluble calcium-P complex and increased soil infiltration that reduces loss of NH₄-N by surface runoff (Favaretto et al., 2006).

The concept of organically produced blueberries is becoming more popular. In a 2006 survey (Files et al., 2008), organic growers indicated that they are selling their produce to wholesalers, farmers markets, restaurants, and bed and breakfast establishments, indicating a wide range of markets are available. The increasing demand for organically grown blueberries is being matched by organic growers (Drummond et al., 2008). While DAP is the recommended chemical fertilizer for non-organic growers, organic growers have no recommended organic fertilizer for lowbush blueberry production. Since DAP is chemically manufactured, it is not approved for use by organic growers. Gypsum is an organically approved soil amendment that may correct leaf nutrient deficiencies and improve plant growth and yield. Cations, such as NH₄ ⁺ and K⁺, on the soil exchange sites are displaced by calcium (Ca) making those cations available in the soil solution for the roots to absorb. The increased concentration of Ca in the soil solution encourages the increase in phosphorus (P) uptake by the roots (Mills and Jones, 1996).

Wild blueberry production follows a 2-year cropping cycle. Berries are harvested in August. In the fall, after leaf drop, or in the spring before bud break, fields are pruned to the ground level by fire or by mechanical pruning with flail mowers. In the spring of the first year of the cycle (prune year), prior to shoot emergence from the underground rhizome sites, fertilizer application and weed control practices are performed. Flowering, fruit set, and ripening occurs in the second year of the cycle (crop year). Sanderson et al. (1996) evaluated the effect of gypsum at 2,000, 4,000, 6,000, or 8,000 kg·ha⁻¹ over three cropping cycles on blueberry nutrient uptake, growth, and yield at six Prince Edward Island, Canada wild lowbush blueberry fields. The treatments were only applied during the prune year of the first cropping cycle. Yield was increased by gypsum at three of the six fields and gypsum at 4 t·ha⁻¹ was the most effective rate, increasing marketable yield by an average of 47% compared to the control at the responsive fields.

The environmental issues concerning lowbush blueberry production, the need to find alternative fertilizers for organic production of the wild lowbush blueberry, and the positive response to gypsum in Prince Edward Island blueberry fields justifies evaluating gypsum under Maine soil conditions. The objectives of this study were to (1) evaluate the efficacy of gypsum as an alternative fertilizer to DAP for Maine lowbush blueberry soils, and (2) determine the most suitable rate of gypsum for correcting nutrient deficiency, improving plant growth, and increasing blueberry yield.

MATERIALS AND METHODS

Two commercial wild lowbush blueberry fields in the prune year were selected at Addison and Belfast, Maine. Within each field, treatment plots measuring 0.91 m × 15.24 m were established with 1.83 m alleyways between each plot. Treatments included a control (no fertilizer application), gypsum (1,121, 2,242, 3,363, or 4,484 kg·ha⁻¹) or diammonium phosphate (DAP; 448 kg·ha⁻¹) applied in May 2009. All gypsum and DAP treatments were applied using a Gandy Model 42 drop spreader (Gandy Company, Owatonna, MN, USA).

Belfast Field

Gypsum and DAP were applied pre-emergent on May 5, 2009. To determine their effects on nutrient uptake, composite leaf samples were taken from 50 randomly selected stems in each treatment plot on July 6, 2009, after the stems had stopped elongating, at the tip dieback stage (Trevett, 1972). Leaf tissue samples were dried at 70°C and ground to pass through a 40-mm mesh sieve. All leaf nutrient concentrations except nitrogen were measured at the Maine Agriculture and Forestry Experiment Station (MAFES) Analytical Laboratory, Orono, ME using inductively coupled plasma emission spectroscopy (ICP-OES) following the procedure of Kalra and Maynard (1991). Leaf nitrogen concentrations were measured using a CN analyzer (CN-2000, Leco Corporation, MI, USA). Ten soil sample cores within each treatment plot were collected after leaf samples were taken on July 6, 2009 using a standard 2-cm-diameter soil sample tube to a depth of 7.6 cm. Soil pH, organic matter, cation exchange capacity, and soil nutrient concentrations were determined. Soil organic matter content was measured by loss on ignition at 375°C. Soil nutrients were extracted in pH 4.8 ammonium acetate (Modified Morgan) and measured using ICP-OES. Stem characteristics were measured on stems collected on November 1, 2009 from within four 0.023 m² quadrats per treatment plot. Stem density, height, branching, and flower bud formation were measured. Berry yield was estimated using a mechanical harvester on August 6, 2010 by harvesting a 0.61-m strip in the 0.91-m-wide plots.

Addison Field

Both gypsum and DAP were applied pre-emergent on May 20, 2009. Composite leaf samples from 50 randomly selected stems in each treatment plot and ten soil sample cores per treatment plot for nutrient analysis and soil characteristics, respectively, were collected on July 14, 2009 and analyzed as described above. Stem samples were collected on October 28, 2009 within four 0.023 m² quadrats per treatment plot for the analysis of stem characteristics. Berry yield was collected on August 10, 2010 using a mechanical harvester.

Statistical Analysis

Treatments were assigned in a randomized complete block design with six replications. All data were subjected to analysis of variance (ANOVA) using the SAS General Linear Model (SAS Institute Inc., 2006). Data were tested for assumptions of normality and homoscedasticity and transformed when appropriate. All treatments were subjected to the Ryan-Einot-Gabriel-Welsch (REGWQ) multiple range test. Response to increasing rates of gypsum was determined using trend analysis. Percent branching was analyzed as its arcsine, but is presented as original percentage.

RESULTS AND DISCUSSION

Soil Analysis

Application of 4,484 kg·ha⁻¹ of gypsum slightly decreased soil pH when measured in water (H₂O) at the Addison field (Table 1). However, when measured using calcium chloride (CaCl₂) instead of water, pH was not affected by any of the gypsum treatments at Addison or Belfast. The high electrical conductivity of the gypsum-treated soils affects the pH electrodes and creates an anomaly. Calcium from CaCl₂ levels off the ionic strength of the solution, therefore factoring out the salt effect giving a more accurate measure of soil pH. Soil organic matter measured by loss on ignition method ranged from 10.1 to 11.1% at Belfast and 17 to 21% of soil organic matter at Addison. Compared to the control, soil Ca was raised by increasing rates of gypsum at Addison and Belfast, exhibiting quadratic relationships (P = 0.0165 and P = 0.0240 for Addison and Belfast, respectively) (Table 1). Similarly, a positive quadratic relationship between soil sulfur (S) and increasing rates of gypsum was observed at Addison (P < 0.0001) and Belfast (P = 0.0012). The increase in soil Ca and S can be attributed to the Ca⁺⁺ and SO₄ ²⁻ ions from gypsum released through dissolution by water during rain events. DAP had no effect on soil Ca and S at either location. Gypsum did not have effect on soil P at either location. At Addison, neither DAP nor gypsum increased soil P concentrations, compared to the control. A negative quadratic relationship between soil Mg (P < 0.0001, Addison; P = 0.0002, Belfast) and increasing rates of gypsum was observed at both locations. Moreover, a similar relationship was observed for soil Zn at both fields (P < 0.0001, Addison; P = 0.0201, Belfast). DAP did not have any effect on soil Mg and Zn concentrations compared to the control at either location. Soil K, Al, Cu, Fe, Mn, and Na concentrations were not affected by any of the gypsum treatments or DAP at either location (data not presented).

TABLE 1 Effect of DAP and Increasing Rates of Gypsum on Soil Nutrient Concentrations of Two Maine Wild Lowbush Blueberry Fields

Treatments	Rates (kg·ha^−1)	pH (H2O)	pH (CaCl2)	OM (%)^x	Ca (ppm)	S (ppm)	P (ppm)	Mg (ppm)	Zn (ppm)
ADDISON
Control	0	4.57 a ^z	3.93 a	19.87 a	647.20 bc	30.33 ^x e	13.52 a	123.33 ^x a	3.65 ab
DAP	448	4.59 a	3.96 a	17.08 a	575.00 c	34.33 e	15.30 a	100.33 ab	3.47 abc
Gypsum	1,121	4.58 a	4.04 a	20.80 a	1023.30 ab	72.67 d	13.87 a	94.00 ab	4.13 a
Gypsum	2,242	4.49 ab	4.03 a	18.23 a	913.20 abc	142.00 c	11.72 a	53.50 bc	2.85 dc
Gypsum	3,363	4.33 ab	3.97 a	20.15 a	1155.50 a	223.17 b	14.27 a	44.33 c	3.18 bcd
Gypsum	4,484	4.23 b	3.93 a	18.80 a	1252.00 a	387.67 a	12.15 a	36.17 c	2.60 d
Gypsum Trend Analysis		Q ^y	NS	NS	Q	Q	NS	Q	Q
BELFAST
Control	0	4.73 a	4.12 a	10.52 a	294.83 ab	81.33 bc	11.90 b	43.50 ab	3.15 ab
DAP	448	4.69 a	4.10 a	10.13 a	252.00 b	50.67 c	13.78 a	46.33 a	2.52 ab
Gypsum	1,121	4.67 a	4.15 a	10.93 a	389.33 ab	110.00 ab	12.47 ab	35.33 abc	3.30 a
Gypsum	2,242	4.66 a	4.19 a	11.10 a	433.17 a	137.33 a	11.92 b	31.00 bc	3.10 ab
Gypsum	3,363	4.63 a	4.17 a	10.28 a	427.50 a	156.00 a	11.90 b	26.33 c	3.05 ab
Gypsum	4,484	4.62 a	4.19 a	10.85 a	449.83 a	179.67 a	11.68 b	23.83 c	2.80 b
Gypsum Trend Analysis		NS	NS	NS	Q	Q	NS	Q	Q
^zMeans within a column for each location not followed by the same letter are significantly different at P < 0.05 level using the REGWQ test.
^yLinear (L) and Quadratic (Q) Trend analysis done excluding DAP treatment at P < 0.05 level. NS = Not significant.
^xData at the Addison field was log transformed for analysis but are presented as original means on the table.

Toma et al. (1999) reported an increase in soil exchangeable Ca up to 1.2 m down the soil profile 16 years after gypsum application, indicating its long-term effect. However, Sanderson et al. (1996) reported no residual effect of gypsum on yield production in the second and the third cropping cycles following a single gypsum application in the first cycle. Application of gypsum upsets the soil chemical equilibrium causing higher charged ions (Ca⁺⁺) to be adsorbed by the cation exchange sites and replaces the lower charged ions (NH₄ ⁺, K⁺). Through time, most of the free Ca ions from gypsum will move from the topsoil down the soil profile bringing back the chemical equilibrium in the upper soil horizons. The mass action of Ca ions helped trigger the release of nutrients bound by the exchange sites; with most of the free Ca ions leached down the soil profile as the remaining free Ca ions are unable to release the remaining nutrients tied at the exchange sites. The absence of most Ca ions from gypsum in these upper soil horizons means very little soil nutrients can be released from the exchange sites to sustain the nutrient requirement of the second cycle's plant growth and yield, as reported by Sanderson et al. (1996).

Leaf Analysis

Leaf analysis showed that plants in control plots at Addison were deficient in both N (<1.60%) and P (<0.125%) while plants in control plots at Belfast had more than adequate leaf N (1.79%) and P (0.141%) (Table 2). Gypsum was more effective in raising leaf nutrients at the Addison field than at the Belfast field, perhaps due to the higher OM content at the Addison field. At Addison, all gypsum treatments raised leaf N compared to the control exhibiting positive linear (P = 0.0468) and quadratic (P = 0.0006) relationships. No relationship was found between leaf N concentration and increasing rates of gypsum at Belfast. Positive linear (P = 0.0028) and quadratic (P = <0.0001) relationships between leaf P concentrations and increasing rates of gypsum were found at Addison. Only the 3,363 and 4,484 kg·ha⁻¹ rates of gypsum were able to raise leaf N above the leaf N standard (1.6%) established by Trevett (1972), while 2,242 kg·ha⁻¹ was sufficient to raise leaf P up to the leaf P standard. At the non-deficient Belfast field, gypsum had no effect in raising leaf N or P, suggesting a differential response to gypsum application between the fields. Application of gypsum at a Canadian blueberry field that was deficient in P but not N only raised leaf P (Sanderson and Eaton, 2004). Our results are similar in that gypsum only corrected leaf nutrient deficiency at a nutrient deficient field. Application of DAP increased leaf N and P compared to the control at both fields.

TABLE 2 Leaf Nutrient Concentrations and Yield of Lowbush Blueberry as Affected by Increasing Rates of Gypsum and DAP at Two Maine Wild Lowbush Blueberry Fields

Treatments	Rates (kg·ha^−1)	N (%)	P (%)	Ca (%)	Fe (ppm)	B (ppm)	Zn (ppm)	Mg (ppm)	Yield (kg·ha^−1)
ADDISON
Control	0	1.44 c ^z	0.112 ^x c	0.401 b	30.35 b	18.95 b	12.83 b	0.169 a	2948.8 ^w b
DAP	448	1.72 a	0.138 a	0.370 b	29.72 b	18.10 b	14.92 ab	0.150 a	5684.2 a
Gypsum	1,121	1.56 b	0.124 b	0.463 a	33.97 ab	23.62 ab	15.52 a	0.159 ab	4117.5 ab
Gypsum	2,242	1.57 b	0.125 b	0.479 a	31.80 ab	22.58 ab	15.77 a	0.161 ab	3120.8 b
Gypsum	3,363	1.61 ab	0.129 b	0.505 a	32.18 ab	23.77 ab	15.55 a	0.154 ab	4291.4 ab
Gypsum	4,484	1.64 ab	0.129 b	0.489 a	35.17 a	27.47 a	16.55 a	0.153 ab	3178.2 b
Gypsum Trend Analysis		L, Q ^y	L, Q	Q	NS	Q	L, Q	Q	NS
BELFAST
Control	0	1.79 b	0.141 b	0.412 bc	30.40 b	14.00 a	19.25 a	0.153 a	2878.9 a
DAP	448	2.04 a	0.157 a	0.385 c	32.98 b	16.32 a	19.35 a	0.144 ab	4022.5 a
Gypsum	1,121	1.81 b	0.146 ab	0.442 ab	29.70 b	16.07 a	20.52 a	0.146 ab	3696.3 a
Gypsum	2,242	1.86 b	0.149 ab	0.480 a	30.52 b	15.70 a	20.02 a	0.146 ab	3002.5 a
Gypsum	3,363	1.82 b	0.150 ab	0.481 a	34.30 ab	16.15 a	20.27 a	0.133 b	3058.1 a
Gypsum	4,484	1.90 b	0.152 ab	0.467 a	42.28 a	16.53 a	19.70 a	0.130 b	2606.5 a
Gypsum Trend Analysis		NS	NS	Q	Q	NS	NS	Q	NS
^zMeans within a column for each location not followed by the same letter are significantly different at P < 0.05 level using the REGWQ test.
^yLinear (L) and Quadratic (Q) Trend analysis done excluding DAP treatment at P < 0.05 level. NS = Not significant.
^xLeaf P data at the Addison field was transformed by square transformation for analysis but are presented as original means on the table.
^wYield data at the Addison field was log transformed for analysis but are presented as original means on the table.

A positive quadratic relationship between leaf Ca and increasing gypsum concentration was observed at both fields (Addison P = 0.0011; Belfast P = 0.0005; Table 2). Sanderson et al. (1996) reported a positive linear relationship between leaf Ca and increasing rates of gypsum (2,000–8,000 kg·ha⁻¹). At Belfast, the increasing rates of gypsum and leaf Fe concentrations showed a positive quadratic (P = 0.0253) relationship; this trend was not observed, however, at Addison. Wild lowbush blueberry grower-submitted leaf samples analyzed at the MAFES Analytical Laboratory revealed a majority of the fields are deficient in leaf Fe (Smagula, 2008). However, raising leaf Fe to the sufficiency level (50 ppm) proposed by Trevett (1972) did not result in an increase in yield suggesting the leaf Fe standard is too high (Smagula, 2008). In our study, none of the gypsum treatments increased leaf Fe to the Trevett (1972) standard at either location. DAP did not raise leaf Ca or Fe compared to the control at Addison or Belfast. Boron deficiency was observed at both fields. A positive quadratic (P = 0.0319) relationship was observed between leaf B concentrations and increasing rates of gypsum at Addison while gypsum treatments had no effect on leaf B at Belfast. DAP did not have any treatment effect on leaf B concentrations at either location. Smagula (2008) reported that increasing leaf B concentration above the standard (24 ppm; Trevett, 1972) in a deficient field did not increase blueberry yield suggesting the standard is too high. The relationship between leaf zinc concentrations and increasing gypsum rates at the Addison field was both linear (P = 0.0269) and quadratic (P = 0.0078); no relationship was seen at Belfast.

DAP did not raise leaf Zn compared to the control and gypsum treatments at Addison or Belfast. At both locations, leaf Mg concentrations showed a negative quadratic relationship with increasing rates of gypsum (P = 0.0023, Addison; P = 0.0017, Belfast). This is similar to the relationships found between soil Mg and increasing rates of gypsum at Addison and Belfast. The decrease in leaf Mg is weakly correlated with the decrease in soil Mg in Belfast (r ² = 0.3246, P = 0.0003) but not at Addison (r ² = 0.0259, P = 0.3487). Leaf Mg concentration was not reduced below the standard (0.13%) by any of the gypsum treatments in both locations; therefore, Mg deficiency is not a concern in these fields.

Stem Measurements

Gypsum treatments did not affect stem or flower bud density, branched stem length, or number of unbranched stem flower buds at either field. A positive quadratic (P = 0.0240) relationship in stem branching with increasing gypsum rates was observed at Addison while no significant treatment effect was seen in Belfast (Table 3). Sanderson et al. (1996) reported that stem branching was not raised by gypsum treatment but in their study leaf tissue analysis indicated N was not deficient. No significant treatment effect on stem length was observed at Addison while a positive linear (P = 0.0390) relationship between stem length and increasing gypsum rates was observed at Belfast. At Addison, stem length, branching, and branched stem length was significantly raised by DAP, compared to the control. Taller stems are desirable because they can hold the berries high from the ground making it easier to harvest using a mechanical harvester. Increased stem branching has been shown to increase potential yield since the total number of flower buds increase with stem branching (Smagula and Fastook, 2009). At Belfast, stem length and branched stem length were raised by DAP, compared to the control.

TABLE 3 Effect of DAP and Increasing Rates of Gypsum on Lowbush Blueberry Stem Characteristics at Two Maine Wild Lowbush Blueberry Fields

Treatments	Rates (kg·ha^−1)	Stem density (number per 0.023 m^2)	Branched stem length (cm)	Flower bud density (number per 0.023 m^2)	Number of unbranched stem flower bud	Percent branching	Stem length (cm)
ADDISON
Control	0	31.25 a ^z	8.10 ^x b	52.00 a	1.31 a	25.70 b	8.35 b
DAP	448	39.30 a	10.90 a	70.00 a	1.18 a	41.66 a	10.24 a
Gypsum	1,121	36.75 a	9.72 ab	62.67 a	1.19 a	35.58 ab	9.23 ab
Gypsum	2,242	38.25 a	8.62 ab	58.21 a	1.14 a	32.32 ab	8.38 b
Gypsum	3,363	35.80 a	8.50 ab	62.33 a	1.24 a	39.70 a	8.70 ab
Gypsum	4,484	29.38 a	8.85 ab	56.38 a	1.25	36.51 ab	8.88 ab
Gypsum Trend Analysis		NS ^y	NS	NS	NS	Q	NS
BELFAST
Control	0	35.00 a	8.32 b	55.75 a	1.03 b	58.80 a	9.20 c
DAP	448	28.54 a	13.60 a	67.92 a	1.13 ab	59.94 a	13.27 a
Gypsum	1,121	33.79 a	10.71 b	63.17 a	1.15 ab	53.59 a	11.78 ab
Gypsum	2,242	30.67 a	9.77 b	47.08 a	0.95 b	60.30 a	10.42 bc
Gypsum	3,363	33.92 a	10.41 b	50.33 a	0.85 b	54.12 a	10.42 bc
Gypsum	4,484	33.79 a	10.59 b	70.38 a	1.66 a	54.79 a	12.00 ab
Gypsum Trend Analysis		NS	NS	NS	NS	NS	L
^zMeans within a column for each location not followed by the same letter are significantly different at P < 0.05 level using the REGWQ test.
^yLinear (L) and Quadratic (Q) Trend analysis done excluding DAP treatment at P < 0.05 level. NS = Not significant.
^xBranched stem length data at the Addison field was log transformed for analysis but are presented as original means on the table.

Berry Yield

Gypsum treatments did not increase blueberry yield at either field. In an earlier report by Sanderson et al. (1996), lowbush blueberry yield was raised at three of six fields in response to application of 4,000 kg·ha⁻¹ gypsum. In a later study, Sanderson and Eaton (2004) reported no effect of gypsum (4,000 kg·ha⁻¹) on lowbush blueberry yield for two consecutive cropping cycles at five lowbush blueberry fields at Prince Edward Island and Nova Scotia. Smagula and McGovern (in press) found that gypsum at 4,000 kg·ha⁻¹ increased leaf N and P concentrations in the deficient field but not in one that had sufficient concentrations of these nutrient elements. Combining DAP and gypsum was more effective in raising leaf N and P concentrations than DAP alone. This did not necessarily result, however, in an increase in yield. In our study, while gypsum had no effect on blueberry yield, application of the recommended rate of DAP (448 kg·ha⁻¹) increased blueberry yield at the Addison field by about 93% compared to the control.

CONCLUSIONS

As suggested in the literature, gypsum can supply Ca without affecting soil pH. Soil pH should be measured in gypsum-treated plots using CaCl₂ instead of water to make valid comparisons to untreated control plots. Soil Ca, S, and P concentrations were raised by application of gypsum. As reported by Sanderson et al. (1996), not all wild lowbush blueberry fields responded positively to gypsum application; in our study, the field in Addison responded, but the one at Belfast did not. Only at the Addison field, which was deficient in nutrients, was gypsum effective in increasing leaf N and P up to the Trevett (1972) standards, with the 3,363 kg·ha⁻¹ rate correcting both nutrient deficiencies. The same rate was effective in increasing stem branching at Addison to a level comparable to DAP application. Use of gypsum as an alternative fertilizer in Maine blueberry fields should be used with caution. It appears that gypsum is only effective in raising leaf N or P concentrations when the field is deficient in N or P.

Application of gypsum at a 3,363 kg·ha⁻¹ rate, 637 kg·ha⁻¹ less than the recommended Canadian rate (4,000 kg·ha⁻¹), should be enough for Maine lowbush blueberry fields. Gypsum can be used by organic lowbush blueberry growers that cannot use commercially produced fertilizers, such as DAP, for correcting leaf nutrient deficiency. In addition, growers that have fields with plants having leaf deficiency of either N or P can use gypsum to correct the deficiency without adding more N or P through application of DAP. Application of gypsum, however, did not increase blueberry yield at either field. The use of DAP for lowbush blueberry production is still recommended unless organic guidelines need to be followed. The reason gypsum is effective in raising yield in some fields and not in others is not clear and warrants further investigation.

ACKNOWLEDGMENTS

We wish to acknowledge the financial support of the Maine Wild Blueberry Commission and Hatch Act. The technical help of Kristen McGovern and the cooperation of blueberry growers Delmont and Marie Emerson, and Harold Morse are greatly appreciated. This manuscript is publication number 3123 of the Maine Agriculture and Forest Experiment Station.