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The influence of foliar and soil fertilization of humicacid on yield and quality of pepperYasar Karakurt a , Husnu Unlu b , Halime Unlu b & Huseyin Padem ba Suleyman Demirel University, Bahce Bitkileri, Suleyman Demirel Universitesi, ZiraatFakultesi, Isparta, 32260, Turkeyb Horticultural Sciences, SDU Ziraat Fakultesi Bahce Bitkileri Bolumu, Suleyman DemirelUniversity, Isparta, Turkey

Version of record first published: 22 Jul 2009.

To cite this article: Yasar Karakurt, Husnu Unlu, Halime Unlu & Huseyin Padem (2009): The influence of foliar and soilfertilization of humic acid on yield and quality of pepper, Acta Agriculturae Scandinavica, Section B – Soil & PlantScience, 59:3, 233-237

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ORIGINAL ARTICLE

The influence of foliar and soil fertilization of humic acid on yield andquality of pepper

YASAR KARAKURT1, HUSNU UNLU2, HALIME UNLU2 & HUSEYIN PADEM2

1Suleyman Demirel University, Bahce Bitkileri, Suleyman Demirel Universitesi, Ziraat Fakultesi, Isparta, 32260 Turkey, and2Suleyman Demirel University, Horticultural Sciences, SDU Ziraat Fakultesi Bahce Bitkileri Bolumu, Isparta, Turkey

AbstractHumic acid (HA) is the result of organic matter decomposition and is beneficial to plant growth and development. Theobjective of the study was to find the influence of foliar and soil HA application on fruit quality and yield of organicallygrown pepper. Pepper plants were treated with soil and foliar HA applications at various concentrations (0 ml/l, 10 ml/l,20 ml/l, 30 ml/l, and 40 ml/l). Starting four weeks after planting, HA was applied via spraying and/or drenching to the plantroot area three times during the growth period at 15-day intervals. HA treatments had no significant effect on fruit firmness,fruit length, or diameter. Total and reducing sugar contents significantly increased in response to both foliar and soil HAtreatments. Moreover, HA application significantly influenced total chlorophyll content and this effect was mainly onchlorophyll b content. Foliar 20 ml/l and soil 20 ml/l HA application resulted in the highest total chlorophyll content. Foliarand soil HA applications also led to significantly higher mean fruit weight, and early and total yield than for control. Thestudy demonstrates that both soil and foliar HA treatment might successfully be used to obtain higher fruit yield and cansignificantly enhance fruit quality in organically grown pepper.

Keywords: Capsicum annum, hormone-like activity, organic agriculture, sugars, yield.

Introduction

Humic acids (HAs) increase growth and yields of

various crops including vegetables (Hayes & Wilson,

1997; Padem et al., 1997; Atiyeh et al., 2002;

Zandonadi et al., 2007). Several mechanisms, one

of which was their positive effects on nutrient uptake

of vegetable crops (Akinremi et al., 2000; Cimrin &

Yilmaz, 2005; Zandonadi et al., 2007), have been

suggested to account for this stimulatory effect of

HAs. It has also been demonstrated that HAs could

serve as growth regulators to control hormone levels,

enhance plant growth and increase stress tolerance,

improve soil physical properties and complex metal

ions (Stevenson, 1982; Serenella et al., 2002). They

have lower stability constants than synthetic chelates

for metals, thus enhancing metal activity in solution

(Mackowiak et al., 2001). HAs might provide pH

buffering because of a large number of weakly acidic

functional groups (carboxylic and phenolic) forming

the molecule (Thurman, 1986; Mackowiak et al.,

2001).

Moreover, studies explaining the effects of HA

suggested that HAs demonstrate their effects

through increasing enzyme catalysis, enhancing re-

spiration and photosynthesis, and stimulating nu-

cleic acid metabolism (Dell’Agnola & Nardi, 1987;

Nardi et al., 1988; Muscolo et al., 1999; Serenella

et al., 2002). Although positive influences of HAs on

plant growth and development have been well

established for many species (Adani et al., 1998;

Buckerfield et al., 1999; Padem & Ocal, 1999;

Atiyeh et al., 2000, 2002; Dursun et al., 2002;

Turkmen et al., 2004), their effects on fruit yield and

quality especially in organically grown crops have not

received much attention. Therefore, in this study we

determine the influence of soil and foliar HA

application on fruit yield and quality of organically

grown pepper under greenhouse conditions.

Correspondence: Y. Karakurt, Suleyman Demirel University, Bahce Bitkileri, Suleyman Demirel Universitesi, Ziraat Fakultesi, Isparta, 32260 Turkey.

E-mail: karakurty@hotmail.com

Acta Agriculturae Scandinavica Section B � Soil and Plant Science, 2009; 59: 233�237

(Received 10 January 2008; accepted 19 February 2008)

ISSN 0906-4710 print/ISSN 1651-1913 online # 2009 Taylor & Francis

DOI: 10.1080/09064710802022952

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Material and methods

The study was performed at the Research Station of

the Faculty of Agriculture, Suleyman Demirel Uni-

versity, Isparta, Turkey, in a greenhouse in 2006 and

was repeated in 2007. Organically grown pepper

[(Capsicum annum L.) cv. Demre sivrisi] seedlings

were purchased from a commercial seedling produc-

tion company (Bircan Tarim Co., Antalya, Turkey)

and were planted on 16 May 2006 and 17 May

2007, in rows 80 cm apart with an intra-row spacing

of 40 cm. Prior to planting, cattle manure (50 t/ha)

containing 1.4% N, 1.3% K2O, 2.2% P2O5, with an

electrical conductivity (EC) value of 4.2 dS/m, was

applied to the plots in both years. Since pepper

plants were grown organically, no chemical fertilizers

were used. The soil physical and chemical properties

of the experimental area are summarized in Table I.

Plants were exposed to 0, 10, 20, 30, and 40 ml/l

HA (providing 0, 300, 600, 900, and 1200 ml/ha

HA) solutions prepared with deionized water con-

taining 0.02% Tween 20 (Sigma Chemicals, St.

Louis, MO, USA) as surfactant. HA solution con-

tained 22% HA and was obtained from Cukurova

Tarim Lombrico, Adana, Turkey. Four weeks after

planting, HA application was performed via spraying

three times during the growth period at 15-day

intervals utilizing a hand-held sprayer. During eve-

ning hours, both lower and upper leaf surface were

sprayed until totally wetted in order to provide

maximum absorption (Hull et al., 1975). Moreover,

plant root area was drenched with similar concentra-

tions of HA solutions three times during the growth

period at 15-day intervals four weeks after planting.

Plants sprayed with 0.02% Tween 20 (prepared in

deionized water) and wetted with the same

amount of deionized water were used as the control

(0 ml/l HA). A randomized complete-block design

with three replications was used, and each replica-

tion consisted of 10 plants.

Standard cultural practices were applied homo-

geneously through all plots in both years. The

temperature during the experimental period varied

from 12.4 to 35.8 8C and from 13.8 to 39 8C for the

first and the second experiment, respectively.

Harvesting was performed 14 times at green-

ripening stage using the criteria described by Sevgi-

can (2002) from 18 July to 30 October in 2006, and

from 15 July to 27 October in 2007. Immediately

after harvest, fruit firmness, fruit diameter, fruit

length, mean fruit weight, and early and total yield

were determined. The early yield was obtained by

weighing out the yield harvested during the first

30 days of the harvest period (Alan & Padem, 1994).

Fruit firmness was determined for 20 fruits from

each sample using a Chattillon hand penetrometer

(Model DPP 1000, John Chatillon and Sons, New

York) with a 0.6 mm probe. Measurements were

performed at the center of each fruit. The maximum

force (N) required to reach the bioyield point was

recorded.

Five hundred grams of fruit tissue from each

treatment were frozen in liquid nitrogen and utilized

for the determination of total and reducing sugars

contents, chlorophyll a and b, and total chlorophyll

contents. Chlorophyll a, b, and total chlorophyll

contents were determined as described by Akcin

(1980).

Determination of total soluble and reducing sugars

Mesocarp tissue (5 g) from 20 fruits of each

treatment was homogenized in a Polytron homo-

genizer with 20 ml of 95% ethanol for 2 min. The

homogenates were incubated in a boiling water-bath

for 10 min. After cooling to room temperature the

extracts were centrifuged at 8000 g for 15 min and

the supernatants were passed through GF/C filter

paper (Whatman). The pellets were re-extracted

with 20 ml of 80% ethanol at 25 8C and the

supernatants were combined. Total soluble sugars

and reducing sugars were determined as described

by Dubois et al. (1956) and by Honda et al. (1980),

respectively. Glucose was utilized as a standard for

both assays.

Statistical analyses

Statistical analysis was performed using the GLM

procedure of SAS (SAS, 1985). Data from both years

were pooled and subjected to analysis of variance

(ANOVA) to compare the effects of HA treatments.

The means were separated using Student Newman

Keuls test at the 5% level of significance.

Results and discussion

Tables II and III show the effects of humic acid (HA)

treatments on the yield and quality characteristics of

Table I. Soil physical and chemical properties of the experimental area.

PH

CaCO3

(g/kg)

EC*106

(dS/m)

Sand

(%)

Clay

(%)

Silt

(%)

OM

(g/kg)

P

(mg/kg)

K

(cmol/kg)

Ca�Mg

(cmol/kg)

Fe2O3

(ppm)

N

(ppm)

8.0 254 355 16.5 40.2 43.3 21.4 13.30 1.65 34.24 3.30 15.3

234 Y. Karakurt et al.

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pepper fruit. Data represent the average of the data

obtained in 2006 and 2007. Pepper fruit yield was

significantly influenced by soil and foliar HA treat-

ments (Table II). The highest yield was determined

from 20 ml/l foliar treatment (73.8 t/ha). Parallel

results were obtained in tomato fruit by Atiyeh et al.

(2002) who determined that HA treatment at

different concentrations significantly improved to-

mato yield. Moreover, improvement in fruit growth,

yield, and fruit quality characteristics in response to

HA treatment were also reported by Adani et al.

(1998) and Padem and Ocal (1999). On the other

hand, Dogan and Demir (2004) reported that

tomato yield was not significantly influenced by the

addition of HA to aggregate culture, possibly sig-

nifying that HA is effective in an intricate medium

such as soil. Both soil and foliar HA applications did

not significantly affect fruit diameter or fruit length

(Table II) in contrast to the findings of Yildirim

(2007) who determined a significant increase in fruit

diameter and length in response to HA application in

tomato fruit. However, the author did not find any

significant difference between soil HA application

and control with regard to fruit diameter. The

variable responses obtained in both studies might

result from the use of different species, cultural

(organic vs. conventional farming), and climatic

conditions and HA source (Arancon et al., 2006).

Mean fruit weight demonstrated similar responses to

foliar and soil HA applications with the exception of

foliar 40 ml/l treatment, which did not significantly

affect mean fruit weight as compared with the

control. Increasing soil and foliar HA concentrations

resulted in a significant enhancement in mean fruit

weight as compared with the control, but as the

concentration of HA increased further, the mean

fruit weight decreased, especially in response to

foliar 40 ml/l HA. It has been pointed out that the

characteristic growth-response curves as a conse-

quence of exposing plants to humic substances

display progressively increased growth with increas-

ing concentrations of humic substances, but there is

usually a decrease in growth at higher concentrations

of the humic materials (Chen & Aviad, 1990;

Zandonadi et al., 2007). Early yield of pepper fruit

was also significantly influenced by HA treatment,

confirming the findings of Atiyeh et al. (2002) and

Dogan and Demir (2004). With the increase in HA

concentration early yield increased significantly as

compared with the control, but it decreased with

higher concentrations of foliar HA application;

Table II. Yield and fruit characteristics of pepper in response to foliar and soil fertilization with various concentrations of HA. Data

represent the means9standard deviations.

Humic acid

treatment (ml/l) Yield (t/ha) Early yield (t/ha) Mean fruit weight (g) Fruit firmness (N) Fruit diameter (mm) Fruit length (cm)

Control 57.097.0 c 16.793.1 c 13.890.5 c 10.690.9 ns 13.590.2 ns 16.090.8 ns

Foliar 10 72.597.9 ab 23.7 93.4 ab 15.790.5 a 11.191.1 ns 13.790.8 ns 17.290.6 ns

Foliar 20 73.895.5 a 25.094.3 a 15.690.2 a 10.390.5 ns 13.490.5 ns 17.490.9 ns

Foliar 30 69.695.2 ab 21.692.4 b 15.391.1 ab 10.090.7 ns 13.491.3 ns 17.790.8 ns

Foliar 40 57.297.0 c 18.692.1 bc 14.390.5 bc 1.090.3 ns 13.490.6 ns 17.290.5 ns

Soil 10 61.199.2 bc 24.795.1 a 15.690.4 a 9.890.8 ns 13.290.4 ns 16.890.6 ns

Soil 20 63.794.1 abc 23.192.6 ab 15.390.8 ab 9.690.3 ns 13.390.3 ns 17.191.2 ns

Soil 30 62.996.4 abc 23.692.8 ab 15.290.2 ab 10.090.7 ns 13.290.2 ns 16.591.0 ns

Soil 40 60.392.4 bc 20.490.8 b 15.290.4 ab 9.590.2 ns 12.690.5 ns 16.990.3 ns

Means within each column followed by the same letters are not significantly different at the 5% level of significance. ns: Nonsignificant.

Table III. Chemical composition of pepper fruit in response to foliar and soil fertilization with various concentrations of HA.

Humic acid

treatment (ml/l)

Total soluble sugars

(mg/g)

Reducing sugars

(mg/g)

Chlorophyll a

(mg/mg)

Chlorophyll b

(mg/mg)

Total chlorophyll

(mg/mg)

Control 208.7915.3 bcd 34.3392.1 cd 0.3890.06 ns 0.3690.09 cd 0.7490.16 c

Foliar 10 228.192.6 ab 40.9095.3 abc 0.5190.08 ns 0.4290.08 cd 0.9390.16 bc

Foliar 20 213.599.4 bc 39.4595.1 abcd 0.4990.08 ns 0.5690.04 b 1.0690.10 b

Foliar 30 212.6916.4 bc 33.6592.6 cd 0.4590.10 ns 0.4290.06 cd 0.8790.16 bc

Foliar 40 205.499.3 cd 36.4594.7 bcd 0.5490.11 ns 0.3390.10 d 0.8790.09 bc

Soil 10 217.2910.5 bc 45.1594.0 ab 0.4190.06 ns 0.3990.05 cd 0.8090.14 c

Soil 20 241.899.7 a 46.4196.5 a 0.5690.08 ns 0.7990.06 a 1.3590.10 a

Soil 30 211.1911.7 bcd 35.9795.8 bcd 0.5190.03 ns 0.4690.06 bcd 0.9790.07 bc

Soil 40 191.699.0 d 31.2496.3 d 0.4190.06 ns 0.5090.09 bc 0.9190.15 bc

Data represent means9standard deviations. Means within each column followed by the same letters are not significantly different at the 5%

level of significance. ns: Nonsignificant.

Humic acid affects pepper fruit yield and quality 235

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20 ml/l foliar HA application resulted in the highest

early yield (25 t/ha).

Soil or foliar HA application did not change fruit

firmness significantly (Table II). Firmness levels

remained at relatively comparable levels in all treat-

ments. Both total soluble and reducing sugar con-

tents were significantly influenced by HA treatments

(Table III). The highest total soluble and reducing

sugars were obtained from soil 20 ml/l application;

30 or 40 ml/l soil and foliar HA applications

significantly reduced both reducing and total sugars

as compared with the other HA treatments. Fruit

chlorophyll content is considered as an important

quality characteristic of pepper fruit, being mainly

responsible for the green color of the fruit and thus

affecting their appearance and consumer acceptance

(Lightbourn et al., 2006). Total chlorophyll content

significantly increased in response to both foliar and

soil HA treatments, suggesting a significant improve-

ment in the green color of pepper fruit. The highest

total chlorophyll content was achieved with soil

20 ml/l HA followed by foliar 20 ml/l. The change

in total chlorophyll content in response to HA was

mainly due to the change in chlorophyll b content

since there was no significant effect of HA on

chlorophyll a content. Chlorophyll b content showed

a significant increase in response to both foliar and

soil HA treatments increasing to more than two-fold

with soil 20 ml/l HA and to 1.55-fold with 20 ml/l

foliar HA.

Conclusively, the results show that both foliar and

soil HA application result in higher yields and

quality in organically grown peppers. Hormone-like

activity of Has, which is reported to be concentration

specific (Mackowiak et al. 2001; Serenella et al.,

2002; Zandonadi et al., 2007), and enhanced

absorption of mineral nutrients due to increases in

cell permeability (Valdrighi et al., 1996; Dursun et

al., 2002; Zandonadi et al., 2007) could be respon-

sible for the stimulatory effect of HA on fruit yield

and quality of organically grown pepper. It is also

hypothesized that plant growth hormones may be

adsorbed onto humic fractions and thus influence

plant growth and development in a combined

hormonal/humic effect (Atiyeh et al., 2002). This

hypothesis was supported by Canellas et al. (2000)

who demonstrated that there were exchangeable

auxin groups in the macrostructure of humic acids.

Other mechanisms suggested to be responsible for

this stimulatory effect of humic substances at low

concentrations propose an ‘‘indirect action’’ on the

metabolism of soil microbial population, and soil

physical conditions (Albuzio et al., 1989; Casenave

de Sanfilippo et al., 1990; Chen & Aviad, 1990;

Muscolo et al., 1996, 1999; Zandonadi et al., 2007).

Thus, it is concluded that both foliar and soil HA

application may be recommended for obtaining

better quality and yield in organic production of

pepper.

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activation. Planta, 225, 1583�1595.

Humic acid affects pepper fruit yield and quality 237

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