Response of Clementine Citrus var. Cadoux to Foliar Potassium Fertilization; Effects on Fruit Production and Quality

Foliar K increases fruit yield of clementine var. Cadoux. Photo by authors.

Abstract

Clementine var. Cadoux is a citrus fruit that is well appreciated by consumers. However, this cultivar tends to produce fruits of small to medium size that are less accepted commercially. In this field study, we evaluated the effects of various rates and frequencies of foliar potassium (K) fertilization, as either potassium nitrate (KNO₃) or potassium sulfate (K₂SO₄), on fruit production (fruit size, weight and yield) and quality parameters (skin thickness, firmness, color index, maturity index, juice content, acidity and total soluble sugars). Application rates of tested foliar fertilizers were 5 percent and 8 percent KNO₃, and 2.5 percent and 4 percent for K₂SO₄, applied either two or three times during fruit growth on orchards of three planting densities (D₁: 6 x 6 m, D₂: 5 x 6 m and D₃: 6 x 3.5 m tree spacing). The levels of K in leaves of clementine var. Cadoux increased by up to 40 percent two weeks after the last foliar K application. Fruit weight increased with K application rate and frequency. K fertilization treatments in three foliar applications showed the best percentages of fruits of extra size class, whatever the source of K (KNO₃ or K₂SO₄) or plant density. Foliar applications of K increased fruit color, firmness and rind thickness. Fruit juice content, acidity and total soluble sugars were slightly increased by foliar K application. Raising K concentration and the number of foliar applications increased tree fruit production. In terms of efficiency of foliar K fertilizers, 4 percent K₂SO₄ in three applications resulted in a maximum gain of fruits per kg of foliar fertilizer.

Introduction

The citrus industry plays an important role in Moroccan agriculture. Morocco is the world’s fourth largest fresh citrus exporter and the third for clementine (USDA, 2010). Citrus exports amount to around 500,000 tons per year, generating annual revenue of 3 billion dirhams. An area of 85,000 ha is devoted to citrus cultivation with an annual production of 1,300,000 tons of fruit. In social terms, the citrus sector provides more than 21 million workdays and is the main source of income for 13,000 farming families.

The national average yield of citrus is 17 tons per hectare, which is considered low compared with other major citrus producing countries. The identified constraints to citrus productivity are ageing orchards, scarcity of water, management inefficiencies and losses due to pests and diseases.

Given the global market requirement for quality citrus fresh fruit in the context of globalization and the emergence of new citrus producing countries, Morocco is expected to review its citrus sector strategy to improve its competitiveness. The Morocco Green Plan agricultural strategy considers the citrus industry as a pilot sector and provides a plan for renewal of old orchards and greater utilization of drip irrigation.

Increasing citrus exports is very often restricted because fruit quality is not high enough to meet the standards imposed by foreign markets. Citrus packing stations, for example, record high reject rates of above 25 percent. Several factors are responsible for this, such as fruit damage from pests and diseases, inadequate mineral nutrition, etc. For clementines and mandarins, it is mainly small fruit size which is considered the principal reason for rejection. Several techniques are used to combat this, such as application of growth hormones, tree pruning etc. The final fruit size in a given year depends on several factors, including tree fruit load, precipitation, fertilization program, tree pruning and combination of variety/rootstock. Of all these factors, fertilization practices are probably the easiest to control. Potassium (K) fertilization, in particular, is known to greatly affect fruit quality.

Citrus fruit quality for the fresh market, in the case of Moroccan exports, is judged by fruit size and skin texture. K deficiency leads to the production of small fruits with thin skin, while an excess of K results in production of large fruit with thick skins and a coarse texture. Regarding the quality of the juice, excess K induces high acidity. K fertilization is thus an important tool to optimize the quality of citrus fruit and juice.

Until recently, fertilization practices were primarily directed towards obtaining high yields and it is only recently that interest has grown in the positive effect of fertilization on citrus fruit quality, particularly in relation to phosphorus (P) and K. Foliar fertilization with K is a practice that now offers a means of reaching these goals of high yield and improved fruit quality.

The research results presented in this article reveal the improvement of both production and fruit quality of clementine citrus var. Cadoux in response to foliar K fertilization by application of different rates and frequencies of potassium nitrate (KNO₃) or potassium sulfate (K₂SO₄).

Material and methods

Experimental site

The experiment was conducted in 2007 at Sidi Allal Tazi experiment station of INRA, located in the Gharb plain of Morocco. The soil type is clay and the citrus clementine variety used was Cadoux, grafted on citrange Carrizo. Healthy and homogeneous trees, aged 23 years, showing no signs of mineral deficiency were selected for this experiment. The experimental design was a split plot design with three blocks, three increasing planting densities (D₁: 6 x 6 m, D₂: 5 x 6 m and D₃: 6 x 3.5 m tree spacing) (large plots) and nine foliar K fertilization treatments (small plots).

Soil analysis indicated that the different horizons had sufficient and acceptable contents of organic matter, P and K for production of clementine (Table 1).

Experimental treatments


Table 1. Soil pH, organic matter, P and K content of the experimental site.
Soil horizon	pH (water)	Organic Matter	P₂O₅	K₂O
cm		%	ppm
0-30	7.7	2.1	41.8	497
30-60	7.9	1.6	11.9	319
60-90	8.2	1.4	4.9	229

Experimental treatments consisted of different K fertilization rates (concentration) and a number of foliar applications (two or three times) for a given planting density (D₁, D₂ or D₃). Two sources of K (soluble mineral fertilizers) were tested: KNO₃ and K₂SO₄. Application rates of tested foliar fertilizers were 5 percent and 8 percent KNO₃, 2.5 percent and 4 percent for K₂SO₄. The control was sprayed with water alone. Concentrations of KNO₃ applications correspond to those recommended in the literature (Obreza et al., 2008; Abd-Allah, 2006; Erner et al., 2004; Erner and Ya’acov, 2004). The applied rates for K₂SO₄ correspond to a K content equivalent to that contained in KNO₃. The dates of foliar applications were as follows: July 16, August 3 and August 21, 2007. At a given application date, each tree was sprayed with ten liters of the foliar K fertilizer.

Measurements

The measured production parameters in this study were fruit size, weight and yield. Monitored fruit quality parameters were skin thickness, firmness, color index, maturity index, juice content, acidity and total soluble sugars. Skin thickness was measured on ten fruit skins using a caliper. Fruit firmness was measured with a penetrometer (Koehler, France). Color measurements were determined using a colorimeter (Minolta CR-400; Konica Minolta Sensing, Inc) from which maturity index was derived. Juice content was extracted from ten fruits, acidity was determined by Sodium hydroxide (NaOH) titration and total soluble sugars were measured using a digital refractometer (PAL-1, Atago Co., Japan).

Results and discussion


Table 2. P, K, Ca and Mg levels (percent dry matter) in leaves of clementine var. Cadoux before and two weeks after the last foliar K application.
Treatments	P		K		Ca		Mg
Treatments	Before	After	Before	After	Before	After	Before	After
	% of DM
Control	0.16	0.14	1.88	1.60	4.04	4.76	0.28	0.23
5%-KNO₃ x 2	0.15	0.16	1.84	2.32	4.12	4.16	0.25	0.26
5%-KNO₃ x 3	0.18	0.15	1.88	2.48	4.00	3.60	0.22	0.23
8%-KNO₃ x 2	0.13	0.16	1.76	2.20	5.32	4.24	0.27	0.26
8%-KNO₃ x 3	0.12	0.16	1.72	2.08	4.44	3.96	0.26	0.26
2.5%-K₂SO₄ x 2	0.18	0.17	1.88	2.64	3.56	3.92	0.25	0.25
2.5%-K₂SO₄ x 3	0.19	0.15	2.04	2.28	4.24	4.32	0.38	0.27
4%-K₂SO₄ x 2	0.13	0.17	1.80	2.48	4.28	3.24	0.25	0.23
4%-K₂SO₄ x 3	0.16	0.15	1.76	2.24	3.68	3.88	0.23	0.23

Effects of foliar K fertilization on mineral elements content in leaves

The levels of K in leaves of clementine var. Cadoux increased two weeks after the last foliar K application. This increase was observed in all treatments except in the control where a slight decrease in leaf K content was noticed (Table 2).

The increase of K concentration in the leaves of fertilized trees was independent of the source of K (KNO₃, K₂SO₄) and the number of foliar applications. The increase in K after foliar K applications reached 40 percent, while in the control that content dropped 15 percent from its level before foliar fertilizer applications.

This marked increase in K content of citrus leaves indicates the effectiveness and speed of absorption of K when applied as a foliar fertilizer to leaves (Opazo and Razeto, 2001) as compared to conventional soil application.

Levels of P and magnesium (Mg) were not affected by foliar K applications. In all treatments, Mg concentrations were below optimum before and after foliar applications of K, while calcium (Ca) content was at optimal levels.

Effects of foliar K fertilization on citrus fruit parameters

Fig. 1. Effects of foliar K fertilization on fruit weight of clementine var. Cadoux: (A) Effect of K source, (B) effect of application rate, and (C) effect of number of applications.

Fruit weight

The highest average weight, about 96 g per fruit, was recorded in the medium planting density (D₂: 6 x 5 m) while the lowest average weight, about 91 g, was obtained with the highest density (D₃: 6 x 3.5 m). Regardless of planting density, the control treatment had the lowest values of fruit weight (Fig. 1).

At the low density D₁, treatment with 8 percent KNO₃ in two or three foliar applications, proved most effective in improving average fruit weight. These treatments had respective weights of 98.2 and 98.6 g (Fig. 1). Two foliar applications recorded the lowest average fruit weight compared to other treatments.


Table 3. Distribution of fruit number (%) of clementine by size class in response to foliar K fertilization for the low (D₁), medium (D₂) and high (D₃) planting density.
Treatments	Cal 1-3 57 to 63 mm			Cal 4 51 to 56 mm			Cal 5 44 to 50 mm			Cal 6 41 to 43 mm			Out of range <40 mm
	D₁	D₂	D₃	D₁	D₂	D₃	D₁	D₂	D₃	D₁	D₂	D₃	D₁	D₂	D₃
	%
Control	7.8	9.2	7.6	68.9	52.8	30.5	23.3	11.5	24.2	0	15.3	24.6	0	11.2	13.2
5%-KNO₃ × 2	22.2	17.3	14.4	67.8	72.2	63.3	10.0	7.1	4.3	0	3.3	3.2	0	0	14.7
5%-KNO₃ × 3	63.3	60.8	54.2	36.7	37.4	30.2	0	1.3	3.2	0	0.5	11.5	0	0	1.0
8%-KNO₃ × 2	21.1	16.2	12.6	64.4	66.2	64.2	14.4	15.2	13.4	0	2.3	3.3	0	0	6.6
8%-KNO₃ × 3	73.3	62.9	57.3	26.7	28.4	32.6	0	7.8	7.2	0	0.8	2.8	0	0	0
2.5%-K₂SO₄ × 2	20.0	22.4	16.2	71.1	58.7	61.4	8.9	9.2	6.5	0	7.2	8.2	0	2.5	7.7
2.5%-K₂SO₄ × 3	62.2	55.3	48.7	37.8	31.5	36.6	0	5.2	7.2	0	4.3	5.3	0	3.7	2.2
4%-K₂SO₄ × 2	24.4	19.3	14.6	67.8	65.2	51.4	7.8	8.2	5.2	0	5.2	16.2	0	2.1	12.6
4%-K₂SO₄ × 3	63.4	56.2	47.5	36.7	38.7	42.2	0	3.4	5.2	0	1.7	3.2	0	0	1.9

In terms of efficacy, treatments with high concentrations of K (8 percentKNO₃ or 4 percent K₂SO₄) in three foliar applications are to be recommended to increase weight and size of clementine fruit.

Fruit size

K fertilization treatments in three foliar applications showed the best percentages of extra size class (Cal 1-3), whatever the source of K (KNO₃ or K₂SO₄) or plant density (Table 3). The treatment 8 percent KNO₃ x 3 gave the highest percentage of fruit in the extra size class in all planting densities. For the three planting densities (D₁, D₂ and D₃), the 8 percent KNO₃ x 3 treatments, respectively, produced 73, 63 and 57 percent of fruits in the larger Cal 1-3 class, compared to the 7.8, 9.2 and 7.6 percent recorded in the control.

Fig. 2. Effects of foliar K fertilization on fruit size of clementine var. Cadoux : (A) Effect of K source, (B) effect of K concentration, and (C) effect of number of applications.

Trees sprayed with only two foliar applications were markedly less effective in improving clementine fruit size when compared with those that received three applications (Fig. 2).

These results show the effectiveness of foliar K fertilization in improving fruit size of clementine (Photo 1).

It should be noted that an increase in planting density also induces a decrease in fruit size. It is therefore wise to choose a low planting density (D₁ = 6 x 6 m) to reach a level of profitable, commercial grade fruit (Cal 1-3 and 4).

Our results confirm those in the literature (Embleton et al., 1975, Wei et al. 2002; Obreza et al., 2008) which show that increasing the rate and frequency of foliar applications of K is accompanied by an increase in citrus fruit size.

The coloration of the fruit rind

In general, whatever the planting density, all treatments of foliar K fertilization induced a redder color of the fruit rind than the control (Photo 1 and Photo 2).

The Color Index (CI), measured with a colorimeter (Model CR-400, Konica Minolta Sensing, Japan) and whose value reflects the intensity of the orange-red, was significantly reduced as plant density increases. The recorded values are respectively 5.52, 4.80 and 3.99 for D₁, D₂ and D₃.

All foliar K fertilization treatments have a greater average value of CI than that of the control, indicating that foliar applications of K had a positive impact on improving fruit color of clementine var. Cadoux. Generally, foliar applications of KNO₃ were more effective in improving fruit color than K₂SO₄ (Photo 1).

Photo 1. Effect of foliar K fertilization on fruit size and color of citrus clementine var. Cadoux
(densitiy D₁, 6 x 3.5 m).

In the low density D₁, treatments 5 percent KNO₃ x 3 and 2.5 percent K₂SO₄ x 2 had the greatest CI values, with 6.3 and 6.4 respectively.

For the medium density D₂, treatment 8 percent KNO₃ x 2 was superior in terms of fruit color improvement when compared to other treatments. Its color index (CI = 5.5) was significantly higher than that of treatment 2.5 percent K₂SO₄ x 3 (CI = 4.2).

For the high density D₃, treatment 5 percent KNO₃ x 3 induced the highest value of color index (CI = 4.6) whereas treatments 8 percent KNO₃ x 2 and 2.5 percent K₂SO₄ x 3 had the lowest values.

A positive effect of foliar K fertilization on citrus fruit color is reported in the literature (Hellali, 2002; Erner et al., 2004; Obreza et al., 2008). It is also reported that high concentrations of K applied to citrus induced green fruits compared to the application of low concentrations (Koo, 1988). This latter effect was observed in the present experiment with the 4 percent K₂SO₄ treatment in two applications which gave the lowest values of color index and caused a delay in fruit coloration (Photo 1).

It can be concluded that foliar fertilization of KNO₃, at 5 and 8 percent in three foliar applications, and those based on K₂SO₄ at 2.5 and 4 percent respectively, in two and three foliar applications, are most effective in improving fruit color of clementine var. Cadoux.

Fruit firmness

Photo 2. Improvement of fruit color by foliar K fertilization and persistence of green color in the control (left). Photo by the authors.

Whatever the planting density, the control treatment had higher values of fruit firmness, with 405, 366 and 402 g/0.5 cm² for densities D₁, D₂ and D₃ respectively. In addition, all treatments of foliar K fertilization had acceptable fruit firmness, which varied between 300 and 415 g/0.5 cm².

Thickness of the fruit rind

The average fruit rind thickness of all treatments is 11.5 mm. Foliar K fertilization slightly increased rind thickness as K concentration, or the number of foliar applications, increased.

Our results are consistent with those reported in the literature, which indicate that increasing K, applied as foliar fertilization, induces an increase in the citrus fruit rind thickness (California Fertilizer Association, 1998; Obreza et al., 2008).

This increase in thickness of the skin of the fruit is of importance as it provides resistance to insects.

Fruit juice content

In all treatments, the fruit juice content met the standards for export of the fruits of Moroccan clementine (a minimum of 40 percent).

In the low density D₁, the increase in the number of foliar applications, for a given concentration of either KNO₃ or K₂SO₄, is accompanied by an increase in fruit juice content. This increase was 0.5 percent between treatments 5 percent KNO₃ x 3 and 5 percent KNO₃ x 2; and 1.8 percent between 8 percent KNO₃ x 3 and 8 percent KNO₃ x 2. A slight decrease in fruit juice content of 0.92 percent is recorded between treatments 4 percent K₂SO₄ x 3 and 4 percent K₂SO₄ x 2.

For the medium density D₂, the juice content decreased with the increasing number of foliar applications for low K concentrations but increased with the number of foliar applications for high concentrations of K.

For the density D₃, the juice content is highest in the 4 percent K₂SO₄ x 2 treatments with a value of 45 percent.

Acidity (A) of the juice

The acidity of the juice of clementine var. Cadoux for all foliar K fertilization treatments meets the export standards of between 0.8 and 1.5 percent. In all treatments, acidity was less than 1.1 percent. Increasing the number of foliar K applications, however, causes a slight increase in acidity in the fruit juice.

These results are consistent with those of the literature which indicates that increased levels of K in leaves, due to an increase in concentration and/or number of foliar applications, induces an increase in acidity of citrus fruit (Erner et al., 2004; Obreza et al., 2008).

Fruit total soluble sugar content (TSS)

The export market for Moroccan clementine citrus requires a minimum content of total soluble sugars (TSS) of nine percent. In all foliar K fertilization treatments, and in the control, TSS contents were much higher than stated standards for exports of clementine.

Foliar K fertilization slightly improved total soluble sugar content of the fruit with better efficiency for KNO₃ in comparison to K₂SO₄ (Fig. 3).

Fig. 3. Effects of foliar K fertilization on fruit TSS of clementine var. Cadoux: Effect of K source, concentration and number of applications.

Index of fruit maturity (TSS/A)

According to export standards, a minimum maturity index of 7 is required for Moroccan clementine although some importing countries require a value between 7 and 7.5.

In all foliar K fertilization treatments, the fruit maturity index exceeded 10.6. This index was lower in treatments which received KNO₃, and higher in treatments that received K₂SO₄ as a source of K for foliar application.

Fig. 4. Effects of foliar K fertilization on fruit yield (kg tree^-1) of clementine var. Cadoux: Effect of (A) K source, (B) K concentration, and (C) number of applications.

Effect of foliar K fertilization on fruit yield

The fruit production of the different planting densities were respectively 174, 160 and 116 kg per tree for the low (D₁), medium (D₂) and high density (D₃). In terms of fruit production per hectare, yields of densities D₁, D₂ and D₃ were 48.1, 53.7 and 55.3 mt ha^-1 respectively.

Concerning the effect of foliar K fertilization, our results clearly indicate that raising the K concentration and the number of foliar applications increased tree fruit yield (Fig. 4B and Fig. 4C; Photo 3).

For treatments that received two foliar applications, raising K concentration did not induce any significant increase in production.

These results are consistent with the literature that indicates that increasing frequency or concentration of K foliar application on citrus, three to five weeks after the fruit size reached a diameter of 20 to 25 mm, leads to increased production (Erner et al., 2004; Obreza et al., 2008).

Within the planting densities D₁ and D₂, 8 percent KNO₃ and 4 percent K₂SO₄ treatments with three foliar applications were the most effective in improving fruit yield of clementine var. Cadoux. The recorded yields for these treatments in D₁ were 204 and 195 kg/tree respectively, while the control produced only 160 kg/tree. This increase in production was 21.5 percent and 17.6 percent respectively for treatments 8 percent KNO₃ x 3 and 4 percent K₂SO₄ x 3 over the control.

For the medium density D₂, the treatment 8 percent KNO₃ x 3 was the most effective as it increased fruit yield by 21.2 percent over the control (184 kg/tree versus 145 kg/tree). The 5% KNO₃ x 3 and 4 percent K₂SO₄ x 3 treatments have generated a marked production increase of 16.5 percent (173 and 175 kg/tree respectively) when compared to the control.

For the high density D₃, treatment 5 percent KNO₃, 8 percent KNO₃ and 4 percent K₂SO₄ in three foliar applications appear to be the most effective in improving fruit production of clementine var. Cadoux. There was no significant difference between these two treatments and the recorded average production was 130 kg/tree, an improvement of 18 percent when compared to the control.

Photo 3. Control - Trees representative of the effects of foliar K fertilization on fruit yield of clementine var. Cadoux (density D₁, 6 x 3.5 m).
Photo by the authors.

Photo 3. 5%-KNO₃ × 3 - Trees representative of the effects of foliar K fertilization on fruit yield of clementine var. Cadoux (density D₁, 6 x 3.5 m).
Photo by the authors.

Photo 3. 4%-K₂SO₄ × 3 - Trees representative of the effects of foliar K fertilization on fruit yield of clementine var. Cadoux (density D₁, 6 x 3.5 m).
Photos by the authors.

Production gains and efficiency of KNO₃ and K₂SO₄ foliar application on clementine var. Cadoux

Treatments 5 percent KNO₃, 8 percent KNO₃, 2.5 percent K₂SO₄ and 4 percent K₂SO₄, with three foliar applications, were selected for an evaluation of their efficiencies because of their positive effects on fruit production and quality parameters of clementine var. Cadoux.

Treatment 8 percent KNO₃ with three foliar applications had an average production gain of 12-13 mt ha^-1 over the control in the three planting densities (Table 4), while the treatment 2.5 percent K₂SO₄, with three foliar applications, gave the lowest fruit production gains.

In terms of efficiency of foliar K fertilizers, the treatment 4 percent K₂SO₄ in three applications resulted in a maximum gain of about 28 and 24 kg of fruits per kg of foliar fertilizer, respectively for densities D₁ and D₂ (Table 4).

Conclusion


Table 4. Production gain and marginal productivity of citrus clementine var. Cadoux with three foliar K applications of KNO₃ and K₂SO₄.
K source and concentration	Planting density	Yield	Applied fertilizer	Production gain	Efficiency
		mt ha^-1	kg ha^-1	mt ha^-1	kg fruit per kg fertilizer
Control	D₁	44.4	-	-	-
	D₂	48.4	-	-	-
	D₃	50.5	-	-	-
5%-KNO₃ × 3	D₁	50.5	415.5	6.1	14.7
	D₂	57.7	499.5	9.4	18.7
	D₃	59.4	714.0	8.9	12.4
8%-KNO₃ × 3	D₁	56.5	664.8	12.1	18.2
	D₂	61.4	799.2	13.1	16.4
	D₃	63.6	1,142.4	13.0	11.4
2.5%-K₂SO₄ × 3	D₁	47.0	207.7	2.7	12.7
	D₂	53.2	249.7	4.8	19.3
	D₃	54.6	357.0	4.1	11.4
4%-K₂SO₄ × 3	D₁	53.9	332.4	9.5	28.6
	D₂	58.2	399.6	9.9	24.7
	D₃	60.9	571.2	104	18.2

Foliar application of K, either as KNO₃ or K₂SO₄, increased fruit production and quality parameters (weight, size, color, rind thickness, juice content and maturity index) of clementine citrus var. Cadoux.

On the basis of yield gain, fertilizer efficiency and profitability, the combination of D₁ spacing (6 x 6 m) sprayed with 4 percent K₂SO₄ three times, gave the best return and is recommended for foliar fertilization of clementine var. Cadoux.

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