Research Article

Effect of Planting Density and Rhizospheric Administration of Humic Acid on Growth and Yield Parameters of Potato (Solanum tuberosum L.)

Ali Amer Dahham1 , Aws Jawad Kadhim2 , Ali Ibadi Manea2,and Sarmad Salim Ali2

1Department of Horticulture, College of Agriculture, University of Al-Qadisiyah, Diwanyah - Iraq; 2Department of Horticulture, College of Agriculture, University of Karbala, Iraq.

Received | April 27, 2025; Accepted | May 20, 2025; Published | July 24, 2025

*Correspondence | Ali Amer Dahham, College of Agriculture, University of Al-Qadisiyah, Diwanyah - Iraq; Email: [email protected]

Citation | Dahham, A.A., A.J. Kadhim, A.I. Manea and S.S. Ali. 2025. Effect of Planting density and rhizospheric administration of humic acid on growth and yield parameters of potato (Solanum tuberosum L.). Sarhad Journal of Agriculture, 41(3): 1206-1213.

DOI | https://dx.doi.org/10.17582/journal.sja/2025/41.3.1206.1213

Keywords | Potato production, Planting distance, Biostimulant, Humic acid, Plant growth traits, Potato yield parameters

Copyright: 2025 by the authors. Licensee ResearchersLinks Ltd, England, UK.

This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

Introduction

Potato, Solanum tuberosum L., belongs to the family Solanaceae and is one of the main perennial herbaceous vegetables serving as the staple food for billions of people worldwide, including Iraq (Al-Sharifi et al., 2020; Mishra et al., 2024). Potatoes constitute the daily food for more than 80% of the world population and are a key source of carbohydrates, proteins, and other nutrients. It is a rich source of vitamins and minerals (Navarre et al., 2009). Potato is an important economic crop worldwide, and ranks at fourth after wheat, rice, and maize. Its global production exceeds 300 million tons per annum, while its production and area under potato cultivation in Iraq is about 300 thousand tons and 18780 hectares, respectively (Abed and Ahmed, 2023; FAO, 2024).

The cultivation and productivity of any agricultural crop including potato are affected by the mechanical treatments such as planting method, planting depth, and planting distance. Planting distance have a significant impact on all crop growth traits. According to the literature, the potato yield was obtained with greater plant to plant distance as it provides ample space for root spread, vegetative growth and tubers development of potatoes (Chehaibi et al., 2013; Al-Sharifi et al., 2021; van Dijk et al., 2022). Plant to plant and row to row distances play a vital role in the production of tuber crops. Narrowing these distances to get higher plant density results into increased competition (for water, light, and nutrients) between the crop plants, leading to the poor growth and development and the accumulation of dry matter, which leads to poor crop production (Pandey et al., 2015). Planting distance has a significant impact on the economically important traits such as tuber size, tuber quality, and total yield. It significantly affects the growth, plant height, tuber size, and yield of potatoes (Nxumalo et al., 2020; Regasa et al., 2022; Abewoy, 2024).

Moreover, the productivity of crops and their vegetative and productive traits are greatly influenced by the physical characteristics of the field soils (Vaičys and Mažvila, 2009; Khalouf et al., 2019; Sainju et al., 2022). Improving the soil chemical and physical characteristics exert a positive impact on the growth traits of potato crop (Setiyo et al., 2016; Al-Sharifi et al., 2021; Wang et al., 2022). One of way to improve the soil physico-chemical properties is the amendments and supplementation of soil profiles with biofertilizers and bio-stimulants (Mahapatra et al., 2022; Chaudhari et al., 2023; El-Shaieny et al., 2023; Rajesaheb et al., 2025). A variety of biofertilizers and bio-stimulants have been shown to enhance potato growth traits, and tuber yield both in terms of quantity and quality (Jardin, 2015; Mohaseb et al., 2018; Gleń-Karolczyk et al., 2022; Karak et al., 2023; Zarzecka and Gugał, 2024).

Humic acid is one of the organic bio-stimulants used to stimulate plant growth and improve crop production (Baligah et al., 2022). Humic acid administration instead of chemical fertilizers has many advantages for agricultural crops such as it increases the nutrient supply, tolerance to abiotic stress, and high-quality yield (Qin and Leskovar, 2020; Liu et al., 2024; Shahrajabian and Sun, 2024). It improves the crop growth and increases yield and quality (Zhou et al., 2019; Omar et al., 2022). The use of humic acid via soil amendments improves the soil physical and chemical properties and the absorption of essential nutrients by plants including potatoes (Man-hong et al., 2020; Bayat et al., 2021). Humic acid may increase the starch content of tubers, dry matter accumulation, tuber production, and increase soil nutrients and nutrient uptake by plants, which increases the nutrient content of tubers, including tuber size (Suh et al., 2014; Al-Bayati et al., 2019; Şanlı et al., 2024).

In light of the aforementioned importance of planting distance and soil amendments by biostimulants, this study aimed to determine the effect of different planting (plant to plant) distance and the application of different concentrations of humic acid on some vegetative and growth traits and yield components of potato crop under field conditions.

Materials and Methods

This field experiment was performed to assess the effect of planting distance and humic acid on the growth and yield parameters of the potato crop. The experiment was carried out under the Tomato and Potato Development Project in one of the fields of the Agricultural Department located 25 km north of Hillah City, Babylon, Iraq, during the autumn season of 2023-2024. The experiment was conducted following the Complete Randomized Block Design (RCBD), and each treatment was replicated thrice. Treatments included two planting distances i.e. A1= 25 cm, and A2= 35 cm, and three humic acid concentrations i.e. B1= 0 g L-1, B2= 5 g L-1, and B3= 10 g L-1.

Soil samples were taken randomly from the field before planting and were pooled to make a composite soil sample to analyze the soil characteristics (Table 1). The chemical and physical characteristics of soil under trial were analyzed in the Muradiyah Research Station of the Babylon Agriculture Directorate following protocols described previously by Chapman (1965), and Page et al. (1982).

The field soil was plowed perpendicularly and was divided into three blocks (replicates). Each replicate included 6 experimental units, and each experimental unit included 3 rows with a length of 3.5 m and a width of 0.75 m. The potato variety Borin was sown in this experiment. Tubers were planted at a distance of 25 cm with 42 tubers, and at a distance of 35 cm with 30 tubers in one experimental unit. Humic acid concentrations were made by dissolving pure humic acid in water and applying it under the tubers, @ 100 ml per tuber. The application was repeated three times during the first growing season at 10 days after the tubers emerged, the second at the tuber formation stage, and the third at the tuber size enlargement stage. The processes of weeding, irrigation, and control were made when needed.

 

Table 1: Physico-chemical properties of the soil under trial determined at the time of seedbed preparation.

Soil characteristics	Values	Units
pH	7.9	-
Electrical conductivity (EC)	3.5	m3/ds
Soil organic matter	11.3	g kg-1
Available Nitrogen	13.4	mg kg-1
Available phosphorus	5.6	mg kg-1
Available potassium	192.0	mg kg-1
Bulk density	1.13	g cm3
Sand	155	g kg-1
Silt	600	g kg-1
Clay	245	g kg-1
Soil texture	Silt loam

 

After the tubers were fully matured, they were harvested, and the following traits were measured, i.e., number of leaves, number of main branches, number of buds per plant, plant height, chlorophyll content, total and marketable yield per plant, and total and marketable yield per hectare. Data was analyzed by the R program (version 4.1.2, R, Vienna, Austria) using analysis of variance (ANOVA), and the Least Significant Difference (LSD) post-hoc was used to compare the treatment means at a standard probability level (α = 0.05).

Results and Discussion

Effect of planting distance and humic acid application on potato plant characteristics

Plant height (cm): There was no significant effect of the planting distances on the average plant height (Table 2). However, the application of humic acid significantly impacted the plant height, as the treatment B3 (10 g L-1) resulted in the highest average value (49.7 cm) of plant height as compared to the control treatment, which resulted in the lowest value (43.2 cm). Similarly, a significant effect on the average height of potato plants was shown by the binary interaction between the planting distances and the application of humic acid (Table 2). The interaction treatment A1×B3 resulted in the highest average value (52.5 cm) compared to A1×B1, which resulted in the lowest value (Table 2).

 

Table 2: Impact of planting distance and humic acid application and their interactions on the vegetative growth traits of potato (Solanum tuberosum L.) crop.

Treatments	Plant height (cm)	Number of branches (n)	Number of leaves per plant	Chlorophyll content in leaves (SPAD)
A1 (25 cm)	45.1	2.04	26.6	48.1
A2 (35 cm)	45.0	2.22	33.4	44.8
LSD (at α = 0.05)	N.S.	N.S.	6.30	N.S.
B1 (0 g L-1)	43.2	1.80	28.1	44.2
B2 (5 g L-1)	42.3	2.53	32.1	47.4
B3 (10 g L-1)	49.7	2.07	29.8	47.7
LSD (at α = 0.05)	5.96	0.64	N.S.	N.S.
A × B Interaction
A1×B1	40.6	1.63	25.5	39.3
A1×B2	42.2	2.67	27.3	45.5
A1×B3	52.5	1.83	26.9	49.7
A2×B1	45.7	1.97	30.6	49.1
A2×B2	42.5	2.40	37.0	49.4
A2×B3	46.9	2.30	32.7	45.7
LSD (at α = 0.05)	8.56	0.90	10.91	9.36

 

A = plant to plant distance, B = humic acid concentration, NS = non-significant (P ≥ 0.05).

 

Number of main branches per plant: There was no significant effect of the planting distance on the number of main branches (Table 2). The application of humic acid B2 (5 g L-1) resulted in a significantly higher branching number per plant (2.53 branches plant-1), while the control resulted in the lowest branching value of 1.80 branches plant-1. The two-way interaction between the planting distances and humic acid applications increased the branching number, as treatment A1×B2 produced the highest and significant value of 2.67 branches plant-1 as compared to the control A1×B1, which had the lowest value for 1.63 branches plant-1 (Table 2).

Number of leaves per plant: In this case, the planting distance of 35 cm resulted in a significantly higher value for the number of leaves per plant (33.4 leaves plant-1). No significant difference was observed between the treatments of humic acid application on the number of leaves per plant. The two-way interaction between planting distances and the application of humic acid A2×B2 resulted in the highest value (37.0 leaf plant-1), as compared to the lowest value exhibited by the control treatment.

Chlorophyll content in leaves (SPAD): Data regarding the soil plant analysis development (SPAD) value as a non-destructive measure of leaf chlorophyll content showed that there was no significant impact of both factors, i.e., planting distances and the application of humic acid, on the chlorophyll content of potato leaves (Table 2). However, the interaction between them significantly affected the chlorophyll content of leaves, as A1×B3 resulted in the highest value (49.7 SPAD), as compared to the control, for which the lowest value was recorded for this trait.

Effect of planting distance and humic acid application on potato yield parameters

Number of tubers per plant: There was a significant effect of the planting distances on the average number of tubers per plant (Table 3). Treatment A2 (35 cm) showed a significant increase in the number of tubers (5.31 tuber plant-1), and the application of humic acid also exhibited significant differences, as the humic acid treatment B3 (10 g L-1) showed the highest significant value (5.27 tuber plant-1), as compared to the lowest value (4.19) of the number of tubers showed by the control treatment. Treatment A2×B1 resulted in the highest significant value of 5.80 tuber plant-1, which differed significantly from most treatments.

Total potato yield (g plant-1 and ton ha-1): There was a significant impact of the planting distance factor on the total yield of potato, either per plant or per hectare (Table 3). The planting distance of 35 cm (treatment A2) resulted in the highest yield rate of 416 g plant-1 and 16.64 tons ha-1. However, no significant difference was seen regarding the humic acid application treatments on the total potato yield values. The interaction treatment A2×B1 resulted in the highest total plant yield (433 g plant-1) and yield per hectare (17.31 ton ha-1), and it was statistically at par with the rest of the treatments (Table 3).

Marketable potato yield (g plant-1 and ton ha-1): Regarding the marketable potato yield either on per plant basis or on a hectare basis, the treatment A2 (planting distance of 35 cm) resulted in the highest and significant value for the marketable potato yield of 363 g plant-1 and 14.51 tons ha-1 (Table 3). However, there were humic acid treatments did not show any statistical difference for their impact on marketable plant yield. The interactions of A2×B1 and A2×B3 resulted in the highest values of 368 and 361 g plant-1 and 14.73 and 13.85 tons ha-1 with significant differences from the rest of the treatments (Table 3).

 

Table 3: Impact of planting distance and humic acid application, and their interactions on the yield of potato (Solanum tuberosum L.) crop.

Treatments	Number of tubers (plant-1)	Total plant yield (g plant-1)	Marketable plant yield (g plant-1)	Total yield (ton ha-1)	Marketable yield (ton ha-1)
A1 (25 cm)	4.47	294	245	11.60	9.60
A2 (35 cm)	5.31	416	363	16.64	14.51
LSD (at α = 0.05)	0.78	71.5	71.3	3.021	2.820
B1 (0 g L-1)	4.19	353	296	14.11	11.82
B2 (5 g L-1)	4.97	346	289	13.93	11.57
B3 (10 g L-1)	5.27	366	328	14.31	12.77
LSD (at α = 0.05)	0.965	N.S.	N.S.	N.S.	N.S.
A × B Interaction
A1×B1	4.73	273	223	10.92	8.92
A1×B2	4.00	291	220	11.81	8.78
A1×B3	4.67	319	294	12.07	11.08
A2×B1	5.80	433	368	17.31	14.73
A2×B2	4.87	401	359	16.05	14.36
A2×B3	46.9	2.30	32.7	45.7	13.85
LSD (at α = 0.05)	8.56	0.901	10.91	9.36	8.27

 

A = plant to plant distance, B = humic acid concentration, NS = non-significant (P ≥ 0.05).

 

This in-situ study was done to assess how planting distance and supplementation of humic acid can enhance or impact the yield parameters and plant growth traits of the potato crop. Results of the study exhibited a significant effect of both factors on most of the plant traits and yield parameters. It was observed that the number of branches of the potato plant was less at the short distance (25 cm), and the number of leaves per plant increased significantly at the wider distance (35 cm). It would be due to the reason that shorter planting distances increased the density of plants, which enhanced the competition of plants for soil nutrients and sunlight, and thus the growth of fewer stems and branches. It means short planting distance of 25 cm enhanced the competition of plants for sunlight and thus, led to long potato plant stems and fewer branches compared to other potato plants maintained at 35 cm planting distance (Nxumalo et al., 2020; Abewoy, 2024). Competition for sunlight in potato and other plants resulted in taller and more branched plants than usual as a result of phototropism (Vinterhalter and Vinterhalter, 2015). Conversely, Mvumi et al. (2018) reported that plants planted at wider distances had lower heights and more branches than plants planted at narrow distances. This is due to the better or sufficient availability of nutrients, water, and sunlight, as the plants at wider distances have less competition and grow more buds and leaves (Craine and Dybzinski, 2013).

The increase in plant vegetative characteristics may also be attributed to the effect of direct administration of humic acid affecting various vital processes and its important role in improving cell division and elongation (Nardi et al., 2021). These processes include respiration, photosynthesis, protein synthesis, increased enzymatic reactions, and vital activities such as respiration and photosynthesis, thus stimulating and increasing the number of main branches (Canellas and Olivares, 2014). This is consistent with what was found by Al-Jumaili et al. (2010), Suh et al. (2014), and Lazzarini et al. (2022) that the humic acid application led to an increase in the main stem numbers per plant.

In addition, the planting distance of 35 cm also significantly affected all yield parameters of the potato plants, i.e., number of tubers per plant, per plant total and marketable yield, and total and marketable yield. The reason is that the better availability of sunlight in plants led to an increase in the photosynthesis process on one hand, and an increase in the root area on the other hand, which led to an increase in biological processes and cell division, and thus the accumulation of dry matter and starch (Haverkort, 2007; Schulz et al., 2019). This is consistent with what was narrated by Mvumi et al. (2018) and Derebe and Kola (2024). The interaction between planting distances and humic acid application also affected the crop. The interaction of 35 cm planting distance and 5 g L-1 humic acid application significantly increased the values for all crop traits. These results corroborate the findings of Alenazi et al. (2016) and Man-hong et al. (2020), which reported that the potato growth was improved by the treatment or application of humic acid, because it improved the photosynthesis system and improved the productivity of fresh tubers in insufficient irrigation conditions. Haider et al. (2015) and Andrejiová et al. (2023) found that humic acid treatment increased the yield of potato tubers compared to untreated plants. The results of Mahmoud and Hafez (2010) and Keskin and Akınerdem (2021) demonstrated that humic acid causes an increase in fresh weight, size, and quality of tubers by increasing the concentration of humic acid used. This is consistent with the results that indicated that humic acid treatment increases the activity of antioxidants to avoid oxidative stress caused by disease infection and thus a significant increase in the total and marketable potato yields as recorded in treatments A2×B1 and A2×B3 (Lazzarini et al., 2022; Şanlı et al., 2024).

Conclusions and Recommendations

Overall results of this field study demonstrated that potato plants planted at shorter planting distances would result in plants with fewer branches and leaves, while the wider planting distances exhibited better plant growth traits and yield components. Moreover, the rhizospheric administration of water-soluble humic acid either alone or in combination with wider planting distance increased the vegetative growth traits of the potato plants, and exerted a significant and positive effect on the quantitative yield components of the potato crop as well.

Novelty Statement

This field experiment was performed to assess the effect of planting distance and humic acid on the growth and yield parameters of the potato crop.

Author’s Contribution

Ali Amer Dahham, Aws Jawad Kadhim, Ali Ibadi Manea and Sarmad Salim Ali: Conceptualization, methodology, formal analysis, investigation, resources, data curation, project administration.

Aws Jawad Kadhim: Writing—original draft preparation.

Conflict of interest

The authors have declared no conflict of interest.

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