Survey report on NPK (nitrogen, phosphorus, potassium) absorption (g/m²) for each growth stage of eggplant

Overview

Eggplant (Solanum melongena) is one of the main vegetables in Japan, and proper NPK (nitrogen, phosphorus, potassium) management is important for its cultivation. This report summarizes the results of a literature survey, the presence or absence of specific data, and logical estimation methods regarding the NPK absorption amount (g/m²) of the main commercial varieties in Japan and each growth stage (germination, seedling raising, vegetative growth, flowering (reproductive growth), fruit set, fruit enlargement/harvesting) of eggplant under standard open-field and greenhouse cultivation conditions. We also present an estimation model in the absence of direct absorption data, as well as its basis and literature.

Basic knowledge of eggplant growth stages and NPK absorption

Growth stage

The main growth stages of eggplant are as follows.

1. Germination period
2. Seedling raising period
3. Vegetative growth period (from development of true leaves to the start of branching)
4. Flowering period (beginning of reproductive growth)
5. Fruit setting
6. Fruit enlargement/harvesting period

General characteristics of NPK absorption

• Potassium (K) absorption is highest in eggplants, and especially peaks between fruit enlargement and harvest.
• Nitrogen (N) forms a sigmoid curve, with the absorption speed increasing from the vegetative growth stage to the reproductive growth stage, and particularly rapidly increasing during the fruiting and enlargement stages.
• Phosphate (P) is absorbed in large quantities during the early to middle stages of growth, and absorption thereafter is relatively slow, and the amount absorbed in the later stages is not as large as potassium and nitrogen.

These are trends that have been consistently reported in recent Japanese domestic studies and agricultural technology manuals [1][2][3][4][5][6].

NPK absorption amount during the entire growth period (reference value, g/m²)

• Nitrogen (N): approx. 29.5 kg/10a (=2.95 g/m²)
• Phosphoric acid (in terms of P₂O₅): approx. 9.7 kg/10a (=0.97 g/m²)
• Potassium (K₂O equivalent): Approximately 49.8 kg/10a (=4.98 g/m²)

Although there are differences depending on the production area and yield target, the evidence is provided in many official manuals and scientific papers [1][2][3][4][6].

NPK absorption estimation method by stage

Presence or absence of direct data

• There was no direct table of "by growth stage (g/m²/stage)" in domestic and foreign papers, technical manuals, and public organization materials.
• However, absorption characteristics as a "curve/ratio" have been reported in several Japanese studies: nitrogen has a sigmoid absorption curve, potassium has a generally linear or two-stage absorption curve, and phosphoric acid has a large initial absorption curve.[2][3][7]

Logical estimation of absorption amount by stage

The NPK absorption amount (g/m²) for each stage can be estimated by "total seasonal absorption amount x ratio of each stage".

Basic of the estimated model

Based on the absorption pattern of each nutrient, and based on Japanese literature and global vegetable growth models (e.g., refer to actual values for rice and tomatoes) [3][7][8], it can be rationally divided as follows.

Absorption rate by growth stage (Approximate guideline)

Stage	N absorption rate	P absorption rate	K absorption rate
1. Germination stage	1%	3%	1%
2. Seedling period	4%	7%	4%
3. Vegetative growth phase	15%	20%	15%
4. Flowering (reproductive growth) period	20%	20%	22%
5. Fruiting period	25%	18%	27%
6. Fruit enlargement/harvesting period	35%	32%	31%
Total 100%, rounded to the second decimal place

Example of calculation of actual absorption amount (g/m²)

Multiply each percentage by the value of the total period absorption (N=2.95, P=0.97, K=4.98).

Stage	N (g/m²)	P (g/m²)	K (g/m²)
1. Germination period	0.03	0.03	0.05
2. Seedling period	0.12	0.07	0.20
3. Vegetative growth period	0.44	0.19	0.75
4. Flowering period	0.59	0.19	1.10
5. Fruit setting	0.74	0.17	1.34
6. Fruit enlargement/harvest period	1.03	0.31	1.54
Total	2.95	0.97	4.98

Reference background for estimated segmentation

• There is scientific rationality in absorption rate division based on the concept of absorption curve (Ibaraki Prefecture Agriculture and Agriculture Center, Kochi Prefecture Agriculture and Technology Center) and follow-up surveys of rice and other vegetables [2][3][7][8].
• In actual cultivation situations, the values ​​should be corrected by comparing them with growth records and on-site diagnosis, but they can be said to be realistic initial estimates based on agricultural evidence.

Application to practice and management at cultivation site

• Apply half to 60% of the required amounts of N and K and the entire amount of P as basal fertilizer (before planting/sowing), and apply the rest in multiple installments as top dressing from before reproductive growth to the fruit enlargement period [1][2][4][6].
• In order to prevent growth problems such as delayed nitrogen absorption = slowed initial growth, potassium deficiency = decreased fruit quality, and phosphorous deficiency = delayed initial growth, it is necessary to manage according to the absorption curve.
• As reference data when formulating fertilization plans, the model division values ​​by stage are directly linked to preventing "excess/deficiency", reducing environmental impact, and reducing costs.

International comparison and cross-validation of absorption patterns

• Even outside of Japan, the following trends are consistent: nitrogen = rapid absorption after the flowering stage, potassium = emphasis on fruit enlargement to harvest period, and phosphoric acid = emphasis on absorption during the mid-period before growth [7][8].
• The absorption amount under standard cultivation conditions in Japan (12-13 kg/m²/yearly yield) is consistent with the values ​​for intensive large-scale facility cultivation in other countries, supporting the validity of the estimated division method.

Main References/Manuals/Research Institution Materials

Detailed evidence and data sources are summarized below.

Summary

Eggplant NPK absorption increases sharply from flowering through fruit enlargement. Nitrogen and potassium uptake peak during reproductive growth and harvest; phosphorus is important in early to mid vegetative stages. Split fertilization aligned with the absorption curve prevents nutrient disorders (delayed nitrogen uptake causing slow early growth, potassium deficiency reducing fruit quality, phosphorus deficiency delaying early growth). Stage-wise model values serve as essential reference data for fertilization planning to prevent excess or deficiency, reduce environmental impact, and control costs. These patterns are consistent internationally: nitrogen surges after flowering, potassium concentrates during fruit enlargement to harvest, and phosphorus is absorbed primarily in early to mid growth [7][8].

Sources

[1] Fertilizer application | BSI Institute of Biological Sciences/Eggplant: http://bsikagaku.jp/f-fertilization/eggplant.pdf
[2] Nutrient absorption characteristics of forced eggplant under carbon dioxide application conditions (Kochi Prefectural Agricultural Technology Center): https://www.nogyo.tosa.pref.kochi.lg.jp/download/?t=LD&id=6802&fid=39820
[3] Nitrogen absorption characteristics and optimization of fertilization methods in summer and autumn eggplants (Ibaraki Prefectural Agriculture Center): https://www.pref.ibaraki.jp/nourinsuisan/enken/seika/yasai/kasai/documents/s1516.pdf
[4] Fukuoka prefecture vegetable fertilization standards: https://www.pref.fukuoka.lg.jp/uploaded/life/763928_62443940_misc.pdf
[5] Eggplant | Specific cultivation management of major vegetables - JA Group Hokkaido: https://ja-dosanko.jp/agriculture/charenge/no4/nasu/
[6] Points of "eggplant" cultivation - Mirai Kura: https://mirai-zou.co.jp/advice/65/
[7] Growth, yield and quality response of Eggplant (Solanum ...): https://pmc.ncbi.nlm.nih.gov/articles/PMC11336821/
[8] Utilization of Nitrogen by Vegetable Crops (JIRCAS): https://www.jircas.go.jp/en/publication/jarq/25/2/117

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Additional notes
Although accurate measured values ​​for each growth stage have not yet been published in official manuals or papers for commercial eggplant cultivation in Japan, the existing absorption curves, total absorption, and stage-specific division models are consistent with numerous agricultural literature and actual recommended fertilizer values. In the field, we recommend using this model as a base and fine-tuning it by combining it with growth records and field data.