Comprehensive Survey of NPK (Nitrogen, Phosphorus, Potassium) Absorption (g/m²) by Growth Stage for Onion

Overview

Onion growth is broadly divided into the following four stages (common in Japanese and international agricultural guidelines):

Germination / Seedling stage (Germination/Seedling stage)
Vegetative growth (Vegetative/Leaf and Stem Elongation)
Bulb initiation and enlargement (Bulb Initiation and Enlargement)
Flowering / Seed production (Flowering/Seed Production)

These names and divisions match Japanese literature and serve as the basic unit for fertilization management and analysis [1][2].

NPK Absorption Characteristics by Growth Stage

Absorption Peak

Onion NPK absorption concentrates in the latter half of growth, especially the 30–40 days from bulb initiation to 60 days before harvest. About 80% of total uptake occurs in this period, consistent across official Japanese literature (Hokkaido Agricultural Research Center, etc.) and international guidelines [1][2][3].
Only a small amount (less than 5–10% of total) is absorbed during germination and early vegetative growth; main uptake shifts when leaves are fully expanded and bulb enlargement begins.

Absorption Pattern Overview (Including Quantitative Values)

Typical total NPK uptake per crop cycle (standard cultivation, average yield) reported in Japanese and international literature:
- Nitrogen (N): 10–17 g/m²
- Phosphorus (P): 2.5–4.5 g/m²
- Potassium (K): 4.5–8 g/m² [3][4][5]
These are calculated from soil analysis, leaf analysis, and nutrient removal measurements in field trials and systematic surveys.

Estimated NPK Absorption by Growth Stage (g/m²) and Basis

Stage Allocation (Aggregated from Japanese and International Evidence)

Absorption proportions are extracted mainly from field experiments and fertilization guidelines in Japan and abroad [1][2][3][6][7].
Representative standard values (example: 10 g N/m², 3 g P/m², 6 g K/m²) with stage allocation:

Growth Stage	N uptake	P uptake	K uptake	Share (%)	Main absorption characteristics and basis
Germination / seedling	~0.3–0.5	~0.1–0.2	~0.3–0.5	2–5%	Very low uptake; undeveloped root system [1][3]
Vegetative growth	~1.2–2.0	~0.3–0.6	~0.9–1.6	12–20%	Leaf growth and tillering start; chloroplast production increases [1][3][6]
Bulb initiation and enlargement	~8–13.5	~2–3.6	~3.6–6.4	70–80%	Main uptake period; bulb growth drives nutrient demand [1][3][6][7]
Flowering / seed production	Trace	Trace	Trace	Few % or less (0–5%)	Seed production only; usually nutrient redistribution period [1][3]

Standard Estimation Example (10 g N/m², 3 g P/m², 6 g K/m² Model)

Germination / seedling (2–3 weeks):
- N: 0.3–0.5 g/m²
- P: 0.1–0.2 g/m²
- K: 0.3–0.5 g/m²
Vegetative growth (leaf and stem elongation):
- N: 1.2–2.0 g/m²
- P: 0.3–0.6 g/m²
- K: 0.9–1.6 g/m²
Bulb initiation and enlargement (30–40 days from 60 days before harvest):
- N: 8–13.5 g/m²
- P: 2–3.6 g/m²
- K: 3.6–6.4 g/m²
Flowering / seed production (seed crop cultivation):
- Uptake is nearly zero; new nutrient uptake is very limited (mostly redistribution from leaves to bulb) [3][6]

Values may vary somewhat by production area, cultivar, and fertilization (high-yield cultivation may reach 20–25 g N/m²) [5].

Main scientific basis

Hokkaido Agricultural Research Center data repeatedly states that "about 80% of total uptake concentrates in the 30–40 days from 60 days before harvest (bulb enlargement period)" [1].
Overseas university publications [2][3] and global fertilizer company guides [4][6][7] confirm similar content.
After leaf lodging and senescence, new aboveground uptake stops and already absorbed nutrients are preferentially translocated to the bulb [1][6].

Estimation Method and Basis

General Scientific Estimation Procedure

When stage-by-stage uptake is not explicitly stated, the following methods are scientifically justified:

Use maximum yield total uptake (g/m²) as reference
→ Apply absorption standard for yield equivalent to 1000 g/m² (e.g., N 10 g/m²) [2][4].
Apply absorption pattern (% allocation) from agricultural guidelines or papers to total uptake
→ Example: "80% during bulb formation period" [1][3][6].
Split estimate by stage days or biomass increment
→ Rich growth observation and sampling data (widely used in Japan) [2][3].

Example: Calculation

If total N uptake is 12 g/m² (standard cultivation),

Germination (3%): 0.36 g/m²
Vegetative growth (15%): 1.8 g/m²
Bulb initiation and enlargement (80%): 9.6 g/m²
Flowering (2%): 0.24 g/m²

This method is used similarly in Hokkaido Agricultural Research Center [1], Pacific Northwest Extension [2][3], and ICL/Haifa international fertilization guides [4][6][7] for local fertilization design and tracking analysis.

Direct Stage Measurements and Estimation Accuracy

Direct measurements (g/m²) per stage are available only in some field trials and academic papers (time-series leaf sampling, nutrient analysis). However, most modern guidelines use allocating total uptake by the above pattern % as the mainstream, realistic, and scientifically supported approach [1][2][3][5].
Variation in uptake peak timing and surrounding periods exists, but Japanese and international literature largely explains patterns with the above divisions [1][2][6][7].

Conclusion

Onion NPK absorption concentrates in bulb initiation and enlargement (about 30–40 days from when bulb enlargement begins to before harvest), about 80% of total; uptake through vegetative growth is only about 20%.
Detailed stage allocation can be estimated with high precision by applying defined % to total uptake.
Japanese and international literature used in this survey show scientific consensus on stage allocation basis, patterns, and estimation methods, with high field practicality.

Comprehensive Survey of NPK (Nitrogen, Phosphorus, Potassium) Absorption (g/m²) by Growth Stage for Onion ​

Overview ​

NPK Absorption Characteristics by Growth Stage ​

Absorption Peak ​

Absorption Pattern Overview (Including Quantitative Values) ​

Estimated NPK Absorption by Growth Stage (g/m²) and Basis ​

Stage Allocation (Aggregated from Japanese and International Evidence) ​

Standard Estimation Example (10 g N/m², 3 g P/m², 6 g K/m² Model) ​

Estimation Method and Basis ​

General Scientific Estimation Procedure ​

Example: Calculation ​

Direct Stage Measurements and Estimation Accuracy ​

Conclusion ​

Sources ​