Nitrogen-phosphate-potassium fertilizers (NPK)

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Compound fertiliser (NPK fertiliser) is a kind of chemical fertiliser containing three main nutrients, namely nitrogen (N), phosphorus (P) and potassium (K), which is scientifically proportioned to meet the nutritional requirements of crops at different growth stages. The following is a specific analysis of its nature and application:

First, the nature of compound fertiliser**
1. Chemical properties
Nitrogen (N): in the form of ammonium nitrogen (NH₄⁺), nitrate nitrogen (NO₃-) or amide nitrogen (e.g., urea), promoting leaf growth and protein synthesis.
Phosphorus (P): mostly in the form of water-soluble monoammonium phosphate (MAP) or diammonium phosphate (DAP), promotes root development and flowering.
Potassium (K): usually potassium chloride (KCl) or potassium sulphate (K₂SO₄), enhances resistance and fruit quality.
Stability: soluble in water, but may absorb moisture and clump (need to add anti-caking agent).

2. Physical Properties
Appearance: granular (common diameter 2-4mm), colour varies according to formulation (e.g. grey, brown, etc.).
Ratio: 15-15-15 (balanced), 20-10-10 (high nitrogen type), 10-20-20 (high phosphorus and potassium type).

3. Environmental characteristics
Excessive use may lead to soil acidification (especially with ammonium nitrogen) or salinisation.
Slow-release compound fertiliser can reduce nutrient loss and improve utilisation.

Application of compound fertiliser
1. Applicable crops
Field crops (rice, wheat): commonly used high nitrogen type (such as 25-10-5) to promote tillering.
Economic crops (fruit trees, vegetables): high potassium type (such as 16-8-24) to enhance fruit sugar and colour.
Flowers and lawn: balanced type or high phosphorus type (e.g. 10-20-10) to promote flowering and root growth.

2. Precautions for use
Soil test: choose the ratio according to the soil deficiency (e.g. choose high potassium type for potassium-deficient soil).
Application method:
Base fertiliser: deep application and mulching to avoid phosphorus fixation.
Follow-up fertiliser: side or hole application, combined with irrigation to improve efficiency.
Avoid mixing: Ammonium nitrogen fertiliser should not be mixed with alkaline fertiliser (e.g. wood ash) to prevent ammonia volatilisation.

3. Special types of application
Nitro-sulphur based compound fertiliser (containing nitrate nitrogen): suitable for fast nitrogen supplementation of dryland crops (e.g. vegetables).
Humic acid compound fertiliser: to improve the stagnant soil and enhance the ability of water and fertiliser retention.

Third, the advantages and disadvantages of compound fertiliser
- Advantages:
Balanced nutrients, reducing labour costs for multiple applications.
Uniform particles, easy to mechanise application.
- Disadvantages:
Fixed ratios may not be able to meet the needs of the entire life cycle of the crop (need to be paired with a monomaterial fertiliser).
Some low-end products contain chlorine (e.g. KCl), and chlorine-avoidant crops (e.g. tobacco, potatoes) need to use potassium sulphate type.