KNOWLEDGE · FERTIGATION
Choosing NPK for drip fertigation
A drip system changes the specification. Nutrients must dissolve completely, the solution must stay stable for hours, and the salt load must stay low enough not to burn roots at the emitter. Three numbers decide whether a grade is suitable: insoluble content, electrical conductivity, and nitrogen form.
1. Insoluble content: the clogging risk
Every gram of undissolved material that enters a drip line ends up somewhere in the tape or at an emitter. Once flow becomes uneven, yield differences appear across the field and are hard to diagnose later. A grade intended for fertigation should be specified with a water-insoluble content figure — typically ≤0.2% for a fully water-soluble product.
Note that "high water-soluble P" is not the same as "fully water-soluble NPK". A granule can carry 90% water-soluble P and still contain other insoluble components. Ask for the water-insoluble figure for the whole product.
2. Salinity: protect the root zone
Fertigation concentrates salts exactly where roots are densest. Chloride-based grades and high-EC formulations raise the osmotic pressure around the emitter. On sandy soils with low buffering, split the daily dose into two smaller injections rather than one large one.
3. Nitrogen form: nitrate for cool soils
Nitrate nitrogen is taken up directly; ammonium and urea nitrogen must first be converted, which depends on soil temperature and microbial activity. In cool seasons, a nitrate-based NPK delivers a faster and more predictable response in a drip system.
| Check | Target | Why |
|---|---|---|
| Water-insoluble content | ≤0.2% | Prevents emitter clogging |
| Chloride | Low or nil | Reduces salt load at the root zone |
| Nitrogen form | Nitrate share as high as practical | Uptake in cool soils |
| Dissolution time | Fully dissolved before injection | Avoids sediment in the tank |
Tower-granulated grades dissolve faster
Tower granulation forms a rounder, more uniform granule than prilling or compaction, and the granule dissolves quickly in the mixing tank. In practice this shortens the preparation cycle and reduces the residue left at the bottom of the tank — both operational benefits that are easy to underestimate at scale.
Practical protocol
- Dissolve a 1 kg sample in 100 litres of the actual irrigation water and check for residue
- Measure the EC of the final solution; keep it within the crop's tolerance
- Flush the lines with clean water after each injection cycle
- Test one block for a week before converting the whole farm
Frequently asked
Can I use ordinary NPK in a drip system?
Risky. Even a product with high water-soluble P can contain insoluble components that settle in the lines. Use a grade specified as fully water-soluble for fertigation.
What EC is acceptable?
It depends on the crop and the substrate. As a rough guide, keep the nutrient solution below 2.5 dS/m for sensitive crops and below 4 dS/m for tolerant ones.
Is MKP suitable for drip?
Yes, MKP is fully soluble and is commonly used for the phosphorus and potassium component of a fertigation programme.