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Views: 0 Author: Site Editor Publish Time: 2026-08-07 Origin: Site
What Is Monopotassium Phosphite?
Monopotassium phosphite (MKPI) is an inorganic salt with the chemical formula KH₂PO₃ and CAS number 13977-65-6. It appears as a white crystalline powder, is odorless, and exhibits exceptionally high water solubility — approximately 2,200 g/L at 25 °C. The compound belongs to the phosphite salt family, derived from phosphorous acid (H₃PO₃), where phosphorus carries an oxidation state of +3 — a structural feature that fundamentally distinguishes it from conventional phosphate fertilizers.
Agricultural Applications and Crop-Specific Protocols
Monopotassium phosphite is registered for use on a wide range of agricultural, horticultural, and ornamental crops. The table below summarizes recommended application rates and target diseases for major crop categories.
1 Fruit and Nut Crops
Crop | Target Disease / Purpose | Typical Rate (foliar) | Application Timing |
Citrus | Phytophthora root rot, gummosis, brown rot | 2–4 g/L water | Every 21–28 days during high-risk periods; pre- and post-harvest |
Grape | Downy mildew (Plasmopara viticola) | 2–4 g/L | From bud break through veraison; 10–14 day intervals |
Apple / Pear | Fire blight (suppression), Phytophthora collar rot | 2–3 g/L | Bloom through fruit development |
Stone Fruit | Phytophthora root and crown rot | 3–5 g/L (soil drench) | Early spring and post-harvest |
Avocado | Phytophthora cinnamomi root rot | 3–5 g/L (trunk injection or soil drench) | Every 2–3 months; pre-flowering to harvest |
Strawberry | Phytophthora crown rot, Botrytis (suppression) | 2–3 g/L | Vegetative stage through fruiting; 14-day intervals |
2 Vegetable Crops
Crop | Target Disease / Purpose | Typical Rate (foliar) | Application Timing |
Tomato / Pepper | Late blight (Phytophthora infestans), Pythium damping-off | 2–4 g/L | Transplanting through harvest; 7–14 day intervals |
Cucurbits | Downy mildew (Pseudoperonospora cubensis) | 2–3 g/L | First true leaf through harvest; 7–10 day intervals |
Potato | Late blight (Phytophthora infestans), pink rot | 3–5 g/L | Tuber initiation through bulking; 7–14 day intervals |
Lettuce / Leafy Greens | Downy mildew (Bremia lactucae) | 2–3 g/L | Throughout production cycle; 10–14 day intervals |
Onion / Allium | Downy mildew (Peronospora destructor) | 2–3 g/L | Bulb initiation through maturity |
3 Field Crops
Crop | Purpose | Typical Rate | Notes |
Soybean | Phytophthora root rot suppression, K supplement | 2–4 g/L foliar | Apply at R1–R3 growth stages |
Corn / Maize | Stress tolerance, K boost during grain fill | 3–5 g/L foliar | Apply at V6–VT; tank-mix compatible with most fungicides |
Wheat / Barley | Root rot complex suppression | 2–3 g/L foliar | Apply at tillering to boot stage |
Cotton | Pythium and Fusarium suppression | 3–4 g/L foliar | Apply at squaring to early bloom |
4 Turf and Ornamentals
Application | Target | Typical Rate | Notes |
Golf course turf | Pythium blight, brown patch suppression | 5–10 L/ha (as formulated product) | Preventive program; 14–21 day intervals |
Nursery ornamentals | Phytophthora root rot, Pythium damping-off | 2–4 g/L (foliar or drench) | Apply throughout propagation and production |
Landscape trees | Phytophthora trunk canker, root rot | Trunk injection (per label) | Professional application only |
Monopotassium Phosphite vs. Monopotassium Phosphate (MKP)
This comparison is essential reading for any grower or agronomist using these products.
Parameter | Monopotassium Phosphite (MKPI) | Monopotassium Phosphate (MKP) |
Formula | KH₂PO₃ | KH₂PO₄ |
Typical NPK Label | 0-58-38 | 0-52-34 |
Primary Role | Fungicide + biostimulant + K source | Phosphorus + potassium fertilizer |
Phosphorus Available to Plant? | No (must be oxidized by soil microbes first) | Yes — immediately available |
Fungicidal Activity | Yes — systemic Oomycete control | None |
Induced Resistance | Yes — activates SAR pathways | No |
Best Use Timing | Preventive disease programs; stress periods; fruit fill and ripening | Vegetative growth; root development; flowering; phosphorus-deficient soils |
Soil Mobility | High — moves readily through soil profile | Moderate — binds to soil particles |
Phloem Mobility | High — translocates to new growth and roots | Low — mostly xylem-mobile |
Regulatory Status | Fungicide (varies by jurisdiction) | Fertilizer (universal) |
The Practical Strategy
Use MKPI when:
● Oomycete disease pressure is high (wet seasons, humid environments, heavy soils)
● You need a preventive fungicide program with resistance-management benefits
● Crops are in fruit fill/ripening and need supplemental potassium
● Environmental stress (drought, heat) requires systemic resilience support
● You are managing Phytophthora-susceptible perennial crops (citrus, avocado, tree nuts)
Use MKP (phosphate) when:
● Soil phosphorus levels are deficient and crops need immediate P nutrition
● Crops are in vegetative growth, root establishment, or flowering stages
● You are formulating a complete NPK fertigation program
● Phosphorus deficiency symptoms are visible (purpling, stunting)
In integrated programs: MKPI and MKP are not mutually exclusive. Many advanced growers use both — MKP for foundational phosphorus nutrition and MKPI for disease suppression and induced resistance. The key is to understand which product supplies which function and to avoid substituting one for the other.
