Monopotassium phosphate: uses, how to dilute, rates per crop

What monopotassium phosphate is#

Monopotassium phosphate (MKP) is the salt KH₂PO₄, with the nutrient grade 0-52-34: 52% P₂O₅ and 34% K₂O. As elements, that is 22.7% phosphorus and 28.2-28.7% potassium (Haifa and ICL data sheets). It contains 0% nitrogen, so it is not a nitrate fertiliser.

Ukraine's State Register of Pesticides and Agrochemicals records the same substance as «монофосфат калію», «монокалійфосфат», «монокалій фосфат» and «калій дигідрофосфат» (potassium dihydrogen phosphate): names of one fertiliser. The register marks the form in the name: кр. (crystalline), п. (powder), гр. (granulated), кр.п. (crystalline powder).

It contains no chloride and very little sodium: ICL states 150-300 ppm. A 0.1-1% solution has a pH of 4.4-4.8 (Haifa), and the Haifa-MKP safety data sheet (2010) gives pH 4.2-4.9 for a 1% solution. How much MKP dissolves depends on the water temperature. Catalogue section: monopotassium phosphatea.

What it is for and when to apply it#

Makers offer monopotassium phosphate as a source of phosphorus and potassium for stages when the crop needs little nitrogen. Haifa names the start of the season, when the plant establishes its roots, and the productive stages of sugar-rich fruit crops; ICL lists flowering, fruit set, ripening and harvest. It goes on as a foliar spraya or with irrigation water, through fertigationa.

No valid MKP row in the register names an application stage: Nova PeaK and Shifang give a rate, a concentration and a number of applications, and Yara's row for protected crops says "through the season".

Spraying adds to soil fertilisation and does not replace it. Haifa calls foliar MKP an optional supplement to the main fertiliser programme. In University of Saskatchewan trials (Canada, 2016-2017), KH₂PO₄ sprayed on canola, pea and wheat did not make up for a smaller dose of seed-row phosphorus at sowing, and the results varied between fields and years.

Registered in Ukraine: crops, rates, class#

As of November 2026, the public file of the State Register holds seven valid MKP rows, all classed as fertilisers. The table shows six: the seventh's producer is in the Russian Federation, so it is not listed here.

Rows for MKP alone that name field crops give no rate: the product label sets it. Ornamental crops and flowers appear in the Karavan and Shifang rows. The file records no waiting period for any product. It ends with a row dated 24.06.2024, so check the number on the pack against the live register.

Field crops: wheat, maize, sunflower, soybean, rapeseed, beet#

Rows for MKP alone name cereals, industrial crops and spring barley, without a rate. Pro NPK names small-grain cereals, maize, sunflower, soybean and sugar beet, but its rates are shared by twelve grades, and the row does not say which belongs to MKP. Rapeseed appears in it only for seed treatment. So for field crops the product label sets the rate.

Winter wheat. An MKP dose comes from a field trial by Dmytro Motorny Tavria State Agrotechnological University (Southern Steppe, 2019-2021): 1 kg/ha of monopotassium phosphate at BBCH 31, in one tank with urea at 10 kg/ha and magnesium sulphate at 2 kg/ha. Chlorophyll a rose by 12-23% over urea alone; the paper reports no yield.

Maize. Iowa State University sprayed maize with MKP at four stages, from V6 to VT, and recorded a significant yield loss of about 0.31-0.38 t/ha (5-6 bushels per acre, a 1999 trial). Sunflower, soybean, winter rapeseed, sugar beet: we found no trial with an MKP dose and a result.

Potato#

Among the valid MKP rows, only Karavan's (А 06329) names potato, and it gives no rate: the product label sets it. The register states no stage for potato either.

A field result with MKP on potato comes from the University of Florida (UF/IFAS, spring 1996, a commercial farm near Hastings). The potatoes were sprayed twice with a 3% monopotassium phosphate solution: first at 64-70 days after planting, when the plants were blossoming and starting to set tubers, then two weeks later. The solution went on at 80 gallons per acre, about 750 L/ha, which is roughly 22 kg of MKP per hectare per spray (calculation).

The sprayed plots gave 384 US hundredweight per acre against 372 unsprayed, about 43.0 and 41.7 t/ha (calculation). The authors saw a tendency towards slightly higher yields, mainly in the field on a reduced programme with no phosphorus, but the difference was not statistically significant.

Tomatoes#

As of November 2026, tomatoes have two register rows for MKP alone with a rate, and neither names a stage. ICL's US leaflet (2024) advises 2-3 sprays 2-3 weeks apart, the first immediately before full bloom; it is a maker's advice for an American label, not a Ukrainian registration.

At 0.5-1%, Nova PeaK's tomato rate means 50-100 g per 10 L and 500 to 3,000 L of solution per hectare (calculation): vegetable spraying volumes, not field ones. For young leaves Haifa advises 1%, for mature ones up to 2% (see the strengths table below). Calcium nitrate does not go into the same tank.

Cucumbers#

As of November 2026, Nova PeaK is registered for cucumbers at a higher rate than for tomatoes, and Yara's row for protected crops is the same. The register gives no stage. ICL's US leaflet (2024) advises two or more applications, starting just before bloom; it is a maker's advice, not a Ukrainian registration.

Nova PeaK's cucumber rate at 0.5-1% needs 1,000-4,000 L of solution per hectare (calculation). In a glasshouse study in Israel (1992-1994, soilless), MKP proved a very effective source of phosphorus and potassium for cucumber and muskmelon and did not differ from phosphoric acid with potassium chloride (Nerson et al., 1997); the abstract gives no rates.

Pepper, cabbage, onion, garlic and other vegetables#

As of November 2026, the rate for vegetables as a group comes from Shifang (А 09468): 70-100 kg/ha as a 0.07-0.1% solution, 6 times, and for households 7-10 g/m² of the same solution. 7 g at 0.07% is 10 L of solution per square metre, so watering, not spraying (calculation).

The row names the group «овочеві культури» (vegetable crops), not species: pepper, cabbage, onion and garlic are not named in it separately. Pro NPK names pepper and cabbage, but its rates are shared by twelve grades. Yara (А 06699) says "vegetables" with no rate, and Karavan names table beet and carrot, also with no rate.

Neither the register nor any peer-reviewed MKP trial we found gives a spray rate for onion or garlic. In the field, the product label sets it.

Strawberries, other berries and grapes#

As of November 2026, berries fall under Shifang's row for fruit, berry and ornamental crops and flowers: 100-150 kg/ha as a 0.1-0.15% solution, 6 times, and for households 10-15 g/m². That too is about 10 L of solution per square metre, so watering (calculation). The row names the group, not strawberries, raspberries, blueberries or currants by name, and gives no stage.

We found no field trial with an MKP rate for these berries, so in the field the label sets the rate.

Grapes are named in no MKP row, valid or expired, so the register has no rate for them. The grapevine studies concern powdery mildew; they are covered below. Autumn feeding "to prepare for winter" appears in advice pages, but we found no source for it.

Seedlings and transplanting#

No MKP row in the register names seedlings. As of November 2026, the closest register rate is Shifang for household vegetables: 7-10 g/m² as a 0.07-0.1% solution, which is watering (calculation). For seedlings in trays before transplanting, we found no source with a rate.

The University of Florida tested starter solutions at transplanting (UF/IFAS, spring 1996): each plant got 250 ml of solution, on soil very high in phosphorus. On tomatoes, the starter solutions had little effect on yield. On pepper, the best total yields came from ammoniated MKP, ammonium polyphosphate, a mix of the two, or MKP with MAP; plain MKP was not among the best.

Ontario's Ministry of Agriculture (Canada, updated 30.06.2026) writes that a starter fertiliser is mainly for the phosphorus response when transplants go into cool soil. Ammonium nitrogen improves phosphorus uptake, while potassium and micronutrients in a starter are of little benefit and can raise the risk of damage.

How to dilute: per hectare, then per 10 litres#

The label gives the rate in kilograms per hectare, and the water volume sets the concentration. Take the spray volume for field crops from the label.

Shifang's rate works out at 10 L of solution per square metre, so it is watering (calculation). A sprayer at 200-300 L/ha applies 2-6 kg/ha at 1-2%, while Nova PeaK's 5-20 kg/ha at 0.5-1% needs 500-4,000 L/ha, which are vegetable volumes (calculation).

Spoons, water temperature and hard water#

The Haifa-MKP safety data sheet (revision of 19.12.2010) gives a bulk density of 0.8-1.2 g/cm³. So a level teaspoon (5 ml) holds 4-6 g, and a level tablespoon (15 ml) 12-18 g (calculation). A heaped spoon holds more, so it is better to weigh the fertiliser.

In its 2022 and 2025 leaflets, ICL makes its stock solution from 10-15 kg per 100 L of water; at 0 °C, 15 kg may not dissolve fully (calculation). ICL adds MKP to the sprayer tank once the tank is half full of water, with the agitator running.

Clemson University's Cooperative Extension (US, 2024) writes that phosphate fertilisers are especially prone to precipitate when irrigation water is high in dissolved calcium and magnesium, and especially at pH 7.5 and above. Check such water with a jar test.

What monopotassium phosphate can share a tank with#

Makers consider MKP compatible with most fertilisers and pesticides, with exceptions; a new tank mixa is checked in a jar first.

• <strong>Iron, manganese, zinc, copper:</strong> chelates only (Haifa, ICL).
• <strong>Copper fungicides:</strong> no; ICL forbids them because MKP is acidic.
• <strong>Urea, potassium sulphate:</strong> yes (the Tavria trial; Haifa's two-tank scheme).
• <strong>Herbicides:</strong> MKP acidifies the solution, and sulfonylureas break down faster in acid water: half of a dose of tribenuron-methyl hydrolyses in 1 day at pH 5 and in 15.8 days at pH 7 (PubChem). The herbicide label decides; check the pesticides in the <a href="/en/calculators/tank-mix-compatibility">compatibility calculator</a>.

The jar test is mixed at the working concentration; if the solution turns milky, the fertilisers are not applied together (Purdue). Clemson advises leaving it overnight: cloudiness or a deposit means the system is likely to clog.

Magnesium sulphate and monopotassium phosphate#

On magnesium sulphate, the sources disagree:

• <strong>Haifa</strong> advises against mixing MKP with magnesium fertilisers, in foliar feeding and in fertigation alike.
• <strong>Yara</strong> warns only against concentrated magnesium fertilisers.
• <strong>Haifa's two-tank scheme</strong> for fertigation dissolves magnesium sulphate and MKP in the same tank B, provided the solution's pH does not exceed 5.
• <strong>The Tavria trial</strong> applied MKP at 1 kg/ha, magnesium sulphate at 2 kg/ha and urea at 10 kg/ha in one tank.
• <strong>Purdue University (2017)</strong> writes that magnesium from Epsom salt forms an insoluble compound when it meets the phosphate in a complete fertiliser, and advises applying magnesium sulphate separately from complete fertilisers.

Haifa advises against adding magnesium fertilisers, Yara only against concentrated ones, so do a jar test first. In a working solution, Ukrainian researchers did combine the two. Run the test at exactly the working concentration and leave it overnight.

Boric acid and monopotassium phosphate#

Boron is not on the incompatibility lists of Haifa, ICL or Yara: those name calcium, magnesium, non-chelated metals and copper products.

Purdue University's Cooperative Extension (2017) counts boron among the elements that are insoluble with phosphate in a foliar solution, especially when not chelated, and advises against applying a high-phosphorus fertiliser together with such a solution. The note does not name MKP or boric acid.

Conference slides from the FREP programme of the California Department of Food and Agriculture (2017) say the opposite: orthophosphate at neutral pH requires fully chelated micronutrients in concentrated blends, except boron and molybdenum.

None of these sources tested MKP with boric acid, so only a jar test answers it: the same water, the same fertilisers, the same concentration, left overnight.

Powdery mildew: what the research found#

Monopotassium phosphate is registered in Ukraine only as a fertiliser: no MKP row in the register is a fungicide registration. The powdery mildew studies we found all come from one group, M. and R. Reuveni (Israel, 1995-2000); the figures below are from the papers' abstracts.

On field-grown Chardonnay vines, seven sprays of 0.025 M K₂HPO₄ and KH₂PO₄ solutions went on 14 days apart, starting from 10 cm shoots; for MKP that is about 0.34% (calculation). The abstract's results are for K₂HPO₄, not MKP. On Chardonnay berries the phosphates worked about as well as the fungicide pyrifenox; on mango and nectarine, systemic fungicides worked better, and alternating the phosphates with them improved control. The phosphates damaged young nectarine leaves.

On young cucumber plants grown in hydroponics, a 1% MKP solution protected the leaves for up to 21 days after inoculation and cut sporulation by 72.3% (2000). Downy mildew is not mentioned in the abstracts. These are research findings, not advice to treat powdery mildew with MKP.

MKP, potassium sulphate and the nitrates#

MKP supplies phosphorus and potassium without nitrogen. Potassium sulphate supplies potassium and sulphur without phosphorus, so where phosphorus is needed it cannot replace MKP. Monopotassium phosphate dissolves about twice as well (22.6 g per 100 g of water at 20 °C against 120 g/L at 25 °C). The acidity of potassium sulphate varies between products (Haifa); K-Leaf gives pH 2.9 in a 1% solution. The two can share a tank: in its two-tank scheme, Haifa dissolves both in tank B.

The nearest substitute by composition is a pair: 0.85 kg of MAP 12-61-0 and 0.65 kg of potassium sulphate give the same P₂O₅ and K₂O as 1 kg of MKP, plus about 0.10 kg of nitrogen and 0.12 kg of sulphur (calculation). Check the labels to see whether the pair is registered for your crop.

When MKP does harm, and common mistakes#

A yield loss in the field was recorded on maize: sprays from V6 to VT (Iowa, 1999). ICL advises against applying MKP under severe heat or water stress and writes that under certain conditions it can injure leaves. The Haifa-MKP safety data sheet (2010) does not classify the product as hazardous under the EU CLP Regulation; in Ukraine's register, the MKP rows carry class 3 or 4. No row rules out a crop.

• <strong>A garden concentration on a field sprayer.</strong> 7-10 g per 10 L is watering; in the field, work from the label's kg/ha and your own spray volume.
• <strong>Calcium nitrate in the same tank.</strong> Haifa, ICL and Yara advise against it.
• <strong>Magnesium in a stock solution without a test.</strong> Jar test first.
• <strong>Spoons for field rates.</strong> Weigh.
• <strong>Hard water without a test.</strong> At pH 7.5 and above, phosphate settles out more easily.
• <strong>Spraying fruit trees more often than every 7 days.</strong> For fruit trees, ICL's US leaflet (2024) states a minimum of 7 days between applications.