Watermelon Nutrient Deficiency: Signs, Causes and the Fertilizer That Fixes It
Watermelon nutrient deficiency is one of the first things to check when the leaves of your pakwan (watermelon) turn yellow, purple or burnt at the edges. On Philippine farms the usual ones are nitrogen, potassium, magnesium, calcium, boron, phosphorus and zinc. This guide shows where each one starts, how to be sure, and the fertilizer that fixes it.
Start with one question: did the change begin on the old leaves or on the young ones? Nitrogen, potassium, magnesium and phosphorus move inside the plant, so a shortage shows on the old, lower leaves first. But calcium, boron and zinc stay where the plant first put them, so the young leaves and the growing tip show it first. After that, the color and the pattern narrow down which watermelon nutrient deficiency you have, and a soil test confirms it. Each one below has its signs, its causes and its fix.
Watermelon nutrient deficiency signs, old leaves and young leaves
Shows first on the old leaves
| Nutrient | Where it starts | What you see | Fix with |
|---|---|---|---|
| Nitrogen | old, lower leaves, the whole plant | Old leaves pale and yellow up the runner, with small, misshapen fruits. | Urea 46 0 0, Ammonium sulfate 21 0 0 |
| Potassium | old, lower leaves, leaf edges and tips | Old leaves scorch and cup up, and fruits have poor flesh and weak flavour. | Muriate of potash 0 0 60, Sulfate of potash 0 0 50 |
| Magnesium | old, lower leaves, between the veins | Mature leaves yellow in the middle with green edges, and the yellow turns red and dies. | Magnesium sulfate (Epsom salt), Dolomite |
| Phosphorus | old, lower leaves, the whole plant | Older leaves turn purple along the veins and young leaves are small, stiff and dark. | Complete 14 14 14, Ammonium phosphate 16 20 0 |
Shows first on the young leaves
| Nutrient | Where it starts | What you see | Fix with |
|---|---|---|---|
| Calcium | fruit, grain or tubers, young leaves and the tip | A dark rot at the flower end of young fruits, opposite the stem. | Agricultural lime, Dolomite |
| Boron | young leaves and the tip, fruit, grain or tubers | Young fruits drop, and fruits show yellow streaks, corky scurf and cracks. | Borax, Solubor (sodium borate) |
| Zinc | young leaves and the tip, between the veins | Young leaves pale with green veins, small and bunched on short runners. | Zinc sulfate |
Before you buy fertilizer for a watermelon nutrient deficiency
So use the table to pick the likeliest watermelon nutrient deficiency, then read its section below. Also keep in mind that dry soil, standing water, cold nights, a pest or a disease can look the same. That is why it pays to check the roots and the whole field first.
Nitrogen deficiency in watermelon
Older leaves turn pale, yellow and drop, and the yellowing moves up the runners. Vines are stunted, and fruits are pale, small, thick skinned or misshapen, so yield falls. You see it most at this stage: vine growth and fruit set.
Why nitrogen deficiency happens in pakwan
Sandy soils, heavy rain or irrigation, low organic matter and no side dressing before fruit set.
How to tell it apart
Magnesium shortage leaves the veins green. Fusarium wilt makes whole runners wilt and the stem turns brown inside. To be sure, old leaves yellow first. Soil test and leaf tissue test.
What to apply for nitrogen deficiency
Side dress urea or ammonium sulfate based on the soil test, before the fruits form. For a quick fix, give a urea leaf spray at 10 grams per liter of water (160 grams in a 16 liter sprayer) every 15 days. Side dress 5 to 10 grams of Urea 46 0 0 a plant, about 1 to 2 bags per hectare, every two to three weeks from flowering, and water it in.
- Urea 46 0 0
- Ammonium sulfate 21 0 0
- Complete 14 14 14
- Calcium nitrate
Sold here with nitrogen: Atlas Granular Urea (46 0 0) by Atlas Fertilizer, Crown Champion Ammonium Sulfate by Aqua Hydroponic Prime Ventures (21 0 0 with 24 percent sulfur), Agripower 46 0 0 (Locally Blended) Coated by Green Acres Agri Tech and Calcipro by Ramgo (15.5 0 0 with 18.8 percent calcium). Check the FPA registration on the bag and follow the label rate. For the how and when, see our guide to urea fertilizer, and the page on nitrogen deficiency in every crop. In anee.io, Anee, the smart farm technician, reads a photo of the leaf and tells you which nutrient is short.
To keep it away, apply some of the nitrogen before planting, then side dress the rest in one or two doses. Avoid heavy late nitrogen. Keep organic matter up with compost, manure or crop residues.
Potassium deficiency in watermelon
Older leaves yellow and scorch, then curl upward or cup, with sunken dark spots along the midrib. Browning spreads until leaves dry out. Shoots die back, stems weaken and fewer female flowers form. Fruits can have poor flesh texture, off flavour or spotted skin. You see it most at this stage: fruit filling.
Why potassium deficiency happens in pakwan
Sandy or acid soils, heavy rain or irrigation, and fertilizing with urea only.
How to tell it apart
Gummy stem blight and anthracnose also brown the leaves, but as spreading spots that start anywhere on the leaf. To be sure, soil test for potassium and a leaf tissue test.
What to apply for potassium deficiency
Apply muriate of potash based on the soil test, preferably in split doses. For immediate relief, give a morning spray of sulfate of potash at 10 grams per liter of water (160 grams in a 16 liter sprayer) with a sticker. Keep soil pH above 6 with lime or dolomite. A usual dose is 1 to 2 bags of Muriate of potash 0 0 60 or Sulfate of potash 0 0 50 per hectare, split from flowering to fruit fill.
- Muriate of potash 0 0 60
- Sulfate of potash 0 0 50
- Potassium nitrate 13 0 46
Sold here with potassium: Atlas Muriate of Potash (MOP) by Atlas Fertilizer (0 0 60), Simplot Sulfate 0 0 50 by Nutranta, Potassium Nitrate by Parafield (13 0 46) and Qrop Mix Production 14 0 23 plus 13 Cao by Farmtech Agriland (14 0 23 with 9.3 percent calcium). Check the FPA registration on the bag and follow the label rate. For the how and when, see our guide to potash fertilizer, and the page on potassium deficiency in every crop.
To keep it away, give potash at planting and again as fruits enlarge. Keep organic matter up with compost, manure or crop residues.
Magnesium deficiency in watermelon
Mature leaves turn yellow in the middle with green edges and green veins. The yellow areas turn red and develop spotted dead patches, older leaves shrivel and drop early, and yield falls. You see it most at this stage: fruit enlargement.
Why magnesium deficiency happens in pakwan
Sandy acid soils, dry years, and heavy potash, lime or ammonium fertilizer.
How to tell it apart
Nitrogen shortage yellows the whole leaf. Manganese toxicity on very acid soil gives bronze spots on old leaves. To be sure, soil test for magnesium, potassium and pH, and a leaf tissue test.
What to apply for magnesium deficiency
Try magnesium sulfate in a band near the plants at about 80 kg per hectare. Keep the pH near 6.5 and feed Epsom salt through drip lines when needed. Leaf sprays seldom cure a serious shortage on their own. Do not apply magnesium with potash. For quick help, spray Magnesium sulfate at 1 to 2 percent (160 to 320 grams in a 16 liter sprayer) two or three times, 10 to 14 days apart.
- Magnesium sulfate (Epsom salt)
- Dolomite
Sold here with magnesium: Dolomite by A6 Agrisource (24 percent calcium and 11 percent magnesium), Agsmart Kieserite Granule (20 percent sulfur and 16.3 percent magnesium), DDDD Planter's Choice Magnesium Sulphate Monohydrate by Modern Time (15.1 percent magnesium) and Zetryl MAG 9 by Zetryl Chem (9 percent magnesium). Check the FPA registration on the bag and follow the label rate. For the how and when, see our guide to complete fertilizer, and the page on magnesium deficiency in every crop.
To keep it away, use dolomite on acid soil before planting. Test the soil every two or three years.
Calcium deficiency in watermelon
Developing fruits rot at the blossom end, opposite the stem, starting as a sunken, leathery patch. Young leaves show yellow stripes along the midrib, crinkle and scorch at the edges, and new leaf tips hook. Growing points can die back, roots are stunted, and fruits may be small or tasteless. You see it most at this stage: early fruit growth.
Why calcium deficiency happens in pakwan
Uneven or too little watering, high humidity, poor root growth or anything else that slows water flow, which leaves the fruit short of calcium for a while. Heavy ammonium fertilizer, root injury and waterlogged soil make it worse.
How to tell it apart
Poor pollination gives shrivelled, misshapen young fruits without a leathery rot. Fruit rots from fungi are soft and mouldy. To be sure, look for the rot on young fruits after dry spells. Soil test for calcium and pH.
What to apply for calcium deficiency
Water steadily and well, avoid hurting roots or waterlogging the soil, and go easy on ammonium fertilizer. On acid soil, try lime at about 250 kg per hectare when pH is 5.5 to 6, or about 840 kg per hectare in two doses when pH is 3.5 to 4.5. Gypsum on soil that is not acid. Lime 2 to 4 weeks before planting. Calcium sprays go on young fruit from fruit set.
- Agricultural lime
- Dolomite
- Gypsum
- Calcium nitrate
Sold here with calcium: Micronized Activated Calcite by CMCS Agricultural Products (37.6 percent calcium), Dolomite by A6 Agrisource (24 percent calcium and 11 percent magnesium), Titon Pro by Vast Agro (27.2 percent calcium, 6.1 percent silicon and 3.6 percent magnesium) and Calcipro by Ramgo (15.5 0 0 with 18.8 percent calcium). Check the FPA registration on the bag and follow the label rate. For the how and when, see our guide to fertilizer for tomatoes, and the page on calcium deficiency in every crop.
To keep it away, lime acid soil before planting and keep watering even. Test the soil every two or three years. Keep potash in balance with magnesium and calcium.
Boron deficiency in watermelon
Young leaves are malformed and buds die, making the plant bushy. Young fruits drop or abort, and fruits show mottled yellow streaks, corky scurf on the skin, cracks, small size and early fall. You see it most at this stage: flowering and fruit set.
Why boron deficiency happens in pakwan
Sandy soils, overlimed soils, low organic matter and drought.
How to tell it apart
Poor pollination also drops young fruits but leaves stay normal. Thrips scarring on fruit skin is silvery rather than corky streaks. To be sure, soil test for boron and a leaf tissue test. In a soil test, less than 0.5 ppm of boron is low.
What to apply for boron deficiency
Try borax in the soil at about 10 kg per hectare or a borax spray at 1 gram per liter of water (16 grams in a 16 liter sprayer). Do not mix borax with calcium or ammonium fertilizer. Give boron before flowering: flowers and fruit set need it most.
- Borax
- Solubor (sodium borate)
Sold here with boron: Etifert 11 (Borax Decahydrate) by Zetryl Chem (11 percent boron), Fertibagra 15G by Unilox Far East Resources (14.6 percent boron), B17 (Boric Acid) by Newman Chemicals (17 percent boron) and Better Yield by Bacphil Planters Fertilizer (0 0 0.9 with 10 percent boron). Check the FPA registration on the bag and follow the label rate. For the how and when, see our guide to foliar fertilizer, and the page on boron deficiency in every crop.
To keep it away, add a small boron dose to the basal fertilizer on sandy fields. Test the soil every two or three years. Keep organic matter up with compost, manure or crop residues.
Phosphorus deficiency in watermelon
Older leaves turn purple, especially along the veins. Shoots are short and thin, young leaves are small, stiff and dark green, leaves look dull and shrivel, and vigour and yield drop. You see it most at this stage: early vine growth.
Why phosphorus deficiency happens in pakwan
Acid or iron rich soils, low organic matter, cold or waterlogged soil, and no phosphorus at planting.
How to tell it apart
Root knot nematodes also stunt vines. To be sure, soil test for available phosphorus and pH.
What to apply for phosphorus deficiency
Apply phosphorus in the hill based on the soil test. Use single superphosphate for a quick fix and rock phosphate or bone meal for the long term. Uptake is best near pH 6.2 to 7.0. A usual basal dose is 4 to 6 bags of Complete 14 14 14 per hectare, 10 to 20 grams a hill, mixed into the soil at transplanting.
- Complete 14 14 14
- Ammonium phosphate 16 20 0
- Single superphosphate
Sold here with phosphorus: Philphos 18 46 0 (Di Ammonium Phosphate), Atlas Super Gro (16 20 0) by Atlas Fertilizer, Amigo Planters Granular Single Super Phosphate by La Filipina (0 18 0 with 11 percent sulfur) and Triple Super Phosphate by Newman Chemicals (0 46 0). Check the FPA registration on the bag and follow the label rate. For the how and when, see our guide to 16 20 0 fertilizer, and the page on phosphorus deficiency in every crop.
To keep it away, use complete fertilizer or 16 20 0 in the hill at planting.
Zinc deficiency in watermelon
Young leaves yellow between the veins and can turn white. Leaves are pale with green veins and smaller, the spaces between leaves are short, and leaves form distorted rosettes. You see it most at this stage: vine growth.
Why zinc deficiency happens in pakwan
High pH or overlimed soils, heavy phosphorus and waterlogged soils.
How to tell it apart
Mosaic viruses also give small, mottled leaves, often with aphids around. To be sure, soil test for zinc and pH, and a leaf tissue test. In a soil test, less than 0.8 ppm of zinc is low.
What to apply for zinc deficiency
Try zinc sulfate in the soil at about 20 kg per hectare. In severe cases a morning spray of zinc sulfate at 2 grams plus calcium carbonate at 2.5 grams per liter of water (40 grams in a 16 liter sprayer). Do not apply zinc and phosphate together. Give the soil dose at planting or when the rains begin, and sprays on young growth.
- Zinc sulfate
Sold here with zinc: Harvester Zinc Sulphate Heptahydrate by Universal Harvester (22 percent zinc), Tierramino ZN by Unilox Far East Resources (7 0 0 with 10 percent zinc), Semia by SCL Filipinas (20 percent zinc) and Omex Zynergy by Jardine Distribution (3.8 percent zinc and 2.1 percent copper). Check the FPA registration on the bag and follow the label rate. For the how and when, see our guide to fertilizer with zinc, and the page on zinc deficiency in every crop.
To keep it away, keep soil pH between 5 and 7.5. Test the soil every two or three years.
A soil test is the surest check for any watermelon nutrient deficiency. Before you buy, ask your Municipal Agriculture Office where to send a soil sample. The DA regional soil laboratories test soil, and the result says how much of each fertilizer your pakwan needs. That saves money, because extra fertilizer is money lost.
Open the Nutrient Deficiency Finder, pick pakwan, say where the leaves changed first and what you see, and get the fertilizers sold here that fix it.
- Anee, the smart farm technician, reads a photo of the leaf
- The cropping calendar dates every fertilizer application of your season
Watermelon nutrient deficiency questions
Why are my pakwan leaves turning yellow?
What fertilizer fixes nitrogen deficiency in watermelon?
What fertilizer fixes potassium deficiency in watermelon?
What fertilizer fixes magnesium deficiency in watermelon?
How do I know which nutrient my pakwan lacks?
Find the missing nutrient and fix it
With Libre + Anee, read the whole guide: how to tell the deficiency from look alikes, how to be sure, what to apply, and the fertilizers sold here that supply it.
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