Tomato Fertilizer 4-18-38 | Chelated Micronutrients | Hydroponics, Soil & Foliar Application
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Low nitrogen. High phosphate. Very high potassium. Engineered for the flowering-through-harvest window in tomatoes, peppers, eggplants, and other fruiting nightshades — with EDTA-chelated micronutrients in every bag. 100% water-soluble. CDFA registered. Independently lab tested for heavy metals — results consistently well below required limits.
Find your size → Calculate how much I need38%
Soluble potash (K₂O) — dominant for fruit sizing and quality
18%
Available phosphate (P₂O₅) for bloom and reliable fruit set
4%
Controlled nitrogen — energy directed to fruit, not foliage
100%
Water-soluble — safe for drip, NFT, Dutch buckets, foliar
Every bag of 4-18-38 dissolves to a clean 100%-soluble feed. Pick the size that matches your operation — from a few container tomatoes on the patio to a half-acre Dutch-bucket greenhouse.
| Bag Size | Soil / Container Plants | Hydroponic Reservoir Feeds | Best For |
|---|---|---|---|
| 2 lb | ~150 plants/season (3 g per plant, 3 feedings) | ~200 gal at fruiting rate (0.5 lb/100 gal) | Patio gardeners, 4–8 plants in containers |
| 5 lb | ~375 plants/season | ~500 gal at fruiting rate | Most popular |
| 25 lb | Up to ~2,000 plants/season | ~2,500 gal at fruiting rate | Market gardeners, mid-size greenhouses |
| 50 lb | Commercial-scale | ~5,000 gal at fruiting rate | Best value |
The 4-18-38 ratio is tuned to the unique nutritional demands of fruiting nightshades during flowering, set, and fruit fill — not the lush vegetative growth that high-nitrogen all-purpose fertilizers push.
Beefsteak, cherry, Roma, heirloom, grape. The flagship crop — the K₂O:N ratio is built around tomato fruit physiology.
Bell, jalapeño, habanero, sweet, hot. Peppers benefit from the same low-N, high-K balance during fruiting.
Standard, Asian, Italian varieties. Heavy potassium demand during fruit fill makes 4-18-38 a strong fit.
White, red, fingerling. Useful where soil test indicates phosphorus and potassium are warranted — soil test before applying.
The salsa verde staple. Similar nutritional profile to tomatoes — high K supports fruit husk filling.
Goji, ground cherry, pepino, tamarillo. The chelated micronutrient package covers the trace needs of less-common crops.
Tomatoes and their cousins don't need the same thing every week of the season. Once flowering begins, the nutrient demand shifts — and so does this formula.
Tomatoes are among the most potassium-demanding crops, especially during fruit fill. Potassium is known to activate more than 60 plant enzyme systems and directly drives fruit sizing through cell expansion, flavor through sugar transport, and firmness through cell-wall strength — all of which contribute to better post-harvest shelf life.
Phosphorus drives ATP synthesis (the plant's energy currency), nucleic acid and phospholipid production, and active root growth — all of which peak during reproductive growth. Elevated P₂O₅ during this window supports robust flowering and reliable fruit set.
Excessive nitrogen during fruiting causes lush foliage at the expense of fruit production and delays ripening. The 4% here keeps photosynthetic leaves green without diverting energy away from developing fruit. Combined with the 38% K₂O, the 9.5:1 K:N ratio fits standard fruiting-stage tomato programs.
Iron, manganese, zinc, and copper are EDTA-chelated to stay plant-available across a wider pH range than sulfate forms. Boron (boric acid) and molybdenum (sodium molybdate) round out the trace nutrient package — supporting pollination, cell-wall formation, and nitrogen metabolism.
Equally at home in a watering can, a backpack sprayer, a drip line, a Dutch bucket, an NFT channel, or a DWC reservoir. No grit, no settling, no clogged emitters when pre-dissolved — the entire bag goes into solution.
Registered with the California Department of Food and Agriculture. Independently lab-tested for heavy metal content — results consistently well below required limits. Manufactured in Madera, California by a family-owned operation since 1989.
K+
The ionic form plants absorb — freely soluble and mobile in solution
Unlike nitrogen, which plants use to build new tissue, potassium isn't a structural component at all. It functions as an ionic regulator inside the plant — a kind of traffic controller that opens and closes guard cells, activates enzymes, and keeps water and sugars moving through the vascular system. Adequate potassium is closely associated with fruit size, sugar transport, firmness, flavor development, and post-harvest shelf life in fruiting crops.
Tomatoes and their relatives are unusually potassium-hungry. The fruit functions as both a sugar sink and a potassium sink — sugars produced in the leaves get loaded into the phloem and shipped to the developing fruit, and that loading process is potassium-driven. K-deficient plants tend to accumulate sugars in leaves instead of partitioning them to harvestable organs, which is the opposite of what a tomato grower wants.
All three macronutrients in 4-18-38 are delivered in immediately plant-available forms: nitrogen as a mix of ammoniacal and nitrate species, phosphate from monopotassium phosphate (the cleanest soluble P source), and potassium from both nitrate and sulfate sources. The micronutrient package leans on EDTA chelation for iron, manganese, zinc, and copper — keeping those metals in solution across the pH 5.5–6.5 window typical of hydroponic reservoirs and well-managed garden soils, where sulfate forms can begin to tie up. (For correction in higher-pH soils, DTPA or EDDHA iron sources hold availability further into alkaline territory.)
For a deeper read on the underlying chemistry, see What Does Potassium Do for Plants? and What's the Function of Phosphorus in Plants?
Five tabs covering hydroponics, foliar, soil and container, drip fertigation, and field-scale commercial. Rates assume RO or rainwater for hydroponics — reduce Cal-Mag stock and Calcium Nitrate when using tap or well water with 30+ ppm calcium.
Quick answer: 0.25–0.75 lbs of 4-18-38 per 100 gal depending on growth stage, paired with a separate calcium source. Maintain pH 5.8–6.3, EC 1.4–3.2 by stage.
| Growth Stage | 4-18-38 | Cal-Mag Plus stock | pH | EC Target |
|---|---|---|---|---|
| Seedling / Early Veg | 0.25 lbs | 250 ml | 5.8–6.2 | 1.4–1.6 EC (700–800 PPM*) |
| Vegetative | 0.5 lbs | 500–1,000 ml | 5.8–6.2 | 1.8–2.2 EC (900–1,100 PPM*) |
| Flowering / Fruiting | 0.75 lbs | 1,000–1,500 ml | 5.8–6.3 | 2.4–3.2 EC (1,200–1,600 PPM*) |
Quick answer: 1 lb of 4-18-38 per 100 gallons of spray solution (~1 tsp per gallon) is the maintenance rate. Apply early morning or late afternoon only, below 85°F leaf temperature.
| Purpose | 4-18-38 Rate | Frequency | Notes |
|---|---|---|---|
| Maintenance / boost | 1 lb / 100 gal (~1 tsp/gal) | Every 10–14 days | Spray to glistening, not runoff |
| Pre-bloom push | 1 lb / 100 gal | Once, 7–10 days before first flowers | May add Cal-Mag stock 5 ml/gal to tank |
| Bloom support | 0.5 lb / 100 gal | Weekly during early flowering | Avoid open flowers; may add Cal-Mag stock 5–10 ml/gal |
| Deficiency correction | 1.5–2 lbs / 100 gal (max) | Every 5–7 days, max 3 sprays | Test small area first. Apply below 85°F leaf temp only. |
Quick answer: 3 g (~½ tsp) per plant every 4–6 weeks during the growing season, or 0.5–1.5 lbs per 100 sq ft for raised beds, paired with a separate calcium source.
| Application Method | Rate | Timing |
|---|---|---|
| Pre-plant incorporation | 1.5 lbs / 100 sq ft | 1–2 weeks before transplant. Add 2–3 lbs Gypsum / 100 sq ft for calcium. |
| Side-dress (vegetative) | 0.5 lb / 100 sq ft, banded 4″ from stem | At first flower, at first fruit set, and 3 weeks later |
| Container — at potting | 0.5 tsp / gal of soil volume, mixed in | At potting; top-dress monthly |
| Container — liquid feed | 0.5–1 tsp / gal water | Every 2 weeks during active growth |
| Soil drench (transplant) | 0.25 lb / 100 gal | At transplant; repeat in 2 weeks |
| Soil drench (vegetative) | 0.5 lb / 100 gal | Every 7–14 days |
| Soil drench (fruiting) | 0.75–1.0 lb / 100 gal | Every 7–14 days during fruit fill |
Quick answer: 0.5–1.0 lbs of 4-18-38 per 100 gal of irrigation water for weekly feeds, or 25–50% of that rate for continuous fertigation through every irrigation.
| Program | Rate | Frequency |
|---|---|---|
| Weekly feed (typical) | 0.5–1.0 lb / 100 gal | 1× weekly during vegetative and fruiting |
| Continuous fertigation | 0.125–0.5 lb / 100 gal (25–50% of weekly rate) | Every irrigation cycle |
| High-yield fruiting push | 1.0 lb / 100 gal | Weekly during peak fruit fill; back off if EC trends above target |
Quick answer: Field-scale tomato programs typically apply 200–500 lbs of 4-18-38 per acre per season, split across the fruiting window. Always confirm with a current soil test and crop removal estimate.
| Soil Test / Crop Demand | 4-18-38 / Acre / Season | Split |
|---|---|---|
| Light (high baseline soil P and K) | 200–275 lbs | Side-dress at first fruit set, repeat 3 weeks later |
| Standard (medium-testing soil) | 275–400 lbs | 3–4 splits across fruiting |
| Heavy (high-yield with drip irrigation) | 400–500 lbs | Continuous fertigation through fruit fill |
4-18-38 dissolves cleanly in cold water with zero residue. Whether you're feeding three patio tomatoes or running a Dutch-bucket greenhouse, the steps are the same — pre-dissolve, add calcium source separately, apply, water in. The calculator handles the math for whichever method you're using.
Always pre-dissolve each fertilizer in a separate small bucket of water before adding to the reservoir or watering can. Calcium sources and high-phosphate fertilizers must never be combined in concentrated form — doing so causes calcium phosphate precipitation.
Fill your reservoir or container with fresh water (RO or rainwater for production hydroponics). Add Calcium Nitrate solution first (if using), stir thoroughly. Add Cal-Mag Plus stock next. Add dissolved 4-18-38 last, stir until fully dispersed.
Adjust pH to 5.8–6.2 for seedling and vegetative stages, 5.8–6.3 for flowering and fruiting. Verify EC matches your target growth stage before introducing plants. For soil growing, aim for soil pH 6.2–6.5.
Top off the reservoir with pH-adjusted water as plants drink it down. Replace solution completely every 7–14 days in recirculating systems. For soil and container growing, water thoroughly after each fertilizer application to move nutrients into the root zone.
Greenway makes several specialty crop fertilizers. They look similar on paper but solve different problems. Here's how 4-18-38 stacks up against the closest siblings.
| Product | NPK | K₂O:N Ratio | Best Fit | When to Pick This Instead |
|---|---|---|---|---|
| Tomato Fertilizer 4-18-38 (this product) | 4-18-38 | 9.5:1 | Tomatoes, peppers, eggplants, tomatillos — flowering through fruit fill | — |
| Pepper & Herb 11-11-40 | 11-11-40 | 3.6:1 | Peppers with sustained N demand, woody Mediterranean herbs | Higher sustained nitrogen for hot peppers and aromatic herbs |
| Strawberry Fertilizer 8-12-32 | 8-12-32 | 4:1 | Strawberries, blueberries, raspberries, all berry crops | Berries instead of nightshades — ratios are tuned to berry physiology |
| Cucumber Fertilizer 8-16-36 | 8-16-36 | 4.5:1 | Cucumbers, melons, squash, gourds, all vining vegetables | Vine crops with longer continuous fruit-set windows |
| Lettuce Fertilizer 8-15-36 | 8-15-36 | 4.5:1 | Leafy greens, lettuce, spinach, herbs | Foliage crops where you want measured nitrogen without pushing bolting |
| Grow Green 4-2-6 | 4-2-6 | 1.5:1 | Vegetative-stage hydroponic feeding for any crop | Early vegetative growth before flowering begins |
A simple yes/no read. If most of the left column matches your situation, 4-18-38 is the right call. If you're nodding along to the right column, take a look at the alternative linked there instead.
4-18-38 covers NPK and the chelated micronutrient package. To build a complete nightshade program, add calcium (always separately from this formula) and magnesium where your water source is short. Here are the four most common companions.
Calcium, magnesium, and chelated iron in one bag. The default calcium pairing for hydroponic growers and container gardeners on RO water.
Hydro alternative15.5% N + 19% Ca. Water-soluble calcium source with a small nitrogen contribution. Always add to the reservoir first, before any phosphate source.
Soil calcium23% Ca + 18% S. Pre-plant soil amendment that builds calcium without raising pH. The right pick for raised-bed and in-ground tomato gardens.
Magnesium boostUSP-grade magnesium and sulfur. Useful where soil or water tests low in magnesium — interveinal yellowing on older leaves is the classic deficiency sign.
Concentrated fertilizer salts — including this one — need to be handled with the same care you'd give any soluble plant nutrient. The five rules below cover most of what goes wrong in practice.
The questions we hear most often by phone and email. If yours isn't here, reach out at questions@greenwaybiotech.com.
Tomatoes are among the most potassium-demanding crops, especially during fruiting. Potassium is known to activate more than 60 plant enzyme systems and directly controls fruit size and weight, sugar content and flavor, and post-harvest shelf life. The 38% helps ensure your plants have consistent potassium availability through the critical fruiting period when demand peaks.
For a deeper read, see our article What Does Potassium Do for Plants?
Tomatoes prefer slightly acidic conditions. For hydroponics, maintain pH between 5.8–6.2 during seedling and vegetative stages, and 5.8–6.3 during flowering and fruiting. For soil growing, aim for 6.2–6.5. pH directly affects nutrient availability — outside these ranges, key nutrients like iron and phosphorus can become locked out even when present in the solution.
Blossom end rot is caused by insufficient calcium reaching the developing fruit — which, in most home-garden and hydroponic cases, is a transport-and-water-management problem rather than a soil calcium shortage. Inconsistent watering, big swings in soil moisture, and rapid vegetative growth that outpaces calcium uptake can all trigger BER even when calcium is present in the root zone. This formula doesn't contain calcium because calcium and phosphate should not be mixed together in concentrated stock solution — combining them directly can form insoluble calcium phosphate that becomes unavailable to plants. Any high-phosphate fertilizer should be paired with a separate calcium source applied independently.
This formula can still be part of a reduced-BER approach: the controlled nitrogen level helps limit excessive vegetative growth that could otherwise compete for available calcium, and consistent feeding keeps the plant's overall mineral balance on track. Supplement with a dedicated calcium source and maintain consistent, even watering — calcium uptake depends on steady moisture as well as availability.
Recommended calcium sources:
Yes. This formula is well suited to all nightshade family plants including tomatoes (beefsteak, cherry, Roma, heirloom), all pepper types (bell, hot, sweet), eggplants (standard and Asian varieties), tomatillos, and specialty crops like goji berries. It can also be used on potatoes, though potato programs often benefit from a current soil test given their distinct yield and tuber-quality requirements. Larger plants like beefsteak tomatoes may need roughly 25% more fertilizer than cherry tomatoes due to their greater biomass.
Note: sweet potatoes are not nightshades and have different nutritional needs — this formula isn't recommended for them.
Three concrete reasons. First, potassium drives turgor-pressure-related cell expansion — bigger cells mean bigger fruit. Second, the same potassium gradient pulls sugars from leaves into developing fruit through the phloem, so well-fed plants partition more of their photosynthetic output into the harvest rather than into vegetative growth. Third, potassium is involved in pectin cross-linking and cell-wall structure, which directly affects firmness, crack resistance, and how long the fruit holds up after picking. If you've grown tomatoes that taste flat or split as they ripen, K supply during fruit fill is one of the first things to investigate. For a deeper read, see What Does Potassium Do for Plants?
4-18-38 is hygroscopic, meaning it absorbs moisture from the air. Keep the bag sealed tight between uses and store in a cool, dry place out of direct sunlight. If it clumps from moisture exposure, simply break it up — the efficacy isn't affected. Keep out of reach of children and pets.
The low 4% nitrogen is intentional. During fruiting, excessive nitrogen causes lush foliage at the expense of fruit production and delays ripening. The 4% provides enough nitrogen to maintain healthy green leaves for photosynthesis without diverting energy from fruit development. For more on nitrogen's role, see What's the Function of Nitrogen in Plants?
Calcium and phosphate should not be mixed together in concentrated stock solution — combining them directly can form insoluble calcium phosphate that becomes unavailable to plants. Since 4-18-38 contains 18% available phosphate (P₂O₅), calcium is best applied separately. Pre-dissolve each independently before combining in the working solution. Use Cal-Mag Plus 2-0-0 or Calcium Nitrate 15.5-0-0 as a companion product.
Yes — 4-18-38 is 100% water-soluble and well suited to NFT, DWC, Dutch bucket, ebb and flow, and Kratky systems. The standard rates are 0.25 lbs / 100 gal for seedling stage, 0.5 lbs / 100 gal for vegetative, and 0.75 lbs / 100 gal for flowering and fruiting, paired with a separate calcium source. See the Application Rates section for the full table including EC and pH targets.
For a full hydroponic tomato program, see our step-by-step guide to growing hydroponic tomatoes.
4-18-38 is a synthetic blended fertilizer — not OMRI listed. If you're certified organic or growing strictly to organic standards, you'll want to use mineral-based amendments instead: bone meal for phosphate, K-Mag (sulfate of potash magnesia) for potassium and magnesium, and a sulfate-form calcium source. For home gardens that aren't strictly organic, 4-18-38 is independently lab-tested for heavy metals with results consistently well below required limits, and CDFA registered.
2 lb for the patio. 5 lb for the home garden. 25 lb for the market garden. 50 lb for the field. Same 4-18-38 formula in every bag, dissolved in cold water, clean to the last drop. Free shipping in the continental US on orders over $100, backed by our 90-day guarantee.
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