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Tomato Fertilizer 4-18-38 | Chelated Micronutrients | Hydroponics, Soil & Foliar Application

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Weight: 1 Pound

Greenway Biotech · Made in California since 1989

Tomato Fertilizer 4-18-38.
Built for the fruiting window.

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 need

38%

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

01 / Choose your size

Right-sized
for the job.

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.

Coverage by bag size for Tomato Fertilizer 4-18-38
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
02 / Ideal applications

One bag.
Six nightshade crops.

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.

Tomatoes

Beefsteak, cherry, Roma, heirloom, grape. The flagship crop — the K₂O:N ratio is built around tomato fruit physiology.

Peppers

Bell, jalapeño, habanero, sweet, hot. Peppers benefit from the same low-N, high-K balance during fruiting.

Eggplants

Standard, Asian, Italian varieties. Heavy potassium demand during fruit fill makes 4-18-38 a strong fit.

Potatoes

White, red, fingerling. Useful where soil test indicates phosphorus and potassium are warranted — soil test before applying.

Tomatillos

The salsa verde staple. Similar nutritional profile to tomatoes — high K supports fruit husk filling.

Specialty Nightshades

Goji, ground cherry, pepino, tamarillo. The chelated micronutrient package covers the trace needs of less-common crops.

03 / Why 4-18-38

Built for the
fruiting window.

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.

38%

Potassium dominant — sized for fruit, not foliage.

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.

18%

Available phosphate (P₂O₅) for bloom and set.

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.

9.5:1

K₂O:N ratio — potassium nine times nitrogen.

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.

6×

Six chelated and mineral micronutrients in one bag.

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.

100%

Dissolves clean in cold water. Zero residue.

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.

CDFA

Registered, lab-verified, and heavy-metal tested.

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.

04 / The science

Why potassium matters most.

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?

05 / Application rates

Pick your use.
Get your rate.

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.

Hydroponic — rates per 100 gallons of reservoir

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.

📋 RO / rainwater is assumed below. With tap water of 30+ ppm calcium, reduce Cal-Mag Plus stock by 30–50% and Calcium Nitrate by ~25%. Always verify with a meter rather than dosing by the table alone.
Hydroponic application rates — Tomato Fertilizer 4-18-38 per 100 gallons
Growth Stage 4-18-38 Cal-Mag Plus stock pH EC Target
Seedling / Early Veg0.25 lbs250 ml5.8–6.21.4–1.6 EC (700–800 PPM*)
Vegetative0.5 lbs500–1,000 ml5.8–6.21.8–2.2 EC (900–1,100 PPM*)
Flowering / Fruiting0.75 lbs1,000–1,500 ml5.8–6.32.4–3.2 EC (1,200–1,600 PPM*)
*PPM scale note. Values shown use the 500-scale (EC × 500). On the 700-scale, multiply EC by 700 (e.g., 2.4 EC = 1,680 PPM). EC is the more accurate measure — use it when possible. Cal-Mag Plus stock = the entire 5 lb bag dissolved in 2 gallons of water; the ml values above are doses of that prepared stock per 100 gallons of reservoir. For commercial production, leaf tissue testing during fruit fill is the most reliable confirmation that the program is hitting its targets.
RO / rainwater: supplement with Calcium Nitrate. When using RO or rainwater, Cal-Mag Plus alone may not deliver enough calcium for fruiting crops at peak load. Add Calcium Nitrate at 0.25–0.40 lbs / 100 gal during vegetative growth, increasing to 0.55–0.75 lbs / 100 gal during fruiting. Pre-dissolve Calcium Nitrate separately and add it to the reservoir first, before Cal-Mag Plus or 4-18-38. If final EC exceeds your target, reduce the 4-18-38 by 10–20%.

Foliar — rates per 100 gallons of spray solution

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.

⚠️ Foliar burn risk. Do not exceed 2 lbs per 100 gallons without first testing on a small area. Apply early morning or late afternoon only — never in direct midday sun or when temperatures are above 85°F. Do not combine Cal-Mag Plus concentrate with 4-18-38 concentrate — pre-dissolve each separately into the diluted spray tank.
Foliar application rates — Tomato Fertilizer 4-18-38 per 100 gallons of spray solution
Purpose 4-18-38 Rate Frequency Notes
Maintenance / boost1 lb / 100 gal (~1 tsp/gal)Every 10–14 daysSpray to glistening, not runoff
Pre-bloom push1 lb / 100 galOnce, 7–10 days before first flowersMay add Cal-Mag stock 5 ml/gal to tank
Bloom support0.5 lb / 100 galWeekly during early floweringAvoid open flowers; may add Cal-Mag stock 5–10 ml/gal
Deficiency correction1.5–2 lbs / 100 gal (max)Every 5–7 days, max 3 spraysTest small area first. Apply below 85°F leaf temp only.

Soil & container — per plant and per square foot

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.

⚠️ Calcium supplementation required. 4-18-38 does not contain calcium because calcium and phosphate should not be combined in concentrated stock solution — mixing them directly can form insoluble calcium phosphate that becomes unavailable to plants. Supplement with Cal-Mag Plus, Calcium Nitrate, or Gypsum, especially during fruiting. Consistent, even watering is equally important — calcium uptake depends on steady moisture as well as availability.
Soil and container application rates — Tomato Fertilizer 4-18-38
Application Method Rate Timing
Pre-plant incorporation1.5 lbs / 100 sq ft1–2 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 stemAt first flower, at first fruit set, and 3 weeks later
Container — at potting0.5 tsp / gal of soil volume, mixed inAt potting; top-dress monthly
Container — liquid feed0.5–1 tsp / gal waterEvery 2 weeks during active growth
Soil drench (transplant)0.25 lb / 100 galAt transplant; repeat in 2 weeks
Soil drench (vegetative)0.5 lb / 100 galEvery 7–14 days
Soil drench (fruiting)0.75–1.0 lb / 100 galEvery 7–14 days during fruit fill

Drip fertigation — continuous and weekly feed

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.

📋 Keep calcium sources (Cal-Mag Plus, Calcium Nitrate, gypsum slurry) in a separate stock tank from 4-18-38. Pre-dissolve 4-18-38 fully before injecting through drip lines — undissolved particles can clog emitters even when the product is 100% soluble.
Drip fertigation rates for Tomato Fertilizer 4-18-38
Program Rate Frequency
Weekly feed (typical)0.5–1.0 lb / 100 gal1× weekly during vegetative and fruiting
Continuous fertigation0.125–0.5 lb / 100 gal (25–50% of weekly rate)Every irrigation cycle
High-yield fruiting push1.0 lb / 100 galWeekly during peak fruit fill; back off if EC trends above target

Commercial / field-scale — per acre

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.

📋 Rates below are starting ranges based on medium-testing soils. They are not a substitute for a soil test, tissue testing during the season, or local extension guidance. Crop removal estimates for tomatoes can range from 150–350 lbs K₂O per acre depending on yield target.
Commercial field rates for Tomato Fertilizer 4-18-38
Soil Test / Crop Demand 4-18-38 / Acre / Season Split
Light (high baseline soil P and K)200–275 lbsSide-dress at first fruit set, repeat 3 weeks later
Standard (medium-testing soil)275–400 lbs3–4 splits across fruiting
Heavy (high-yield with drip irrigation)400–500 lbsContinuous fertigation through fruit fill
For commercial programs, contact us at questions@greenwaybiotech.com for crop-specific guidance and to discuss your soil test results.
06 / How to use & calculate

Dissolve it.
Drench or spray.
Done.

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.

  1. 01

    Pre-dissolve in a separate bucket.

    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.

  2. 02

    Add in the right order.

    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.

  3. 03

    Check pH and EC.

    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.

  4. 04

    Monitor and refresh.

    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.

07 / Compare

4-18-38 vs
other crop formulas.

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.

Comparison of Tomato 4-18-38 against other Greenway crop-specific fertilizers
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
08 / Decision

Is this the right
fertilizer for you?

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.

Best Choice For

  • You're growing tomatoes, peppers, eggplants, tomatillos, or other fruiting nightshades
  • You're in the flowering through fruit-fill window (or about to be)
  • You're running a hydroponic system, raised beds, containers, or a drip-irrigated field
  • You want a single bag that covers macros plus chelated micros
  • You're willing to supplement calcium separately (Cal-Mag, Calcium Nitrate, or gypsum)
  • You value CDFA registration and independent heavy-metal testing

Consider Another Product If

10 / Safety & handling

Read this before
you mix.

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.

  • Never combine calcium and 4-18-38 in concentrated form. Pre-dissolve each separately, then combine in the diluted reservoir or spray tank. Direct mixing causes calcium phosphate precipitation and locks up both nutrients.
  • Wear gloves and eye protection when handling dry powder or concentrated solutions. Avoid inhaling dust — measure in a ventilated area or wear a dust mask.
  • Foliar: below 85°F leaf temperature, early morning or late afternoon only. Never spray in direct midday sun or when plants are drought-stressed. Test on a small area before applying any new rate at full scale.
  • Store sealed, cool, and dry. 4-18-38 is hygroscopic — it will absorb humidity from the air if left open. If it clumps, break it up; efficacy is unaffected. Keep out of reach of children and pets.
  • If exposure happens: eyes — flush with clean water for 15 minutes. Skin — wash with soap and water. Ingestion — do not induce vomiting; contact Poison Control (1-800-222-1222). See the SDS in the Documents section for complete safety information.
11 / FAQ

Common questions.
Honest answers.

The questions we hear most often by phone and email. If yours isn't here, reach out at questions@greenwaybiotech.com.

Why is the potassium (38%) so high in this formula?

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?

What pH should I maintain for tomatoes?

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.

How do I help prevent blossom end rot?

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:

Can I use this for all tomato and pepper varieties?

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.

Why does potassium matter so much for fruit quality?

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?

How do I store the powder?

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.

What role does nitrogen play at just 4%?

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?

Why doesn't this fertilizer contain calcium?

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.

Can I use 4-18-38 in hydroponics?

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.

Is 4-18-38 safe for organic gardens?

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.

12 / Documents

Lab-tested.
State-registered.

Three documents you may want for your records, your buyer, or your certifier. The SDS is required by OSHA for any concentrated fertilizer; the label and heavy-metal analysis are provided for transparency.

Ready to feed your fruiting tomatoes?

Pick your bag. We'll ship it.

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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