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

It depends on how you grow. In hydroponics and containers on RO or rainwater, yes — those water sources carry little or no calcium, so a dedicated calcium source is important, and Calcium Nitrate 15.5-0-0 is the standard choice (with Cal-Mag Plus for magnesium as needed). On hard tap or well water, your water may already supply calcium — test first and reduce inputs. In raised beds and in-ground soil, Gypsum is often the better calcium fit, applied per soil test.

Not in concentrated stock. Calcium and phosphate react to form insoluble calcium phosphate that precipitates out and locks up both nutrients. You can use them together in the same reservoir when added sequentially into diluted water: pre-dissolve each separately, add the calcium nitrate solution first and stir, then add the 4-18-38 last and stir. Never combine the two as concentrated solutions in the same stock tank.

4-18-38 is built for the reproductive window. Start the transition around pre-bloom — roughly first-flower — and run it through fruit set and fruit fill. For seedlings and early vegetative growth, use the lower rates, or feed a more balanced formula like Grow Green 4-2-6 until flowering begins, then move up to 4-18-38 as the plant sets fruit.

No — not by itself for most hydroponic fruiting programs. 4-18-38 supplies nitrogen, phosphate, potassium, and chelated micronutrients, but it deliberately contains no calcium (calcium and phosphate precipitate together in concentrated solution). For a complete program, pair it with a separate calcium source — Calcium Nitrate and/or Cal-Mag Plus — and add magnesium if your water source runs short.

Several things move EC: your starting water (RO reads near zero, tap and well water start higher), the calcium and magnesium products you add, your meter's scale (500-scale vs 700-scale give different PPM numbers for the same EC), solution temperature, and meter calibration. The EC targets in the table are the final measured value after everything is mixed — treat them as a target to adjust water toward, not a number the 4-18-38 fraction alone will produce. When in doubt, trust a freshly calibrated EC meter over the table.

Tomatoes are among the most potassium-demanding crops, especially during fruiting. Potassium activates dozens of plant enzyme systems and supports fruit size and weight, sugar content and flavor, and post-harvest shelf life. The 38% helps ensure 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 solution.

Blossom end rot is associated with insufficient calcium reaching the developing fruit — which, in most home-garden and hydroponic cases, is a transport-and-water-management issue rather than a soil calcium shortage. Inconsistent watering, big swings in soil moisture, rapid vegetative growth, root injury, and excess nitrogen can all contribute even when calcium is present in the root zone. This formula contains no calcium because calcium and phosphate should not be mixed in concentrated stock — combining them directly can form insoluble calcium phosphate. 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.

Recommended calcium sources:

• Cal-Mag Plus 2-0-0 — well suited to hydroponic systems and container growing
• Calcium Nitrate 15.5-0-0 — water-soluble option with a small nitrogen boost
• Gypsum (Calcium Sulfate) — calcium source that won't alter soil pH
• Bone Meal 3-15-0 — slow-release organic calcium and phosphate for soil gardens

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 supports the water relations and cell turgor that keep fruit firm — while the pectin cross-linking and cell-wall stability behind crack resistance and shelf life are driven mainly by calcium, which is why 4-18-38 is paired with a separate calcium source. If you've grown tomatoes that taste flat or split as they ripen, both K supply during fruit fill and calcium/water management are worth investigating. For a deeper read, see What Does Potassium Do for Plants?

The low 4% nitrogen is intentional. 4-18-38 is designed as the high-phosphate, high-potassium part of a fruiting program, not the whole nitrogen program. During fruiting, excessive nitrogen causes lush foliage at the expense of fruit and delays ripening. In hydroponics, most complete programs also add Calcium Nitrate separately, which contributes additional nitrate nitrogen along with calcium — so keeping the base nitrogen low leaves room for that separate source without pushing the plant into excessive leafy growth. 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. Feed per plant — about 2.8 g at seedling/vegetative, 4.5 g at flowering, and 6.8 g at fruiting — paired with a separate calcium source, then top up water to hit your final measured EC. 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, 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.

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.