Cal-Mag Fertilizer

Cal-Mag Fertilizer is a supplement rather than a complete feed, and it runs at every watering in spring and summer at 1/2 to 1 teaspoon per gallon. In fall and winter, ease back to every 2–4 weeks. Use it alongside your regular fertilizer, since it supplies calcium, magnesium and micronutrients rather than a full NPK diet. For plants in coco coir or containers, and for heavy feeders under strong light, the every-watering routine keeps both nutrients steadily available instead of letting them run short between feeds.

Cal-Mag Fertilizer corrects the two secondary nutrients that run short together most often: calcium and magnesium. Calcium builds cell walls and holds new tissue together, so a shortfall shows in distorted growing tips and the blossom-end rot that sinks into tomatoes, peppers and squash. Magnesium sits at the center of chlorophyll, so a shortage shows as yellowing between the veins on older leaves. It’s most useful for plants in coco coir and containers, which run low on both, and for heavy feeders, used alongside your main fertilizer.

It helps, but it’s often not the whole answer. Blossom-end rot is usually an uptake problem: uneven watering stops your plant moving calcium into the fruit even when calcium is present in the soil. Steady, deep watering and mulch are the first fix. Cal-Mag Fertilizer supplies 2% calcium for the cases where the soil or medium is genuinely short, which is common in coco coir and containers, and used at every watering it keeps calcium available as fruit forms. If watering swings between dry and soaked, supplementing alone frequently won’t stop the rot.

Usually yes. Coco coir holds onto calcium and magnesium and can lock them away from your plant, so cal-mag deficiency is common in coco even when you’re feeding regularly. Cal-Mag Fertilizer is made for exactly that gap: add it at every watering, which is how most coco growers use it, at 1/2 to 1 teaspoon per gallon alongside the regular feed. If you grow in coco or a similar inert medium, adding it from the start heads off the distorted tips and interveinal yellowing before they appear.

New growth and fruit that develop after you start will show the difference; existing blossom-end rot, damaged tips and yellowed leaves are already affected and won’t reverse. With Cal-Mag Fertilizer in the routine and watering kept even, later growth and fruit come in healthier over the following weeks. The small nitrate nitrogen fraction and the micronutrients are available immediately. Because these deficiencies build slowly, steady use through the season prevents them more reliably than reacting once symptoms appear.