What Is the Best Fertilizer for Cucumbers?

Cucumbers (Cucumis sativus) belong to the cucurbit family, along with pumpkins (Cucurbita maxima), squash and gourds (both Cucurbita pepo). Popular annual vines grown throughout the United States, they produce best during long, warm summer days. Standard cucumber vines grow rapidly and need vertical support. Dwarf bush cucumbers support themselves, and they work well in small garden plots or containers. No matter the type of cucumber vine grown, organic soil amendments meet their special nutrient requirements best.

Cucumbers have low nitrogen requirements, but they need high potassium and high phosphorus levels. With commercial fertilizer formulas, this means the first of the three numbers on the package should be much lower than the other two. Knowing your soil type helps determine how much and what nutrients your cucumbers need. Sandy soils leach vital substances and become nutrient-poor quickly. Heavy soils can lock nutrients up. Adding compost before planting improves most garden soil. The organic matter enriches light sandy soils and lightens heavy clay soils.

Beware of overfeeding your cucumbers with general, all-purpose formulas that contain higher nitrogen levels. They may encourage growth, but not the way you want. Since cucumbers have low nitrogen requirements, fertilizers high in nitrogen spark growth spurts that detract from the fruit. Instead of producing blossoms and fruit, nitrogen-fed cucumbers put their energy into growing vines, leaves and shoots. High-nitrogen fertilizers can also cause cucumber flowers to not open, and cause flowers do open to not set fruit.

Well-aged compost is the best source of nutrients for cucumbers. Compost only has 2 percent nitrogen, and it releases slowly over many years. Compost won’t cause runaway vegetative growth at the expense of fruit. Instead, it adds nutrient reserves that stay available in soil long term. Compost can be applied yearly as mulch or worked into your soil without causing excess nutrient buildup. It also supplies phosphorus, potassium and micronutrients that cucumbers need. Compost mulch also helps keep down competition from weeds, so cucumbers get soil nutrients. If you don't make your own compost, you can buy commercially bagged compost or bulk compost at most garden centers and nurseries.

Feed container cucumbers by mixing compost with your potting soil. You can also add a timed-released, low-nitrogen, high-potassium pelleted fertilizer with a N-P-K ratio similar to 2-3-6. Apply 1 tablespoon per pot at planting, and again when you see the first true leaves on your cucumbers. For large containers over 12 inches in diameter or multiple plants in one pot, increase the amount accordingly. After cucumbers show true leaves, apply water-soluble, low-nitrogen, high-potassium fertilizer weekly. Apply weekly fertilizers at half strength, mixing 1/2 tablespoon of fertilizer with 1 gallon of water. Always wear gloves and protective eyewear when working with fertilizers.