Rich, well-aged compost is the best food for cucumbers. Cucumbers belong to the cucurbit family, along with pumpkins, squash and gourds. Grown for pickling or salads, cucumbers produce best during long, warm days. Standard cucumbers vine and grow rapidly and need vertical support. Bushier dwarf cucumbers take less staking and work well in small garden plots or containers. No matter the type, cucumbers have special nutrient requirements that are best met by organic soil amendments.
Cucumbers have low nitrogen requirements and high potassium and phosphorus requirements. Commercial fertiliser formulas can have too high a nitrogen content for cucumbers. Knowing your soil type helps determine how much and what type of nutrients your cucumbers need. Sandy soils leach vital substances and become nutrient-poor quickly. Heavy soils can lock nutrients up. Most garden soils are helped by adding compost before planting. Adding compost or other organic matter enriches light sandy soils and lightens heavy clay soils. Container cucumbers will need feeding too because many potting soils have little nutrient value.
Beware of overfeeding your cucumbers. It will encourage them to grow -- but not in the way you want. Since cucumbers have low nitrogen requirements, fertilisers high in nitrogen will spark a growth spurt. Instead of producing blossoms and fruit, overfed cucumbers will put their energy into growing as many leaves and shoots as possible. If you use fertilisers too high in nitrogen, flowers that do open can abort and not set fruit.
The best fertiliser for cucumbers is well-aged compost. Compost generally has 2 per cent nitrogen and breaks down over a 15-year period. It won't cause overfeeding and runaway vegetative growth at the expense of fruit. Compost adds reserves of nutrients that stay available to cucumbers over many years. Compost can be applied yearly as mulch or worked into your soil without causing excess nutrient build-up. It supplies phosphorus and potassium that cucumbers need along with many micronutrients. Adding compost, depending on how much is used, reduces or eliminates chemical fertiliser use. To know what the possible nitrogen, phosphorus and potassium content of your compost is, you need to know what was used to produce it. If you make your own compost, you'll know what went into it and what nutrient levels you can expect to have available to your cucumbers.
Feed container cucumbers by mixing compost with your potting soil. If you don't want to mix in compost with your potting soil, use time-release low-nitrogen, high-potassium pelleted fertiliser. Apply it when you see the first true leaves on your cucumbers. You can apply liquid low-nitrogen, high-potassium fertiliser weekly after cucumbers show true leaves. Always follow the label directions for container size and crop. Penn State Extension recommends a 1-1.5-3 NPK ratio for cucumbers.