Cucumbers are very heavy feeders. They require phosphorus, potassium, calcium, magnesium, and sulfur in particularly large amounts. Cucumbers are particularly sensitive to high levels of boron, whether in the growing medium or the water supply. The best balance of macro nutrients (Nitrogen, Phosphorous, and Potassium) is 5:6:4 respectively.
HOW TO FEED CUCUMBERS
When the plants have 5 or 6 true leaves and are in their permanent location, you can begin a monthly program of side dressing with a mixture of composted manure and composted blood meal. A foliar spray may be made from diluted liquid fish emulsion and seaweed combination and applied to the plants every other week during the growing season.