Vegetables are classified into three categories as to their nutrient requirements:
- Heavy: cabbage, Irish potatoes, lettuce, onions, sweet potatoes, tomatoes.
- Medium: herbs, asparagus, okra, beans, green peas, beets, pepper, cantaloupes, pumpkin, carrots, radish, corn, sweet, cucumbers, squash, eggplant, Swiss chard, watermelon and all greens.
- Light: Southern peas.
Fertilizer labels contain three numbers that correspond to the percentage of nitrogen (N), phosphorous (P205) and potassium (K20) in the bag. The nutrients will always be listed in this order and in a ratio. A 10-10-10 fertilizer means it has equal amounts of N, P and K. And 5-10-15 means there is twice as much P as N and three times as much K as N.
Why should I buy fertilizer with lesser amounts of nutrients? Why not just buy the highest analysis possible?
Not all of the plants in your garden are alike. Tomatoes flower and produce fruit. Greens just produce leaves. Potatoes flower on top and produce tubers underground.
Use even-number fertilizers on lettuce and other leafy garden crops.
The even analysis (8-8-8, 10-10-10) is used mostly for nonflowering or leafy plants such as lettuce and the greens family. With these, you want to produce a lot of edible foliage.
The other analysis (5-10-15, 4-8-12) is used to grow fruiting plants such as tomatoes and peppers. The high first number is N which produces vegetative growth.
Now, nothing is wrong with vegetative growth. You need it for the leaves to produce chlorophyll for the plant. However, too much N, especially at the wrong time, can cause the fruit on tomatoes and peppers to fall off.
Tomato's Main Concern
Remember, the tomato plant isn't really interested in producing a nice, big, juicy fruit for you to enjoy. It's just trying to produce seed to perpetuate itself.
In the grand scheme of things, putting out a high-N fertilizer signals the fruiting plants to use this extra-N to grow vegetatively. They will slough off their fruit and start growing larger leaves at the expense of fruiting.
Timing the addition of N as a side-dressing is critical, too. The plant needs fertility for growth, so put the fertilizer out at planting. But don't add any more until after the plant flowers and the fruit grows to the size of a dime. Then side-dress with extra nitrogen for the next fruit set.
In short, customize the fertilizer for the plant. The plant will appreciate it.