All plants need three major nutrients: nitrogen (N) for leaves and stems; phosphorus (P) for roots, fruit, and seeds; and potassium (K) for all around health. The numbers on a bag of fertilizer tell you what percentage of each of these nutrients the bag contains. They are called NPK numbers, and they are always in that order. Thus a 100-pound bag of 10-6-4 has 10 pounds of N, 6 pounds of P, and 4 pounds of K. The rest is mostly inert filler. Small amounts of many trace elements, such as iron, magnesium, and zinc, are also present; but unless the soil in your area lacks these, they are of little concern.
Leaf crops, such as lettuce and spinach, need plenty of N. Root crops, such as beets and carrots, should be given little N and some P or they may produce many leaves and small roots. Corn needs N and lots of P. Fruit crops, such as tomatoes and peppers, do best with a balanced fertilizer that leans a little toward the K side.
Plants don’t care whether their nutrients come from organic or chemical fertilizers. Organics, such as manure or dried blood (for N) and bone meal (for P), release their nutrients more slowly than chemicals and are less likely to damage roots. (The K of well-burned wood ashes, however, is released quickly.) Most organics also improve the soil’s texture. Chemicals usually work faster and their contents are precisely measured. Applying fertilizer Dig slow-acting organic fertilizers, such as manure or partly decayed compost, into the soil in the fall for next spring’s planting. Well-decayed manure and compost can be applied in early spring. Apply chemical fertilizers according to the manufacturer’s directions. When in doubt, remember that too little is safer than too much.
For crops grown from seed, mix chemical fertilizer thoroughly into the top 3 or 4 inches of the soil when you prepare it for planting.
When setting out young established plants, dig holes 2 to 3 inches deeper than needed, work a handful of fertilizer into the bottom, and cover with soil. Or after setting a plant out, give it a drink of liquid fertilizer, called a starter solution.
Long-term crops, such as squash, corn, tomatoes, and cucumbers, benefit from a midseason side dressing of high-N fertilizer. Sprinkle it along the rows at least 4 or 5 inches from the stems, or encircle each stem with a small trench and fill it with fertilizer. Mix in the fertilizer, then water.