We are so lucky to live in Napa Valley. The soil is rich; we call it healthy. Being predominantly clay, it has great nutrient-holding capacity. With some compost to balance its structure, we're able to grow an amazing variety of fruits, vegetables, flowers and almost every other kind of plant suitable for our climate zone.

      Sunset's Western Garden Book tells us that we live in zone 14, an area of hot, dry summers and temperate winters with occasional frosty nights. I find this to be a most forgiving area because it doesn't seem to matter if I forget to water or fertilize on schedule. My plants do well in spite of my negligence.

     My roses thrive although I haven't fed them anything but a spring dose of compost and aged steer manure for almost 15 years. When I plant vegetables, I mix in compost, chicken manure and aged steer manure and that's it. No more fertilizer for the rest of the season.

     The harvest no longer surprises me. I am blessed with huge zucchini, enough tomatoes to feed the neighbors and foot-long cucumbers. The tomatoes won't win any prizes, but they are just the right size for me. Even though I'm lazy or forget to add fertilizer, I know the soil will come to my rescue.

     I don't have to run to the store to load up on a lot of different fertilizers because the soil already contains 13 essential nutrients in mineral form. Because plants take up nutrients only in soluble form, if I don't water or it doesn't rain, the nutrients just remain in the soil undissolved. 

     The primary nutrients, required by all plants, are nitrogen, phosphorus and potassium. Nitrogen, the nutrient plants need most, is necessary for green growth. Phosphorus and potassium boost flower and fruit production and encourage strong roots.

     Vegetables and annuals use large quantities of nitrogen because they must grow, flower and produce seed in one short season. Gardeners who grow vegetables or annuals usually supplement the soil with organic or synthetic nitrogen fertilizer.

      If over-applied, nitrogen can filter down through the soil, away from root tips, where it can't be used. This excess nitrogen can pollute our water supply. Too much nitrogen also encourages green growth at the expense of flowers, fruits and vegetables, and it can invite disease.

      Napa Valley soils usually have enough phosphorus and potassium, but if you use supplements, place them close to the root zone. Unlike nitrogen, these two nutrients do not move much in the soil.

     Read the labels on any fertilizers you buy. Labels always display the NPK ratio, the percentage of nitrogen (N), phosphorus (P) and potassium (K) that the fertilizer contains. A fertilizer labeled as 10-5-5 contains 10 percent nitrogen, 5 percent phosphorus and 5 percent potassium.

      In the fall, avoid fertilizing with nitrogen because it promotes tender new growth that could be damaged by frost. A more useful fall fertilizer might read 0-10-10: no nitrogen, but 10 percent each of phosphorus and potassium for promoting strong roots and good bud production.

      Secondary nutrients in the soil include calcium (Ca), magnesium (Mg) and sulfur (S).

Calcium is essential in cell wall development. Calcium deficiency reduces bud growth and causes blossom end rot in tomatoes, peppers and melons. Magnesium is an essential part of photosynthesis. Sulfur raises soil acidity.

     Micronutrients, also known as trace elements, are as essential to plant health as the more familiar macronutrients. Like pieces in a puzzle, each has a job to do. There are seven micronutrients: zinc (Zn), iron (Fe), copper (Cu), manganese (Mn), boron (B), molybdenum (Mo) and chlorine (Cl). If soil is deficient in any of these nutrients, plants will display irregularities.

      Vegetables such as tomatoes, cucumbers, eggplant, corn and melons use lots of nitrogen, phosphorus and potassium, so after harvesting these summer crops, you should replace these nutrients. Cover crops such as clover, rye, beans and peas will return nitrogen to the soil.

     Plant a row or two of fast-growing fava beans in September when you plant a winter vegetable garden. Harvest the young beans for appetizers, chop the rest of the plant up, and till it back into the bed. Soil microorganisms and earthworms will gradually break down this green plant material, renewing your soil and rewarding you with a wealth of vegetables when you replant.