By David Alosi, U. C. Master Gardener

Standing in front of the fertilizer section of any nursery or garden store can be a bewildering experience. So many brands and so many specialized uses...how can you possibly make a choice? And what do those numbers on the packages mean? Do your plants even need feeding?

Lacking a chemistry degree, you may find all of this a little bewildering. All plants need some nutrients for healthy growth and for flower and fruit production. According to the California Master Gardener Handbook (a great resource for Napa Valley gardeners), there are 17 essential plant nutrients. Three of them—carbon, hydrogen and oxygen—come from air and water. The other 14 are divided into 3 groups: primary nutrients, secondary nutrients and micronutrients.

The numbers on the bags represent the percentages by weight of the three primary nutrients: nitrogen (N), phosphorus (P) and potassium( K). A bag labeled 15-15-15 has equal percentages of each. Fertilizers may be labeled "complete" if they contain all three of these major needs, but they may still lack secondary nutrients and micronutrients. Some specialized fertilizers contain a single nutrient.
Plants that are slow growing, stunted or have yellow-green leaves (especially on older leaves) may have a nitrogen deficiency. Poor fruit set or seed development and blotchy yellowing between the veins of the leaves can indicate lack of phosphorous. Potassium deficiency has similar symptoms but is less common in Napa Valley as the mineral is abundant in most local soils.

The secondary nutrients are calcium, magnesium and sulfur. Lack of calcium can cause many common problems on vegetables and fruits, such as blossom-end rot on tomatoes. Curling leaves can signal magnesium deficiency. The micronutrients include boron, chlorine, copper, iron, manganese, nickel, zinc and molybdenum. They are needed only in very small quantities.
A little fertilizer may be good, but that doesn't mean a lot is better. Too little results in poor plant growth, but too much is a waste of money and can cause serious plant injury. Too much nitrogen causes excessive dark-green growth and can even burn plants and cause them to need a lot more water. On the other hand, nitrogen leaches easily from the root zone, especially in potted plants, and is the most common element that needs to be replenished. It is wise to follow the instructions on the fertilizer package.

Many books and web sites have great photographs of nutrient deficiencies in plants. For starters, try this North Carolina State University site: http://www.ces.ncsu.edu/depts/hort/floriculture/def/mimulus.htm. Soil testing can also help you determine what fertilizer you need and how much. Home soil test kits aren't that precise but will get you in the ballpark. Some laboratories offer an inexpensive soil-testing service. Call the Napa County Master Gardener help desk or check our web site (phone number and web address below) for a list of labs. The California Master Gardener Handbook suggests that you can perform your own inexpensive soil diagnosis by planting corn. It is an excellent indicator of soil nutrient deficiencies. For an illustrated diagnosis chart from the University of Minnesota, visit www.extension.umn.edu/cropenews/2002/nutrientdeficiencyflowchart.pdf.

Soil testing will also tell you the pH of your soil, reflecting its relative acidity or alkalinity. This greatly influences the availability of nutrients. You can apply all the fertilizer in the world, but plants may not absorb it if the soil pH is too high or low. Most nutrients are unavailable to the plant unless the soil pH is between 5.5 and 7.5. You can adjust the soil pH with amendments if it is outside this range.

Some fertilizers, such as cottonseed meal, add organic matter to the soil, which can help improve the soil structure. Because these organic fertilizers are usually slower to release their nutrients, they last longer and are less likely to burn plants from over-fertilization. Liquid fertilizers are faster acting.

Proper watering is an important aspect of fertilization. Yellow leaves can be caused by over-watering or under-watering. Waterlogged soil will cause symptoms of iron deficiency no matter how much iron you apply. Additionally, cold weather (especially this year) can cause your lemon trees to have yellow leaves even if they have adequate fertilizer.

Generally, you need more fertilizer for intensive gardening such as vegetable production, where the plants are constantly removing nutrients. Plants that require a lot of water also tend to need frequent feeding. Established trees and shrubs in the landscape may need little or no fertilizer. While you are out enjoying your garden, stand back and take a good look at your plants. You will soon develop an eye for determining if they are healthy and happy or in need of a meal.