Fertilizers Demystified

With spring about here, it is time to start fertilizing our garden plants, whether they are landscape shrubs, shade trees, lawns, roses, fruit trees, berries, grapes or vegetables. But many people are confused by the huge number of different fertilizers to choose from. I hope to help clear up a little of the mystery.

There are 18 elements essential to plant growth: carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, boron, chloride, cobalt, copper, iron, manganese, molybdenum, nickel and zinc. Generally, the first three elements carbon, hydrogen and oxygen come from the air or through watering and usually don’t need to be added. The elements boron through zinc are micronutrients and are needed in such small quantities that adding them is not usually necessary, however in alkaline soil environments, such as here in the High Desert, iron and zinc may be insufficient and may need to be added depending on the specific plant. The macronutrients needed in larger amounts include nitrogen (N), phosphorus (P), and potassium (K), plus secondary nutrients calcium, magnesium and sulphur.

That brings us to the N, P, and K – in that order. These correspond to numbers that come on the front of the fertilizer packages, such as those pictured. The numbers correspond to the ratio of the specific nutrients in the package. Nitrogen is responsible for strong stem and foliage growth; phosphorus aids in root growth, plus flower and seed production; and potassium is responsible for plant health and disease resistance.

So, keeping in mind the specific plant and what you are trying to achieve, you can pick the right fertilizer. Nitrogen is the most likely nutrient to be deficient, but if you get too much, your plants may get too leafy at the expense of fruits and flowers. Too much tender new growth can also be targeted by sucking insects such as aphids. If nitrogen is applied too late in the growing season, the lush new growth may be subject to frost damage. Phosphorus is rarely deficient, but if you are looking to get more flowers, fruit or stronger roots it may be needed. If you want more resistance to disease or to hot and cold temperatures, a little potassium might be good, but mot too much or it can interfere with the absorption of calcium and magnesium. I have had many people over the years mention that their tomato plants were huge and green, but not producing tomatoes. The most likely problem is that the plant was getting too much nitrogen and/or not enough phosphorus. The remedy is to cut back on the nitrogen fertilizers and add a little phosphorus. Natural sources of phosphorus include bone meal and bat guano, both are available as packaged fertilizers via nurseries and catalogs.

There are all kinds of choices available from chemical fertilizers to all-natural and organic. Inorganic/chemical fertilizers are made from synthetic substances and are available for fast or slow release. Organic/natural fertilizers come from living or once living organisms and can include manure, fish emulsion, bone meal, blood meal, kelp meal and more. Plants don’t care where their nutrients come from, so whether to use chemical or natural is personal preference – my preference being natural/organic. It is very important to always follow the package directions for amounts to use and application methods – as the saying goes, more is not always better.

There are many fertilizer formulations from “general purpose” with nearly equal proportions of N, P and K, to “special purpose” fertilizers such as high-nitrogen fertilizers (example 29-2-4) for green, quick-growing lawns, or a higher phosphorus formulation (example 6-10-4) for getting more flowers and fruit. Others include special formulations for the specific needs of palms, citrus, blueberries and more.

Determining nutrient deficiencies is also a way to figure out what fertilizers are needed. There are a number of books available, including the “Sunset Western Garden Book”, which has a small section on fertilizers with an illustration describing what symptoms to look for and which nutrient will remedy the problem. The book also offers a lot of other useful gardening information.

If you are unsure of what nutrients your soil needs, soil test kits are available at garden centers and catalogs. Territorial Seed Company – territorialseed.com – offers a couple of different kits depending on the extent of testing you want to do.