Warning – I am about to rant just a tiny bit about how erroneous people are who speak of fertilizer as “plant food.” This is an oxymoron and flat wrong to boot.
Plants make all of their own food by doing photosynthesis.
Now – so that plants can do photosynthesis and thus feed themselves, plants need specific minerals (chemical elements) from the soil. That is what fertilizer is all about.
You should think of fertilizer as the vitamins & minerals that plants need so they can make their own food. And while they are at it, plants make food and oxygen for us too!
Essential Chemical Elements
The essential elements plants need for life are oxygen (O), carbon (C), and hydrogen (H), which they get from air and water. Then there are all those other chemical elements that are essential for a healthy life and so the plant can complete all life stages such as forming flowers and making fruit. Nutrients are divided into macronutrients and microronutrient based on the amount needed by the plant. All are equally essential.
Almost all fertilizers contain the macronutrients that plants need for life: nitrogen, phosphorous, and potassium, or by chemical symbols – N, P, and K. You can purchase fertilizer in more forms than you can shake a stick at, and they come with a bewildering variety of labels, but – by government mandate – the N-P-K amounts will be listed, and in that order.
Plants are complicated living beings, and need far more than the six basic elements for life. I could list them all and discuss them, but it would take pages. These additional elements are what makes hydroponics, aquaponics, and even vertical and container gardening so challenging. Often a single element lacking (like magnesium) can mean failure. This is also why many people are proponents of using organic fertilizers rather than synthetic fertilizers, because the required trace elements are generally in there.
When to Fertilize
Fertilizer should be provided to landscape plants when they are actively growing but not too close to the time when it might freeze in your area. Fertilizer will stimulate new growth and such tender growth is more susceptible to frost damage. Fertilize at least one month before “first frost” – which is an average date and your local cooperative extension should have the information. There is also a “last frost” date after which you can set out heat-loving vegetables like tomatoes. Last frost marks the time to fertilize again in spring.
Plants that are actively producing flowers, fruits, or nuts should be fertilized carefully. If a nitrogen rich fertilizer is applied, plants may drop many of the flowers or fruit they have started and switch to growing leaves. Fruiting fertilizers carry their own hidden problems, in that too much may cause the plant to drop an number of developing fruit and just concentrate on making a few really large ones. Do read and follow label directions. If in doubt, use less than they call for.
Some plants are stunted by fertilizer. Members of the legume or pea family work with soil bacteria to take nitrogen out of the atmosphere. If you give them nitrogen-rich fertilizer they may stop growing.
Cacti and other succulents have slow metabolisms. Always use fertilizer at half the recommended dose on them.
Fertilizer should not be applied to newly planted plants. Fertilize at least two weeks after plants are rooted in and established. Use root growth promoters if you desire, but not general fertilizer.
Less is More in the Desert
Always read and follow label directions for any chemical compound, and this includes fertilizer application. Too much fertilizer can kill the plant you wanted to help. If you are in doubt, err on the side of caution. Apply half strength fertilizer twice. Wait two weeks in between time for plants to show you that you got the dose right or wrong.
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More about fertilizer for your landscape in this book – Month by Month Gardening for Arizona, Nevada, and New Mexico (Cool Springs Press). This link is to Amazon and if you buy the book there the Horticulture Therapy non-profit “Tierra del Sol” may get a few pennies at no extra cost to you.
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