Plant Nutrients: Strategies for Your Tucson Garden | Home & Garden

Plants are remarkable creatures. From thin air, water and sunlight they create their own food and structural elements. They also need some basic nutrients, which they get from the soil and water.

Three nutrients essential to plants are familiar to most gardeners – nitrogen, phosphorus and potassium, the NPK listed on all fertilizers. In addition to these three, plants also need calcium, magnesium and sulfur. Together with N, P and K, these six elements make up the macronutrients required by all plants. These nutrients have a number of functions in a plant:

  • Nitrogen: Promotes plant growth, especially green foliage. This macronutrient is commonly required in our soils due to the low organic content of native desert soils.
  • Phosphorus: Needed for root and seedling growth, flower and fruit growth. Also helps with disease resistance. This is another macronutrient that may be in short supply in our native soils.
  • Potassium: Needed for overall growth, especially in fruit trees, and disease resistance.
  • Calcium: plant structure and growth. Abundant in our soils.
  • Magnesium: Essential for leaf growth and sugar synthesis. Particularly important in fruiting plants such as tomatoes.
  • Sulfur: Also important for green growth and fruit development.

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Plants also need small amounts of micronutrients in order to fulfill their cell functions and to grow happily. Micronutrients include iron, chlorine, zinc, molybdenum, boron, manganese, copper, sodium and cobalt. Most of these are found in abundance in our native desert soils. However, due to the alkalinity of our desert soils, these micronutrients are not always present in a plant-available form. As such, plants (especially non-native ones) sometimes need extra iron or zinc. For example, citrus trees growing on our soils need regular fertilization with nitrogen, iron and zinc to stay healthy and produce good fruit.

Fertilizers can be natural or synthetic, and can be either water soluble (and therefore released into the soil immediately) or slowly released. My article on natural fertilizers covers some of the options for gardeners who prefer natural fertilizers. The best fertilizers for your plants are complex, organic molecules found in compost and composted manure. You can read more about this in my article on organics and fertilizers.

If you are planting in the ground, you can add organic matter such as compost, manure, or vermicompost (earthworm droppings) to the top of the soil. The creatures in the soil ensure that these are built into the deeper soil layers over time. However, as expected, this will take some time. It also needs water, as these same creatures usually rest or walk during very dry periods. So if you want to improve your soil, you need to add both organic matter and water.

If you’re having trouble getting plants to grow well in your soil, consider a soil test. This tells you the amount of nutrients and micronutrients in your soil, as well as pH and other properties. Most plant experts will tell you not to apply nutrients haphazardly unless you know for sure what your soil’s nutrient deficiencies are. For a list of labs that can perform the testing for you, check out this helpful list from the University of Arizona Extension Office.

Planting in a container gives you more control. There are many plant- and flower-specific potting soils on the market with various slow-release fertilizers incorporated. You can also make your own mix, like this one recommended by Thrive and Grow Gardens. Container plants can have a lot of organic matter added to the soil, unlike soil plants. For in-soil plants, you want to limit the organic matter you add to just the top layer. This is because your soil volume decreases as the organic matter breaks down (i.e. is processed by soil organisms and their nutrients are taken up by your plants). You don’t want your planted tree to suddenly fall into its planting hole! However, since you will be replenishing your vegetable containers each growing season, it’s okay to have lots of organic matter.

For more information on fertilization, nutrients, and soil testing, check out these helpful handouts from the University of Arizona Extension Office:

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Do you have gardening topics you want the Tucson Garden Guide to cover? Email me at dheusinkveld@tucson.com with your suggestions and questions. Thank you for reading!

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