Nutrient-rich soil

DESERT GARDENING 101

Just like humans and animals, plants need varying combinations of nutrients to grow and reproduce. Native plants have evolved to survive in native soils without supplemental feeding. (Proper watering will do more to enhance their health than fertilizer.) However, some non-natives, such as fruit trees, roses, turf, flowers and vegetables, may benefit from careful fertilizer applications.

Gardeners who regularly incorporate plenty of compost into garden soil and who layer organic mulch around plants help to build healthy soil over time. That soil may supply plants with all the nutrition they need with little, or even no, extra feeding. If your soil hasn’t yet reached that stage, the following information will help you understand the role of fertilizers.

NPK Ratio
Plants require relatively large amounts of nitrogen, phosphorus and potassium, which are known as macronutrients. Fertilizer packages are labeled with three numbers, called the NPK ratio, which corresponds to the percentage by weight of nitrogen (N), phosphorus (P) and potassium (K) in the product.

For example, a bag of ammonium sulfate (21-0-0)—a chemical fertilizer—contains 21-percent nitrogen, 0-percent phosphorus and 0-percent potassium. A container of fish emulsion (5-2-2)—an organic fertilizer—contains 5-percent nitrogen and 2-percent each of phosphorus and potassium.

Although interactions among nutrients and plants are complex and not yet completely mapped out by scientists, the following generally hold true: Nitrogen produces foliage growth, phosphorus helps set flowers and fruits, and potassium promotes overall vigor and resistance to disease. Put into practice, this would mean that if you are seeking a bumper crop of flowers and fruits from such plants as bulbs or tomatoes, for instance, you would want to select a formulation that is lower in nitrogen and higher in phosphorus, such as 5-10-5. Lawn fertilizers have a higher nitrogen content to promote foliage growth and typically come in a ratio of 3-1-2, such as 21-7-14.

Micronutrients
Elements that plants need in trace amounts are referred to as micronutrients and include boron, chlorine, copper, iron, manganese, molybdenum and zinc. In our desert soils, micronutrients usually are available in sufficient amounts for most plants, and micronutrient deficiencies are rare.

If you suspect a problem, review University of Arizona Cooperative Extension’s Guide to Symptoms of Plant Nutrient Deficiencies at ag.arizona.edu/pubs/garden/az1106.pdf. Some fertilizers contain micronutrients, which will be listed on the label. Compost, fish emulsion and kelp are other micronutrient sources.

TIP:
Overfeeding herbs with nitrogen fertilizer reduces their natural flavors and scents. Instead, layer compost around your herb plants as a mulch. As it decomposes, it releases sufficient nutrients to grow tasty and aromatic herbs.