Plants need both major and minor nutrients. Minor nutrients (also called “trace elements”) are usually supplied by the soil, but plants use major nutrients in large enough quantities that we should add them regularly.
Of the major nutrients, nitrogen (N) is associated with green, vegetative growth; phosphorus (P) is associated with flowering; and potassium (K) aids root development. However, that’s an oversimplification. All three major nutrients are necessary, to some degree, for all aspects of healthy plant growth. If plants don’t have enough nitrogen or potassium, they’re unlikely to flower well, even though there’s plenty of phosphorus in the soil.
All different kinds of Fertiliser
There is no one best fertiliser formulation for annuals or perennials. Many work well. However, choose a fertiliser that’s higher in phosphorus and potassium than nitrogen to encourage blooming and discourage excessive leafy growth. If you’re growing perennials primarily for foliage, use a balanced fertiliser with equal parts N-P-K.
Fertilise annuals throughout the growing season, and perennials from spring through midsummer. Stop fertilising most perennials after midsummer so they’ll slow their growth and prepare for winter dormancy. Perennials that are still putting on new growth in fall will be vulnerable to winter injury.
Some newer annuals do better with frequent fertilising (every 10 to 14 days), particularly if they’re growing in containers. An easy way to provide enough nutrients is to mix slow-release fertiliser pellets into the soil when you plant. The pellets will release nutrients into the soil for about three to six months.
Ironite is a soil supplement
Ironite is a soil supplement and fertiliser used primarily on golf greens and athletic fields. Made from silver-mine tailings (what remains after ore is processed), Ironite contains iron plus secondary and trace minerals that help plants grow. A soil test can give you information about the nutrients in your soil. If your soil is deficient in minerals, you could work Ironite into the soil at planting time or add it later in the season. Despite some controversy, Ironite is widely available at home and garden centres.
The reason it’s somewhat controversial is that it contains small amounts of arsenic and lead (both as components of other minerals). According to Ironite’s producers, neither is released to your plants or to the environment, but some environmentalists and organisations discourage the use of this product.
What about alkaline soil and fertiliser?
Desert areas like yours often have alkaline soil (soil with a pH higher than 7.0). It’s easier to choose native plants that are well-adapted to local conditions than to fight nature to acidify your soil. Plant choices depend on where you live and how alkaline your soil is.
Alkaline-tolerant perennials include Russian sage (Perovskia atriplicifolia, Zones 4 to 9), globe thistle (Echinops giganteus, Zones 4 to 8), and Oriental poppy (Papaver orientale, Zones 3 to 9). For more choices, check with local nurseries.
To make soil less alkaline, incorporate elemental sulfur or sulfur-containing fertilisers such as ammonium sulfate or iron sulfate. Granular sulfur is cheapest, but also works slowest. In fact, reducing soil pH is always slow—it often takes several years. Your success will depend, too, on what type of soil you have. It’s easier to acidify sandy soils than heavy clay soils.
Incorporating organic matter such as peat moss or cottonseed meal will help acidify the soil. In a pinch, you can spray plants suffering from iron chlorosis (a condition in which high soil pH causes leaves to yellow and eventually turn brown, particularly between the veins) with liquid chelated iron. It will give them a quick green-up, but it’s a temporary fix.