Landscaping is an odd agricultural industry in that many different species of plants grow as near neighbors. To achieve desired effects, you might plant azaleas near junipers or yews and rhododendrons in close proximity to viburnums or tulips or lilies.
In both of these examples, species with very different soil requirements are expected to survive and thrive. Junipers, boxwoods and viburnums prefer a soil with a pH above 6.0. Azaleas and rhododendrons perform best at a pH around 5.0. The numbers are only one digit apart, but in pH units they are exponentially different. A soil with a pH of 5.0 is 10 times more acidic than a soil with a pH of 6.0. The nutritional and cultural needs of species are often ignored.
Recently a landscape contractor informed me of the loss of 50 azaleas he had installed last summer in a mixed planting with junipers. None of the azaleas survived the winter. Soil tests indicated the soil had a pH of 6.7, with only three percent organic matter.
This spring he was able to pull each azalea plant from the ground without effort. The high pH and low organic matter prevented them from rooting into the new soil. In other words, all of the azalea roots remained in the original root ball; the plants dried out during the winter months. Weather records show little rain or snow from October 2017 until March 2018.
Azaleas, rhododendrons, mountain laurel, blueberries, andromeda and the like grow at their best in soils with a pH of 5.0 because they require lots of iron and can only absorb nitrogen in the ammonium form. This is why you fertilize them with acidic fertilizers such as ammonium sulfate.
Arborvitae, boxwood, cherry laurel, holly, pines, spruce and yews perform best in soils with a pH of 6.0 and above. These species do not require as much iron but have a greater demand for calcium, magnesium and nitrogen in the nitrate form. They can absorb nitrogen in the ammonium form but grow more efficiently when the available nitrogen is nitrate nitrogen.
You can satisfy the nutrient needs of diverse species by increasing the organic matter concentration of the soil to eight percent or above. Mature compost, which has an organic matter concentration between 40 and 50 percent, will do the trick. Blend one-third by volume compost with two-thirds by volume soil to satisfy the needs of diverse plant neighbors.
Increasing organic matter will also make iron available at a high pH due to the humic and fulvic acids released by the compost. As the compost decomposes and releases its nutrients, nitrogen first appears in the ammonium form in adequate amounts to satisfy the needs of the azaleas and related species. At a pH between 5.5 and 6.0, the ammonium is slowly converted to nitrate nitrogen.
You won’t know how well you’re doing unless you have your soil tested. If you start with very acidic soil with a pH of 5.0 or below, and both calcium and magnesium levels are low, you will need to amend with calcium sulfate (gypsum) and magnesium sulfate (Epsom salts).
Continue mulching with compost to keep your landscape happy. Never use hardwood-bark mulches as they infuse the soil with toxic levels of manganese.
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