pH—Acidity/Alkalinity
Our natural soil in Tucson is for the most part alkaline. In slightly alkaline soil, some nutrients such as calcium, phosphorus, potassium and magnesium are more available to plants but other nutrients such as iron and trace minerals become insoluble so they are less available. In highly alkaline soil, phosphorus becomes unavailable. Nitrogen becomes unavailable when the soil becomes either too alkaline or to acidic.
As one would expect, plant native to the Tucson area are adapted to alkaline soil but many species that have been imported from elsewhere need more acidic soil. When planning a layout for gardens, plants that thrive in alkaline soil should be grouped together and plants that thrive in acidic soil should be grouped together. Planning more alkali-loving plants will save work and money later.
To lower pH (more acidic), add sulfur, ammonium sulfate, or iron sulfate. If, for some reason, the soil becomes too acidic, the pH can be increased (more alkali) by adding ground limestone. Either way, avoid adding more than is necessary for the health of the plant not only to save money but also because it could harm the plants. More isn't better.
When adding materials to adjust the pH, water the soil well first the day before then mix in the amendment and water again to prevent areas of high concentrations.
For native plants, the pH of native soil is not likely a problem but for acid loving plants such as Azaleas or Hibiscus, the pH should be tested while adjusting the chemistry and then every few years or if the leafs turn pale with dark veins.
Iron Chlorosis
In addition to pH problems, pale leaves with dark veins can also be a sign of iron deficiency called iron chlorosis. Most of our soil has plenty of iron but it isn't available because of the alkalinity, so chelated iron must be added. Chelated iron is iron that is bound to some other substance, probably organic, that makes it available to the vegetation even in high pH soils. Iron chlorosis is a common problem in citrus trees.
Salinity
Salinity is a measure of salts in the soil. Our soil tends to have a lot of salts and again, native vegetation has adapted to salty soils but imported plants such as roses may not fare as well. The problem is exacerbated by the high salinity of our water that we use to irrigate the gardens, salts present in fertilizers, and high salt levels in pool water when we backflush into the soil. If you've ever heard the admonition to "salt the ground so nothing grows", you can probably imagine the effects of excessive salts on many non-native plants. Salts draw up water and will dehydrate tender roots.
Salts dissolve in water. That said, it's logical that the cure for excessive salt is to wash it out of the soil either by one annual heavy watering (recommended for citrus trees) or better yet, improve drainage if that's possible. This is one reason why raised beds and container gardening are so popular here.
Nutrients
Although most of the mass of plants consists of carbon, hydrogen and oxygen removed from carbon dioxide in the air and from water, other nutrients are also very important and any one that is deficient is a limiting factor in the health of the plant.
The three major plant nutrients (primary macronutrients) are nitrogen (N), phosphorus (P), and potassium (K). In fertilizer, the percentage of each of these are expressed by three numbers in the form N—P—K. Actually, phosphorus is represented as the amount of phosphorus pentoxide (P4O10) if the phosphorus were in that form and potassium is represented as the amount of potash or potassium oxide (K2O) if the potassium were in that form. A 24-8-16 fertilizer has 24% nitrogen, 8% phosphorus pentoxide, and 16% potash supplying potassium and the remaining 52% is called ballast or filler. All three are necessary for plant health, but nitrogen is more associated with new green growth of foliage; phosphorus is more associated with encouraging rooting, blooming and fruit production; while potassium is more associated with cold hardiness, disease resistance, and general health.
The secondary macronutrients which are required in similar quantities are calcium, magnesium, and sulfur which are usually available in sufficient amounts for native vegetation in our alkaline soil, however additional sulfur may be beneficial for or even necessary for acid-loving plants.
Some of the most common micronutrients which are required in only very small amounts include iron (Fe) and zinc (Zn). Deficiencies of most micronutrients are uncommon but, as mentioned previously, the iron in our soil may not be readily available for plants due to alkalinity.
Calcium Carbonate and Caliche
Because we only get about 12 inches of rain per year in Tucson, calcium carbonate in our soil is not dissolved and washed down very far into the soil. As the soil dries between rainfalls, water rises back toward the surface pulling calcium carbonate back up from lower levels. As a result, our soil tends to accumulate calcium carbonate near the surface. Calcium carbonate acts like a mortar to hold together gravel, sand, silt, and/or clay to form a very hard layer of caliche (or hardpan). This layer of caliche obstructs drainage because water does not penetrate well and it prevents roots from reaching supplies of water and other nutrients. The poor drainage tends to cause a buildup of salts which makes life difficult for many non-native plants. Calcium carbonate also lowers the soil pH (more basic/alkali) which reduces the availability of iron.
Caliche can be the most difficult problem to resolve in our gardens. It may be several feet down or just a few inches down and it may be a few inches thick or several feet thick and there may be multiple layers. The traditional method of dealing with caliche is to break it up, remove it, and replace it with soil and organic matter. Sometimes digging drainage holes through the caliche is sufficient.