Recommendations For Amending The Soil

Overview
The soil in Phoenix Arizona is composed mainly of clay and has large deposits of calcium carbonate, making it very alkaline. The calcium carbonate also forms layers of concrete hard caliche which can make it impossible to hand dig a hole in some locations. A jackhammer is the tool of choice in these cases. The soil is actually sufficient in almost all basic nutrients necessary for plants with the exception being nitrogen. There is sufficient iron but it is often chemically unavailable to plants because of the high alkalinity. The water here is also fairly alkaline and salty which compounds the Ph problem. Organic materials are lacking in the soil, which reduces the quality of the soil structure in general. Following is a list of soil amendments that can help with these problems.

Generally, I like to start out with the basics, only compost and water, and see how a plant does. Newly planted plants shouldn't be fertilized anyways so this phase can last a couple of years. Many plants will do very well with just the basics and will never need anything else, while other plants will do better with some stronger organics or slow release fertilizer. Stronger organics are things such as fish meal, and sea weed etc.

So, in general, strive for a minimal amendment environment, be patient, and if you do decide to fertilize, do it in moderation.

Compost
Compost is partially decomposed organic matter and is the most organic of the organics. I consider it my number one, best soil amendment. It can be purchased or made in one's yard out of old plant matter. Making your own generally isn't worth the trouble, and you can still consider yourself environmentally friendly by donating your yard waste to the city. Many cities have special trash barrels for this purpose.

Mixing compost in with the backfill soil, when planting, benefits the plant by creating food for microorganisms that will continue to decompose the organic material. Decomposition produces nutrients for the plant and also creates acid, which will offset the soil alkalinity. Compost also changes the structure of the soil, making it more permeable to air and water. Since most nursery plants come in potting soil, mixing compost with the native soil, in the backfill, will create a transitional area for the plant's roots.

Soil Salinity, Deep Soaking and Gypsum
The phrase "salty soil" is a term used to describe soil that has a level of salts dissolved in it that is detrimental to plant growth. There are many different chemical compounds that are classified as salts. Generally speaking, salts are molecules that disassociate when they come into contact with water. By mixing into the water, the salt reduces the ratio of actual water molecules that are present within the liquid.

Water salinity is important to plants because it affects the ability of a plant's roots to take up water. Plants normally regulate how much water they have in their system by actively drinking through their roots. When the water surrounding a plant's roots becomes too salty the salty water does not actually have a lot of water molecules within it, relative to the fresher water within the plant.

Water, like all liquids seeks to evenly distribute itself. The scientific term for this is osmosis. This means that the lower concentration of water molecules outside of the roots draws water from the roots with such force that the plant is less able to take in water. The plant then starts to show signs of drought stress. Plant growth is first slowed, because the plant is using a large part of its energy trying to take in water. If these conditions continue the plant will begin to conserve water by allowing its extremities to dry out, which will be first evident on its leaf tips, referred to as salt burn, followed by major branches, and so on.

Salt burn is one of the number one problems the exotic plant gardener will have in Phoenix, and can often kill salt sensitive plants. Keep in mind that fertilizers make the soil more salty, so it is often best to not use them at all on salt sensitive plants.

The best way to minimize salt buildup around a plant is to periodically deep soak/leach the soil. Leaching is done by placing a hose in the basin and letting it flood slowly for many hours. This procedure can be done preventatively for salt sensitive plants 2 to 4 times a year. Sometimes, leaching with water alone, is all that is needed. If this is not effective enough one can try leaching in combination with gypsum.

A fascinating fact is that gypsum is also a salt. So, how is it that adding gypsum can help to reduce the saltiness of a soil? The short answer to this question is that gypsum helps to wash away salts in sodic soil.

A soil is sodic when it has an excessive number of sodium ions in it. In Arizona, the water is high in sodium causing our soils to be sodic. Sodium ions cause a problem for plants because they bond themselves to clay particles and make the soil less permeable, meaning that water and almost every other substance has trouble passing through it. Washing away salts in sodic soils is therefore difficult.

When gypsum dissolves in water, calcium ions are released which displace sodium ions from their bonding sites in the soil. Unlike the sodium ions, the calcium ions open up the soil. A thorough soaking can now wash away excess salts, including any gypsum ions that did not find a bonding site. Interestingly, gypsum is not a harmful salt when thoroughly leached because it permits itself to be quickly washed away. Keep in mind, that using too much gypsum will necessitate more leaching to wash it way so moderate amounts should used.

Formulations of garden gypsum also contain sulfur, which helps to acidify the soil. To leach with gypsum, sprinkle a thin layer of it in the watering basin around the plant. This layer should be just deep enough to hide the ground, similar to a light dusting of snow. The next step is to thoroughly soak in the gypsum by flooding the area around the plant for many hours.

Some may wonder why caliche, which is calcium carbonate, does not supply the calcium needed to displace the sodium in the soil. The reason is that the calcium in caliche is strongly bonded and very insoluble, so it does not become an ion.

Water softeners add sodium to the water, so softened water should never be used for watering plants.

Here are a number of articles that were drawn upon to write this section on soil salinity.

Australian Academy of Science

City of Mesa

North Dakota State University

Western Farm Press

Chelated Iron
The alkalinity in Phoenix soil can make iron unavailable to plants. Chelated irons, such as Ironite, add a form of iron to the soil that is still available regardless of the alkalinity. Plants deficient in iron can get very unhealthy and exhibit signs of chlorosis. Plants that are over watered also have trouble taking up iron, so be sure that the plant is being watered properly before applying iron.

Chemical Fertilizers
Nitrogen can be added to the soil using standard chemical fertilizers. The strength and components of the fertilizer should be chosen to suit the plant it will be used on. Fertilizers containing sulfur will help to lower the soil Ph of the soil temporarily. It is best to go easy on the chemical fertilizers because it is very easy to burn a plant beyond the point of recovery. Furthermore, plants that dislike alkaline soils are more easily burned. Fertilizers take a long time to give results and can even take up to a month to burn a plant. So, patience is truly a virtue in this case. Many a gardener has killed a plant by fertilizing and then two weeks later fertilizing again because nothing seems to happening.

Dry vs. Water Soluble Chemical Fertilizers
Dry fertilizers tend to burn plants more than water soluble ones do. This is the reason water soluble fertilizers are recommended for house plants. If dry fertilizers are used, take care to apply the fertilizer at the drip line of the plant rather than getting close to the trunk. It is much more likely a plant will be killed if its trunk is burned.

Slow release dry fertilizers can be a conservative alternative as well. In fact, I have become a very big fan the Osmocote fertilizers for acid loving plants, because they reliably slowly release Nitrogen to the soil plus help reduce alkalinity.

Often it is a good idea to not fertilize a new plant at all and see how it does. Plants without enough nitrogen decline slowly so there is plenty of time to diagnose what's wrong with them and possibly give them a little boost.

Fertilizing in the fall
It is better not to fertilize plants in the fall. Plants grow much slower during the winter and will therefore salt burn much more easily. Also, since less irrigation is needed during the winter, the washing away of harmful salts, which are byproducts of fertilizer, is reduced.

Chemical Fertilizers And Fruit Flavor
It is not recommended to fertilize a plant when it has fruit developing on it. Fertilizer generally shocks a plant even if the effect is not noticeable. When a plant is shocked it goes into emergency mode and pulls sugars and other good stuff out of its fruits. The result is bland flavorless fruit. The closer the fruit is to being ripe the more dramatic this effect. The best time to fertilize a plant is just prior to when it flowers.