Recommendations For Amending The Soil

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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. There is sufficient iron in the lower desert soil 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. Information on diagnosing what a plant needs can be found in the Vitamin and Nutrient Deficiency Symptoms in Plants page.

Organic Vs. Chemical Amendments
Organic gardening has become a very hot topic these days and for good reason. If the number one way of killing plants is overwatering, then the number two way is over fertilizing them. Organic gardening generally involves introducing substances to the soil that will naturally break down into nutrients that will feed the plant, while chemical fertilizers add nutrients in a more rapid and direct manner.

Rather than debate which is better, which would take pages to do, I would rather convey my general philosophy. The approach I currently recommend could be called ChemOrganic.

Generally, I like to start out with the basics, only mulch/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 emulsion, apple cider vinegar, and sea weed etc. I have become such a big fan of fish emulsion that I have dedicated a separate page to it.

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

Mulch/Compost is partially decomposed organic matter and is the most organic of the organics. I consider it my number one, best soil amendment.

Compost can be purchased in stores but I have found the store bought stuff to be unsatisfactory. First of all, it can contain extra ingredients such as gypsum. While I have found gypsum to be very useful as a fix for high sodium levels in the soil, it is not something you want to be constantly sprinkling on your plants. Constantly applying gypsum will make your soil salty. The second problem with store bought compost is that it often has large sticks, rocks and other debris in it.

I recommend making your own mulch/compost. The complete composting approach with the rotating barrel and fully composting of everything seems a bit too slow and high maintenance for me, so I use the "mulch pile" approach. I take grass clippings, tree trimmings (small branches), and leaves and throw them in a pile and let them age some. I'll occasionally turn over the pile to distribute the decay.

To fertilize my trees, I put a two inch layer of this green/brown mix below their canopy, taking care to leave some space between the mulch and the trunk. It's generally not healthy for a tree to have it's trunk buried. The material that is still green especially the grass clippings are a bit "hot", both chemically and physically, and can burn plants if piled on so I sprinkle them in and mix them up with the rest of the brown black material. In fact, the green stuff is actually full of nitrogen so I'd be losing some of that Nitrogen if I fully composted it. I have found that even totally fresh grass clippings can be used on plants if they are treated like a fertilizer instead of a mulch and sprinkled down in very small quatities.

Mulch spread around the plant in this manner will help prevent the growth of weeds and will make the pulling of weeds easier. However, do not be too generous with the mulch because it can cause some problems if overused. Too much mulch spread on the surface can act as a water barrier, either keeping the plants soil soggy or preventing rain from penetrating because it acts like a sponge. Generally, a couple of inches of mulch on the surface is usually sufficient, and it should be allowed to decompose and thin out before the next application.

If you need more mulch than your yard can provide, landscaping companies will happily dump a couple hundred pounds of it in your driveway, for free.

Sand, To Improve Aeration
The Phoenix area has a very dense clay soil which packs together tightly and becomes like soup when wet, preventing soil aeration. To help make the soil around a plant more permeable to air one can mix in several pounds of a large grained sand during planting. The typical construction sand you can get at your local big box store works well. The need to mix in sand depends on the type of plant, and how often it is watered. Plants that need improved aeration the most are the ones that prefer the soil to be constantly moist but are still sensitive to the soil being too soggy. When the soil is too wet for a plant it will typically get chlorotic, meaning its leaves will become an unhealthy yellow color. I have found that lychees, cherries of the rio grande, and joboticaba plants benefit from having sand mixed into their backfill during planting. If the plant is already in the ground, a tool such a hammer drill with a large bit (1 inch plus) can be used to drill holes around the perimeter of the tree which can then be filled with sand. I did such a drill and fill operation around my, already planted, lychee tree and its health improved substantially.

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 in the case of salty soil 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.

Sodicity - Australian Academy of Science

Salinity and Sodicity - North Dakota State University

Gypsum - Western Farm Press

Soil Salinity - Wikipedia

Salt Damage - Wester Australia Dept. Of Agriculture

Chemical Fertilizers
Nitrogen, phosphorus, potassium, and other nutrients 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.