Although the climate in Southern California is ideal for growing plants, there is a common belief that most of the soil is poor.
Not so, says Jim Barry, a Southland soil and tree expert. According to Barry, much misinformation exists concerning Southern California soils.
Barry cites three commonly held myths about Southland soils:
1--Most are composed of hard clay and are almost impossible to work with. "Wrong," said Barry, "in fact, City Hall in Los Angeles sits on some of the best possible soil."
2--Adding sand to clay soils will improve them. "Don't do it," said Barry, "although it sounds like a good practice, you end up with an unmanageable, concrete-like mess."
3--Southern California soils lack nutrients and require heavy applications of fertilizers.
"Not true," said Barry. "Most Southland soils are fairly rich in nutrients and don't require much except for nitrogen and iron."
Barry, who has spent 16 years working with and testing Southern California soils, heads Mobile Soil Labs and Barry Tree Service. He is a certified arborist and also a horticulture instructor at Cal Poly Pomona.
According to Barry, most Southland soils area generally either alluvial or marine. Alluvial soils are ideal for optimal plant growth. They are generally found in the low foothills, in basin areas such as the Los Angeles Basin, and near the dry river areas of the Los Angeles, San Gabriel and Santa Ana rivers. These soils are generally sandy loam-type soils.
Marine soils are not necessarily found near the ocean--they are located throughout Southern California, because millions of years ago much of the Southland lay under oceans. They are difficult soils and tend to be alkaline, saline--or both.
The culprit in these soils is a buildup of calcium carbonate. This type of soil tends to bind up nutrients so that they are not readily absorbable by plant roots. Most clay soils are marine soils. Incidentally, Barry said clay isn't all bad; some clay is important in soils because it holds in nutrients. The ideal sandy loam soils contain about 10% clay.
Barry said that another soil problem occurs because most of the water delivered to Southern California has a relatively high salt content, which can damage plants. The water doesn't necessarily start out salty, but when it travels in canals through hot deserts or inland valleys, a significant amount evaporates, leaving a fairly high salt content.
This high-saline water is acceptable for use on plants if the soil is porous enough to allow it to be flushed out periodically with deep irrigation. However, if water penetration is inadequate, it is possible that plant damage will occur.
If you live in an area that has marine-type soils that are clay-like, alkaline, saline, or all of the above, Barry offers suggestions.
Clay soils, which are also usually alkaline can be improved by the addition of organic materials and gypsum. These two materials loosen the soil structure and allow for better penetration of water. With proper water penetration, soils can be flushed of accumulated salts.
Which organic materials should be used? Barry recommends homemade compost or commercial products containing humus materials. Barry suggested avoiding cow or horse manures as they are typically high in salts.
According to Barry, you should use 1 cubic yard of organic materials per 1,000 square feet of soil surface. Work the materials deeply into the soil.
If your soil is also alkaline (it can be tested with inexpensive kits available at most nurseries), add sulfur to it. Over a period of time, sulfur will acidify the soil and lower the alkalinity. This will allow for better absorption of soil nutrients by plants.
If your plants turn yellow with iron chlorosis (iron isn't properly absorbed by the roots), Barry suggests adding iron sulfate to the soil. It both acidifies the soil and adds iron to it, getting quicker results than soil sulfur. Follow label directions for application rates.
As for fertilizers, Barry said that most Southern California soils have good nutrient content. Of the three basic nutrients--nitrogen, phosphorous and potassium--only nitrogen usually needs to be added on a regular basis. Indeed, many commercial growers use only nitrogen fertilizers.
Of the secondary and trace elements, usually only iron needs to be added to the soil. And this isn't because iron is lacking in the soil, it's because the soil's high alkalinity prevents it from being absorbed by plant roots.
How often should plants be irrigated? According to Barry, it depends upon your soil type. Heavy clay soils retain water and don't need to be irrigated as often as light sandy soils.
Barry said that probably 80% of plants are watered either too often or incorrectly. The key when irrigating is to water deeply; as deep as the plant roots extend. This practice leeches the salts from the soil. By watering deeply, and not so often, the soil is allowed to dry out somewhat so plant roots can get sufficient air.
In his role as an arborist, Barry has seen many trees that have died because of constantly soggy roots.