Soil is brown, right? Usually. But perhaps you have also seen gray soil, whitish soil, red soil, black soil or even kind of a bluish soil. What does it all mean?
Delving into my Master Gardener references, I found some answers in the University of California Division ofAgriculture and Natural Resources Leaflet 2280.
Although color does not influence how productive a soil might be, it is an indicator, and we can learn a lot about the quality of the soil from that one observation. Color is one of the most useful characteristics in the classification and identification of soils.
We can usually determine what color soil is just by looking, and generally this observation is sufficient. But if a color description is required for analysis or communication, the exact shade can be matched to the Munsell Soil Color Chart, which provides a standard method for describing soil color.
In Napa Valley and throughout most of California, brown and gray soils comprise the largest swaths of landscape. Although these soils are moderately low in organic matter and often high in clay, they include some of our most productive alluvial soils.
Alluvial soils are created on valley floors by the movement of water. One property owner's erosion is another lucky person's alluvial soil. Nearly all the soils in the central and coastal valleys, including the Napa Valley, are alluvial and extremely useful for growing intensive crops.
Soils in the Central Valley tend to be gray. On the west side of the valley,soil texture is coarse to medium from granitic alluvium. The east side of the valley features medium- to fine-textured soil due to layering with sedimentary alluvium.
Still, color is only one indicator. These two areas show wide variation in productivity and other important characteristics.
Black soils are usually high in organic matter, but high is relative. Some black soils test relatively low in organic matter and relatively high in mineral content. Others occupy the other end of the spectrum—squishy black muck that can contain up to 50 percent organic matter. Peat bogs fit this description.
Black soils often have a strong granular structure. In coastal valleys, they can form on top of native grasslands. In other areas, black soils form under poorly drained conditions and range from peaty to mostly clay in texture. With proper identification and good management, these soils can be highly productive for vegetables and field crops.
Red soils can be beautiful but are often deficient in nutrients needed for healthy plant life. Usually lacking in nitrogen, essential for strong leaf development, red soils are also frequently deficient in zinc, sulfur and phosphorus. These nutrients are all necessary for the proper development of plant leaves, roots and fruits, so anyone attempting to garden in red soil will probably need to amend generously.
Why are red soils so problematic for gardening? Generally they are older soils that have gone through intensive weathering. Some of that rich alluvial gold in the valley soil probably came from old red bluffs that were robbed of their riches. This explains why bluffs with exposed edges and timberlines that have been exposed to hot summers and drenching rains are often red hued. The soil has been washed away by wind and rain down to clay pans or hard pan.
White, light gray or gray soils usually have a preponderance of sand or lime. If you rub these soils through your fingers, they feel gritty and sandy. While these soils often drain freely, they can have difficulty holding water and nutrients long enough for your plants to absorb them. In some chalky or sandy soils, iron deficiency can be a problem. This deficiency is a common problem for orchard crops but can also be problematic for other food and ornamental crops.
And then there is the blue or blue-gray mucky soil that smells bad and can have a sewer- like odor. Often this condition is the result of poorly aerated subsoil. Organic matter doesn't have enough oxygen to completely breakdown the materials.
These incompletely digested soils are not healthy for plants. Dissolved materials and gases in these soils are toxic to plant roots. To rehabilitate blue, smelly soil, extensive aeration is needed to complete biodegradation and provide a healthy environment for plant growth.
Most of the soils in Napa County are predictably brown. But the occasional pockets of different colors now speak to me in a different way. And now I understand what they are saying.
Workshops: U. C. Master Gardeners of Napa County will host a workshop on “Home Vineyard: Part 1” on Saturday, February 27, from 9:30 a.m. to 2:30 p.m. at the University of California Oakville Experimental Station, 1380 Oakville Grade Road, Oakville. What to do, what to look for, and what to plan for in the vineyard between February and August. Workshop will be presented in two parts. The morning (9:30 a.m. to 11:30 a.m.) will be classroom discussion. The afternoon (12:30 p.m. to 2:30 p.m. will be a field trip to a local vineyard. On-line registration (credit card only); Mail-in/Walk-in registration (cash or check only).
U. C. Master Gardeners of Napa County will host a workshop on “Irrigation Hands On” on Saturday,February 27, from 9:30 a.m. to 11:30 a.m., at the University of California Cooperative Extension office, 1710 Soscol Avenue, Napa. Learn how to modify your current irrigation system to make it more efficient and effective. There will be demonstrations and hands-on learning about irrigation controllers, sprinklers, drip systems, rain water capture and grey water systems. For the hands-on segment, bring garden gloves to protect your fingers and a pair of scissors or garden shears .On-line registration (credit card only); Mail-in/Walk-in registration (cash or check only).
Master Gardeners are volunteers who help the University of California reach the gardening public with home gardening information. U. C. Master Gardeners of Napa County ( http://ucanr.org/ucmgnapa/) are available to answer gardening questions in person or by phone, Monday, Wednesday and Friday, 9 a.m. to Noon, at the U. C. Cooperative Extension office, 1710 Soscol Avenue, Suite 4, Napa, 707-253-4143, or from outside City of Napa toll-free at 877-279-3065. Or e-mail your garden questions by following the guidelines on our web site. Click on Napa, then on Have Garden Questions? Find us on Facebook under UC Master Gardeners of Napa County.