- Author: Kim Schwind
Every gardener knows that weeds are just plants in the wrong place. Webster's dictionary defines a weed as “a plant that is not valued where it is growing and is usually of vigorous growth; especially: one that tends to overgrow or choke out more desirable plants.”
In nature, weeds play an important role. They can resist conditions like drought, acidic soil, lack of humus, and mineral deficiencies. Weeds protect our topsoil from eroding away in heavy rains and strong wind. They provide a cover and shade for soil microbes and insects.
Many years ago various weeds were used for food and pharmaceutical products. Currently, foraging for wild foods and mushrooms has made a comeback. Weeds in the legume family fix nitrogen in the soil and are often used as a cover crop to help enrich the soil over winter. Weeds have also been used to help indicate the presence of ground water. Curly dock is a weed that's typically found in wet saturated soils.
The primary value of weeds, wrote the eminent U.S. botanist Frederick Clements in 1920, is to “reveal information about the health and pH of our soils.” For example, certain species are confined to acidic soils and others to alkaline.
The use of weeds as soil indicators is not a new idea. Many early North American immigrants to the eastern United States chose land for their farms according to the weeds, plants, and trees that it supported. Conifers were characteristic of sandy, acidic soils that had little agricultural value. Birch, beech, maple, and hemlock indicated fertile soil. They learned that the tall-grass prairies were suitable for cereals, hay, and orchards. The bunch grass regions were better suited to wheat and grass.
If we look at our own gardens we can use our weeds to tell us what we can do to improve our soil. For instance, if our lawn is being taken over by clover we can note that clover thrives with low levels of nitrogen in the soil. We can remedy the problem by applying a nitrogen fertilizer (a 16-16-16 fertilizer is a good balance for turfgrass).
Ehrenfried E. Pfeiffer, a European scientist and student of Rudolf Steiner, wrote an entire book on this subject in the 1950's: “Weeds and What They Tell Us” (still in print). According to Pfeiffer, sorrel, plantain, horsetail, and knotweed are found in acidic soils. Dry soils with very little humus might support mustard, thistle, broom, and St. John's wort.
Sandy soils will have goldenrod, aster and toad flax. Alkaline soils support chicory, spotted spurge, sagebrush and woody aster. In heavy clay or compacted soil you might see morning glory, plantain, Bermuda grass, chickweed, and dandelion. Dandelions also indicate low calcium in the soil.
When you learn the type of soil some of these common weeds prefer you may be able to make corrections in the soil based on the information some weeds are giving you. If you really want to know, a soil analysis or at least a pH test by a local agricultural laboratory will provide factual information to accurately guide the use of soil amendments. Sometimes, improving drainage by adding well-balanced compost, organic manure, or employing cultural practices may be all that is required to improve your soil's tilth and nutrition.
Identifying the weeds in your garden can be fun! In some cases knowing what their presence indicates may help you manage your soil. Controlling weeds by hand weeding or with herbicide before they seed will reduce future populations if done consistently from year to year. One year's uncontrolled weeds can produce seven years seeds! You may even develop a new appreciation for weeds.
For more information about weeds or help identifying them see:
UC Statewide Integrated Pest Management (IPM): Weeds
UC IMP Weed Photo Gallery
UC Weed Research & Information Center
Ehrenfried E. Pfeiffer, Weeds and What They Tell Us.
If you have a gardening question or problem, call the Master Gardener Hotline at (530) 538-7201 or email firstname.lastname@example.org.
The UC Master Gardeners of Butte County are part of the University of California Cooperative Extension (UCCE) system, serving our community in a variety of ways, including 4H, farm advisors, and nutrition and physical activity programs. Our mission is to enhance local quality of life by bringing practical, scientifically-based knowledge directly to our community. To learn more about UCCE Butte County Master Gardeners and their upcoming events, and for help with gardening in our area, visit https://ucanr.edu/sites/bcmg/.
By Brent McGhie, Butte County Master Gardener, September 20, 2013
To grow, plants must absorb nutrients from the soil. Not only must these nutrients be present in sufficient quantities, they must also be available for uptake by the plant. Nutrient availability is primarily determined by soil texture and pH. Soil texture is the amount of sand, silt, clay and organic matter in the soil. Soil pH is a measurement of the degree of soil acidity or alkalinity; this measurement has the greatest effect on nutrient availability in the soil.
pH is measured using a scale from 0 -14, with 0 being the most acidic, 7 being neutral and 14 being the most alkaline. Most plants grow best in a pH range of 5.5 to 7.5 (medium acidity to very slightly alkaline). In this pH range, essential nutrients are available in chemical forms that plant roots can absorb. At higher or lower pH levels, some nutrients form solid precipitates that cannot be dissolved in water, and thus are not available for absorption by roots.
Soil pH can also affect the solubility of harmful mineral elements. For example, in very acidic soils (low pH) the essential elements of nitrogen, phosphorus, potassium, sulfur, calcium magnesium, and molybdenum may be unavailable, while toxic levels of aluminum and manganese might be present. Soils with high alkalinity can reduce availability of nitrogen, phosphorus, iron and manganese resulting in deficiencies, and they may contain excessive levels of soluble salts or sodium, both of which are harmful to plants.
When a soil’s pH is too low (acidic) it can be adjusted upward with the addition of wood ashes or lime. Some forms of lime that can be added to the soil include calcium carbonate, calcium hydroxide, calcium oxide, or dolomite. Sulfur is used to lower the pH of alkaline soils (those with pH above 7.5). Elemental sulfur, gypsum, ammonium sulfate, and Epsom salt are sources of sulfur normally available for purchase.
Here in Butte County, low soil pH is most likely to be encountered in the foothill areas including Paradise, Magalia, east Oroville, and Forbestown. There are limited areas on the valley floor near Gridley and Biggs, and on the south and west margins of Durham and Chico where more alkaline soil conditions (above pH 7.5) are found. If you feel that pH and nutrient availability may be an issue in your garden soil, it is a good idea to have the soil tested. Simple pH test kits are available in garden stores; alternatively, soil samples can be analyzed by commercial Agricultural Laboratories, which can be found listed in the yellow pages of the phone book under Laboratories-Analytical, or in online directories.
Soil texture affects the soil’s ability to retain water and nutrients. Generally speaking, soils with large amounts of clay or organic matter tend to hold water and nutrients more effectively than sandy soils. Sand does not hold nutrients very tightly, so as water drains through sandy soil, it tends to carry nutrients along with it. This process, known as leaching, carries nutrients out of the root zone and makes them unavailable to plants. Clay, on the other hand, has the ability to attract and hold nutrients in the soil and thus fewer nutrients are lost when water drains through clay soils. However, too much clay in the soil can cause other problems. For example, clay soils have a tendency to be waterlogged during wet weather and rock hard when they dry out.
If a soil has too much sand or clay, the most cost-effective remedy is to add organic matter. Organic matter in the soil mimics the positive effects of clay (improved water and nutrient retention) without the disadvantages. Organic materials are most effective when the soil is amended at least 30% by volume. The effect of adding organic material is not permanent because it decomposes with time. Coarse materials decompose more slowly and last longer in the soil. Also, organic material should be composted before incorporating it into the soil because microbes feeding on uncomposted material can actually remove important nutrients from the soil.