Soil Fertility and What Weeds
Can Tell Us

Soil fertility is much more than the macronutrients of nitrogen-phosphorus-potassium. The bigger picture as mentioned here is how much organic matter is in your soil to support microbial life and making available all nutrients, macro and micro.

You can also contact your local county extension office for more information on soil types in your area. Soil test kits are inexpensive and can be found online or any garden supply retailer.

Getting to know your weeds can be instructive in knowing what amendments to add to increase soil fertility that make it harder (or less desirable) for the weeds to survive and easier for your vegetables to thrive.

In assessing soil, we usually talk about whether it's alkaline or acidic. This is where the pH scale comes in. This scale refers to the parts of hydrogen ions in an element; in this case, the soil. The concentration affects the availability of nutrients to plant roots.

A Little About pH
  • The pH scale runs from 0-14. With 7 being neutral, anything below that is acidic, above 7 is alkaline.

  • Areas that receive high rainfall (Eastern U.S. and Pacific Northwest) tend to have slightly to moderately acid soil.

  • This results from leaching of the more alkaline calcium and magnesium salts from the soil. Plants favoring an acidic environment will be found within the pH range of 5.0-7.0.

  • Areas of very little rain to desert conditions will be moderate to highly alkaline. Alkaline-loving plants will be found between the 7.0-7.5 pH range, generally.

  • Areas receiving light to moderate rainfall (central parts of U.S.) are generally pH neutral.

  • Some weeds will grow anywhere whether fertile or very little soil fertility, thus there will be a few showing up in several soil types. Purchasing a pH litmus test kit can be helpful in your assessment.

    Chronic Dry Soil

    mullein plant


    Mullein can be left, if you prefer, for herbal use or dug up after a good soaking in the fall, before it goes to seed.

    G.J. Chafaris/EB Inc. common mullein. [Photograph]. In Encyclopædia Britannica. Retrieved from

    prickly sowthistle

    Prickly Sowthistle

    This weed's flower stalk can be 12-15", but the flower head looks very similar to the dandelion's.

    Also can be dug out after a good soaking, before it develops its 70,000 seeds per plant!. This is a perennial plant. If you don't get all of the taproot, you'll see it again in the spring!

    wild carrot or queen anne's lace

    Queen Anne's Lace
    (wild carrot)

    Digging them out during their first year is best as they don't produce seed the 2nd year. On the upside,QAL also acts as a trap plant for insects that would otherwise attack your prized flowers and veggies.

    Remediating Chronic Dry Soil

  • Adding humus-rich compost helps to retain moisture

  • Incorporating mulched leaves, composted manures mixed with approximately 10%-20% of worm compost

  • Applying in the fall so winter rain can aid in decomposition will go a long way in reclaiming this soil situation and increase soil fertility.

    Low Soil Fertility/Compacted Soil



    This weed grows in all areas of the U.S. Favors low calcium, dry areas and low levels of organic matter. If it's in a lawn area outside your garden beds, it will find its way into your beds. As crabgrass favors low calcium, like dandelions, you may want to try this organic tip.

    shepherd's purse
    Shepherd's Purse

    Can be found in sunny, moist to dry, rich, disturbed soil, but will also grow in partly shaded, extremely poor soils.

    A single plant may produce a couple thousand seeds, and the seeds may remain viable for many years. To control this plant, pull it up before it goes to seed, and use mulch where possible, as the seeds remain dormant in the dark.
    Photographed by Eike Wulfmeyer,
    2004-MAY-22, Cologne (Germany)

    Wild carrot or Queen Anne's Lace

    black medic clover
    Black Medic

    This is a type of clover, but it develops a burr-like seed head (see cluster on left side of enlarged image). One plant can grow out in all directions or just one for a distance of 2 ft or more. It is relatively easy to pull out.

    Remediating Compacted Soil

    Compacted soil can happen with any kind of daily foot traffic (especially when ground is wet) and most definitely by motorized vehicles (tractors, lawn mowers, etc).

    Break up the top 2” of soil using a spading fork (no shovel), preferably in the fall. Adding a 2” layer of composted manure and leaves mixed with
    10% -20% worm compost will go a long way in changing the texture and soil fertility.

    With the added microbes and worms aiding in soil fertility and helping to break up the soil, you don't need to break out the old rototiller or spend hours double-digging. Churning the soil like that only harms the existing microbiology, particularly fungi.

    Adding 23-30 lbs of gypsum per 1000 sq.ft. will help to loosen things up.

  • not a quick-acting agent, but over 2-3 years will naturally penetrate the compacted strata.

  • will allow air and water into the newly created spaces.

  • also contains some calcium and sulfur which are plant beneficial.
  • Follow in the Spring with more composted material added to the surface of the bed and planting can be done directly in this top layer, adding a handful of worm compost to each transplant or sprinkling along the seed trench.

    Fertile/Acidic Soil

    Finding weeds growing in this soil type, especially in larger numbers can indicate a good amount of organic matter.

    Dandelion grows in areas low in calcium, but has a long tap root which helps to aerate the soil and bring up minerals from the subsoil, making the young spring leaves chock full of minerals (if used as an edible herb).

    Tip: For a chemical-free dandelion deterrent, apply a fungi-rich compost or compost tea to the plant. The fungus exudes a slime which dissolves the calcium in the soil, making it available to the dandelion(s) and eliminates them.

    stinging nettle

    Stinging Nettle

    An herbaceous shrub that grows all over the world. Grows well in nitrogen-
    rich soil. Can grow up to 4 feet high. Make sure to wear heavy-duty gloves on when digging this out.

    Prof. Dr. Otto Wilhelm Thomé Flora von Deutschland,
    Österreich und der Schweiz 1885, Gera, Germany

    Red or white Clovers- these plants have nitrogen-fixing nodes that transport nitrogen from the air into the soil, a natural way of increasing soil fertility.

    I transplant the clovers to my chicken pastures as their value to soil fertility is too great to just throw on the compost pile. If left in your garden until Spring, they can be turned into the soil as a green manure.


    Lambsquarter or Pigweed

    This is a relative of Amaranth. The seed head on the weedy cousins produces thousands of tiny seeds too. Root system is quite shallow, so hoeing or pulling them up while their small will very easily eradicate them from your garden area. Disturbing the soil surface in the Spring usually will take care of any that are sprouting.

    Source=Wikipedia. Own work by uploader Author=6th Happiness


    I let these come up in my raised beds and harvest them throughout the summer for salad greens and omega-rich greens for the chickens.

    Carl Heilman Photography
    common purslane. [Photograph].
    In Encyclopædia Britannica. Retrieved from

    Remediating Fertile/Acidic Soil

  • Increasing humus in the form of either thermophilic or worm composts (better is a combo of both with 10-20% of worm compost)

  • Highly acidic soil should have between 7-9 lbs of calcitic lime/100 sq. ft applied in the fall. This is a slow-release form comprised of calcium carbonate. may also be used and can be purchased at most feed stores.

  • Mildly acidic soil requires about 5 lbs/100 sq. ft.

  • Dolomitic limestone contains calcium and magnesium. Using this form of lime would be most beneficial if your soil was Mg deficient. Doing a soil test would answer that.

  • Wood ash from a wood-burning stove or fireplace is a natural, fast-acting alkalizer. It is generally recommended that no more than 25 lbs of ash/1000 sq. feet should be applied and even that may be too much if you're only trying to achieve a small pH increase. If that's the case, only use half the rate. Mix in well and test. Remember, you can always add, you can't take away.

    Since soil fertility is dependent on all essential nutrients getting to the roots, adding microbes through the use of worm compost is the most cost effective and natural way of releasing those nutrients.

    Acidic/Clay Soil

    As stated before, areas that receive high rainfall (Eastern U.S. and Pacific Northwest) tend to have slightly to moderately acid soil. In addition, wherever you find pine and oak trees, you will usually have acidic soil.



    Another clover-type similar to Black Medic.
    Even though their nitrogen-fixing is important for soil fertility, one plant can easily take over a garden area. With a good soaking sorrel is easy to remove.

    Walter Chandoha oxalis europaea. [Photograph]. In Encyclopædia Britannica. Retrieved from sorrel

    annual bluegrass

    Annual Bluegrass

    This stuff pops up all over the place and can set several generations of seed in one season.

    Flipping them out of the ground with a 3-pronged hoe and exposing the roots to the sun, will quickly turn this weed into organic matter for the compost pile.

    TIP: Making your plant spacing a little closer together can form a shade canopy which makes it harder for the grass (or any weed) to sprout.

    After your plants are 4-6" tall, apply a 2" layer of mulch which works wonders in keeping all things weedy from sprouting.

    With their long tap root, they help by forcing clay soils to open, making pathways for water and air.

    broadleaf plantain
    Plantain or Buckhorn

    This weed loves moist, fertile places. They have a very short root system, but hold their ground tenaciously, making it almost impossible to pull out if the soil is on the dry side. Watering the area and using a weed digger pops them out. Best to get them before the flower stalk appears.

    Wild carrot (Queen Anne's Lace)

    Remediating Acidic/Clay Soil

    To increase soil fertility and change pH, follow recommendations for fertile/acidic.

    Alkaline Soil

    Generally speaking, arid or desert areas produce alkaline soil.

    Wild carrot (Queen Anne's Lace)


    It only takes a few plants of this weed to overtake a bed! I'm usually pulling it out in May-June.
    With the mulch layer I've been putting on the beds in Fall, I'm seeing less and less of it, but it does create a lot of biomass that can be tossed on the compost pile or chopped up and fed to the worms or chickens.

    spotted spurge
    Spotted Spurge

    A summer annual weed that grows out in many directions from a shallow tap root, forming a dense mat of vegetation. Very easy to pull out.

    Remediating Alkaline Soil

  • Adding thermophilic compost mixed with worm compost to lower pH.

  • Adding gypsum, calcium sulphate or powdered elemental sulphur will also help lower pH, depending on how alkaline the soil and how acidic the amendment.

    For identification of weeds specific to California, U.C.Davis has an informative site.

    These non-domesticated plants will always be with us. Most have herbal qualities, some are a real problem. It's just a matter of extricating the most invasive of these plants and controlling the rest so they don't take over.

    As I have noted above, most, if not all of them are soil builders in one way or the other and aid in soil fertility. Finding the balance of peaceful co-existence with our weedy friends might be the better part of gardening valor.

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