When Nature speaks, we are wise to listen. Weeds have an indispensable purpose in our ecosystem. Nature uses them to tell us about the state of our soil fertility and the nutrients lacking, or overabundant in our soils. The weeds we are accustomed to despising can actually be our allies. In fact, weeds are not the problem. Rather, they are only a symptom of a larger problem.
Weeds have numerous purposes. They are nature’s way of covering bare soil to prevent erosion due to wind and water. They also exist to correct soil fertility deficiencies. For example, the common dandelion is an indication of a Calcium deficiency and Potassium excess in the soil. The dandelion has a very long taproot that can grow as deep as three feet into the soil, retrieving Calcium from below the root zone, and bringing it to the surface where it will be deposited as the plant decays. Canada Thistle tells us we may have a Calcium deficiency along with excess Phosphorous and Potassium. Many broadleaf weeds thrive in soil conditions where available Potassium exceeds available Phosphorous. (Ideally, we want Potassium to equal Phosphorous by weight.)
If you were to observe vacant land over time, you would see a “weed progression” indicating the nutrients lacking in the soil during a given period of time. Whatever nutrients are deficient, weeds needing to capture those nutrients will be dominant. Let’s say those nutrients are Calcium, Phosphorous, and Copper and you are having trouble with Jerusalem-oak and Oakleaf (Goosefoot). When enough Calcium, Phosphorous, and Copper have been brought to the surface and released, those weeds will be suppressed by others seeking nutrients deficient at THAT point in time, and on it goes.
If you were to have a soil sample analyzed it would most likely confirm the deficiencies of Calcium, Phosphorous, and Copper. Additionally, if you balanced the minerals in the soil and corrected the above deficiencies, the Jerusalem-oak and Oakleaf would not thrive. With soil nutrients in optimum balance, weeds will still exist but won’t be an onerous, insurmountable difficulty.
Weed seeds are tiny in most cases and don’t have the food store to sustain them during early growth like larger crop seeds do. Therefore, the weed seeds depend on soluble fertilizers for their early growth and will respond to those fertilizers by proliferating. Having optimum soil mineral balance will give crops what they require without the need to apply soluble salt fertilizers (NPK).
When addressing weeds in the garden or crop field it is best to do so before they go to seed. If you can, get them early when they are “in the white.” Whether you cultivate, cut, or mow them leave the residue on the soil surface. It will keep the ground covered and return nutrients to the soil as the weeds decompose.
Further problems may arise when adding a soil amendment without a soil test. In one instance, a farmer growing alfalfa added nearly 1000 lbs./acre of Potassium sulfate after having heard it would grow better alfalfa. Dandelions began to grow during the first year and after several years; the dandelions took over the alfalfa. Too much Potassium interfered with the uptake of Calcium in the root zone. The dandelions grew to reach deep into the soil to bring up available calcium. This illustrates why it is so important to get a soil test before adding anything to your soil. You must know where you are to determine where you need to go. It also demonstrates the effects of adding too much or too little of one mineral on other minerals.
We’ve just scratched the surface on the subject of weeds and what they can tell us. In addition to indicating soil mineral imbalances, an excess of weeds can help us improve our farming and gardening practices by encouraging the use of cover crops or fostering effective grazing practices. There are a number of excellent books on the subject of weeds. If you wish to investigate further, we highly recommend the following:
McCaman, Jay L. When Weeds Talk. 2nd ed. of Weeds and Why They Grow.
Retitled, When Weeds Talk. MO, 2013. Print.
Pfieffer, Ehrenfried E. Weeds & What They Tell Us. 3rd ed. Floris Books, 2012. Print.
Walters, Charles. Weeds, Control Without Poisons. 2nd ed. Acres USA, 1991/1999. Print.