Weeds -  What Are They Telling Us?

Weeds are a nuisance in our fields and the reason why we own a sprayer or hoe, if you have vegetable, tobacco or organic fields.  But have you ever stopped to look at which weeds you have or thought about why they grow where they do. Weeds do have a purpose, so what are the weeds in our fields trying to tell us? 

Weeds are good indicators of soil fertility. The roots of weeds are actually nature’s way of attempting to correct whatever soil problem exists.  Weed roots help to bring the lacking nutrient to the surface or aid in breaking up compacted soil.  Let’s take a look at a few common weeds and what they are telling us.


Dandelions grow where Calcium is low. Their deep tap roots bring calcium to the surface.  If you have a continuous Dandelion problem in a field you need to increase soil Calcium near the surface.

Common Ragweed indicates drought and poor moisture reserves in the soil causing a short fall in Potassium.  Increasing Potassium, Manganese, and Copper reduces ragweed pressure.

Black Nightshade can indicate several soil fertility problems: low calcium availability, low Phosphorous, and high Magnesium.  High Manganese and high Aluminum also increase nightshade.

Quackgrass is often present where there is low Phosphorous and compaction.  Soil aeration through tillage or cover crop roots and a good calcium supply reduce this weed.

Red Root Pigweed often grows where there is low calcium and high Potassium.  This weed indicates an imbalance in the calcium to potassium ratio in the soil.

Sow Thistle tells us that Calcium is low and there is an excess supply of Magnesium.

Yellow Foxtail and Fall Panicum:  Both of these annual grasses often grow where the soil is acidic, telling us that pH is Low.  Lime is needed to bring soil pH into optimum range.

Next time you are travelling across your fields in the sprayer, take note of which weeds are growing and where they are growing.  Correcting the soil problems indicated by the weeds can both increase yields and reduce sprayer passes.

Resources: A&L Soil Fertility Course notes; Weeds and Why They Grow, by Jay L. McCaman