Dean Kreager, OSU Extension Educator ANR, Licking County
(Previously published in Farm and Dairy: May 6, 2021)
If plants 
could talk, we could learn a lot, and our jobs as stewards of the land would be much easier. When we go to the doctor because we are sick, we do not sit quietly and expect the doctor to know how we feel and then tell us how to get better. We need to provide information that will help with the diagnosis.
But since plants cannot talk, our job is difficult when we try to locate the source of a problem, such as low productivity or an infestation of weeds.
Recently, one of my colleagues, Ed Brown, suggested a method of taking stock of what is growing in your pasture. Knowing what plants are growing in your pastures is an important first step in listening to what the pasture is telling you. Varieties of plants or changes in these populations from year to year can provide important clues.
Indicators
Indicator plants are plants that can provide suggestions of issues in the soil. Often, perennial weeds can be our best indicator plants. These plants are living in a condition that has allowed them to survive for multiple years.
Annual plants only need conditions that allow them to make it through one growing season, but their ability to come back for multiple years can also suggest problems.
Identifying and inventorying these plants can be an additional tool to use when managing your pasture.
While I would not suggest that these plants take the place of soil testing, they could hint that a soil test is needed to interpret what the indicator plants are telling us. By testing soil and continuously monitoring the stock of plants present, we can document actual improvements over time.
Much of the information on indicator plants dates back many years, but there continue to be studies from universities that support many of the old findings.
Examples
Here are some examples of indicator plants, though there are many more. Broadleaf plantain may indicate compacted soil with low fertility. Broomsedge is often an indicator of low phosphorous, which may be due to low pH. Burdock can indicate low calcium and high potassium.
Curly dock often indicates wet or compacted soils, as well as low calcium and extremely high magnesium, phosphorous and potassium. Knapweed does well with low calcium and very low phosphorous.
Oxeye daisy likes low phosphorus, high potassium and high magnesium. Common mullein often indicates low pH rocky soils. Redroot pigweed can indicate too much iron or to little manganese, but it may also indicate high potassium and manganese and low phosphorous and calcium, and is often an indicator of fertile soil.
Managing weeds
Usually, our goal is to find a way to remove weeds from our pastures. They typically reduce productivity and compete against the desired forages. But research has shown that simply removing those weeds without addressing why they grew there in the first place will only provide a short-term solution, as the weeds will likely return.
Taking stock of weeds and listening to what they are telling us about soil conditions can be an additional tool in our toolbox. We do our best to provide a good representative soil sample to learn about the fertility status on our pastures. These samples are great for providing averages.
But maybe, for example, we did not pull a sample from a particular area, and we realize that area is covered with broomsedge. Going back and taking a soil sample in that area could be valuable in both getting rid of the weed problem and improving fertility.
Pick up a good weed identification guide and do not be afraid to contact your local extension educator for assistance with identification. There are also several university-based guides for assistance with what these weeds may be telling you. Listening to those indicator plants is one more tool for managing your pastures.