Can weeds tell you about soil health issues? Well, yes and no.
I’ve got a lot of experience with weeds since I’ve spent the last 15 years teaching cattle, sheep, goats and bison to eat them under all kinds of soil conditions in and all kinds of landscapes. My observation is that spotted knapweed does well in all kinds of soils. So does Canada thistle. In fact, most of the weeds I’ve worked with grow in a wide variety of locations and soil types. So I’m a little doubtful that they can tell us much about the nutrients of the soil they’re growing in.
Mostly, weeds are indications of past management, as the chart below indicates. (Thanks to Greg Brann for sharing this.) Areas that have been over grazed have more weeds due to soil compaction and lack of competition. Some plants grow well in low fertility soils – like pastures and hay fields that haven’t been fertilized or limed in some time. Some of the examples you’ll find under these conditions are shown in the chart below. (Just click on the chart to download a larger, more readable version.) In addition to showing us what we might see in compacted soils, overgrazed pastures, and areas with low fertility, it also shows you what grows in wet or flooded soils, and shows you what plants look like when they’re suffering from nutrient deficiencies.
What Should You Do When You Find Weeds?
If you have weeds that indicate compacted soils, you need more roots, both fibrous and tap roots. Start by maintaining living roots in your soil year-round by allowing plants to recover longer between grazing and mowing so that roots can recover as well. All plants help reduce issues with compaction but the following plants are especially known for decreasing soil compaction:
• Cool season annuals: forage radish and cereal rye
• Cool season perennials: alfalfa, chicory, red clover and sweet clover
• Warm season annuals: sorghums
• Warm season perennials: native warm season grasses like big bluestem, little bluestem, indiangrass, switchgrass and eastern gamagrass. Bermudagrass is tolerant of overgrazing and rather drought tolerant but doesn’t have as deep a root system as natives and needs nutrients for production.
If your pastures have been overgrazed, provide more rest. Lowering stocking rate will remove stress on the the grass, the soil, the livestock and you. Here are some other practices that can help:
• Feeding over winter,
• Increasing the number of paddocks and rotating more often,
• Leaving minimum heights of grass for soil protection, improved infiltration, lower soil temperature and improved water management
How to Find Out About Your Soil
If you really want to know what’s going on underground, I’d suggest a couple of resources. First, you can find out about your soils using the Web Soil Survey from the Natural Resources Conservation Service. Not only can it tell you what type of soil you have, but it will also note potential deficiencies and estimate the amount of forage you can grow. Here’s an On Pasture article that includes instructions for using it.
Gather soil samples and send them off for analysis! There’s no better way to find out what’s going on underground. For instructions on how to collect soil samples and send them, check our article, “The Thrill of Soil Sampling!” If you’re not sure where to send your soil sample, check in with your local Natural Resources Conservation Service Office, your local Conservation District, or an extension professional in your state.