Over the past few weeks, I’ve written about what cover crops like Sunn Hemp, Tepary Bean, and Cowpea leave behind in the soil and how their nutrient contributions stack up in standard soil tests. But it wasn’t until we looked at the Haney Soil Test results from March 2025 that we could truly see the biological influence each of these summer cover crops had on the soil. In this post, I’m sharing new insights drawn from those results and why I believe every grower should consider this test when evaluating cover crop performance.
Why the Haney Test?
Unlike standard chemical soil tests that only measure nutrient availability, the Haney Test adds a biological lens. It measures microbial respiration (CO₂-C), available organic nitrogen (Haney N), and gives an overall Soil Health Score. These indicators help us understand how biologically active the soil is and how much of the nutrients are likely to cycle into plant-available forms.
For organic and sustainable systems, this is vital. We’re not just feeding the crop—we’re feeding the soil.
All Plots Started Equal
Just to set the stage: all test plots had a rye cover crop terminated in early spring 2024 and were kept bare and weed-free through summer. The only difference among plots came when Sunn Hemp, Tepary Bean, or Cowpea was planted in August 2024. The check plot remained bare.
Sunn Hemp had the highest CO₂-C (43.21 ppm), strong Haney N (74.74 lbs/ac), and the highest Soil Health Score (9.41). It fed the microbes and left behind a soil system ready to cycle nutrients. If you’re planting a high-demand crop like corn or grain sorghum, Sunn Hemp sets the table biologically.
Cowpea followed closely with a CO₂-C of 32.08 ppm, Haney N of 71.50 lbs/ac, and a Soil Health Score of 7.80. Reliable, balanced, and consistent—it’s a solid choice for improving soil function while conserving moisture and nutrients.
Tepary Bean, despite good forage quality and tissue N content (3.02%), showed low microbial activity (CO₂-C of 13.75 ppm) and the lowest Soil Health Score (6.43). It may decompose slower or produce compounds less favored by microbes. That’s not necessarily bad—it might serve longer-term fertility, but it’s not the best option for short-term nutrient release.
Check Plot (bare fallow) showed high mineral N (83.73 lbs/ac) and decent CO₂-C (37.14 ppm), but that’s misleading. There was no cover crop to feed soil life or cycle nutrients—just unutilized residuals from last year. Long term, this approach does not build soil health.
The biological boost from a cover crop can be measured and managed. Without the Haney Test, we’d only be guessing how much nitrogen or biological activity remains from cover cropping. We tell growers: don’t plant blind—use this test to make more informed fertility and management decisions.
Sunn Hemp again proved why it’s a leading summer cover crop for southern systems. Cowpea is a great second choice when water is limited or biological stimulation is still desired. Tepary Bean may have a role in longer rotations but isn’t the best for quick turnover systems.
Final Thought
We use cover crops for more than just erosion control. They’re engines of soil biology, nutrient cycling, and resilience. The Haney Test gives us a dashboard to read those engines.
If you’re not already using it, this is your sign: test for biology, not just chemistry.
I was scrolling through my LinkedIn this morning (Monday, July 15, 2024) and saw a post by Dr. Joseph Burke that I just had to check out!
Just click on the picture to read the full research paper!
I am going to cut through all the information in the full-text and give you a look at the mini version. Let’s start with the abstract from the first page.
Abstract: By improving soil properties, cover crops can reduce wind erosion and sand damage to emerging cotton (Gossypium hirsutum L.) plants. However, on the Texas High Plains, questions regarding cover crop water use and management factors that affect cotton lint yield are common and limit conservation adoption by regional producers. Studies were conducted near Lamesa, Texas, USA, in 2017–2020 to evaluate cover crop species selection, seeding rate, and termination timing on cover crop biomass production and cotton yield in conventional and no-tillage systems. The no-till systems included two cover crop species, rye (Secale cereale L.) and wheat (Triticum aestivum L.) and were compared to a conventional tillage system. The cover crops were planted at two seeding rates, 34 (30.3 lbs./ac.) and 68 kg ha (60.7 lbs./ac.), and each plot was split into two termination timings: optimum, six to eight weeks prior to the planting of cotton, and late, which was two weeks after the optimum termination. Herbage mass was greater in the rye than the wheat cover crop in three of the four years tested, while the 68 kg ha (60.7 lbs./ac.) seeding rate was greater than the low seeding rate in only one of four years for both rye and wheat. The later termination timing produced more herbage mass than the optimum in all four years. Treatments did not affect cotton plant populations and had a variable effect on yield. In general, cover crop biomass production did not reduce lint production compared to the conventional system.
This last statement, “cover crops did not reduce lint production,” is hugely significant and yet it is exactly what many organic cotton producers have been saying for years!
Temperature and Rainfall data during the study
To continue the “mini version” of the research let’s turn to the Summary and Conclusions on page 9 of the research paper.
The semi-arid Texas High Plains presents challenging early-season conditions for cotton producers. Cover crops can help mitigate erosion and protect cotton seedlings from wind and sand damage without reducing yields compared to conventional practices if managed appropriately. Effective cover crop management is needed to optimize cotton lint yield compared to conventional tillage systems. We focused on three cover crop management practices: species selection, seeding rate, and termination timing. With regard to species selection, rye produced greater herbage mass in three of the four years. The seeding rate had less of an effect on herbage mass; doubling the seeding rate from 34 to 68 kg ha (30.3 – 60.7 lbs./ac.) did not contribute to increased herbage mass. This change in seeding rate only causes an increase in seed costs, and this trend held true for both species and termination timings. Termination timing had the most significant effect on herbage mass, with a two-week delay in termination timing, increasing herbage mass production from 44 to 63%. At the targeted termination time of six to eight weeks before planting, rye and wheat experienced increased growth as they transitioned from vegetative to reproductive growth. This critical period makes termination timing an essential aspect of herbage mass management. Termination timing can also impact the carbon-to-nitrogen ratio, where higher C:N at later growth stages can increase N immobilization. While water availability or allelopathy concerns are cited as risks for cotton germination and emergence when using cover crops, cotton plant populations were not affected in this study.
Cotton lint yields were not impacted by increasing cover crop herbage mass, except in 2018, when greater wheat biomass resulted in decreased lint yield compared to the conventional system. In each year, wheat or rye at a 34 kg ha (30.3 lbs./ac.) seeding rate and optimum termination timing resulted in cotton lint yields not different than the Conventional Treatment. While yield potentials can differ between years depending on precipitation and temperatures, effective cover crop management can help sustain cotton lint yields when compared to conventional treatments. Rye seed tends to cost more than wheat, but it grows more rapidly and could be terminated earlier to allow for increased moisture capture and storage between termination and cotton planting. (below is the final sentence in the paper and summarizes well the entire study)
“This research demonstrates that with effective cover crop management, the implementation of conservation practices can be successful in semi-arid cropping systems.“
Cover crops play a pivotal role in sustainable agriculture by enhancing soil health, managing pests and diseases, and improving overall crop yield resilience. Cover crops can be any non-harvested crop used primarily to protect soil from erosion during off-season periods, provide actively growing roots to extract and stabilize nutrients that might be otherwise vulnerable to leaching or volatile loss, and increase levels of SOM to promote soil physical properties and C sequestration. Cover crops have other values to farmers, as some crops can also be harvested for forage or seed or to diversify the cropping system to suppress diseases, obtain other crop rotation benefits, improve off-season access to fields, or extract water during wet periods.
As a source of additional C delivered to soil during non-cash-crop growing periods (e.g., in fall and winter in many temperate regions), cover crops are particularly effective in supplying soil microorganisms with readily available carbon sources from both root exudates during growth and C-rich crop residues upon termination. Several studies have found greater soil organic carbon sequestration with implementation of cover crops (Poeplau and Don, 2015). Often combined with no-tillage, management of cropland with cover cropping can enhance soil organic C sequestration due to addition of organic materials growing directly on land rather than imported from another location.
In the summer we plant sorghum sudangrass (top picture) for weed control because it has an allelopathic effect on weeds (click that link to read about it) and it shades any weeds coming on later. It is a vigorous and versatile cover crop that stands out for its exceptional contribution to soil health and weed suppression. Its rapid growth and dense canopy make it highly effective at outcompeting weeds, thus reducing the reliance on herbicides. This competitive growth habit is instrumental in shading out weeds, significantly lowering weed biomass and seed bank potential in the soil. Beyond weed control, sorghum sudangrass excels in improving soil structure and health. Its deep and extensive root system breaks up compacted soil layers, enhancing soil porosity and aeration. This root action not only facilitates better water infiltration and storage but also promotes the activity of beneficial soil organisms by increasing organic matter and available nutrients in the soil profile. Just remember the allelopathic effect (preventing weeds or the crop growing) last for 10-14 days after soil incorporation!
The benefits of sudangrass extend to its role in adding organic matter to the soil when it is mowed and incorporated as green manure. This process means making sure the plant is in a 30-40:1 Carbon to Nitrogen ratio. The decomposition of sudangrass residue releases significant amounts of nutrients, especially nitrogen, which are then available for subsequent crops, thereby improving soil fertility. Additionally, sudangrass has been noted for its biofumigant properties, particularly when specific varieties are used. The breakdown of its tissues can release compounds that suppress soil-borne pathogens and nematodes, further promoting a healthy soil environment conducive to high-yielding crops. However, it’s important to manage sudangrass properly, as allowing it to reach maturity (beyond the 40:1 carbon to nitrogen ratio) can result in a tough, woody residue that is slower to decompose and might interfere with planting subsequent crops.
Sunn Hemp
Sunn hemp (picture above) is increasingly recognized for its substantial benefits as a cover crop, particularly in warm climates where it thrives. One of the key advantages of incorporating sunn hemp into crop rotations is its ability to rapidly accumulate biomass, which, when turned into the soil, significantly enhances soil organic matter. This increase in organic matter improves soil structure, water retention, and nutrient availability, leading to a more fertile and resilient soil ecosystem. Moreover, sunn hemp is an excellent nitrogen fixer, capturing atmospheric nitrogen and converting it into a form that subsequent crops can easily absorb. This natural fertilization process reduces the need for synthetic nitrogen inputs, lowering production costs and minimizing environmental impact.
However, while sunn hemp offers numerous benefits, there are also challenges associated with its cultivation. One potential issue is its allelopathic properties, which can inhibit the germination and growth of subsequent crops if not managed properly. This is due to compounds released by sunn hemp into the soil that can affect sensitive plants, or it can work to keep weeds out! Additionally, sunn hemp may pose a risk of becoming invasive if not carefully controlled. This risk underscores the importance of implementing appropriate management practices, such as timely mowing or incorporation into the soil before seed set, to prevent unwanted spread. Despite these challenges, the benefits of sunn hemp, particularly in terms of soil health enhancement and its role in sustainable agriculture practices, often outweigh the potential drawbacks, making it a valuable tool in the arsenal of organic farmers aiming for weed control and soil health benefits.
Good video about Sunn Hemp from Missouri research!
Cowpea
Cowpea (Vigna unguiculata) (picture above) serves as an excellent cover crop in a variety of agricultural systems, providing multiple benefits for soil health and weed management. Its ability to thrive in poor soil conditions, coupled with a relatively low requirement for water, makes cowpea a robust choice for enhancing soil fertility and structure, especially in regions prone to drought. As a leguminous plant, cowpea enriches the soil with nitrogen through symbiotic nitrogen fixation, a process where bacteria in cowpea roots convert atmospheric nitrogen into a form that plants can use. This natural fertilization boosts the nutrient content of the soil, reducing the need for synthetic fertilizers and thereby lowering agricultural input costs.
In terms of weed control, cowpea’s rapid growth and dense foliage provide an effective cover that suppresses weed emergence by significantly reducing light penetration to the soil surface, thus minimizing the growth opportunities for unwanted plants. The shading effect also helps in retaining soil moisture, further supporting the growth of the cowpea while inhibiting weed development (this effect is not nearly as effective because it is a shorter plant). Additionally, when cowpea is incorporated into the soil as green manure after its growth cycle, the organic matter added to the soil improves soil structure, enhances water retention, and stimulates the activity of beneficial microorganisms. However, it’s important to manage cowpea cover crops effectively to prevent them from becoming a weed themselves, as their vigorous growth can sometimes lead to challenges in controlling their spread if not timely mowed or incorporated into the soil. Overall, cowpea stands out as a versatile and beneficial cover crop, contributing to sustainable agricultural practices by improving soil health, enhancing nutrient availability, and providing effective weed suppression.
Winter Cover Crops
Winter cover is more difficult because we typically start to get land ready about the time our cover crops start to grow in February/March. Winter cover is almost always a small grain and most of the time we use a “combine run” wheat or oat since they are cheaper with a planting of turnips or daikon radish or both.
Cereal Rye
Cereal rye (not ryegrass), scientifically known as Secale cereale (pictured above), serves as an exceptional cover crop for a multitude of reasons, pivotal for enhancing agricultural sustainability and soil health. One of its foremost benefits is its robust root system, which significantly improves soil structure and enhances water infiltration. This characteristic is particularly valuable in preventing soil erosion and runoff, thus protecting water quality in the surrounding environment. Additionally, cereal rye’s ability to uptake residual nitrogen from the soil makes it an excellent tool for nutrient management, reducing the risk of nitrogen leaching into water bodies and thereby mitigating the environmental impact of nitrogen fertilizers.
Moreover, cereal rye acts as a natural weed suppressant due to its quick germination and fast growth, outcompeting weeds for light, nutrients, and space. The crop’s residue also provides a mulch that further suppresses weed growth and retains soil moisture, which is particularly beneficial in dryland farming systems. Furthermore, by providing a habitat for beneficial insects and microorganisms, cereal rye enhances biodiversity and contributes to the overall health of the agroecosystem.
This picture is from Carl Pepper near O’Donnell on the South Plains. It was planted last September into cotton plants. Seeding rate is 4.5 lbs. of Rye and 4.5 lbs. of Barley with 1 lb. of purple top turnips burned in the freeze.Holds the soil, uses very little if any moisture and is cheap to establish.
Short video of Roller Crimping a rye cover crop at pollination
Mustards
Using mustards as a cover crop is a practice rich in benefits for sustainable and organic agriculture. Mustards contribute significantly to soil health and pest management strategies without reliance on chemical inputs. They are known for their rapid growth, which quickly covers bare soil, reducing erosion and suppressing weed growth through competition. The deep rooting of mustards helps break up compacted soil layers, enhancing water infiltration and aeration for future crops. Perhaps most notably, mustards possess biofumigant properties; when incorporated into the soil, they release natural compounds that suppress a variety of soil-borne pathogens and pests (click here for a great project with mustard seed meal). This dual action of physical soil improvement and chemical pest suppression makes mustards an invaluable tool in the organic farmer’s toolkit, promoting a healthier, more productive soil ecosystem and paving the way for successful crop rotations.
“Caliente Rojo” mustard is a variety specifically bred for its biofumigation properties, which can play a significant role in organic agriculture, particularly in disease management and soil health improvement.
Biofumigation Properties: “Caliente Rojo” mustard, when incorporated into the soil, releases isothiocyanates (ITCs), which are naturally occurring compounds found in Brassica plants. These compounds have been shown to suppress a wide range of soil-borne pathogens, including fungi, bacteria, nematodes, and some weed species.
Soil Health Improvement: Beyond disease suppression, “Caliente Rojo” mustard contributes to soil health by adding organic matter, improving soil structure, and enhancing microbial activity. This leads to better water infiltration, aeration, and nutrient cycling in the soil.
Growth Habit: It has a fast growth rate, which quickly provides ground cover, reducing soil erosion and weed growth. Its deep rooting system can also help in breaking up compacted layers of soil, improving root penetration for subsequent crops.
Sowing: It is typically sown in the fall or early spring when the soil can be worked. The planting rate and spacing should be adjusted based on the specific goals (biofumigation, erosion control, etc.). Typical planting rate is 8 lbs./ac. but can be lower.
Incorporation: For biofumigation, the mustard should be mowed or chopped and immediately incorporated into the soil while it is still fresh. This action releases the biofumigant compounds.
Irrigation: After incorporation, irrigating the area can help in releasing the biofumigant compounds more effectively as they hydrolyze in the presence of water.
Vetch
Common vetch (Vicia sativa) and hairy vetch (Vicia villosa) are leguminous cover crops celebrated for their multifaceted benefits in sustainable agriculture. These species excel in nitrogen fixation, a process where atmospheric nitrogen is converted into a form that plants can use, enriching the soil and reducing the need for synthetic fertilizers. This attribute makes them particularly valuable in crop rotations, especially preceding nutrient-demanding crops. Hairy vetch, with its robust growth and cold tolerance, is particularly noted for producing a significant amount of biomass, which can improve soil structure and organic matter content.
Both common and hairy vetch exhibit vigorous root systems that enhance soil health by increasing porosity and water infiltration, thereby reducing erosion and improving drought resilience. Their dense foliage serves as an excellent weed suppressant by outcompeting weed species for sunlight and nutrients, which can lead to reduced herbicide reliance. Upon termination, the biomass of these vetch species acts as a natural mulch, conserving soil moisture and further suppressing weed growth. Additionally, the flowers of vetch attract beneficial insects, including pollinators and predatory insects, which contribute to the biodiversity and resilience of agroecosystems.
Hairy vetch, in particular, stands out for its ability to thrive in a wide range of soil conditions and its notable winter hardiness, making it an excellent choice for cover cropping in cooler climates where other legumes might fail to establish or survive. Hairy vetch will produce more residue than common vetch 1/3 to 1/2 more. Common vetch does tend to reseed and establish easier in a pasture system compared to hairy vetch. When used in a no-till farming system, the decomposing vetch residue can release nitrogen slowly over time, closely matching the nutrient uptake patterns of subsequent crops. This synchrony minimizes nitrogen leaching and maximizes nutrient use efficiency, showcasing the role of vetch not only in enhancing soil fertility but also in promoting more sustainable and environmentally friendly farming practices.
Wheat (Triticum spp.)
Advantages: Wheat is widely adaptable, with a deep root system that improves soil structure and enhances water infiltration. It’s excellent for erosion control and can be a good scavenger of residual soil nitrogen, reducing nitrate leaching. Wheat also serves as a decent biomass producer in cooler climates.
Best For: Erosion control, nitrogen scavenging, and when a crop that can survive a wide range of conditions is needed.
Oats (Avena sativa)
Advantages: Oats are fast-growing and establish quickly, providing rapid ground cover to outcompete weeds and reduce erosion. They produce significant biomass, which can improve soil organic matter. Oats also die off in freezing temperatures, which makes them easy to manage in the spring.
Best For: Quick cover to outcompete weeds, adding organic matter to the soil, and as a winter-kill cover crop in regions with cold winters.
Barley (Hordeum vulgare)
Advantages: Barley establishes quickly and can provide a good ground cover and weed suppression. It’s more drought-tolerant than oats and can be used in areas with lower water availability. Barley also contributes to soil health by adding biomass and improving soil structure.
Best For: Fast establishment, drought-prone areas, and effective weed suppression.
Triticale (× Triticosecale)
Advantages: Triticale, a wheat and rye hybrid, combines the best traits of both parents. It offers a robust root system, excellent biomass production, and good tolerance to both poor soil conditions and colder temperatures. Triticale is also notable for its nutrient scavenging ability and can be used to improve soil fertility.
Best For: Biomass production, nutrient scavenging, and versatility in both cold and marginal soil conditions.
Daikon Radish or Tillage Radish
Daikon radish, often referred to as tillage radish, has gained popularity as a cover crop for its unique ability to improve soil structure and health through natural biotillage. Characterized by its rapid growth and large, penetrating taproot, tillage radish drills through compacted soil layers, creating channels that enhance air and water infiltration. This deep penetration also helps to break up hardpans, reducing the need for mechanical soil tillage, hence the name “tillage radish.”
One of the standout benefits of tillage radish is its capacity to capture excess nutrients from the soil profile. The deep roots absorb nitrogen and other nutrients, which are then stored in the plant’s tissue. When the radishes decompose in the spring, these nutrients are released back into the soil, becoming available for the next crop. This nutrient recycling can improve crop yields while reducing the risk of nutrient runoff into waterways, contributing to more sustainable farming practices.
Tillage radish also contributes to weed suppression. The rapid, dense canopy formation shades out weeds, reducing their ability to establish. This effect can carry over into the spring, providing a cleaner start for the next crop. Additionally, the decaying radish residue leaves behind significant organic matter, contributing to soil organic matter content and overall soil health. This organic matter feeds soil microorganisms, which play a critical role in maintaining soil fertility.
Moreover, the winter die-off of tillage radish eliminates the need for chemical or mechanical termination, simplifying spring field operations. This characteristic makes it an attractive option for farmers looking to reduce labor and input costs associated with cover crop management. The holes left by the decomposing radishes can also improve soil aeration and provide pathways for the roots of subsequent crops, potentially enhancing root development and access to deep soil nutrients.
Purple Top Turnip
Purple top turnip is a cover crop that has been used for years in Texas. The seed is relatively cheap, serves as winter grazing if needed, grows fast and adds lots of organic matter. It is known for its rapid growth and adaptability to a wide range of soil types, this cover crop is an excellent choice for farmers looking to enhance soil structure, suppress weeds, and improve nutrient cycling within their farming systems. The large, leafy greens of the purple top turnip create a dense canopy that can quickly cover the ground, effectively suppressing weed growth by outcompeting weeds for sunlight and nutrients.
Texas A&M AgriLife Organic 