Understanding the nutrient content of cover crops can be challenging because it depends on many different factors. Grasping everything that could affect the nutrient value of cover crops can be like trying to drink from a fire hose, says soil health specialist Jim Hoorman, but there are ways to simplify and understand how to determine what nutrients cover crops deliver to cash crops. Hoorman says microbes and cover crops are the main keys to making all nutrients more plant-available.
“Each microbe is just a soluble bag of fertilizer,” Hoorman says. “Get as many microbes in your soil as possible. The way to do that is by having a living root in the soil year-round to help feed microbes and mycorrhizae.”
Relative Plant Concentration of Nutrients
Hoorman has been studying soil health for more than 40 years. Formerly a soil health specialist with Ohio State University Extension as well as the USDA-NRCS, he currently operates his own soil health research group, Hoorman Soil Health Services.
Hoorman says carbon and oxygen make up roughly 90% of the nutrient content of a plant followed by hydrogen, which is typically 5-6%. After that, it gets a bit more complex. Nitrogen (N) and potassium (K) are usually the next highest concentrated nutrients — each about 1-1.5%.
But Hoorman says one of the most important nutrients to monitor is calcium, because although it only makes up roughly .5% of the plant’s nutrients, it can activate 146 different enzymes in the plant — which helps with photosynthesis and other metabolic processes that contribute to overall soil health.
NITROGEN CONTENT. The chart shows Ohio State University research that depicts the total amount of nitrogen (N) in the soil compared to the N contribution of legume and clover species measured in pounds per acre. Hoorman says crimson clover likely didn’t perform as well due to salt content in the soil that was used for the trials. Crimson clover is a salt-sensitive crop. Rafiq Islam, Ohio State University Extension
Several other micronutrients in plants, such as chlorine, iron, boron and manganese, also activate enzymes. These micronutrients combine with a protein to form an enzyme and ultimately increase the biological processes in the soil.
“A lot of these micronutrients and macronutrients are associated together, and you don’t want to separate them out,” Hoorman says. “They need to be combined together to be effective.”
Maximum nutrient availability varies based on pH level, according to Hoorman. Specifically, he says iron and manganese need careful attention. Iron is more available at a pH ranging between 4-6.5, for example.
“You don’t necessarily want your pH levels to be that low in general,” Hoorman says. “But what happens is right around the rhizosphere, the bacteria can change the pH by at least 1 or 2 pH units and create a smaller focused area that has the lower pH needed to maximize the availability of iron and even manganese.”
Boron, copper and zinc are also more available at lower pH levels, whereas nutrients like molybdenum can still be maximized at a pH level as high as 10.
Hoorman says that it’s possible to have too much of one nutrient in the soil, which can then tie up other important nutrients and have a negative effect on the plant. Too much N, P and K can limit the level of plant-available micronutrients in the soil.
Special N-fixing bacteria known as Rhizobia need cobalt to increase nodules 2-3 times, which in-turn can increase yields — especially in soybeans. More nodules on a legume or clover plant can have a strong positive effect on yield, according to Hoorman.
Ideal Soil Conditions
During drought, soil can’t release the adequate amount of plant-available nutrients. This is also true of soils that are too wet.
“The solution is that we need to achieve Goldilocks soil conditions where everything is just right,” Hoorman says. “Oscillating wet and dry conditions will give you the best microenvironment for good plant growth and soil health. We don't want conditions to be too dry, and we don't want conditions to be too wet. Compaction is not good, and soils that are too well-aerated are not good. The soil organic matter is really important in helping to make these conditions so they're just right.”
“The more biomass you have, the more plant-available P you're going to have…”
Certain areas in the soil have macro-aggregates and micro-aggregates. The micro-aggregates tend to be anaerobic, or lacking oxygen. Anaerobic bacteria will break down these nutrients and then flow with rainfall.
“If right next to that you have a root growing in a well-oxygenated area, the root can pick up those nutrients,” Hoorman says. “That's where we get our best growing conditions or as I like to call it — the Goldilocks soil conditions.”
Nitrogen Functions
Cover crops can be grouped into three categories when it comes to N functions. Some cover crops are N makers, some are slow N recyclers, and others are quick N recyclers.
N recycling is the process of converting N into different chemical forms in order to make N more available for the plant. Legumes and clovers tend to be the best cover crops for making N. Grasses are slower N recyclers, while brassicas recycle N quicker. Rhizobia bacteria inoculants are crucial for legume and clovers to be able to make N.
GLYPHOSATE TOXICITY. This graphic depicts the potential negative impacts of glyphosate on the soil and how glyphosate can affect nutrient availability. Hoorman says Roundup can tie up micronutrients, intensify drought stress and have a negative impact on yield. Fluid Fertilizer Forum
Hoorman says many farmers are not aware that natural inoculants produced from growing alfalfa or red clover can last up to 2 years, while soybean inoculants can last as long as 120 days. But when using commercialized planter box products, Hoorman says the timeline is only about 48 hours at the absolute maximum.
“On a lot of our cover crops like crimson clover for example, you’ve got about 48 hours before that inoculant starts to break down,” Hoorman says. “Temperature is very critical. You want to keep it cool and out of the sunlight. If you want to get maximum inoculation on your cover crops and if you want to get a lot of nodules and N produced, you need to inoculate them at planting.”
Alfalfa is one of the best cover crops for producing N as it can produce anywhere from 200-400 pounds. Hairy vetch, balansa clover, cowpeas, sweet clover and Austrian winter peas all have a range of closer to 150-200 pounds of N. Red clover, crimson clover and sunn hemp are estimated to produce 100-150 pounds of N in most cases. These ranges are dependent on climate, planting and termination timing, and a number of other different management factors.
Nutrient Losses
Soil compaction is detrimental to N, P and K.
“Most of your N is tied up in an organic form, and when we have compaction, we end up with high losses due to denitrification,” Hoorman says.
P reacts in a similar way. Whenever compaction occurs, soluble reactive P is lost. Hoorman says in areas where tillage is popular, compaction and N, P and K loss are both much more common.
Hoorman says recent research studies conducted in Ohio included soil testing for K, applying the K to fertilizer, and then vertically tilling it while the soil conditions happened to be a little bit wet.
“When you do that, the soil particles, especially those clay particles, tend to open up and seal off that K,” Hoorman says. “So when they soil tested after they applied the K, they actually found that the K level went down rather than up. This shows you how compaction has a negative overall impact on nutrient efficiency.”
Hoorman says too much P will decrease the amount of plant-available calcium, iron and magnesium. It can also lead to a decrease in plant-available K as well as copper and zinc. Hoorman says it is relatively easy to calculate the P levels in your soils to avoid nutrient tie-up.
“Each microbe is just a soluble bag of fertilizer…”
“It's a fixed relationship,” Hoorman says. “Phosphorus is 0.2% of all the biomass, so you just take the total biomass and multiply that by 0.2%, and that'll tell you a number pretty close to how much P you have. The more biomass you have, the more plant-available P you're going to have.”
Hoorman says the best cover crops for making P available in the soil include sorghum sudan varieties and oats because both are highly mycorrhizal.
Winter annual grasses, radishes and buckwheat are also very good because of their fine root systems. Buckwheat is an especially appealing option because it has a low pH in the rhizosphere and does a good job of increasing plant-available P, according to Hoorman.
Macronutrient Uptake
The best cover crops for N and P uptake, according to Hoorman, include oats, radishes, sorghum-sudan, cereal rye, triticale, red clover and hairy vetch.
Cereal rye, triticale and hairy vetch are especially appealing options because they tend to survive the winter and are good for absorbing manure as well as N and P nutrients and keeping them in the soil long-term.
Radishes can be good for recycling N and P, but Hoorman says they should always be planted alongside a grass cover crop because radishes when planted by themselves can make it easier to lose N from the soil. This is because the deep root system of a radish is meant to take up nitrogen in the form of nitrates from the soil.
Role of Mycorrhizae
Hoorman says mycorrhizae, a group of fungi microbes that live symbiotically with the crops by accessing and supplying needed plant nutrients from the soil to their plant partner, are a focal point of the discussion about nutrient content of cover crops and soil health in general.
According to Hoorman, there may be as many as 250 different species of mycorrhizae and they can have a strong impact on how many pounds of nutrients per acre are available in the soil.
Mycorrhizae are responsible for supplying all nutrients to the soil, according to Hoorman.
“The mycorrhizae are like root extenders,” Hoorman says. “They can extend 6-18 inches from the plant. They’re like trains bringing back the micronutrients to the plant.”
For N, P and K, there can be anywhere from 50-150 pounds per acre of each nutrient in the soil. The secondary nutrients, such as calcium, magnesium, sulfur, copper, boron, molybdenum and chlorine, are closer to 10-50 pounds per acre. And finally, the mycorrhizae is typically responsible for supplying less than 1 pound per acre of iron, manganese and zinc.
Exploring Natural Nitrogen Sources
By Jim Hoorman
Farmers pay a lot of money for nitrogen (N) fertilizer, especially on corn and wheat, but also on vegetable crops like tomatoes, pickles, melons, sweet corn and more. Most N fertilizer is produced from 200 plants world wide using the Haber-Bosch process. Natural gas or coal is combined with atmospheric N using high pressure and high temperatures. The coal or natural gas is a source of hydrogen, while the atmosphere supplies the N to produce ammonia (NH3). About 96% of the N fertilizer is produced this way, but this process results in high greenhouse gas emissions, including methane and carbon dioxide. The need for N fertilizer is currently about 100 billion tons per year.
The atmosphere is a natural source of N at 78%. Worldwide, there may be 4,000 trillion tons of total N atmospheric worldwide or about 34,000 tons of N per acre. Almost all of this N is in in the wrong form for plants. However, lightning can fertilize our crops, adding 1-50 pounds of natural N per acre. Lightning converts N molecules with high temperature by N fixation into nitrites and nitrates, which are called N oxides. When it rains, the nitrates are a common source of N fertilizer. Every single day, almost 10,000 tons of N fertilizer are produced by lightning.
In 2018, researchers from Stanford University began experimenting with plasma to mimic lightning to form N fertilizer. They formed a company called Nitricity. The advantage of producing N fertilizer this way is that it can be produced on farm using solar panels, atmospheric N and water. The good news is that greenhouse gas emissions are much lower, but the final product costs more than regular fertilizer. Most common N fertilizer costs are around 48 cents per pound for anhydrous ammonia, 67 cents per pound for urea, while liquid forms of N cost 72 cents up to $1 per pound. For a small 1-acre system using Nitricity technology, the cost is $3-4 per pound of N, but as the size increases to 25 acres, the cost goes down to about $1.50 per acre. It really depends on how much N is needed per acre. The system can also produce calcium nitrates and potassium nitrate, both common fertilizers. Currently, most of this N is applied in irrigation systems as a low-rate continuous source of N, which is good for efficient use of N fertilizer.
Several companies are looking to use this technology. Green Lightning is another company that uses the lightning process to make N fertilizer by creating lightning in the form of plasma in a controlled environment under the same premise. Air and water is combined in a static electric field for the nitrogen molecule to go through the rearrangement process. It attaches itself to the water, flows through the machine, and drips out the bottom of this machine as nitrite (NO2) and nitrate (NO3).
U.S. researchers are also incorporating genes from Mexican corn into U.S. corn varieties to make N naturally on brace roots. A certain variety of Mexican corn has a gene that allows plants to acquire N from the air through a process called N fixation. It is similar to the process of soybean nodules making N. Mexican corn brace roots secrete a gel-like substance called mucilage. The mucilage creates a low-oxygen, sugar-rich environment that attracts N-fixing bacteria. The bacteria convert the atmospheric N into a form the corn plant can use. In the future, corn may not need much N or may only need starter N to get a good corn crop.
All these new discoveries and innovations may soon help lower the cost of N fertilizer for crop production. However, they may still be a few years away. With corn prices around $3.50 per bushel, farmers are reluctant to plant corn because of the high N fertilizer costs. For farmers who have wheat and have not planted double-crop soybeans, now is an excellent time to plant legumes and clovers as a cover crop into the wheat stubble to generate natural N fertilizer.
Cowpeas, hairy or common vetch, and Austrian winter peas are excellent legumes that may produce up to 150 pounds of N per acre or more if you get a solid stand. Balansa, crimson and red clover can also produce large amounts of natural N fertilizer. All these crops grow better if planted early in August and September to get a good stand. All legume and clover cover crops should be inoculated with the proper rhizobium bacteria to maximize N nodule production. Building soil organic matter is still a great source of natural N, and it keeps the soil and nutrients in place.
Hoorman says plants can allocate up to 30% of their carbon to these mycorrhizae, and in exchange, the mycorrhizae can then acquire 80% of what the plant needs for N and P.
“Mycorrhizae also support free living bacteria as well as bacteria-releasing soluble P,” Hoorman says. “They are extremely good at releasing nutrients. All the studies we have seen show that plants grown with these mycorrhizae inoculants have anywhere from 30-300% more P.”
Potassium, Calcium Uptake
K is used primarily to accumulate other nutrients in the plant. It is unique in that it is not a part of the plant or any of the plant’s molecules. Instead, Hoorman says it functions like an iron pump.
“It’s a lot like oil in a tractor,” Hoorman says. “The tractor is made of metal, and the oil is made of hydrocarbons. What K does, just like the oil, is makes the tractor, or in this case the plant, perform better.”
“We need to achieve Goldilocks soil conditions where everything is just right…”
Legumes and clovers have the highest concentration of K because it is needed to recycle N and to make N. Alfalfa, winter peas, cowpeas, hairy vetch, common vetch and clovers are all good sources of K.
Having too much calcium, N or P can decrease the level of plant-available K in the soil. On the other hand, excess K can lead to decreased boron, which indirectly leads to a decrease in plant-available calcium.
“You need to have boron and calcium together,” Hoorman says. “Boron is the bus driver to get the calcium into the plant. Adequate calcium also helps decrease the chances of diseases like powdery mildew, pythium, sclerotina, fusarium, leaf and fruit scab, among others.”
Hoorman emphasizes that nutrients in the soil are intertwined and can directly or indirectly affect other nutrient levels. Adequate K levels also lead to more plant-available iron and manganese, for example.
The best cover crops for calcium uptake include radishes, oats, sorghum-sudan as well as most grasses. Legumes, clovers and brassicas, such as kale or rapeseed, are good for boron uptake.
Glyphosate Impacts
Hoorman says it’s important to be mindful of the impacts that glyphosate can have on nutrients and yield.
“Glyphosate is like a chelator, and it will tie up a lot of your micronutrients,” Hoorman says. “When it gets in the plant, even in Roundup Ready corn and soybeans, you can see a negative effect because when you fully apply glyphosate, it can move throughout the plant, and it chelates those micronutrients, which is going to intensify drought stress.”
Because glyphosate can move through the chutes and root tips of the plant, it ties up nutrients anywhere it goes within the plant, according to Hoorman.
He also says it’s not just the glyphosate that can be damaging, but often the byproducts of the glyphosate as it breaks down. Some of these byproducts will last a long time in the soil — sometimes as long as multiple years.
Glyphosate toxicity can hurt N-fixing microbes and mycorrhizae. It can also hurt earthworms and has the ability to compromise the plant defense mechanisms, which increases the risk of fusarium, pythium and other plant diseases. For this reason, Hoorman says it is best to tread lightly with glyphosate applications and be careful not to use too much.
