Nutrient Cycling in the Environment

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“The nation that destroys its soil destroys itself.” This quote by Franklin D. Roosevelt simply explains the importance of managing soil quality. This becomes extremely applicable to farmers who are trying to maximize crop production, which can be achieved by maximizing the productivity of their ground. Fields contain much more than just dirt. They’re a complex ecosystem that contains a large amount of diversity when it comes to chemical and biological composition. One major factor in soil’s productivity when related to crop production is the nutrients found in soil. Some nutrients come from organic materials that are naturally occurring, while others are added to the soils because they are deficient. This process becomes a bit complicated when talking about specific nutrient cycling. This post will showcase how nutrients move throughout the environment while shining a light on the importance of managing soil nutrients. 

How do nutrients cycle in the soil?

Nitrogen (N)

Nitrogen is a macronutrient required by all plants, and is especially correlated with high yields in corn and soybean production. But first here’s a little bit about the basics of nitrogen in a cropping system! 

  1. Nitrogen gas (N2) is abundant in the air, however it cannot be taken up by plants. The plant available forms of nitrogen is nitrate (NO3) and ammonium (NH4+). 
  2. Nitrate is mobile in the soil profile. Due to this molecule’s negative charge, it repels from negatively charged soil particles and is easily lost to leaching and soil runoff. 
  3. Plants use nitrogen to synthesize amino acids, proteins, and chlorophyll. Ammonium is the easier form of nitrogen to use because it requires less energy in the reduction process. 
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This diagram helps to visualize each process in the nitrogen cycle. Photo from wikiwand.

Now that we understand the importance of nitrogen in crop fields, here’s how it cycles and moves around in the environment! Nin the atmosphere goes through ammonification to become NH4+, which occurs due to nitrogen-fixing bacteria found in the soil. Legume roots have a symbiotic relationship with these bacteria, which adds plant available N to a field. Once in the ammonium molecule, nitrifying bacteria changes NH4to nitrites (NO2) and then nitrates (NO3). From this point, the molecules can either be taken up by plants, processed back to N2 through denitrifying bacteria, or leached with water. If the soil’s natural amounts of nitrogen is insufficient for a specific crop, the producer can apply fertilizers to a field.  It’s important to remember that these processes are constantly changing the chemical makeup of a soil, and that severe weather events could deplete the soil of many plant-available forms of nitrogen. 

Phosphorus (P)

Phosphorus is another essential macronutrient that’s found in phospholipids, lipids, and the backbone of DNA.

  1. Crop grain contains a large amount of phytic acid, which is primarily comprised of phosphorus molecules. 
  2. Phosphate is the plant available for of phosphorus, and the two most common forms of P are HPO4-2 and H2PO4
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This diagram helps to depict and simplify the chemical changes that occur in the phosphorus cycle. Photo from soilmanagementindia.

Unlike nitrogen, phosphorus’s most abundant form is a solid found in the ground. Organic P is created over an extremely long period of time with plant residue, hummus, and microbial biomass. Organic P is turned into a plant usable form through mineralization, and the reverse reaction is called immobilization. Once phosphorus is available and held in solution it can become unavailable by reacting with clay and various mineral surfaces or by binding with cations such as calcium, iron, and aluminum. Phosphorus held in solution is susceptible to leaching, much like nitrogen is. It’s important to know that the main P inputs into ecosystems are derived from fertilizers and plant residue. 

Potassium (K)

Potassium is a macronutrient that’s required for protein and starch synthesis, acid neutralization, enzyme activation, as well as water regulation in plants. 

  1. Plant available potassium is K+. Some soils can contain a lot of potassium, but not in the cation form. 
  2. If a plant is deficient in potassium, it can exhibit stunting, necrosis, lodging, and an overall reduction in yield. 
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This photo showcases the major steps and processes in the potassium cycle. Photo from nutrien-eKonomics.


Potassium starts as a primary mineral such as mica. After years and years of weathering, it changes into a nonexchangeable form. K cations change between nonexchangeable and exchangeable molecules through release and fixation. Once K is in an exchangeable form, it changes into a plant available form through desorption. K in solution is able to be leached, but is much less susceptible than nitrates. In terms of mobility within the soil profile, potassium is immobile. This is because a large percentage of soil K is nonexchangeable, due to its location and attraction to soil minerals. The main inputs of potassium into systems is through fertilizer and plant residues. 

How are these cycles manageable? 

Since these cycles are constant and on-going, it’s crucial for producers to maintain a knowledge of the nutrient levels within their fields and which areas are the most susceptible to losing nutrients faster than others. One way to measure nutrients is to test the plant’s vegetative matter for chemical composition. While these results are helpful and accurate, it doesn’t necessarily provide information for the available forms within the soil. This is where soil testing comes into play! Soil tests can be used to qualitatively measure nutrient levels precisely, which helps to give producers recommendations on management practices in the future years. 

Hopefully this opened your eyes a little to the vast possibilities within soil science, as well as provided a better understanding of some prominent nutrients that cycle through ecosystems!

~Rosie

Soil and Water Conservation Practices – What are They Doing?

Agricultural run-off has been a big talking point in recent years. Many people know bits and pieces of the conversation, but the scope of the issue can be a bit complicated. There are many factors and smaller issues that need attention. So, what is the deal with run-off?

As you may know, Iowa is under national pressure to reduce the amount of nutrients washing off of our land and into the rivers, and ultimately into the Gulf of Mexico. The main issue at the Gulf is hypoxia, which basically means there are too many of specific kinds of nutrients, which promotes algae growth, which in turn chokes out other organisms like fish. Clearly this is not ideal.

The main nutrient that gets the press is nitrogen. Nitrates in the water has been one of the bigger issues people talk about. The main concerns are removing nitrates from drinking water (primarily because of the blue baby syndrome that was a larger problem in the 1950s), issues of environmental stability, and even cost of wasted nutrients on the farm associated with nutrient loss from the field.

The other nutrient we are now paying attention to is phosphorus. This one isn’t quite as popular to talk about in the public space, but it impacts aquatic ecosystems similarly to nitrogen. They both support the growth of algae, sometimes to the point of using up all of the oxygen in the water and killing off native fish.

Nitrogen and phosphorus (along with potassium) are two of the three main macronutrients that plants need to grow. Nitrogen tends to be the nutrient that is applied most to Iowa farm fields, because of how important it is to crop growth. These are naturally occurring elements in the soil and play a vital role in life on Earth.

However, we’ve come to this issue. Aquatic ecosystems are being negatively impacted by these nutrients. Farmers pay to apply these nutrients to their crops and don’t want to lose their investment. How can we reduce those nutrient loads in our waterways and keep nutrients and soil where they belong?

The good news is those questions are being asked, and many programs are underway. The more difficult news is that there is not a one-size-fits-all answer either for fields or for the nutrients themselves.

Let’s back up a little bit and talk about how these nutrients interact with the soil and water. Soil particles are made up of sheets of molecules that when bonded together create an overall negative charge. This works out pretty well because most nutrients have a positive charge in their plant-available form. This means that most nutrients bond with the soil and stay available to the plants living in that soil. Phosphorus is one of these nutrients.

But then we get to nitrogen. It sometimes is in a form with a positive charge (NH4+), but it doesn’t really like to stay like that forever. It gets broken down or may volatilize into a different form. It could be lost into the atmosphere, or it could become NO3-, which is nitrate. When nitrogen changes into its nitrate form, it is no longer attracted to the negatively charged soil and ends up leaching its way through the soil profile along with ground water.

Since nitrogen moves with ground water, the primary way it gets around is through tile lines. Field tile is commonly used in Iowa to help keep excess water from the field. If soil is too waterlogged, the crops may struggle. Thus, Iowa farmers have been tiling their fields for decades to give the water a quick and easy way to escape the field.

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Iowa Learning Farms display highlighting differences between nitrogen and phosphorus loss from a field and how tile lines play a part.

Both of these nutrients cost money to apply. They are important for crop growth. The soil that can hold them is also valuable. Farmers don’t want them to leave their field partially because they don’t want to lose that value, and also because of the negative impacts too much of them can have on other ecosystems. But because these two things come into the waterways in different ways – phosphorus comes into the water with soil that has eroded, and nitrogen through the water itself – they have to be managed in different ways.

So what are some ideas?

There are some practices that can be built or installed to help modify the landscape or the way the elements interact with it. These can cost real money, but in some cases, government programs or even corporate incentive programs can help fund their start up. These can be largely grouped as permanent structures

One cool new idea is saturated buffers. This system uses both buffer strips and tile lines. Buffer strips are areas of natural plants parallel to ditches and waterways left fallow to help filter runoff. Conventional tile systems are installed 3-4 feet deep to catch and take extra water from the field and into a near ditch or waterway. The outlets of these tile lines have conventionally been placed directly into a ditch or waterway uninhibited. However, the idea with a saturated buffer is that if you place the tile line parallel to the ditch or waterway, the soil and plant growth in the buffer strip will filter the soil particles and nutrients in the water before it reaches the waterway. The Agricultural Research Service claims this system filters an average of 42% of the nitrate load from the water.

The saturated buffer idea is similar to the idea of bioreactors. Bioreactors also help filter nutrients from tile line outlets, but with artificially created biological processes. Essentially a pit is filled with organic, carbon-rich materials, like woodchips, where microscopic life can flourish. These microorganisms help break down and filter nitrates from the water introduced to the system. This is a slightly older technology than saturated buffers, but is costlier to implement and will need redone roughly every 15 years.

Buffer strips and bioreactors are considered “edge of field” practices. This means that they don’t need to take up area in a field, but instead use the less productive or less safe land to farm near a water boundary. Though it still may be a hard sell to pull that land out of production, it is argued that those areas likely aren’t making the farmer much money, and proper water management may help make other areas better. Since these practices help filter ground water before it reaches the stream, they are some promising pieces in removing excess nitrates.

Terraces are another way farmers can make changes to the land to slow water runoff. You may have seen pictures of farms in other parts of the world where terraces are cut like stairsteps into a large hill so that the crops can be grown on flat land. Here in Iowa, we use a different kind of terrace that essentially builds up a smaller hill on a broad hill to slow water running off the slope. When the water runoff is slowed, soil particles and nutrients can have more time to settle out in the grassed front and back-slopes of the terrace. For more info on terraces, check out our previous blog post here, Clean Water Iowa, or the Natural Resource Conservation Service.

Avoca Terraces

Terraces placed on the slope protect the soil from erosion in Avoca, Iowa.

Grassed drainageways or grassed waterways are a common sight to see around the hills of Iowa. These are natural waterways that farmers leave in native grasses to slow water flow, intercept soil runoff (to decrease P loss), and hopefully the plant life will also absorb excess nutrients in the water (to decrease N loss). If not left in a grassed waterway, these areas of the field would be susceptible to rill and gully erosion.

There are also cropping practices that can impact soil and water conservation. These can be opted into or out of any given season.

Another very cool idea is cover crops. We’ve written previous blogs about cover crops, but in a nutshell, cover crops are plants grown in a farmer’s field during the off-season (fall to spring) to keep the soil covered and protected during a time it would usually be left bare. Cover crops protect the soil surface from wind and rain, the roots help improve soil structure, they add organic material that makes soil healthier, hold nutrients near the soil surface, help suppress weed growth, and do lots of other cool things. Because cover crops help use nutrients and protect the soil, they can help with both nitrogen and phosphorus loss.

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Soybeans sprouting through a terminated rye cover crop on a strip-tilled farm near Algona, IA.

Our grandfathers and great-grandfathers broke the land using moldboard plows year after year. Eventually we realized that even though tillage can help prevent soil compaction and help with weed control, it has a negative impact on soil structure and makes soil more susceptible to erosion. Because of this, some farmers swung the pendulum to the opposite side of the spectrum and went no-till. This means they never till the soil on their fields. Instead, they are more dependent on herbicide weed control, and use crop rotations and cover crops to help mitigate soil compaction. When the soil is not tilled, the structure becomes stronger, and previous years’ crop roots and stems help protect the soil. This helps prevent soil from washing off the field, thus preventing phosphorus loss.

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This is a field that has been strip-tilled. Strip tillage is a form of conservation tillage where only part of the field is tilled.

If we think of no-till and moldboard plowing as the two far swings of the pendulum in terms of tillage, likely the middle of the road or slightly more toward no-till would be conservation tillage. This is the idea that a field can benefit from tillage for compaction or weed control issues, but can also benefit from increased soil structure and added organic material from less tillage. These practices vary greatly, but likely include less of the topsoil being broken up.

Contour farming is a way to plant a field so that the rows follow around the hills. This means that when the water runs down the hill, it would run perpendicular into the rows instead of parallel between the rows. This should slow the flow of water, giving soil time to settle out of the water before it continues down the hill.

There are many options available for farmers and landowners in the area of soil and water conservation. More research is being done every day for the best and newest ways to decrease risk, decrease cost, and maximize benefit. Because of the nature of these practices (particularly permanent structures and cover crops) is that it costs real money up front to earn intrinsic value later (i.e.: will not earn them a paycheck), it can be difficult to implement new things. However, farmers recognize the good that these things do, and are interested in new cost-share and grant opportunities that can help them do better with what they have.

What do you think the next big idea will be? Maybe you’ll be right!

-Chrissy

Why do they do that? Anhydrous

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Early in the spring and late in the fall it is common to see tractors pulling large white tanks across bare farm fields. So, what are these strange white tanks? What’s in them and why is it applied to fields?

They are anhydrous tanks filled with anhydrous ammonia (NH3) – one of the most efficient and widely used sources of nitrogen fertilizer for agricultural crops like as corn and wheat.

Nitrogen is one of the 17 essential elements required for plant growth. Nitrogen is most commonly found in the atmosphere making up approximately 78% of the air that we breathe. But in the air it is in the form of N2 which is not available to plants to use. Nitrogen is part of chlorophyll which makes plants green and allows them to use sunlight to produce sugars (food) from oxygen and carbon dioxide through photosynthesis. Nitrogen supports strong vegetative plant growth, which is vital for good fruit and seed development.

Plants use nitrogen by absorbing either nitrate (NO3) or ammonium (NH4) ions through their roots. Soybeans and other legume plants can convert atmospheric nitrogen into a usable form because of nitrogen fixing bacteria on their root nodules. Other plants, like corn, need to have an ample supply of available nitrogen in the soil. Farmers can add nitrogen to fields in the form of livestock manure, granular urea, liquid nitrogen (UAN solution), and anhydrous ammonia.

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When making environmentally and economically sustainable decisions about fertilizers, farmers consider the 4Rs best management practices. This helps them select the right fertilizer source and apply it at the right rate, right time, and right placement in the soil.

Anhydrous ammonia is often a preferred nitrogen source for many reasons. It is more concentrated than other forms of nitrogen, containing 82% nitrogen. It is readily available, because it is used in the manufacturing process of other nitrogen fertilizers. It can be applied long before the crop is planted. It is usually the most economical option as well.

Farmers store and transport anhydrous ammonia in liquid form in pressurized tanks. Using an anhydrous applicator pulled by a tractor, the high-pressure liquid converts to a liquid-gas mixture as the pressure drops while traveling from the tank to the knife outlet on the applicator. The knife slices the soil and injects the fertilizer 6 to 8 inches into the soil.

Once in the ground, the ammonia (NH3) ions react with moisture in the soil and convert to ammonium (NH4). Ammonium ions are very stable in the soil. They carry a positive charge and are bonded to negatively charged soil particles like clay and organic matter. These ammonium ions can be taken in by plants and used directly in proteins. Over time, the ammonium converts to nitrate (NO3) which is the form of nitrogen most used by plants for growth and development. Nitrate does not bond to soil like ammonium does and could leach out of the soil and into waterways. Nitrogen fertilizer stabilizers are often added to anhydrous ammonia before application to slow the conversion of ammonium to nitrate, thus helping to reduce nitrogen loss from leaching.

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Because of the stability of anhydrous ammonia (and converting to ammonium) it can be applied in the fall with less potential to leach, volatilize, or to be lost in water runoff than other nitrogen fertilizers. Cooler soil temperatures help keep the ammonium ion stable and so farmers try to apply it in the fall after the soil temperature drops below 50°F. If applied in the spring, it is best to apply it at least 3-5 days before planting to avoid damaging seeds and emerging roots.

Good nitrogen management is critical for growing healthy plants, good yields, and a profitable farm business. Farmers consider crop nutrient requirements, results of soil tests, soil conditions, weather, cost, time, and equipment available before choosing a fertilizer program that is the best fit for their operation.

-Cindy