This is the first in a series of blog posts aimed at helping you better understand soil testing data for improved horse pasture management, healthier horses, and a better environment. In this series, we will explore traditional soil testing data like N, P, and K levels and soil pH, along with less well understood data including micro-nutrient levels and biological measures of soil health.
Sixteen chemical elements are necessary for plant growth. Perhaps none are more challenging from a management perspective than nitrogen.
Why is nitrogen important?
Nitrogen is one of six chemical elements classified as a plant macro-nutrient (the other five macro-nutrients being phosphorous, potassium, calcium, magnesium and sulfur). Macro-nutrients are those elements generally required at the highest levels in soil for pasture grasses to thrive. Nitrogen is a key building block of proteins and nucleic acids in plants, including grasses, and also regulates plants’ ability to use other nutrients.
Where does nitrogen in soil come from?
Prior to human activity and the use of synthetic fertilizers, nitrogen in soil came predominantly from fixation of atmospheric nitrogen through soil microbial activity, including the activity of soil bacteria and mycorrhizal fungi. The natural recycling of nitrogen via animal manure and decomposition of plants and animals were also important sources. While these natural sources of nitrogen remain important today, the use of synthetic fertilizers has also become a major source of soil nitrogen, with both positive and negative consequences.
Is all soil nitrogen the same, and why does it matter?
Nitrogen in soil is found in a variety of organic and mineral forms, with most soil nitrogen present in organic forms. However, for a plant to be able to use organic nitrogen, it must first be converted into a specific mineral form, nitrate, through a process called mineralization.
Mineralization of organic nitrogen occurs via the microbial activity of bacteria and mycorrhizal fungi that live in our soil. The ability for these soil microbes to function properly, and thereby convert organic nitrogen into its plant-usable form, is heavily influenced by several key soil properties, including soil organic content, pH, drainage, and aeration (and its corollary - soil oxygen levels). For example, heavily compacted soils lack optimal microbial activity due to a lack of available oxygen needed by the microbes. Likewise, soils low in organic matter have poor microbial activity because soil microbes rely on this organic matter as fuel to do their critical work converting nitrogen into its plant available form, nitrate.
It’s also important to understand that if overall soil health is out of balance, any nitrogen you apply in the form of fertilizer may not be converted into nitrate, but rather into forms not available to plants. Not only is this a complete waste of time and money, but it also contributes to nitrogen runoff and pollution of our lakes, streams and other water bodies.
If nitrate is the plant available form of nitrogen, why not just rely on use of nitrate fertilizers?
Not all nitrogen in fertilizer comes in the same chemical form. Some fertilizers provide nitrogen in organic forms, or as other mineral forms such as ammonium (NH4), which must be converted into nitrate (NO3) by soil microbial activity before plants can use it. Nitrate fertilizers, therefore, are often recommended for soils with low pH and low organic content, conditions where microbial activity is typically poor.
However, the problem with nitrate fertilizers is that nitrate is a very unstable form of nitrogen in soils. In simpler terms, this form of nitrogen does not stick to the soil very well. It is estimated that no more than 50% of nitrate nitrogen applied as fertilizer material is ever utilized by plants. This also means that nitrate fertilizers pose the greatest risk with respect to pollution of lakes, streams and other water bodies. This includes your well, which can become contaminated by too much nitrogen (see this article by the CDC).
My colleagues at Brookside Labs (who we use as the lab for all our soil testing services) said it best:
“Of all the plant food nutrients essential to the growth and development of plants, nitrogen stands supreme in the role of both good and bad.”
What is the optimal level of nitrogen in soil?
Too little nitrogen and your pastures will suffer from poor grass growth and undesirable weed infestations. Too much nitrogen, however, can lead to decreased forage quality, decreased disease resistance (too much nitrogen has been shown to decrease the thickness of cell walls), and even pastures characterized as being “too lush.” This latter attribute is typically associated with concern over laminitis risk, although soils with nutrient levels that are too low may actually pose a greater laminitis risk, as grasses respond to soil nutrient deficiencies by increasing the levels of fructan and other sugars in the edible portion of their blades (but that’s a topic for another blog post).
So what is the right number? Unfortunately, like so many things in nature, the answer is… “It’s complicated.” For one, ideal nitrogen levels are largely dependent on the form of nitrogen found in the soil. As discussed, organic forms of nitrogen are not the same as nitrate nitrogen. To add to the difficulty, different soil types may require different nitrogen levels. Even within the same soil, ideal nitrogen levels will vary seasonably based on when the grasses need nitrogen for growth and maintenance.
You might also be surprised to learn that most routine soils tests, including those done by most ag extension offices, don’t measure soil nitrogen levels at all. Nitrogen testing is both expensive and difficult. That being said, there are some new testing procedures recently developed to measure soil nitrogen, but they are not yet widely adopted. See https://www.ars.usda.gov/new-test-can-determine-nitrogen-levels-in-soil/.
The obvious follow-up question is, if soil tests don’t measure nitrogen levels, how do they make nitrogen application recommendations? Nitrogen application recommendations are often based on “management goals” rather than measured soil nitrogen levels.
Management goals may include things like what type of grasses you are trying to grow (the example shown on the right shows different nitrogen application recommendations depending on whether you have fescue or bermudagrass pastures), whether you want to maintain or eliminate legumes (such as clover) from your pastures, whether the pasture is for grazing or hay production, and whether you are looking to establish new or manage existing pasture. That’s why such management questions are often asked when filling out a soil test form.
This doesn’t mean that soils testing data is not important when it comes to nitrogen. It still is. For example, if your soils are low in phosphorous or potassium, or have a low pH, nitrogen applications will be ineffective unless you address those deficiencies as well. The key to soil health is the right overall balance in soil nutrients, and you won’t know whether overall soil nutrients are in balance without reliable soils testing data.
Other posts in this series
At Sustainable Stables, we believe that the key to healthy pastures starts with healthy soils. Our soils testing services go beyond simple macro-nutrient (NPK) and pH analysis to include micro-nutrient levels along with biological measures of soil health. Such soil data is key to developing a sustainable management plan to create healthy, resilient pastures. Our testing services can be done independently, or as part of our equestrian property planning and design services. Contact us today to learn more.