“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 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!
- 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+).
- 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.
- 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.
Now that we understand the importance of nitrogen in crop fields, here’s how it cycles and moves around in the environment! N2 in 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 NH4+ to 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 is another essential macronutrient that’s found in phospholipids, lipids, and the backbone of DNA.
- Crop grain contains a large amount of phytic acid, which is primarily comprised of phosphorus molecules.
- Phosphate is the plant available for of phosphorus, and the two most common forms of P are HPO4-2 and H2PO4–.
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 is a macronutrient that’s required for protein and starch synthesis, acid neutralization, enzyme activation, as well as water regulation in plants.
- Plant available potassium is K+. Some soils can contain a lot of potassium, but not in the cation form.
- If a plant is deficient in potassium, it can exhibit stunting, necrosis, lodging, and an overall reduction in yield.
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!