Jay Norton, November 8, 2018. For publication in the Wyoming Livestock Roundup
Fertilizers generally improve soil health when used as part of sustainable agricultural systems, but degrade soil health when used to replace good management. Fertilizer can enable “quick-fix” systems with heavy tillage and without good crop rotations or inputs of organic materials.
In sustainable agricultural systems, the 4 Rs of fertilizer management (Right rate, Right placement, Right time, and Right type of fertilizer) are combined with proven conservation practices in ways that increase crop biomass and conserve soil organic matter. This enhances soil health and reduces fertilizer needs.
The right rate means correctly using soil test results to determine how much nutrient the soil can provide and how much more is required for optimal yield.
The right placement means appropriately injecting, banding, incorporating, or watering in for best root access and minimal loss.
The right time means applying nutrients as close as possible to when plants need them.
And the right source means choosing the right form of dry or liquid fertilizer for the crop and environment.
In natural, undisturbed rangeland soil systems, nitrogen in forms available for uptake by plants and soil microbes is almost always the most growth-limiting nutrient. That’s because nitrogen comes almost solely from breakdown of plant and animal residues. In an undisturbed rangeland, lack of disturbance (tillage) constrains air supply to microbial decomposers. Constrained decomposition means that most N is tied up in organic forms and not available for uptake by plants or microbes. Cycling rates of organic materials can be high, but released nutrients are quickly taken up by diverse plant and microbial communities. We often refer to such systems as “nitrogen limited” because, while there is usually abundant organic nitrogen, forms available for uptake by plants and microbes are usually in short supply. Mycorrhizal fungi, which form symbiotic relationships with plants that increase access to nutrients, are often prevalent in the soil microbial community of undisturbed rangelands.
Fertilizers create a large influx of available nitrogen and other nutrients previously in short supply. This drives huge increases in the number of soil microbes, especially when tillage increases access to air. Numbers of bacteria usually increase while mycorrhizal fungi decrease. The happy microbes rapidly decompose crop residues, converting carbon to carbon dioxide and creating a “carbon limited” system. With a limited supply of carbon, microbes attack soil organic matter that is crucial to soil health.
In a quick-fix system, continuous annual cropping and heavy tillage cause degradation of soil structure and losses organic matter, often leading to erosion. Loss of soil health causes a spiral of decreasing yields and increasing fertilizer needs. The resulting soil system has a small and opportunistic soil microbial community where nutrient availability is often out of sync with crop demands. Expensive fertilizers not taken up by plants or microbes are lost to deep leaching, runoff to surface waters, or as gases to the atmosphere.
Poor fertilizer management also damages soil health by allowing bad farming practices to continue without losing too much yield. Besides on-farm decline in soil health, the lost nutrients, sediments, and organic matter pollute water and air as they contribute to global warming.
On the other hand, good management that combines the 4Rs with conservation farming practices improves soil health, reduces fertilizer demand, removes carbon dioxide from the atmosphere, and mitigates global warming.
Sustainable agricultural systems include practices that increase soil organic matter by either conserving it (slowing decomposition) or adding it, including minimized tillage, cover and green-manure crops, compost application, rotations that include periods of perennial hay or pasture, careful grazing management, and others. In such systems, proper use of fertilizers increase plant biomass production. More plant roots and aboveground residues contribute to soil biota and increased soil organic matter content.
Higher soil organic matter contents support thriving plant and microbial communities that convert available nutrients into organic materials that are easily decomposed. Decomposing plants and microbes release nutrients in ways that coincide with crop uptake, because both plant growth and decomposition increase with increasing temperature and moisture.
With time, practices that increase soil organic matter also decrease the amount of added fertilizer needed per unit of crop yield, but as soil health improves, yields can also increase, so optimal fertilizer rates might stay the same or increase.
The 4 R’s of fertilizer management are crucial to efficient and responsible use of fertilizer as part of a sustainable agricultural production system, whether in pasture, hayland, or cropland.
For more detailed information about managing fertilizers to enhance soil health from a global perspective and about the 4 R’s of fertilizer management, go to my Web site (https://soilmanagement.wordpress.com) and click on Soil Links.
Feel free to contact me at 307-766-5082 or firstname.lastname@example.org to talk about sustainable management or the 4 R’s for your soil.