What’s weighing down your soil?

Authors: Skyler Allison & Nallely Leon, Students in the WSU Research and Extension Experiences for Undergraduates Program Summer 2023; Rachel Weime, Extension Specialist, WSU Extension Walla Walla County; Carol McFarland, Associate in Research with the PNW Farmers’ Network

An image of a dark green background.

With declining soil pH, the aluminum within the soil complex is released into the soil solution, where it affects plant health.

Blessed with fertile soils

In the inland Pacific Northwest, we are blessed with rich, fertile soils. Soils in this region of eastern Washington are predominantly classified as mollisols—a soil order known for being rich in organic matter and agriculturally productive. In fact, we currently know the Palouse region as having the highest wheat-producing counties in the United States. Across the U.S., the average wheat production is approximately 30 bushels per acre of wheat. In the Palouse, the average yield is 44.3 bushels per acre and can average above 80-120 bushels per acre in the higher rainfall zones. However, high wheat yields in the Palouse are in danger of decreasing. Unfortunately, these nationally recognized high yields are being threatened by the invisible land degradation of soil acidity, and the subsequent release of heavy metals that comprise the mineral structure of these valuable soils.

Soil acidity alone can reduce the availability of essential plant nutrients, like phosphorus, create less favorable conditions for microbial communities, and can damage plants by “burning roots.” Another primary concern is aluminum toxicity. Soils in much of the Palouse region are composed of aluminum-silicate clays (Figure 1). The aluminum in these soils is normally embedded in the clay mineral lattice. With declining soil pH, the aluminum within the soil complex is released into the soil solution, where it affects plant health.

Figure 1. Palouse soils are primarily composed of aluminum silicate clays, like kaolinite. These clays are built with two structured layers: one aluminum octahedral sheet (top) and one silica tetrahedral sheet (bottom). With declining soil pH, the aluminum in the octahedral layer can be released from the clay structure, and with it comes three H+ ions (protons) that further exasperate declining soil pH. (Figure from The Evolution of Soil Mineralogy)

Until soil pH becomes too low

Aluminum toxicity can be difficult to identify, as it can manifest in a variety of ways. Living things—from plants to humans—are unable to metabolize heavy metals. When absorbed, aluminum can block plant pathways for other essential nutrients and water uptake. This lack of nutrients can cause yellowing, stunted growth, plant senescence, and death. Aluminum toxicity in plants will often cause root clubbing, bloating, and blackening at the tips of the roots. Overall, these factors can lead to a significant reduction in crop yields because they inhibit the crop’s ability to uptake water and essential nutrients. Different crops and cultivars are impacted at varying levels by aluminum toxicity. Legumes have demonstrated the highest degrees of negative response to acidic and high aluminum conditions, as the environment also has negative implications for the symbiotic microorganisms that help those crops thrive. At the same time, aluminum-tolerant wheat varieties can help reduce the severity of a crop yield gap while other management strategies are also being implemented to remediate the underlying acidification.

Soil testing is a good place to start

Once aluminum is free in the soil, it is nearly impossible for it to be assimilated back into the clay mineral lattice. The best way to deal with free aluminum in soil is prevention: with soil health monitoring and soil testing. There are numerous ways to check soil pH and nutrient levels through both public and private institutions. Soil testing is available to growers through university extension agents, NRCS offices, and agronomists. Although professional methods like those are best for comprehensive soil testing, anyone can monitor their soil's pH at home with the right resources. Soil probes and portable pH meters are the primary tools used in soil pH testing in the field. The cost of these can vary from anywhere from $50 to $500. These tools are great investments that can last years, and enable on-going, site specific, real-time data collection. An excellent outline of best practices around in-field soil pH testing can be found in the WSU Extension Publication “Using a pH meter for in-field soil pH sampling.”

a person holding a soil probe with a small meter at the end.
Researcher Rachel Weime tests the pH of the soil using a handheld pH meter.

Fertilizer applications can impact soil acidity

When testing soil, it is important to take samples from a variety of locations and across the property and test areas of concern separately. This is because of a common phenomenon known as soil stratification. The regular application of ammonium-based nitrogen fertilizers causes localized acidification because the microbially mediated transformation that converts ammonium into plant-available nitrate releases protons (acidity) from the fertilizer into the soil solution. With tillage, mixing incorporates the acidity throughout the plow layer. In no-till systems, the acidity is stratified within the profile at the point of application. So, it is important to test your soil pH not only across a given location but also at multiple depths within the soil profile. Soil is often sampled at 12” depths; for soil pH, a more accurate method is to sample in smaller depth increments, such as 0-6" and 6”-12”, or even 3” increments, if possible.

When ammonium-based fertilizers are applied to the soil, they quickly convert to nitrate. This nitrification process releases excess hydrogen ions (H+) that make soil more acidic. Figure from WSU Extension Publication: Soil pH and implications for management: an introduction

Throughout the 'management zone', soil pH may vary based on rooting depth, soil type, and fertilizer infiltration. Each of these zones are critical to monitor. Soil pH has a large impact on the health of the microbiome within the soil. The bacteria, fungi, and other fauna within soil support plant health by creating symbiotic relationships with plants and mobilizing macro and micronutrients. This life within the soil flourishes within the rhizosphere or the area of the soil where a plant’s roots are present; many of them depend on root exudates from the plants to survive. This biome also depends on a more neutral pH to survive, ranging from 6 to 7.5. The rhizosphere is dependent on the rooting depth of a given crop, and so it is important to consider pH throughout this area, even down to 35 inches.

In the case that soil tests indicate acidic soil levels (pH under 5.5), consulting a local agronomist or extension specialist will help you formulate an action plan.

Mitigation strategies for acidic soil include:

  • Treating the land with lime.
  • Changing the type or reducing the usage of synthetic inputs in a field.
  • Retaining crop residue. incorporating new crops into your rotation.
  • Land management practices make a difference on the ‘master variables’ of soil health - soil pH and soil organic matter.
  • Adding legumes like lentils, chickpeas, peas, or alfalfa into your crop rotation can reduce nitrogen fertilizer inputs.

A highly acidic soil is not a healthy soil, and healthy soils are the foundation of agricultural systems here in the inland Pacific Northwest. Soil provides plants and ourselves with the nutrients that we have depended on for generations. By monitoring our soil health and pH, we can give back to the lands that have taken care of us for hundreds of years and ensure that our highly productive soils can continue to support our agricultural needs for decades to come.

Skyler Allison & Nallely Leon; Rachel Weime; Carol McFarland

Students in the WSU Research and Extension Experiences for Undergraduates Program Summer 2023; Extension Specialist, WSU Extension Walla Walla County; Associate in Research with the PNW Farmers’ Network

This article was published by the Washington Soil Health Initiative. For more information, visit https://wasoilhealth.org. To have these posts delivered straight to your inbox, subscribe to the WaSHI newsletter. To find a soil science technical service provider, visit the Washington State University Extension website or the Washington State Conservation District website.