After the flames: How fire affects soil nutrients

Hundreds of thousands of acres of forest, rangeland and cropland have sadly gone up in smoke this summer in Montana. In addition to the devastating effect on personal property and direct loss of crops and livestock, fire can affect soil properties and soil nutrients. The impact is highly dependent on the fire intensity/duration and the proportion of plant material that is burned. Timber and shrubs will burn hotter and longer with greater impact on soil than range- or crop land. Fast moving grass fires have minimal impact on soil nutrients and soil health compared to slow moving, intense fires in moderate to heavy fuels.

In general, fires reduce the pool of nutrients stored in organic matter, release a flush of plant available nutrients in the short term, and redistribute nutrients through the soil profile. The availability of nutrients, especially nitrogen, is increased after low intensity fires, yet, a portion of nitrogen and sulfur is lost to the air. Although these losses are not trivial and are similar to removal by harvest and losses to wind erosion, they are small compared to the average pool of nutrients in the top six-inches of soil.

Nitrogen can additionally be lost through nitrate leaching, as the burned plant matter creates a large pool of nitrate and few active plant roots are left to take up either the nitrate or soil water. This can have long term impact on the productivity of forest and rangeland ecosystems, but can be minimized or remediated on croplands. The other nutrients such as phosphorus, potassium, magnesium, zinc and manganese are more stable and not lost directly through combustion, but rather through blowing ash, and post-fire soil erosion.

Cropland fires rarely burn hot enough to affect soil organic matter. The bigger concern is loss of surface plant residue, which is very important to reduce wind erosion, and protect against the physical sealing impact of raindrops. Ash particles also contribute to reduced water infiltration as they plug soil pores. All these factors increase the risk of water runoff and soil erosion.

Intense forest and shrubland fires can burn soil organic matter, reducing the pool of nutrients in the soil, soil aeration and water infiltration/retention, and the soil’s ability to hold nutrients coming from ash or fertilizer.

In addition, forest and shrubland fires can create a water repellent layer within the top 2 inches of soil that comes from compounds in the burnt litter, coating soil aggregates or minerals. The depth and thickness of this layer can vary greatly, and it can affect infiltration for several months to years. This layer should not form on grassland or stubble fires.

Fire kills bacteria and fungi at the soil surface but microbes rapidly recolonize from deeper soil layers, except in severe fires where the soil is sterilized several inches deep. Microbial activity can actually increase with the flush of nutrients available after a fire. However, new input of plant material is important to sustain their populations.

Post-fire management includes soil testing to determine nutrient availability, and establishing ground cover where possible. Test for nitrogen, phosphorus, and potassium to calculate fertilizer needs. Because drought preceded fire, it’s likely that many fields have nitrogen that wasn’t used this summer, so less might be needed next spring. When soil sampling burned fields, be sure to select representative sites, avoid areas where there may have been a windrow, bale, or other high accumulation of straw or residue. Spreading manure can be very beneficial post-fire but this is rarely available or reasonable at large scales.

The MSU Soil Fertility Extension website has several publications and presentations on soil testing and calculating fertilizer rates. Contact Clain Jones at or 406-994- 6076 if you have any questions.

MSU Extension offers advice on evaluating soil health

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BOZEMAN – Experts with Montana State University and MSU Extension have recommendations for growers on evaluating soil quality and health.

The concept may seem subjective, but there are ways to measure and improve soil health. It takes time to measure, monitor and manage to improve soil health, but it can be worth the effort for potential benefit in sustainability and productivity.

“With ‘soil heath’ now being a frequently heard term, we want agricultural producers to be aware of what factors contribute to soil heath and how they can be reliably measured,” said Clain Jones, Extension soil fertility specialist at MSU.

Soil productivity is influenced by its chemical characteristics, physical structure and biological activity. Measurements of these properties provide an estimate of the soil’s ability to produce crops. Indicators of soil productivity can be tracked over time, compared in side-by-side fields, or compared to a reference soil and are useful to assess the effect of management or evaluate problem areas.

Chemical soil characteristics, including pH, soil organic matter, nutrient levels and cation exchange capacity are often part of routine soil analyses done by analytical labs. The physical properties such as available water holding capacity (also called plant available water), bulk density, porosity and aggregate stability, are also most reliable if measured by an accredited lab, yet not all labs perform these measurements. Field tests are available for many of these soil properties but they often rely on subjective interpretation of potentially imprecise measurements. Microbial activity is also important, yet has the least defined set of measureable factors by which it can be quantified.

For a quick assessment of soil health, get out a shovel and dig. Compare a cropped soil with undisturbed fence-line soil. How deep do roots go? Does it break apart easily? Does it smell earthy? Is there evidence of worms? Darker color indicates more soil organic matter or soil carbon.

“The shovel test can give the grower an idea of their soil quality and identify what problems they might be facing,” said Jones.

Major steps towards increasing soil health are to reduce tillage, increase crop diversity and reduce fallow time by including alternative crops or cover crops into the rotations.

For more detailed information on soil health indicators and measurements, see Jones’ The Soil Scoop on his website, or contact Clain Jones at or 406-994-6076.

MSU Extension offering new publications on soil nutrient management for forage crops

montana state extension logoBOZEMAN – Forage crops provide substantial income to many Montana farmers. They are also an integral part of livestock production systems. Improvements in forage production through good soil fertility practices have the potential to increase income for farmers and ranchers.

Montana State University Extension has recently published two bulletins, “Soil Nutrient Management for Forages: Nitrogen” and “Soil Nutrient Management for Forages: Phosphorus, Potassium, Sulfur, and Micronutrients.” These publications present soil nutrient management options for Montana forage production systems based on regional research results.

The key to nutrient management for optimal forage yield and quality is to select the right fertilizer source, rate, placement and timing for your operation, known as the 4R concept.

“These are usually interrelated. For example, the right rate, placement and timing are very dependent on the source,” said Clain Jones, co-author and Extension soil fertility specialist in the Department of Land Resources and Environmental Sciences (LRES) at Montana State University. In addition, selecting the right crop and the best management practices to maximize legume nitrogen fixation are also critical. “Getting it ‘right’ not only increases your bottom line, it also protects soil, water, and air resources,” said Jones.

Nitrogen is the most common nutrient that needs to be added for production of forages containing a low percentage of legumes, while phosphorus and potassium are more important for those dominated by legumes. The correct balance of nutrients can influence stand species composition and is important for efficient fertilizer use and forage yield and quality. Fertilizer rates should be based on soil tests or plant tissue concentrations to ensure adequate amounts, yet minimize the risk of forage nutrient concentrations that are toxic to livestock.

Timing of fertilizer application depends largely on the source in order to optimize the amount of nutrient that gets taken up by the crop, rather than lost to the environment. “Nutrient sources that slowly release their nutrients over time, such as manure, phosphate rock or elemental sulfur, can extend benefits over years, while many commercial inorganic fertilizers are more immediately available,” said Jones. Legumes may be the most economical source of nitrogen. “Because fertilizer can become tied up temporarily in the soil and plant material, the economic benefit of fertilization should be evaluated over several years,” said Jones.

Adequate nutrients are key to sustaining stand health and most likely are less expensive than reseeding or interseeding. If stands are largely desirable species, rejuvenating old forage stands with fertilizer is more effective than mechanical rejuvenation methods such as aeration or harrowing. “Well thought out nutrient management on forages can easily pay for itself,” said Jones.

The bulletins are available as printed copies from MSU Extension,, or (406) 994-3273, as well as online at Jones’ webpage at