(par Consequences for organic matter in soils - Environatics (2023)



Another problem with extensive and abundant use of inorganic fertilizers is that their use does not improve soil fertility and structure over the long term. Further, as they are used more and more in place of alternative practices that do build the soil, we become increasingly dependent on their use — an addiction.


Largely from four sources, whose relative importance varies with the ecosystem:

Inputs from the atmosphere (both with precipitation and in dry form)

Weathering of the parent material underlying the soils

Activity of nitrogen-fixing organisms

Decomposition of dead organic matter

Let’s focus on the last of these. How does decomposition affect soil structure and fertility? Following is quite a simplified version of these complex processes.

(1)An organism dies (or parts of it do) and is on or in the soil.

(2)Invertebrates, fungi, and bacteria living in and on the soil consume it.

(3)In so doing, they assimilate many of its nutrients into themselves. This process is called “nutrientimmobilization.” As long as these nutrients are in the bodies of the decomposers, they are not available for plant uptake, hence the term “immobilization;” they are temporarily unavailable. When the nutrients are immobilized, they are in organic form. (Decomposers can also immobilize nutrients that are applied inorganically and tie them up temporarily in organic form. This is particularly common with nitrogen, and particularly when the carbon:nitrogen ratio of the litter is very high, such that nitrogen is in great demand.) You might have experience with this phenomenon if you are a gardener. Have you ever applied an abundance of carbon-rich mulch, such as sawdust, to your garden, hoping to enhance its organic matter content, but then noticed that the plants don’t seem to be producing as well as usual — and may look yellowed? If so, it is likely that the decomposers who were stimulated by the addition of all that good carbon-containing material have immobilized most of the available nitrogen (and possibly other nutrients) in the process of decomposing the material, so that less is available for the plants.

Interesting sidelight: you probably know that the invasive Eurasian grass cheat grass (Bromus tectorum) has taken over vast areas of rangeland, particularly in thewestern US? Well, researchers have learned that it has higher nitrogen requirements than do many of the native rangeland plants, and experiments are underway to take advantage of this. I kid you not — researchers are spreading table sugar on infested rangelands (often in association with other treatments as well), attempting to decrease nitrogen availability through the mechanism I described just above, hoping that this will diminish the competitiveness of cheat grass in these systems. Dr. David Pyke of the USGS (and affiliated with OSU’sDepartment of Botany and Plant Pathology) is involved in some of these research efforts.

(4)The decomposers all tend to be short-lived. They die and are consumed by others, often through a complex succession of organisms. However, gradually, the nutrients that they contained are converted to inorganic forms either through biological action or by leaching by water. This process is calledmineralization; the conversion of organically bound forms of nutrients to inorganic forms as result of inorganic or biological chemical reactions. At this point, the nutrients are available for plant uptake.

Thus, in soils, death, immobilization and mineralization, coupled with plant uptake, are constantly occurring. Nutrients gradually become available from the decay of organic inputs, which act essentially like timed-release fertilizers. This is thenutrient cyclingwith which you are undoubtedly already familiar.

One result of this process is that, in natural ecosystems, nutrient cycling (particularly of nitrogen, which is most commonly a limiting nutrient)– is very “tight.” Not much is lost from the soil because:

(1)it is released slowly,

(2)it is taken up rapidly, and

(3)there is usually little loss of nutrients via erosion in natural ecosystems. Some nutrients are lost, of course, as with erosion of soil, volatilization to the atmosphere, and leaching with water, but losses are usually minor.

Natural ecosystems also generally have a high storage capacity for nutrients. This storage capacity comes largely in and on organic material which is slowly decayed.


The least decomposable material is left behind ashumus, a structureless, dark, chemically complex organic material. Humus is also gradually decomposed, releasing nutrients, but also is continuously added to in intact systems

Humus is vitally important for soils:

(1)It provides nutrients as it is decomposed and an energy source for soil organisms.

(2)It furnishes sites for retention of other nutrients. Humus contains large numbers of charged sites – for example negatively charged sites to which positively charged nutrient ions (such as ammonium [NH4 + ]) adhere.

(3)It increases the water-holding capacity of soil (is “spongy”).

(4)It improves soil aeration, as it is fibrous and porous. This improves the soil environment for many microbes and plant roots, which require aeration.

(5)It increases infiltration of water into the soil (versus run off of water), again because of its porosity.

The latter three all related to aspects of the soil“structure”which are enhanced by organic matter in soils.

So,what does all this have to do with inputs of inorganic fertilizers? First:


(1)Many of our richest farm soils are rich because for thousands of years they supported productive prairies, which accumulated soil organic matter over the ages. Hence, current agriculture in many cases relies on this historical, pre-agricultural accumulation of organic material.

(2)Historically, agricultural systems featured mixed crops and livestock. The animals were used to work the soil (as for plowing) and also grazed on crop residues post-harvest. Their manure furnished organic matter to the soils.

(3)Relatively abundant crop residue was left on the site post-harvest, which also contributed to soil organic material.

(4)Farmers made extensive use of fallow years (years when the soil was left unplanted, to recharge its water and nutrients) and ofcrop rotation(rotating, for example, a legume (whichfixesatmospheric nitrogen) with a grain, and plowing in the legume (or its residue) at grain planting time or before, and thus adding nitrogen and organic material to the soils). The effectiveness of rotation of course depends on the species being used and how much of them is harvested before tillage or re-planting. For example, soy doesn’t add much organic matter to the soil because it doesn’t produce much post-harvest residue – (see Bullock’s article on rotation in thesupplementaryreading list for more information on this).


First, farmers find it easier to regulate precisely the amounts of various nutrients added to the soil by using inorganic fertilizers. They are also easier and less time- and labor-intensive to use than organic sources, and the yield gains tend to be more immediate for inorganics than for organics, which take time to decompose and release nutrients. Other changes in agriculture that contribute to the decreased emphasis on organic sources of fertility include:

(1)Fewer farms mix livestock and crops (there is more specialization in agriculture). Much of the manure from feedlots does get returned to the soil, but only after it has lost, typically, much of its nutrient value. Also, most manure is returned to the fields that are close to the feedlots (which then receive more nutrients than they can retain…) because of the perception that it costs too much to haul it farther — we’ll examine that assumptionlater.

(2)Harvests are often more intensive than in the past. For example, crop residues are removed and used for animal feed off-site, and more and more of them are being used for bio-energy production, straw bale construction, and so forth. Furthermore, the Green Revolution varieties produce less residue than did traditional varieties, in general (recall the high“harvest index”associated with Green Revolution varieties?)

(3)There is decreased use of rotation and fallow. Land tends to be under continuous cultivation of single crops — for example, in 1991, 40% of US corn acreage was being grown continuously in corn.


(1)For many years, US government farm policies essentially blocked farmers from rotating. To receive full crop subsidies and other financial supports, growers had to commit acreages to certain crops, which made rotation a financial liability. (See the National Academy of Science book on thesupplementaryreading list for information on this.) The 1990 and 1996 Farm Bills eased these restrictions a bit, but there are still restrictions. (See2002 Farm Billfor information on the conservation provisions for the 2002 version of the Bill; information on the2008 Farm Billis provided part way down the section of notes that deals with Conservation Tillage.) Subsidies are hugely important in US agriculture — during the 1990’s, ~ 1/4 of net farm income for US agriculture came from direct government payments. However, most rotation crops are not eligible for subsidies — the ’08 Farm Bill greatly limited the number of crops that are eligible for such payments; 90% of payments went to coren, wheat, soy, cottom and rice. (Side note, ~ 70% of these subsidies do not go to regular family farm operations but rather to the largest 10% of producers, which are often corporations such as Cargill. Many small farmers do, however, depend on these subsidies, so can’t afford to use rotation crops.)

(2)There is a perceived need in these times to have every acre be producing food crops every year, which makes rotation with many legumes less attractive than it might otherwise be

(3)Increased mechanization decreased the need to raise rotation crops (e.g., alfalfa) as feed for animals, since fewer farmers used animals to work the farm and there are fewer mixed livestock-crop operations.

(4)Mechanization and the economies of scale associated with specializing worked against rotation. A grower needs less equipment if he/she is only growing corn than if he/she is growing corn and a rotation crop.

(5)Economics have worked against rotating in another way as well, in that rotating requires that the land be used every other year or so for crops that may not bring as much money as the grain crops.


A series of interconnected changes occur as we rely more and more on inorganic inputs and less and less on organics.

(1)Humus decreases because:

There is less organic input

Soil microbes that were nutrient limited can decay what organic matter there is faster when inorganic nutrients are supplied. (This faster decay persists at least until structural problems in the soil [described below] outweigh the nutrient advantages. That is, early on, inorganic inputs increase decomposition rates, but, over time, that often reverses.)

Intensive tillage disrupts soil aggregates (clumps), so there is faster decomposition of organic material. This involves both faster chemical oxidation and enhanced activity by aeration-stimulated decomposers.

A dramatic example of this loss of organic material in agricultural soils is in the midwestern US, whose prairie soils havelost 1/3 – 1/2 of their organic material since they began being cultivated.This is a common pattern: conversion to cropland is almost universally associated with a rapid decrease in soil organic matter and soil nitrogen content.

(2)Lowered organic material (humus) results in:

Decreased input of slow-release fertilizer

Lowered water holding capacity and more runoff of water (which takes with it inorganic fertilizers and soil with its organic material [which is typically concentrated in the topsoil, which is most vulnerable to erosion by runoff]).

Decreased aeration, as the soil loses the structure given by organic material. This of course worsens problems of increased runoff. Poorly aerated soils are also less suitable for beneficial soil organisms, so natural inputs of nutrients via mineralization eventually slow. In addition, poorly aerated soils are not as good for plants, worsening the efficiency of their roots at taking up nutrients

(3)Heavy use of inorganic sources of fertility, in particular nitrogen fertilizers, decreases the efficiency of free-living nitrogen fixers in the soil. Thus, their fixation rates decrease and there is less natural input of nitrogen.

(4)High inputs of nitrogen fertilizers can also result in soil acidification.

Essentially, as growers add inorganic fertilizers without due attention to organics, they step onto a one-way street. The combination of factors described above means that they need to add ever-increasing amounts of inorganic fertilizers to sustain their yields. It is similar to anyaddiction, where increasing amounts of the desired substance are required to achieve satisfaction.

The amounts of inorganic fertilizers required increase because natural inputs of fertility and the nutrient retentiveness of the system diminish:

Humus is lost rapidly (see above)

Biological nitrogen fixation slows

Soils are less retentive of nutrients because: (a) enhanced runoff takes more nutrients off with it (b) lowered organic material means fewer exchange sites in the soil on which nutrients are normally retained, and (c) plant roots are less efficient in poorly aerated soils.

Thus, Green Revolution style agriculture (in company in the US with the requirements of government farm subsidy programs) has set farmers up for a system that requires increasing dependence on inputs of inorganic fertilizers, rather than on methods of insuring soil fertility that are potentially more sustainable and also less energy intensive. (Remember all that fossil fuel energy that is required to makenitrogenfertilizers….)


Which is a consequence of soils having high amounts of organic matter? ›

Increased levels of organic matter and associated soil fauna lead to greater pore space with the immediate result that water infiltrates more readily and can be held in the soil (Roth, 1985).

What is a good level of organic matter in soil? ›

Soil organic matter is the fraction of the soil that consists of plant or animal tissue in various stages of breakdown (decomposition). Most of our productive agricultural soils have between 3 and 6% organic matter.

What percentage volume does organic matter account for in a healthy soil? ›

The typical soil consists of approximately 45% mineral, 5% organic matter, 20-30% water, and 20-30% air.

What are the consequences of lack of organic matter in soil? ›

Loss of soil organic carbon content can limit the soil's ability to provide nutrients for sustainable plant production. This may lead to lower yields and affect food security. Less organic carbon also means less food for the living organisms present in the soil, thus reducing soil biodiversity.

How much organic matter is too much? ›

Too much compost or other organic matter, however, can increase the phosphorus concentration in soils to the point where the element may become a pollutant. So have your soil tested regularly to make sure it holds 20 to 40 pounds per acre of available phosphorus.

What are the environmental factors affecting soil organic matter? ›

Inherent factors affecting soil organic matter include climate and soil texture and clay mineralogy. Climatic conditions, such as rainfall and temperature, and soil moisture and aeration (oxygen levels) affect the rate of organic matter decomposition.

What are soils with 20% organic matter? ›

Soil Organic Matter Levels

If a soil has 20% or more organic material to a depth of 16 inches, then that soil is considered organic and is termed a peat or muck, depending on the extent of decomposition. These soils are described taxonomically as a Histosol (Figure 1).

What percentage of organic matter is needed for a lawn? ›

Ideal organic matter ranges in turf fall within 1% to 2%. In contrast, farmers are customarily trying to find ways to add and increase organic matter accumulations. Ideal organic matter ranges in agriculture are generally as high as 10%.

Is high organic matter in soil good? ›

CHEMICAL: Soil organic matter significantly improves the soil's capacity to store and supply essential nutrients (such as nitrogen, phosphorus, potassium, calcium and magnesium), and to retain toxic elements. It allows the soil to cope with changes in soil acidity, and helps soil minerals to decompose faster.

What is a good average value for the amount of organic matter? ›

The correct answer is 10%. Only 10% of the available energy is transferred to the next trophic level, most of the energy is lost to the environment as heat. 10% can be taken as the average value for the amount of organic matter that is present at each step and reaches the next level of consumers.

Is a soil with greater than 30% organic matter considered to be an organic soil? ›

Soil which contains 30% or more organic matter is considered as organic soil. Organic matter is the organic component of soil which includes residues of dead plants, animals and other organisms. It consists of nutrients necessary for plant growth such as Nitrogen, Phosphorus and Potassium.

What does high soil organic matter mean? ›

Soil organic matter means all living, or once-living, materials within, or added to, the soil. This includes roots developing during the growing season, incorporated crop stubble or added manures and slurries. Back to: Soil organic matter.

What are four important effects of organic matter in soil? ›

Increasing levels of organic matter aid in soil structure, water-holding capacity, nutrient mineralization, biological activity, and water and air infiltration rates.

How does organic matter affect the environment? ›

Organic matter includes any plant or animal material that returns to the soil and goes through the decomposition process. In addition to providing nutrients and habitat to organisms living in the soil, organic matter also binds soil particles into aggregates and improves the water holding capacity of soil.

What are the 5 factors that affect amounts of organic matter in soil? ›

  • Temperature. Several field studies have shown that temperature is a key factor controlling the rate of decomposition of plant residues. ...
  • Soil moisture and water saturation. ...
  • Soil texture. ...
  • Topography. ...
  • Salinity and acidity. ...
  • Vegetation and biomass production.

What destroys organic matter? ›

Decrease in organic matter supply

Burning destroys the litter layer and so diminishes the amount of organic matter returned to the soil. The organisms that inhabit the surface soil and litter layer are also eliminated.

How much water can 1% organic matter hold? ›

Soil scientists report that for every 1 percent of organic matter content, the soil can hold 16,500 gallons of plant-available water per acre of soil down to one foot deep.

How much organic matter can soil hold? ›

Gardeners (and farmers) aim to manage the levels of soil organic matter to get acceptable plant growth, which will typically mean that organic matter levels should be 3-6%.

What are at least 5 environmental problems issues relating to soil? ›

Problem: Overgrazing, monoculture planting, erosion, soil compaction, overexposure to pollutants, land-use conversion - there's a long list of ways that soils are being damaged.

How does soil organic matter affect soil properties? ›

Soil organic matter improves soil structure and thus increases resistance to compaction. Practices such as in-row, non-inversion subsoiling minimize soil-surface disruption and organic-matter losses through decomposition. These practices are preferred over those that invert the soil to alleviate compaction.

What are 5 examples of soil organic matter? ›

It includes living plant roots and animals, plant and animal remains at various stages of decomposition, and microorganisms and their excretions. On farms the main sources of organic matter are plant litter (plant roots, stubble, leaves, mulch) and animal manures.

What is the soil order of a soil with at least 20% organic matter? ›

Histosols are soils that are composed mainly of organic materials. They contain at least 20 to 30% organic matter by weight and are more than 40 cm thick.

Is organic matter usually about 10% to 20% of the total volume of soil? ›

Active organic matter is a small portion - 10 to 20% - of the total organic matter in the soil, but it is an important portion, because it fuels microbial activity and releases nutrients into the soil. Active organic matter contains nutrients that are easy for microbes to digest and use for their metabolism.

How do you increase organic matter in soil? ›

How to add organic matter to soil
  1. use crop residues - chop and leave straw and not just crop roots or stubbles.
  2. grow a cover crop or use green manures to feed the soil.
  3. sow a mixed cover of deep-rooting grasses and herbs, which is particularly effective in compacted soil.

Can I increase soil organic matter by 1% this year? ›

The source of soil organic matter is photosynthesis resulting in plant growth – either root or aboveground. Therefore, the organic matter content cannot increase more than the amount of plant growth that can be produced in a year.

What is ideal organic matter? ›

Most garden and landscape plants perform best when the soil organic matter level is at least 2% (the goal for vegetable and flower beds should be 5%-10%). These soils are loose, easy to prepare for planting seeds and plants and have a large number of earthworms.

Which has the highest amount of organic matter? ›

Soil layers include topsoil, subsoil, and the C horizon. Topsoil has the highest proportion of organic material.

What percentage of organic matter is in muck soil? ›

Muck soils are uniquely different from mineral soils. They may contain 20 to 80% organic matter compared with mineral soils that can contain 1 to 5%.

What 5 factors have the greatest impact on soil formation? ›

Scientists attribute soil formation to the following factors: Parent material, climate, biota (organisms), topography and time.

What is a negative effect of organic matter? ›

A few harmful effects are given below:

(i) Organic matter is an energy and carbon source for many disease organisms, ensuring their longer periods of survival in soils. (ii) Excessive amounts of organic matter create a problem for mixing with the soil thoroughly and obstruct easy planting.

What are the examples of organic matter in environment? ›

Examples of organic compounds are carbohydrates, lipids, proteins and nucleic acids. Since they are comprised of carbon-based compounds they are broken down into smaller, simpler compounds through decomposition when they die. Living organisms also excrete or secrete material that is considered an organic material.

What are the 3 main factors that affect soil strength? ›

Factors Affecting Soil Shear Strength

So the shear strength of a soil depends on the composition of the soil's particles, the amount of water in the soil, and how well compacted the soil is.

What is the most important factor affecting soil? ›

Climate: This is probably the most important factor that can shape the formation of soils. Two important climatic components, temperature and precipitation are key. They determine how quickly weathering will be, and what kind of organic materials may be available on and inside of the soils.

What does increasing amounts of organic matter in soil cause an increase in? ›

Increasing levels of organic matter aid in soil structure, water-holding capacity, nutrient mineralization, biological activity, and water and air infiltration rates.

Which of the following is an effect of organic matter on soil quizlet? ›

Which of the following is an effect of organic matter on soil? Decomposition of plant and animal matter present in soil is largely due to soil microorganisms.

What are the five major factors directly affecting the amount of organic matter in the soil? ›

Scientists attribute soil formation to the following factors: Parent material, climate, biota (organisms), topography and time.

How does organic matter affect soil density? ›

Organic matter increases a soil's ability to hold water, both directly and indirectly. Compaction increases bulk density and reduces total pore volume, consequently reducing available water holding capacity.

What has the most dramatic effect on soil organic matter? ›

Organic matter production and conservation is affected dramatically by conventional tillage, which not only decreases soil organic matter but also increases the potential for erosion by wind and water (Plate 8).

What are two ways organic matter affects soil? ›

CHEMICAL: Soil organic matter significantly improves the soil's capacity to store and supply essential nutrients (such as nitrogen, phosphorus, potassium, calcium and magnesium), and to retain toxic elements. It allows the soil to cope with changes in soil acidity, and helps soil minerals to decompose faster.

How can we improve soil organic matter? ›

How to add organic matter to soil
  1. use crop residues - chop and leave straw and not just crop roots or stubbles.
  2. grow a cover crop or use green manures to feed the soil.
  3. sow a mixed cover of deep-rooting grasses and herbs, which is particularly effective in compacted soil.

How does soil organic matter affect soil texture? ›

Fine-textured soils already have small pores and aggregate more easily, so there are diminishing returns on increased organic matter. More soil organic matter means more soil pores and lower bulk density. Some of those pores are large, which is great for infiltration, but won't increase plant-available water capacity.

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