Soil Organic Matter: Tips for Responsible Nitrogen Management

For soil organic matter to work the way it should, it depends on a careful balance of nutrients and minerals, including one of

Healthy, homegrown carrots in rich soil.

the most important elements — nitrogen. One of the great paradoxes of farming is that lack of nitrogen is regarded as one of the great limitations on plant growth, and yet plants are bathed in it because the atmosphere is 78 percent nitrogen.

Most plants cannot use nitrogen in this form (N2) as it is regarded as inert. It has to be converted into other forms — nitrate, ammonia, ammonium and amino acids for plants to utilize it.

In conventional agriculture most of these plant-available forms of nitrogen are obtained through synthetic nitrogen fertilizers that have been produced by the Haber-Bosch process.

Many experts credit the Haber-Bosch process for producing the nitrogen needed for high-yielding agriculture. Others fur­ther state that without using this energy-intensive method to synthesize ammonia, we will not be able to feed the world. At the same time, the loss of soil fertil­ity is resulting in yield decline around the world. Farmers have to dramatically in­crease synthetic fertilizers and pesticides to maintain yields.

According to the United Nations Mil­lennium Assessment Report (MA Re­port) on the environment, there has been a dramatic increase in the amount of nitrogen fertilizers used and these are causing a range of problems.

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Since 1960, flows of reactive nitrogen in terrestrial ecosystems have doubled, and flows of phosphorus have tripled. More than half of all the synthetic nitrogen fertilizer … ever used on the planet has been used since 1985.

Soluble nitrogen fertilizers from con­ventional farming systems are causing the eutrophication of freshwater and coastal marine ecosystems and acidification of freshwater and terrestrial ecosystems. These are regularly creating harmful algal blooms and leading to the formation of oxygen-depleted zones that kill animal and plant life. The dead zones in the Gulf of Mexico and parts of the Mediterranean are caused by this and other soluble nutri­ents from farming.


Biological Systems

The process of turning nitrogen in the air into plant-available forms occurs naturally in healthy soil systems through a multitude of microorganisms. This is called biological nitrogen fixation and is done by symbiotic organisms such as Rhizobium bacteria in legumes and free-living nitrogen-fixers: azobacters, cyano­bacters/blue green algae and countless thousands of other species of free-living nitrogen-fixers.

This process is strongly associated with the amount of soil organic matter (SOM). Stable soil organic matter will have carbon-to-nitrogen ratios between 11:1 to 9:1. Soil organic matter is the greatest store of soil nitrogen and most of this nitrogen is plant available.

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Minute amounts of useable nitrogen can be fixed by electrical storms and be dissolved in the following rain. This is rarely enough for crop growth and in most areas with heavy or prolonged rain, if the soil has low levels of organic matter, most of these types of N will be leached out of the soil and into our river systems.

Biological fixation is the major source of plant-available nitrogen in natural soil systems.

The issue of soil organic matter and nitrogen continues to be largely ignored by most agronomists and this dates back to the 1840s when the father of synthetic fertilizers, Justus von Liebig, dismissed the roles of humus in plant nutrition.

Professor Albrecht’s Nitrogen Theory

Von Liebig was the first scientist to show that plant growth is dependent on adequate levels of nutrients in the form of ions — cations and anions and this formed the basis of modern agronomy with water-soluble synthetic fertilizers.

Emeritus Professor of Soils at the Uni­versity of Missouri Dr. William A. Al­brecht was the first soil scientist to show the importance of having all the soil minerals in a balanced ratio along with adequate levels of organic matter

Whereas Professor von Liebig felt that organic matter was not important and all necessary plant minerals could be sup­plied by soluble chemical fertilizers, Pro­fessor Albrecht wrote extensively on the importance of organic matter in acting as the primary source for plant nitrogen and as the buffer and storehouse of all the minerals that plants needed along with the importance of the correct soil biology to do this.

Albrecht strongly supported the con­cept of the soil as living body and the fundamental importance of organic mat­ter and soil biology in this process.

In the 1930s he wrote:

Decomposition by microorgan­isms within the soil is the reverse of the process represented by plant growth above the soil. Growing plants, using the energy of the sun, synthesize carbon, nitrogen, and all other elements into complex compounds. The energy stored up in these compounds is then used more or less completely by the mi­croorganisms whose activity within the soil makes nutrients available for a new generation of plants. Or­ganic matter thus supplies the “life of the soil” in the strictest sense. When measured in terms of carbon dioxide output, the soil is a live, ac­tive body. (Albrecht 1938)

Albrecht had science degrees in biol­ogy, agricultural science and botany. His life-long study was devoted to the roles of soil nutrients, soil organic matter and microbiology in producing high-yielding healthy crops. He was one of the first multidisciplinary scientists who took a whole systems approach to agriculture rather than a reductionist approach in the laboratory.

Albrecht also firmly established the link between plant health, particularly the role of soil mineral deficiencies, and the health of the animals and ultimately the humans who fed on the plants and ani­mals. He showed the direct link between poor-quality forage crops and the health of the stock that fed on it. For Albrecht soil health was the fundamental basis of crop health, good yields and animal and human health.

This clearly fits within the organic paradigm of building a healthy soil to grow a healthy plant, rather than the conventional farming paradigm of just adding the soluble nutrients for the plant to take up from the soil solution.

The two critical issues that Albrecht wrote about was to have soils that have adequate amounts of all the minerals that plants need and that these should be in the correct balance or ratios to achieve the highest yields.

While Albrecht wrote about calcium being the most important cation, his pa­pers on organic matter clearly state that nitrogen in the form of nitrate (an anion) is the nutrient that plants needed in the largest quantities, and insufficient nitro­gen was the one of the major limitations in yield.

In addition to carrying nitrogen, the nutrient demanded in largest amount by plants, soil organic mat­ter either supplies a major portion of the mineral elements from its own composition, or it functions to move them out of their insoluble, useless forms in the rock minerals into active forms within the col­loidal clay. Organic matter itself is predominantly of a colloidal form resembling that of clay, which is the main chemically active fraction of the soil. But it is about five times as effective as the clay in nutrient exchanges. Nitrogen, as the largest single item in plant growth, has been found to control crop-pro­duction levels, so that in the Corn Belt crop yields roughly parallel the content of organic matter in the soil. (Albrecht 1938)

Albrecht did his doctorate on soil ni­trogen and legumes and was an expert on the subject. In Albrecht’s writing the nitrate form of nitrogen is the most im­portant of all nutrients for plant growth.

Decades of research shows that nitrate anions, along with other anions, do not have many spaces in the soil where they can adsorb (stick) to be stored for later use by plants. Most of the electrostatic charges on the clay colloids are negatively charged. This means that that they will at­tract and store cations, however they will repel the negatively charged anions. This is one of the reasons why anions like ni­trate, sulfur and boron are readily leached from the soils with low levels of organic matter. The humus in organic matter has charged sites that will attract and store anions like nitrate. The majority of the nitrogen in the soil is stored on humus.

Albrecht’s research showed that soil organic matter is the most important source of nitrogen for plants. He wrote:

Soil organic matter is the source of nitrogen. In the later stages of decay of most kinds of organic mat­ter, nitrogen is liberated as ammonia and subsequently converted into the soluble or nitrate form. The level of crop production is often dependent on the capacity of the soil to pro­duce and accumulate this form of readily usable nitrogen. We can thus measure the activity that goes on in changing organic matter by mea­suring the nitrates. It is extremely desirable that this change be active and that high levels of nitrate be pro­vided in the soil during the growing season. (Albrecht 1938)

Albrecht was the first soil scientist to write widely on the relationship between nitrogen and soil organic matter and showing that the correct way to maintain sustainable fertility was to have farm­ing systems that recycled enough organic matter to have the quantities of nitrogen that are needed by the crop.

The other very important role for or­ganic matter that Albrecht wrote about was its buffering role. While Albrecht wrote widely about the need for the cor­rect percentages and ratio of available cations in soils, he also showed that ad­equate levels of organic matter would act as a buffer where the ratios were not exact and ensure that plants would receive the correct amounts of nutrients. The key was that there were no deficiencies and that there were adequate levels of all the nutrients that plants needed.

Equally important Albrecht showed that adequate levels of nitrogen, calci­um and other minerals were essential to building soil organic matter.

Bacterial activity does not oc­cur in the absence of the mineral elements, such as calcium, magne­sium, potassium, phosphorus, and others. These, as well as the nitro­gen, are important: Recent studies show that the rate of decomposi­tion is reduced when the soil is de­ficient in these elements. In virgin soils high in organic matter, these elements also are at a high level, and are reduced in available forms as the organic matter is exhausted. A decline in one is accompanied by a decline in the other.

It has recently been discov­ered that the fixation of nitrogen from the atmosphere by legumes is more effective where high levels of calcium are present in available form … Thus, if in calcium-laden soils, excellent legume growth re­sults and correspondingly large nitrogen additions are made, such soils may be expected to contain much organic matter. Liberal cal­cium supplies and liberal stocks of organic matter are inseparable. The restoration of the exhausted lime supply exerts an influence on building up the supply of organic matter in ways other than those commonly attributed to liming.

In the presence of lime (cal­cium) the legumes use other el­ements more effectively, such as phosphorus … and probably other nutrients. Thus heavier production results on soils rich in minerals, including more intensive and ex­tensive root development; the most effective means of introducing organic matter into the soil. The presence of large supplies of both organic matter and minerals points clearly to the fact that the soils were high in the latter when the former was produced. (Albrecht 1938)

Biology Fixes Nitrogen into the Soil

The most well-known form of biologi­cal fixation of N for plants is the Rhizo­bium bacteria that forms nodules in the roots of legumes and live symbiotically with them. The Rhizobium transform the N2 in the soil air into forms that plants can use. The legumes in exchange give the Rhizobium a home and glucose.

Researchers are continuing to find that there are an enormous number and types of symbiotic and free-living microorganism species that fix nitro­gen. Unfortunately most agronomy texts will only mention Rhizobium bacteria that live in symbiosis in the nodules of legumes. A few more will mention the free-living nitrogen-fixing organisms such as Azotobacter, Cyanobacteria, Ni­trosomas and Nitrobacter.

Many of these species live in the rhi­zosphere (the zone around plant roots) and help plants take up nitrogen from the soil. Very importantly they are finding that there are multiple species that work in symbiosis to achieve this.

Researchers are also finding new ni­trogen fixing species in the rhizospheres associated with most species including hostile environments like mangroves growing in seawater. Scientists from the Department of Microbiology, The Center for Biological Research in Mexico stated:

These findings indicate that (i) other species of rhizosphere bacte­ria, apart from the common diazo­trophic species, should be evalu­ated for their contribution to the nitrogen-fixation process in man­grove communities; and (ii) the nitrogen-fixing activity detected in the rhizosphere of mangrove plants is probably not the result of indi­vidual nitrogen-fixing strains, but the sum of interactions between members of the rhizosphere com­munity. (Holguin et al 1992)

The critical issue is that the majority of these species are associated with the or­ganic matter cycles of soils. Continuously building and maintaining soil organic matter is the key.

Amino Acids and Soil Nitrogen

A high percentage of the nitrogen in soil organic matter is in amino acid form. Amino acids are some of the most important building blocks of life because they are the basis of DNA, RNA, proteins, hormones and many of vital functions.

Plants generally synthesize the amino acids that they need by combining the nitrate form of nitrogen with the glucose sugar that they form through photosyn­thesis. This is why nitrate is so important.

Until recently scientists believed that plants rarely took up organic nitrogen in the form of amino acids. It was assumed these molecules were too big for roots to absorb. They believed that most of the amino acid nitrogen in the soil was not useful for plants unless it was trans­formed into nitrate.

An extensive body of published sci­ence is showing that amino acids are one of the most important forms of nitrogen, especially in natural systems such as for­ests where in some cases they can be the dominant form of nitrogen.

Scientists are challenging the tra­ditional view on organic nitrogen. Re­searchers from Griffith University in Aus­tralia wrote:

In recent years, there is increas­ing evidence that some plants are able to directly utilize and generally prefer amino acids over inorganic N (e.g. Schimel & Chapin 1996, Lipson & Monson 1998, Näsholm et al. 1998, Henry & Jefferies 2003, Weigelt et al. 2005). This challenges the traditional views of the ter­restrial N cycle that plants are not able to access the organic N directly without depending on microbial mineralization to produce inorgan­ic N and that plants cannot com­pete efficiently with soil microbes for uptake of nutrients from the soil. (Xu 2006)

Researchers are finding an increasing number of crops that readily take up large amounts of amino acids from the soil organic matter.

This emerging body of research is very important as it shows:

  • That the large pool of organic nitro­gen associated with organic matter is readily available to the crop.
  • That these forms of organic nitro­gen are very stable in the soil if or­ganic matter levels are maintained or increased.
  • And most importantly that the crop can access this organic nitrogen at the critical growth or seed produc­tion periods when they need large amounts of nitrogen.

Understanding the Ratios

It is important to get an understanding of the potential for how much nitrogen can be stored in the soil organic matter for the crop to use. Soil organic matter contains nitrogen expressed in a carbon-to-nitrogen ratio. This is usually between 11:1 to 9:1, however there can be further variations. The only way to firmly estab­lish the ratio for any soil is to do a soil test and measure the amounts.

For the sake of explaining the amount of organic nitrogen in the soil we will use a ratio of 10:1 to make the calculations easier.

The amount of carbon in soil organic matter is expressed as soil organic carbon (SOC) and is usually measured as the number of grams of carbon per kilogram of soil. Most texts will express this as a percentage of the soil to a certain depth.

There is an accepted approximation ratio for the amount of soil organic car­bon in soil organic matter. This is SOC × 1.72 = SOM.

The issue of working out the amount of SOC as a percentage of the soil by weight is quite complex as the specific density of the soil has to be factored in. This is because some types of soils are denser and therefore heavier than other soils. This will change the weight of car­bon as a percentage of the soil.

However for the sake of this article we will avoid the complex mathematics and to make these concepts readily under­standable we will use an average estima­tion developed by Dr. Christine Jones, one of Australia’s leading soil scientists and soil carbon specialists.

According to Dr Jones:

… a 1% increase in organic car­bon in the top 20 cm [8 inches] of soil represents a 24 t/ha [24,000 kilograms] increase in soil OC…

Note that kilograms per hectare (kg/ha) is almost identical to pounds per acre. They are close enough so that people not familiar with the metric system can use the U.S. system and it is much the same.

This means that a soil with 1% SOC would contain 24,000 kilograms of car­bon per hectare. With a 10 to 1 carbon to nitrogen ratio this soil would contain 2,400 kilograms of organic nitrogen per hectare in the top 20 cm — which is around 2,400 pounds of organic nitrogen per acre in the top 8 inches of the soil.

Good management of soil organic matter means that the soil around the root layer of the crop will contain amounts of organic nitrogen. It con­tains tons and tons of nitrogen rather than the hundreds of pounds or kilo­grams that are recommended to be add­ed in most agronomy texts. This shows that there is no need for farmers to pay the huge cost to purchase the synthetic nitrogen produced by the Haber-Bosch process. Good farm management will mean that the farms can get considerably more crop-available nitrogen for free.

Building Up Total Soil Nitrogen

The key to increasing soil nitrogen is to increase soil carbon by increasing the SOM levels.

A typical soil is supposed to be 25 percent air, 25 percent water, 45 percent mineral and 5 percent soil organic matter.

The primary reason for good soil aeration is to get oxygen into the roots. Most plants acquire oxygen directly through their roots. What most experts forget is that air is 78 percent nitrogen in the form of the inert N2.

Biologically active soils continuously fix the N2 in the soil air into plant avail­able forms as well as build the total stores of organic N, provided that the systems are continuously fed with organic matter.

The key is the continuous supply of organic matter. How do you get it on to the farm? You grow it. Farm management should be about producing as much bio­mass as possible and avoiding bare earth.

Legumes should be incorporated as much as possible in all rotation systems in cropping and should be a permanent component in all perennial systems such as pastures and orchards.

The aim of the management systems should be to let cover crops get as tall as possible and as mature as possible. This not only produces more biomass on the surface, it ensures that the roots get deep into the soil depositing organic matter as they grow down.

The Importance of Mineral Balance

The efficient production and use of N requires the correct mineral balance. Some of the key nutrients to achieve this are calcium, phosphorus, sulfur, sele­nium, molybdenum and cobalt.

Molybdenum is essential for plants to turn the nitrate and glucose in the leaves into the amino acids — the basis of the proteins, hormones, DNA and other criti­cal components of life. It works as a cata­lyst and without it the plant can’t grow and reproduce (flower, fruit and seed).

Sulfur is critical as it is needed to form the key sulfur-based amino acids such as methionine and cysteine.

Selenium is also critical to forming the sulfur-based amino acids. An emerging body of research shows higher levels of these essential amino acids when soils have good levels of selenium.

Cobalt is needed to help the nitrogen-fixing micro-organisms make vitamin B12. Without it they cannot survive. Low levels of cobalt will significantly reduce the numbers of these organisms.

Calcium is critical to good legume growth and to healthy systems of soil microorganisms.

Phosphorus is very important as it is needed to power the activity of most cells — this includes the cells of the legumes, the cells of the Rhizobium bacteria that live in the root nodules of legumes and fix nitrogen as well the cells of the free-living nitrogen-fixing microorganisms.

Just adding organic matter may not al­ways be sufficient to achieve good results if it does not contain enough nutrients to correct deficiencies. Soil mineral balance is critical to optimizing the fixation of N in the soil and the use of that by the cash and cover crops.

Albrecht wrote about this in the 1930s: It seems logical to ascribe caus­al significance to the minerals in the production of organic matter, whether or not they are effective in preserving it. If the soils that have lost their organic matter are to be restored, the loss of minerals, which has probably been fully as great, must be taken into account, and provision must be made to restore these mineral deficiencies before attempting to grow crops for the sake of adding organic matter. (Albrecht 1938)

Andre Léu first published this in the August 2012 issue of Acres U.S.A. magazine. Leu is the author of The Myths of Safe Pesticides.

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