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When Less is More: Understanding Fertilizer and Solubility

By Lawrence Mayhew

Make the gesture “just a little bit” by squeezing your thumb and index fingers as tight as you can; tighter, tighter — the amount of fertilizer you could hold between your fingers is about the amount dissolved in soil solution … per acre! That’s right; there is very little, if any, dissolved “plant food” in the water of a typical soil.

The amount of plant nutrients dissolved in soil solutions is so small that it is expressed as parts per million (ppm), not hundreds of pounds or tons per acre. While synthetic fertilizers are sold primarily on the basis of their water (aqueous) solubility, the emphasis on aqueous solubility is generally misunderstood and somewhat misguided.

It is generally known that over-application of extremely soluble synthetic fertilizers has been responsible for disrupting ecosystems and numerous environmental problems. What is not generally known is that all highly soluble soil inputs, including sulfates, chlorides and fluorides, disrupt the structure of water molecules, impeding the biochemical energy flows that affect the metabolism of plants, making them more susceptible to insect pressure and diseases and decreased water use efficiency.

It is also a well established fact that highly soluble phosphate fertilizers become “tied-up” soon after application. When there is an overabundance of dissolved phosphates in soil water, the soil system responds chemically by forming more stable forms of phosphorus, usually by chemically combining with calcium cations and complexing with lanthanides (rare earths) and organic matter. All of these materials can release phosphorus as plant nutrients through microbial activity.

Although water is critical to all life forms, there are numerous metabolic pathways in biological systems where it gets in the way and must be pushed aside; it’s called the hydrophobic effect.

The hydrophobic effect is responsible for the fluidity of organisms and the efficient energy flow through all biological systems, where enzymes, microbes, surface activities, and the thermodynamics of the energy available to do “work” (enthalpy) operate in environments where water is squeezed out, making hydrophobic interactions the most important life-sustaining processes, not aqueous solubility.

field to be fertilized
The amphiphilic nature of humic substances allows them to work in water and hydrophobic environments, providing the critical conditions necessary for biological processes when they are closely associated with clays.

The living organisms that provide plant nutrients through geomicrobiological interactions with insoluble soil colloids live in a universe of extremely low concentrations of dissolved nutrient ions in soil water, where some of the most critical steps in the processes of life proceed mainly in the intentional expulsion of water in the process of microbial adhesion to surfaces of soil materials. However, if there is an imbalance in the positive and negative ions dissolved in soil solution, the adhesion is disrupted.

In eco-agriculture, highly soluble fertilizers are either used sparingly or, as in the case of organic agriculture, they are banned or highly restricted. Regenerative agricultural practices use materials that are nearly insoluble in water, such as humates, compost and rock phosphates as interactive components of geological and microbiological processes that provide a balance of nutrients for plants. The role of carbon dioxide (CO2) in soil processes, which has the greatest impact on all soil processes and pH balance, is generally overlooked.

Fertilizer and Solubility: Soil Solution

The water associated with soils contains both dissolved chemicals and undissolved suspensions of colloidal substances. The portion of soil water containing dissolved substances is called the soil solution. Table 1 illustrates the amounts of common plant nutrients dissolved in soil solution. The concentrations of soil nutrients in the table are listed in millimoles (mmol) of dissolved material per liter (L-1) of water and parts per million (ppm). A millimole is 1/1000th of the molecular weight of a substance expressed in gram equivalent weight. For example, the molecular weight of nitrogen is 14, therefore 1 mmol of nitrogen would weigh 0.014 grams. A U.S. dime weighs about one gram.

Using the data for nitrogen in Table 1, the highest concentration of nitrogen reported was 0.46 mmol L-1 of soil water; that’s about 0.006 grams of dissolved N per liter of soil water. Assuming field moisture is about 15 percent (~0.9 inch ft-1) and assuming there is about 1 million pounds of soil in the upper 6 inches per acre, total dissolved nitrogen in soil solution at that depth would be approximately 1 pound in 1 acre. For dissolved phosphorus, the range would be 0.001 to 0.002 pounds in 1 acre!

The accuracy of these data can be questioned, and soil solution concentrations vary tremendously depending on soil types and their water holding capacity; nevertheless, astonishingly little if any dissolved nutrients exist in stable soil systems at any time other than when soluble fertilizers are applied.

Synthetic nitrogen, the most over-applied agricultural chemical, is not applied to improve the quality or fertility of soils; instead it is used primarily as a plant growth stimulant to increase yield. If the old adage that says it takes 1 to 1.25 pounds of nitrogen to grow 1 bushel of corn is true, and if there is only 1 pound of N in an entire acre of soil solution, where do the hundreds of pounds of nitrogen come from?

The answer is the numerous organic compounds produced by biological fixation of nitrogen from the atmosphere and carbon dioxide released by microbes. Look at any chart of the Nitrogen Cycle; you will see that numerous microbial interactions are part of the cycle supplying plant nitrogen needs, but the role of soil CO2 in that cycle is generally ignored.

As you are reading this article, you are breathing in almost 80 percent nitrogen — plants and soil microbes do the same thing, but instead of treating nitrogen like an inert unreactive chemical, soil microbes utilize it. Additionally, soils need to be well aerated because microbes require oxygen to convert atmospheric nitrogen to nitrates and ammonium. Soil microbes may be responsible for providing roughly 80 percent of all the soil nitrogen needs for all plants globally. However, soil compaction and overuse of nitrogen fertilizers are having such a negative impact on nitrogen-fixing microbes that the Earth is to the point where the total fixed nitrogen in contemporary soils is actually less than pre-industrial times. Soil compaction also impedes the transfer of CO2 from the atmosphere into soils and the movement of CO2 generated by soil microbes through the soil.

Hydrophobic Interactions

Conventional dogma considers the water of soils as a place to accommodate dissolved chemicals intended to be passively taken up by plant roots in the soil solution. Although this is true to some degree, overall the uptake of plant nutrients is a highly regulated complex process, especially when nutrient levels are low.

Hydrophobic conditions, where water is squeezed out, allow insoluble nutrients to pass through living membranes in a manner that utilizes the least amount of energy.

Biological membranes make use of enzymes, integrated membrane proteins and amphiphilic molecules such as humates to assist in the movement of nutrients through living membranes. Amphiphilic molecules have both hydrophobic (water shunning) and hydrophilic (water loving) domains in their molecular structure, making them very versatile. Humic substances, many proteins and almost all soil sulfur compounds are complex amphiphilic organic (carbon) substances. Because of these multidimensional interactions, where reliance on water solubility is thermodynamically counterproductive, the ability of microbes to adhere to surface structures of minerals is extremely important because only certain surfaces are recognized by microbes and their enzymes as compatible surfaces for attachment in the process of converting insoluble minerals into plant nutrients. It’s analogous to recognizing a hamburger as food compared to a cow pie; both are about the same color and made of carbon compounds, but you instinctively know the difference.

When soluble fertilizers are over-applied, only a small fraction remains “available” as plant nutrients because they are highly polarized ionic salts that combine with other soil chemicals and minerals. This so-called “tie-up” is actually the insoluble products of chemical reactions within soils to bring unstable chemicals into chemical equilibrium, reducing the toxic properties of soluble fertilizers.

Thermodynamically, these reactions cause energy flows to be more chaotic (entropy), where energy that would otherwise be available for “work” is diverted to stabilizing the system instead.

Eventually, all fertilizers will interact with soil microbial processes that release the tied-up nutrients. Microbial release is assisted primarily by organic matter, especially humic substances, that provide the proper conditions for these natural interactions.

Soil organic matter is the single most important component of soil systems. It is derived from the action of microbes and plant root exudates, providing conditions for the most efficient release of soil nutrients for plant uptake.

As many of you already know, soil colloids (e.g. clays and organic matter) carry an overall negative charge on their surfaces, which can be measured as cation exchange capacity (CEC).

However, many microbes that have to interact with soil colloids are also negatively charged. Their ability to adhere to like-charged soil surfaces seems contradictory because like charges repel, however due to their hydrophobicity, microbes overcome the charge repulsion through a physical force called van der Waals attraction, enabling certain bacteria to attach to mineral surfaces to extract food.

As a general rule, microbes that have plenty of nutrition also have the ability to adhere to surfaces better than microbes that are undernourished, and well-nourished bacteria demonstrate the highest degree of hydrophobic character.

Van der Waals attractive forces are extensively exploited in nature; the ability of spiders to “stick” upside down to a ceiling is a good example. Because van der Waals attractive forces are not electrostatic, meaning they have no charge, they are weak forces easily overcome by the presence of high concentrations of positively charged soil solution ions (cations) from soluble fertilizers, soluble soil amendments or soil acidity. An excess of any cation in the soil solution will cause a reduction in the hydrophobicity of these bacteria and the collapse of van der Waals forces, interfering with the process of bacterial surface adhesion.

Therefore, the bioavailability of nutrients is not totally dependent on water solubility, because in nature it would be far too inefficient to support life based solely on aqueous solubility. Nature works smart, not hard.

Carbon Dioxide to the Rescue

Soil acidification, where the measured soil pH is too low for efficient agronomic production, is the excess of hydrogen (H+) and aluminum (Al3+) cation activity in soil solution. It is typically caused by the overuse of high nitrogen inputs, especially urea and anhydrous ammonia because they dissolve stable soil organic matter.

The reduction of SOM reduces biological activity, which leads to a reduction in microbial CO2 production, increasing the chemical activity of aluminum (Al3+) and acids (H+). Al3+ and H+ take the place of magnesium (Mg2+) and calcium (Ca 2+) 2, 3 cations on clay and organic matter cation exchange sites, causing an imbalance in bioavailable nutrients.

The chemical activity of Al3+ and H+ needs to be controlled in all biological systems to avoid toxicity. The chemical control and balance of these cations is called buffering, where pH is maintained in a more desirable range.

It is generally accepted that humic substances, the most stable form of SOM, and clays, provide a great deal of buffering in soil solutions. However, the role of CO2 is generally overlooked nowadays. Soil-building activities are enhanced when buffered by CO2, one of the products of microbial activity.

The combination of CO2 and water is the first step from which all of the countless forms of organic matter are biologically synthesized in the process of making soil conditions fit for life.

In natural soil systems, the availability of nutrients is controlled by the biological release of organic acids (H+) that are balanced (buffered) by bicarbonates (HCO-) released from plant roots, which are balanced by the microbial release of CO2, which is in balance with soil calcium carbonate (calcite). Carbon dioxide released by microbes, in addition to the CO2 adsorbed from the atmosphere, converts rapidly to carbonic acid upon contact with soil water. Carbonic acid readily dissociates in water into acid H+ cations and anti-acid carbonate CO3 2- anions, counter balancing the acid H+. The rapid back and forth reactions of these ions has a powerful buffering effect on soil solution, helping to keep soil solutions in the range of 6.4 to 6.8 pH; a compatible range for a majority of plants and soil microorganisms.

Additionally, the alkalinity provided by CO2 and water as carbonic acid can move rapidly into the lower subsoil, whereas limestone has to be broken down by soil acidity and may take decades to move into subsoils.

As plant root mass increases, more organic acids are released by plant roots, such as carboxylic acids that increase the soil solution acidity to dissolve insoluble minerals, converting them into bioavailable nutrients.

Balance in pH is restored when carboxylic acids are rapidly consumed by microbes as sources of carbon-based energy, releasing CO2, which in turn produces carbonic acid that buffers the soil solution pH back to a more alkaline condition.

While microbes are dining on these energy-rich, carbon-based acids, they pull some of the geo-available calcium (Ca2+) out of soil solution, combine it with carbonate (CO3 2-) in soil solution making a particular crystalline form of calcium carbonate (CaCO3) called calcite. Calcite is a biomineralized calcium carbonate made by soil microbes that cycles through the whole soil system with the dual role of buffering agent and a bioavailable form of calcium.

Because plant root exudates provide multiple functions, it becomes apparent that plant root mass has a major impact on soil pH buffering, thus regulating the amount of H+ protons that are donated to negatively charged cation exchange (CEC) sites.

As more H+ is donated to the CEC sites, more Ca2+ and Mg2+ cations are released because H+ protons are preferentially adsorbed to CEC sites. The remaining bicarbonate anions participate in regulating the amount of cations released from CEC sites. The combinations of all of these chemicals in the soil solution in the presence of humates are widely recognized as charge-balancing mechanisms.

pH: CEC Balance

The amphiphilic nature of humic substances allows them to work in water and hydrophobic environments, providing the critical conditions necessary for biological processes when they are closely associated with clays.

Both humic substances and clays are water-insoluble soil components with powerful pH-buffering capacities and very high CEC, effectively driving fertility when dissolved nutrients in soil solution are at very low levels.

Because it is critical for soil bacteria to maintain hydrophobic conditions at the negatively charged soil colloidal surfaces, and as microbial hydrophobic conditions are influenced by both H+ hydrogen ion activity (pH) and nutrient cation activity in soil solution (CEC), it becomes apparent that the relationship between pH and CEC can be described as electro-charge balance.

The relationship of soil pH, CEC and soil CO2 was demonstrated many years ago (Figure 1). This balance appears to be critical to soil biological activity because the pH:CEC ratio is currently being used by some practitioners as another effective tool in regenerative crop production where biological activity, organic matter and biocompatible inputs are used to reduce the amounts of highly soluble inputs.

Editor’s note: This article appears in the October 2015 issue of Acres U.S.A.