39 Synthetic Fertilizers’ interaction with soil components

Meenakshi Nandal

Objectives

To study interaction mechanism of fertilizers and soil components To study how these interactions, affect soil health

To study hazards of fertilizer overuse

When we add a supplement in form to fertilizer to soil, careful considerations have to be undertaken to decide which fertilizer would be best suited to the soil and the crop. It does not end here, as after selection of the fertilizers we have to think about what will actually happen after we add this fertilizer to soil. Therefore, a brief knowledge about the effects is important to gain benefits from these resources.

There are five processes that take place when we add fertilizer to soil.

1. Crop absorbs the fertilizer directly
2. It becomes part of the soil reserve by interacting with soil minerals and organic
3. It undergoes leaching from the root zone with help of water
4. It turns into a gas, gets evaporated and lost to the atmosphere
5. It moves off field by soil erosion or runoff

Nitrogen fertilizer: It is the most difficult fertilizer to be managed as all the five major processes are reported in case of nitrogenous fertilizer. The compounds of N fertilizer bear specific behavioral properties and so are quite different from each other. This fertilizer is commonly added in form of ammonium, urea or nitrate. The major portion of added N fertilizer is attacked by soil microorganisms even before uptake by the plant roots. Under optimum condition these microorganisms immobilize some quantity of N by cellular functions and it constitutes some part of the soil organic matter. Phosphate fertilizer: is quicker in reaction, does not spread to larger areas, initially soluble in water and majorly in dissolved form, produces new compounds in soil. The phosphate fertilizers most commonly used are diammonium phosphate and monoammonium phosphate. Eventually, they become less soluble due to quick reactions with clays and soil elements. These compounds then cause slow release of phosphorous extending too many

days or months. In case of P fertilizer runoff by erosion may be a pathway of fertilizer loss as it is tightly bound to the soil particles.

Potassium fertilizer: This fertilizer is most easy and convenient to use and handle as it does not undergo any biological transformations. With quick dissolving property, the k ions either get solubilized in water or displace the cations in clayey soil. The most common K containing fertilizer is potassium chloride and some blended including, nitrate, thiosulfate, sulfate or phosphate.

40.1 Nitrogen Interaction with Soil components

When plants are grown in extremely N deficient atmosphere, the presence of a low NH4+ and NO3- conditions affects various processes in plants. The concentration of NO3- is quite high in soil, as compared to NH4+ so, it freely diffuses along with mass flow to the roots. Plants generally prefer NH4+ as compared to NO3- though the preference varies with ambient pH and temperature. For e.g. Uptake of NO3- declines with a higher uptake of cations instead of anions the major cause being increase of Ca2+ uptake by plants.

The major soil component interaction in nitrogen fertilizers occurs by ammonical nitrogen (NH4+) via adsorption to soil particles. It exists as a positively charged ion and does not undergo leaching in chemical form. It undergoes bacterial transformation and NH4+ is reduced to the NO3- form. This is a rapid process that occurs within 2 to 3 days with soil temperature rising above 50°F and is completely converted within a month or so. Nitrate nitrogen (NO3-) is negatively charged is not adsorbed by soil particles and so is free to be leached from the soil. It also escapes to the atmosphere by denitrification as in soils become saturated with water. When anhydrous ammonia, urea, nitrogen solutions and diammonium phosphate etc. are applied to soil temporarily show an increase in soil pH in application zone. There is release of ammonia gas that can affect seed germination or roots by causing “burns” in the area where fertilizer has been applied. An acid residue is formed where NH4+ is eventually converted to NO3-. Nitrogen gets concentrated under no-till and minimum-till conditions on soil surface and so there is formation of an “acid roof” where the pH may be 0.5 to 1.0 pH units higher at depths as compared to surface. This could negatively impact the effect of fertilizer on root growth. So, a regular pH check in upper layers is necessary.

In case of Urea, the nitrogen portion is entirely water-soluble. When applied to soil, urea-nitrogen is transformed to NH4-N and so is immediately available to plants but major part of nitrogen may be lost through volatilization in form of ammonia. Generally, most of urea is lost within one day or two following application and can about one-third of the N, lost within a week after application to soil. This Loss is enhanced due to warm-moist nature of soils, high pH, low cation exchange capacity (CEC) or sandy soils and surface organic matter. Therefore, incorporation of urea or nitrogen solutions should be done by mechanical mixing or by movement of water.

40.2 Phosphorous

The phosphorus interaction with soils primarily takes place in four different “pools” on the basis of their accessibility to plants (Syers et al., 2008):

1. The first pool -Immediate Available Phosphorous
2. The second pool -held on sites on the surface of soil particles. This phosphorus can be made available for uptake by plants through the phosphorus already present in soil solution, as in the phosphorus concentration reduces in the soil.
3. The third pool -adsorbed strongly to the soil particles and is less frequently extractableby plants but with time can become available to plants.
4. The fourth pool – through soil components and Phosphorous complex, but is less available to plants for uptake.

When soil becomes deficient in soluble P and Phosphorous is added externally in form of fertilizer it moves in soil through the major channel of dissolution. A second process called desorption, is responsible for the detachment or attachment of Phosphorus particles to soil particles, such as clay or specific minerals containing iron or aluminum. Thirdly, the process of weathering can be a major phosphorous source, when it breaks down into soil over a long-time period. Just as soil solution P can be replenished when the concentration of P becomes low, P can be removed from the soil solution if its amount exceeds in the soil solution. Consider the iced tea example. If too much sugar is added to the tea, some of it will not dissolve and will remain in solid form at the bottom of the glass .In the soil solution when concentrations of P are too high, some of the dissolved P will form solid P minerals by a process called precipitation. Soil pH plays important role in precipitation of phosphate, at high pH precipitation can result in the formation of solid calcium phosphate minerals whereas at low soil pH aluminum and iron phosphate minerals precipitate may be formed. Alternatively, P can be removed from the soil solution and attach to soil particles like clays or iron and aluminum-bearing minerals via a process called adsorption.

The fertilizer bound Phosphorus is adsorbed by the soil through roots in form of orthophosphate ions, mainly H2PO4 – and to a lesser extent HPO4 2-. Phosphorus in soil must be in dissolved form in the soil for its uptake by plant roots (Shober, 2012). The dissolved forms of plant-available P in the soil solution are present in form of orthophosphates (H2PO4 – or HPO42–, depending on the soil pH). The orthophosphate (H2PO4-) form in Phosphorus bound fertilizer is majorly absorbed by plants from the soil solution in spite of the phosphorus origin. The negatively charge orthophosphate is not adhered to the soil due to cation exchange capacity (CEC) and does strongly react in the soil, primarily with the  huge amount of iron and aluminum that is present naturally in the soil leading to formation of products that are insoluble and unavailable to plants.

The amount of P dissolved in the soil solution at any particular time is usually very small. From the soil solution P is removed by the plant roots system on the other hand it is again replenished by the residual P in the soil. Applied P fertilizer recovery is influenced both by the rate and quantity of P taken up by the plant. The concentration and P-buffer capacity in the soil solution are the most important factors controlling the P availability to plant roots. The Phosphorous-buffer capacity regulates the replenishment or desorption rate of P in the soil solution and dissipates in soils with a high buffer capacity. Also the root system size and root growth rate and the efficiency with which P is taken up by roots is an important factor along with soil moisture and the extent to which weeds, pests and diseases have been checked. We have earlier discussed how soils microbe’s convert’s organic forms of P in plant residues or organic soil amendments into plant accessible P. The end products so formed are soluble orthophosphates and process is called mineralization. This  orthophosphate form of P can be taken up by plant roots once present in the soil solution. The soil solution can also be replenished from several pools of inorganic (mineral) P in the soil. When we add P containing fertilizer to soil, a very small proportion remains in the soil solution with a small part undergoing initial precipitation reactions in some calcareous soils. The processes of adsorption and then re-absorption makes P rapidly distributed between the readily-available and less readily available pools. The soil pH is the major factor controlling these reactions.

At high or low pH the solubility of Phosphorous becomes very low and the maximum accessibility occurs in the 6.0 to 7.0 pH range. This is another important reason to lime regularly. The solubility of phosphorus in fertilizer varies. Available phosphorus existing in fertilizer legally can be defined as the submission water soluble and citrate solution of the phosphorus. The solubility of phosphorus in water ranges from 0 to 100 percent. The effectiveness of Phosphorous directly depends on amount of water-soluble content. This is especially important for short-season, starter fertilizers, quick-growing, restricted root systems and for areas where rates of phosphorus are less-than-optimum.

In polyphosphate fertilizer about half or three-quarters of the Phosphorous is present in form of chained polymers. The remaining P (orthophosphate) is immediately available for plant uptake. The plant roots and soil microorganisms produce enzymes that break polymer phosphate chains to monomer phosphate molecules. Some of the polyphosphate will decompose without the enzymes. The enzyme activity is faster in moist, warm soils. Within a time period of seven to fifteen days conversion of half of the polyphosphate compounds to orthophosphate occurs. Except under cool and dry conditions where the transformation may take longer time. The polyphosphate fertilizers consist of both orthophosphate and polyphosphate in an optimum combination and so are used as an effective source of fertilizer by plants. Most P-containing fluid fertilizers have ammonium polyphosphate in them. Fluid fertilizers are easy to work with for farmers as they get easily mixed with other nutrients and chemicals so, are commonly used in production agriculture.

The Hazards of Synthetic Fertilizers of Phosphates constitute of less radionuclides, could show increase in radioactivity near phosphate mining areas. Phosphogypsum contains large amount of uranium and is a by-product of process using rock phosphate as raw material. Phosphogypsum stockpiles present a serious environmental problem, with potential hazard for human health and pollution of the groundwater. There is wide spread of the levels of radioactivity of phosphate fertilizers but in the long term they might be of concern because of their potential risk to increase natural radioactivity in agriculture soils. Some rock phosphate fertilizers contain small amounts of the heavy metal cadmium. Because cadmium is highly toxic to humans, there are concerns about its accumulation in agriculture soils and transfer through the food chain (Tirado and Allsopp, 2012).

40.3 Synthetic Fertilizer- Potassium and its interaction with soil

The constant fixation and release of Potassium Fertilizer through various reactions are shown in Figure 40.3. When the potassium fertilizer is applied potassium penetrates into the soil solution, out of this major part goes into the exchangeable and some to the non-exchangeable forms. After crops cause removal of potassium that exists in readily available form, the reactions are reversed and exchangeable potassium moves into the soil solution. As a consequence, constant fixation and release of potassium occurs in the soil. The parent material is broken down by physical, chemical and biological forces during weathering into finer fractions largely sand, silt, and clay sized particles. This breakdown results in the release of several chemical elements, including potassium, and the formation of different clay minerals. The amount of sand, silt and clay fractions present in a soil are defined by the kind of parent material from which the soil was formed, parent material being sandstone, limestone, shale or mica. The amount of fixed Potassium and its release is mainly determined by the analogous amount and the clay minerals present the fractions in the soil. Sand and Silt Fractions — The sand and silt fractions of most soils are composed largely of quartz. The feldspars present in these fractions also contain potassium with other nutrient elements but due to relative larger particle the particles dissipate very slowly with slow rate of potassium release. Also, ability of sand and silt to fix potassium is low because of their physical and mineralogical nature.

Clay Minerals — Clay minerals being the dominant materials present in soil, generally in clay or colloidal fraction are very active in fixing and releasing potassium. The different types of clay minerals vary in their capacity to fix and release potassium. Every clay mineral has its own relevancy towards potassium fixation and release. Additionally, the clay minerals contain varying amounts of native potassium bonded between the clay layers. The illite and vermiculite clays due to their crystal structure and negative charges among the crystals can absorb potassium from the soil solution and entrap it between layers of the clay particle. In the two-adjoining mica or vermiculite layers of silica sheets the potassium cations are fixed or entrapped depending on the relationship of the size of the hexagonal cavities This fixed, or nonexchangeable, potassium is unavailable to plants but releases slowly as there is decrease in levels of exchangeable and soil solution potassium. The kaolinite clay mineral in the soils has less tendency of exchangeable potassium to release compared to mica and vermiculite type clay minerals possessing soils. The soils with montmorillonite mineral contain large amounts of exchangeable potassium, but will fix only a small percentage of it. Therefore, most of the potassium held by montmorillonite clay is in an available form.

The Hazards of Synthetic Fertilizers of Potassium

This product contains about 50% potassium and 50% chloride. So, Potassium chloride contains very high amounts of potassium resulting in an unbalanced phosphate: potash ratio. This ratio ideally ranges from 2:1 (most soils) to 4:1 (grasses). Since plants absorb calcium, magnesium and potassium largely in the ratio in which they are present in the soil, excess application of potassium to soil can cause deficiency of lead and calcium in plants with loss of structure. In plants the root respiration and soil air levels is reduced leading to production of toxic compounds. There is Reduction in soil air and insufficient calcium affecting soil microbes and breakdown of organic matter/ availability of nutrient to plants. In the process of drilling the soil is “close” by potassium as it disintegrates the clay particles and effectively seals the soil. High levels of Potassium can be harmful as being soluble and highly leachable it can affect plant growth if not supplied in optimum quantity. Once applied it is rapidly taken up by the plants. Most of plant experts discourage application of potassium to soil unless soil tests reflect it is needed.

Summary:

We studied the major processes and soil interaction mechanism’s that happen when NPK fertilizers are applied to soil

We studied about the interaction mechanisms of Nitrogen fertilizer with soil

We studied about the interaction mechanisms of Phosphorous fertilizer with soil

We studied about the interaction mechanisms of Potassium fertilizer with soil

We studied the hazards caused by the excessive use of synthetic fertilizers and the harm to plants and ecosystem.

References

1. Shober, L.A. Soils and fertilizers for master gardeners: Phosphorus in the home landscape, florida cooperative extension service. Institute of Food and Agricultural Sciences, University of Florida, 1–3. 2012.
2. Smil, V. Enriching the earth. Fritz Haber, Carl Bosch, and the transformation of world food production. MIT Press, Cambridge, MA, 2001.
3. Syers, J.K., Johnston, A.E. and Curtin, D. Efficiency of soil and fertilizer phosphorus use: Reconciling changing concepts of soil phosphorus behaviour with agronomic information. FAO Fertilizer and Plant Bulletin 18. Food and Agricultural Organization of the United Nations. Rome, 978–992, 2008.
4. Tirado, R. and Allsopp, M. Phosphorus in agriculture problems and solutions. Technical Report (Review), 36, 2012.