What is the specific principle of PVP working in soil?

The core of PVP's (polyvinylpyrrolidone) function in soil lies in its molecular structure (polar groups and polymer chains) and physicochemical properties (water solubility, adsorption, and water retention) . Through "intermolecular interactions" or "physical form manipulation" with soil particles, water, nutrients, and pollutants, it indirectly improves the soil's physical structure, moisture status, nutrient availability, and pollutant activity. The specific mechanism is broken down by core functional scenarios, explaining the effects at the molecular level and soil level step by step:

1. Principle of assisting in preventing soil compaction: regulating the aggregation and bonding of soil particles

The essence of soil compaction is that soil particles (especially clay particles) are tightly aggregated due to electrostatic attraction, water film adhesion, and other factors, resulting in reduced porosity . PVP breaks this process by "dispersing particles and building microstructures." The specific principles are as follows:

• Molecular adsorption and particle surface modification: Reduce direct adhesion of particles.
  The pyrrolidone ring (containing polar amide group -CONH-) on the PVP molecular chain has strong hydrophilicity and adsorption properties. It can be tightly adsorbed on the surface of soil particles (clay, silt particles) through "hydrogen bonds" or "van der Waals forces", forming an ultra-thin polymer protective film (nanoscale) :
  • This film "isolates" adjacent soil particles, preventing them from forming large aggregates due to electrostatic attraction (clay particles are negatively charged and easily absorb cations and approach each other) or water film adhesion (the water film disappears during drying and the particles come into direct contact).
  • At the same time, the "steric hindrance effect" of the PVP molecular chain will cause the adsorbed soil particles to repel each other, reduce the probability of aggregation, maintain the dispersion of the particles (similar to the "lubricant" effect), and reduce the hardness of the compaction after compaction.
• Polymer chain bridging: building a loose micro-aggregate structure and increasing soil pores.
  The long-chain polymer structure of PVP (molecular weight is usually 10,000-1 million Da) can act as a "molecular bridge" to slightly connect dispersed soil fine particles (sand particles, silt particles) into micron-sized micro-aggregates (diameter 10-100μm) :
  • These micro-aggregates are not tightly aggregated lumps, but rather a porous structure formed by loosely connected PVP chains. A large number of "capillary pores" and "ventilation pores" are formed between the aggregates. The capillary pores retain moisture, while the ventilation pores allow air to circulate, preventing the soil from becoming airtight and compacted.
  • Note: The micro-aggregates are "physical temporary structures" with weak stability (they may disintegrate in heavy rain or frequent irrigation). They cannot replace the "water-stable aggregates" formed by organic fertilizers (formed by the cementation of organic matter and resistant to erosion in the long term). They can only alleviate compaction in the short term.
• Water retention and evaporation control: prevent the surface soil from drying out and hardening.
  The hydrophilic group (amide group) of PVP can absorb the free water in the soil to form a hydrogel (water content can reach 10-20 times its own weight) and adhere to the soil surface:
  • Hydrogel can slowly release water, slowing down the rapid evaporation of surface soil water (especially in drought or high temperature environments);
  • The main cause of soil surface compaction is "sudden loss of water leading to particle shrinkage and adhesion". The water retention effect of PVP can maintain the moist state of the surface soil, reduce the formation of dry cracks, and indirectly prevent compaction.

2. Principle of Soil Water Retention: Hydrogel’s “Retention-Slow Release” Water Mechanism

The water retention function of PVP in soil is essentially to achieve "retention" and "slow release" of water through "physical adsorption + gel encapsulation", thereby improving the effectiveness of soil moisture. The specific principles are as follows:

• Molecular-level moisture adsorption: Locking free water
  The amide group (-CONH-) on the PVP molecular chain is a strong hydrophilic group that can combine with free water molecules in the soil (water not adsorbed by soil particles) through "hydrogen bonds", "fixing" the water around the polymer chain to form a "bound water layer";
  • This bound water is not easily lost through transpiration or gravity and can be retained in the soil for a long time, allowing crop roots to slowly absorb it (preventing ordinary free water from evaporating quickly or penetrating into deep soil layers).
• Macro-hydrogel formation: building a "water storage reservoir"
  When the PVP concentration reaches a certain threshold (usually 0.1%-0.5%, based on the dry weight of the soil), the PVP molecular chains after absorbing water will cross-link with each other to form a three-dimensional network structure of hydrogel (similar to a sponge):
  • Hydrogel can "encapsulate" a large amount of water (accounting for 80%-90% of its own weight), forming a "micro water storage reservoir" in the soil;
  • When there is insufficient water in the soil surface, the hydrogel will slowly release water due to the difference in osmotic pressure, replenish the soil solution, maintain a moist environment around the roots, and reduce crop drought stress.
• Reduce soil moisture evaporation: Physical barrier effect
  Hydrogel covers the surface of soil particles or fills the pores to form a "semi-permeable membrane" that prevents the diffusion of moisture inside the soil into the atmosphere and reduces the evaporation rate - experimental data show that adding 0.3% PVP to the soil can reduce the average daily water evaporation by 15%-25% (compared with untreated soil).

3. Principle of nutrient/pesticide slow-release: Polymer chain "encapsulation-adsorption-controlled release" mechanism

PVP can be used as a "slow-release carrier" for water-soluble nutrients (such as urea, potash fertilizer) or low-toxic pesticides in the soil, reducing their leaching loss and extending the action period. The principle is as follows:

• Physical encapsulation: hindering the rapid migration of nutrients.
  The polymer chain of PVP can encapsulate water-soluble nutrients/pesticide molecules in its three-dimensional network structure through the "entanglement" effect, forming a "microcapsule" shape:
  • This coating can prevent nutrients/pesticides from quickly penetrating deep into the soil with rainwater or irrigation water (avoiding leaching loss) and can also reduce their direct volatilization into the atmosphere (such as ammonia volatilization from nitrogen fertilizers);
  • Only when water in the soil slowly penetrates into the packaging structure, or when microorganisms slightly degrade the PVP chains, will nutrients/pesticides be gradually released into the soil solution for crops to absorb or exert their efficacy.
• Chemical adsorption: Enhance the binding force between nutrients and soil.
  The amide group of PVP can adsorb and bind with nutrient ions (such as NH₄⁺, K⁺, PO₄³⁻) through "hydrogen bonds" or "electrostatic effects" , and fix them on the surface of soil particles (through PVP as a "bridge"):
  • This adsorption can reduce the "mobility" of nutrients and prevent them from being lost downward due to gravity;
  • When the nutrient concentration in the soil decreases (absorbed and consumed by crops), the adsorption balance is broken, and the nutrient ions will slowly desorb and re-enter the soil solution, achieving "release on demand".
• Environmentally responsive release: Adapting to soil conditions
  The water solubility and cross-linking degree of PVP are affected by the soil environment (such as pH, temperature, and moisture):
  • When the soil is moist, the PVP chains swell and the rate of release of the encapsulated nutrients accelerates; when the soil is dry, the chains shrink and the release rate slows down, preventing excessive accumulation of nutrients when the crop does not need them.
  • In acidic soil (pH < 6.0), the protonation of the amide group of PVP is enhanced, the adsorption capacity of cationic nutrients (such as K⁺ ) is improved, and the slow-release period is longer.

4. Principles of heavy metal ion adsorption: coordination bond binding and charge neutralization mechanism

PVP can assist in remediating soils that are slightly contaminated with heavy metals (such as Pb²⁺ , Cu²⁺ , and Cd²⁺ ) , reducing their bioavailability (reducing crop absorption). The principles are as follows:

• Coordination bond binding:
  The pyrrolidone ring (containing nitrogen atoms) in the PVP molecule that fixes heavy metal ions has a "lone pair of electrons" and can form a stable "coordination bond" with heavy metal cations (such as Pb²⁺ , Cu²⁺ ) to form a water-insoluble complex:
  • This complex will be adsorbed on the surface of soil particles or remain in the soil surface with the sedimentation of PVP and cannot be absorbed by crop roots (reduced bioavailability);
  • Experiments have shown that 0.5% PVP can reduce the bioavailability of Pb²⁺ in soil by 20%-30% (verified by detecting Pb accumulation in crop roots).
• Charge neutralization: reducing the mobility of heavy metal ions.
  Soil clay particles are usually negatively charged and easily adsorb positively charged heavy metal ions (such as Cd²⁺ ) . However, this adsorption is easily replaced by other cations in the soil (such as Ca²⁺ and Mg²⁺ ) , resulting in the reactivation of heavy metals.
  • The amide group of PVP is positively charged after protonation and can combine with the negative charge of clay particles. At the same time, its coordinated heavy metal ions are "locked" in the clay-PVP complex, reducing the probability of being replaced by other cations and reducing the mobility of heavy metals.

Summarize

The essence of PVP's role in soil is that it uses the "polar groups" and "polymer chains" in its molecular structure to "physical adsorption", "chemical binding" or "morphological regulation" with particles, water, nutrients and pollutants in the soil , ultimately achieving:

• Improve soil physical structure (help prevent soil compaction);
• Improve water effectiveness (water retention);
• Prolonging the action period of nutrients/pesticides (slow release);
• Reduce the biological risk of heavy metals (adsorption and immobilization).

It should be noted that these principles are all based on the "auxiliary" role of PVP - its effect depends on low concentration use, and it cannot replace organic fertilizers, special water-retaining agents, soil conditioners, etc., and is only suitable for specific scenarios (such as seedling cultivation, potted plants, and lightly contaminated soil remediation).