What soil types are not suitable for PVP?

The suitability of PVP (polyvinylpyrrolidone) in soil is highly dependent on the soil's physical and chemical properties (such as particle composition, pH, salinity, and organic matter content) and core issues (such as compaction, water retention, and pollution remediation needs). The following types of soil are generally unsuitable for PVP use or require strict restrictions on its use due to "PVP's inability to address core issues," "prone to negative effects," or "extremely poor economics" :

1. Saline-alkali soil (pH＞8.5, EC＞4 ms /cm): PVP is ineffective and may aggravate salt damage

The core problem of saline-alkali soil is high salt ions (such as Na⁺ and Cl⁻ ) and a high pH value , which leads to soil colloid dispersion, poor permeability, and difficulty for crop roots to absorb water. PVP is not only ineffective in such soils, but may also have negative effects for the following reasons:

• High-salt environment destroys the adsorption and water-retention function of PVP.
  A large amount of cations such as Na⁺ and Ca²⁺ in saline-alkali soil will compete with the polar groups (amide groups) on the PVP molecular chain for binding sites, weakening PVP's ability to adsorb soil particles. The "polymer protective film" that could have been formed cannot be stably attached, and the anti-caking effect is completely ineffective. At the same time, high salt will destroy the three-dimensional structure of the PVP hydrogel, causing its water-retention capacity to drop by more than 50% (it cannot lock in moisture and may accelerate moisture evaporation).
• High pH value inhibits PVP's heavy metal adsorption (if remediation is required).
  If saline-alkali soil is also contaminated with heavy metals, PVP's adsorption of Pb²⁺ and Cd²⁺ depends on "coordination bond binding", and high pH value (>8.5) will weaken the protonation of PVP's amide group, significantly reduce the coordination ability, and even cause the desorption of adsorbed heavy metal ions, which in turn increases the risk of crop absorption.
• fails to address the core issue of saline-alkali soil and may exacerbate salt damage
  . It lacks the ability to reduce salt levels or adjust pH. The key approaches to improving saline-alkali soils are leaching and draining salt, applying gypsum/desulfurized gypsum to reduce alkali levels, and increasing the application of organic fertilizers to improve colloidal structure. Using PVP is not only cost-effective, but its residual polymer chains may also combine with sodium ions in the soil , forming salt- polymer complexes that clog soil pores and further impair permeability.

2. Heavy clay (clay content > 40%): prone to "anoxia and compaction", the effect is far worse than traditional improvers

The core problem of heavy clay is fine particles, small pores, poor air permeability, and easy water accumulation and compaction . Improvement requires "enhancing the stability of the aggregate structure" (such as increasing the application of organic fertilizers and biochar) rather than the short-term dispersing effect of PVP. The reasons why heavy clay is not suitable for PVP are as follows:

• Excessive PVP can easily clog pores and aggravate
  the narrow pores of oxygen-deficient heavy clay. If PVP is used (especially at a concentration > 0.2%), its polymer chains will form an "over-cross-linked gel layer" between soil particles, completely blocking the capillary pores and ventilation pores. After watering, water cannot penetrate and the roots cannot breathe, but instead lead to "anoxic compaction" (crop roots rot and leaves turn yellow), which is more serious than the problem of untreated heavy clay.
• PVP fails to form stable aggregates, and its anti-compaction effect is short-lived.
  The fundamental reason for heavy clay soil compaction is a lack of organic matter, which prevents soil colloids from forming water-stable aggregates. While PVP can disperse particles in the short term, the resulting "microaggregates" are temporary physical structures (disintegrating with heavy rain or irrigation) and cannot replace the "long-term stable aggregates" formed by organic fertilizers. After one to two weeks of use, the soil will compact again, and the PVP residue may increase its hardness.
• The economic efficiency is extremely poor. Traditional improvers are more efficient.
  Heavy clay requires a large amount of improvers to be effective. If PVP is used (cost 20-30 yuan/kg), the dosage per mu needs 300-500 kg (concentration 0.2%), and the cost exceeds 6,000 yuan, which is much higher than organic fertilizer (50-100 yuan/mu) or biochar (200-300 yuan/mu), and the effect is worse, so it is completely impractical.

3. Sandy soil (sand content > 80%): PVP is easily lost, the effect is short-lived and the cost is high.

The core problem with sandy soil is its poor water and fertilizer retention capacity, coarse particles, and weak adsorption capacity , but it is not easily compacted (large pores between particles). Although PVP can retain water in sandy soil for a short period of time, it is generally not suitable for use due to its "easy loss, frequent application requirements, and poor economic efficiency":

• PVP has weak adsorption capacity and is easily lost with rain/irrigation.
  Sandy soil particles are coarse (small specific surface area) and have weak binding force with PVP molecules (mainly relying on weak hydrogen bonds). When watering or raining, PVP easily penetrates into the deep soil with water (beyond the absorption range of crop roots), causing the PVP concentration in the surface soil to drop rapidly - the water retention effect only lasts for 2 to 3 days, and repeated application every 3 to 5 days is required, which is cumbersome.
• Low anti-compaction requirements, PVP's functionality is redundant.
  Sandy soils have large interparticle pores, making "dense compaction" virtually impossible (only minor cracking due to surface drought may occur, without the need for PVP). PVP's core function (anti-compaction) is completely redundant in sandy soils, and its limited water retention function can be achieved through low-cost methods such as straw mulching and humic acid application, without relying on PVP.
• Long-term use may lead to gelation of the surface
  . Frequent application of PVP in sandy soil may cause the PVP that has not been lost to accumulate on the surface, forming a "thin gel layer" - although this layer can retain water, it will hinder the entry of air into the soil, causing surface root hypoxia (such as the blackening of the surface fibrous roots of wheat and corn), which in turn affects crop growth.

4. Soil with extremely low organic matter content (organic matter content <0.5%): PVP cannot function and may affect microorganisms

The core problem of soils with extremely low organic matter content (such as poor, wind-blown sandy soils and bare soils that have been eroded for a long time) is a lack of soil colloids, low microbial activity, and a loose structure (or compacted soils without a foundation for improvement) . PVP is ineffective in such soils for the following reasons :

• Without organic matter support, PVP cannot form microaggregates.
  PVP needs to rely on soil colloids (such as humus) as "anchor points" to form "microaggregates", but soil lacking organic matter has almost no colloids - PVP molecular chains cannot stably combine with soil particles, and either disappear with water or disperse disorderly in the soil, unable to prevent compaction or retain water.
• Inhibits residual microorganisms and aggravates soil impoverishment.
  The number of microorganisms in soil lacking organic matter is already very small (weak decomposition ability), and the high molecular chains of PVP may attach to the surface of microorganisms, inhibiting their metabolic activities (such as decomposing small amounts of organic matter and fixing nitrogen), further reducing soil fertility and forming a vicious cycle of "the more you use it, the poorer it becomes."
• The core of soil improvement is to replenish organic matter. PVP cannot completely replace
  this type of soil. The only way to improve this type of soil is to "add large amounts of organic matter" (such as composting, returning straw to the field, and planting green manure). Once the organic matter content increases to above 1%, additional improvement measures can be considered. Using PVP is not only cost-effective but also delays the core improvement process.

5. Severely heavy metal contaminated soil (heavy metal concentration > 200 mg/kg): PVP adsorption capacity is insufficient, which can easily lead to secondary problems

PVP can only assist in the remediation of mildly heavy metal-contaminated soils (concentration <100 mg/kg) and is completely unsuitable for heavily contaminated soils (such as soils around mining areas, with Pb/Cd concentrations >200 mg/kg) for the following reasons:

• The adsorption capacity is limited and cannot reduce the activity of heavy metals.
  PVP's adsorption of heavy metals depends on the pyrrolidone ring on the molecular chain. The adsorption capacity of a single gram of PVP is only 0.5~2 mg (depending on the type of fruit and vegetable). Severely polluted soil requires extremely high concentrations of PVP (>1%) to adsorb some heavy metals - but high concentrations of PVP will clog soil pores, leading to hypoxia, which will aggravate crop damage.
• It is impossible to completely remove heavy metals, and can only "temporarily fix" them.
  PVP's adsorption of heavy metals is "reversible" (it will desorb in an acidic environment or high concentrations of other cations). If the pH of the soil in heavily polluted soil subsequently drops (such as acid rain), the adsorbed heavy metals will be released again, causing secondary pollution. The problem cannot be fundamentally solved (professional technologies such as "leaching" and "phytoremediation" are required).

Summary: Core characteristics of soils not suitable for PVP use

The key to determining whether a soil is suitable for PVP is whether PVP can address the soil's core issues without causing negative side effects . The following soils meet the core characteristics of being "unsuitable":

• Core problems cannot be solved by PVP (such as "lowering salt and adjusting pH" in saline-alkali soil, "stabilizing aggregates" in heavy clay soil, and "adding fertilizer" to soil lacking organic matter);
• New problems can easily arise due to the characteristics of PVP (such as "hypoxia" in heavy clay soils, "loss and waste" in sandy soils, and "secondary release" in heavily contaminated soils);
• The economic efficiency is extremely poor (for example, heavy clay and sandy soils require a large amount of PVP, the cost of which is much higher than traditional amendments).

The core logic of soil improvement is to "take targeted measures to address the fundamental problems" (such as draining salt from saline-alkali soil and adding organic fertilizer to heavy clay soil). PVP is only an "auxiliary means in special scenarios" and cannot replace traditional improvement measures, let alone be used for the unsuitable soil types mentioned above.