Understanding Soil to Fertilize EffectivelyReinterpreting Soil and Fertilization from the Perspective of Acid–Base Balance

I. The Overlooked “Soil Condition”: Why Is Fertilization Becoming Less Effective?

In many agricultural regions, farmers are not lacking in fertilization experience. Decisions such as when to apply nitrogen, when to supplement potassium, or when to add micronutrients are often based on long-established practices. However, in reality, many fields show no corresponding improvement in crop performance despite increasing fertilizer inputs year after year. Instead, problems such as greater yield fluctuations, reduced stress resistance, and more frequent pest and disease outbreaks have emerged, leading to a situation where fertilization becomes increasingly inefficient.

The underlying cause of this phenomenon does not lie solely in fertilizer quality, but rather in changes occurring within the soil itself—the system responsible for nutrient retention, transformation, and supply. Soil is never a static growing medium; it is a dynamic system undergoing continuous chemical reactions, biological activity, and structural evolution. Soil acidity and alkalinity (pH) is one of the key indicators regulating the functioning of this system.

When soil pH deviates from the range suitable for crop growth, the chemical forms, mobility, and root uptake efficiency of nutrients change fundamentally. Under acidic or alkaline conditions, nutrients may become fixed or their transformation pathways restricted. Even when sufficient or high-quality fertilizers are applied, mismatches between soil environment and nutrient form may prevent nutrients from being converted into plant-available forms, ultimately resulting in nutrient waste and reduced fertilization efficiency.

From the perspective of sustainable agriculture, truly efficient fertilization management should shift from the superficial logic of “supplying what the crop lacks” toward addressing the more fundamental question of whether the soil itself provides conditions conducive to efficient nutrient uptake. Only by first improving soil condition and then applying nutrients precisely can both nutrient use efficiency and crop productivity be enhanced.

II. How to Determine Whether Soil Is Acidic or Alkaline?

1. What Is pH, and Why Can Small Numerical Differences Have Major Effects?

Soil pH is used to measure the intensity of acidity or alkalinity in soil solution. Its scale is logarithmic rather than linear. In simple terms, a change of one pH unit corresponds to approximately a tenfold change in hydrogen ion activity in the soil solution, which is why soils that appear similar on the surface may differ greatly in nutrient availability and crop performance.

In general, soil pH can be classified into three ranges:

• pH < 7: acidic soils

• pH ≈ 7: neutral soils

• pH > 7: alkaline soils

It is important to emphasize that different crops have distinct pH adaptation ranges. There is no single pH value that is universally optimal for all crops; suitability must be assessed based on specific crop requirements.

2. Preliminary Field Assessment

In routine field management, farmers may use low-cost methods such as pH test strips, simple household reaction tests (e.g., vinegar or baking soda), or field observation to conduct preliminary screening of soil acidity or alkalinity. These approaches can quickly identify cases where soil pH deviates significantly from normal ranges and provide general direction for further management. However, they should not be used directly as the basis for fertilization or soil amendment decisions.

This limitation arises from the inherent complexity of soil environments. First, soils exhibit natural spatial variability, meaning pH values can differ within the same field. Second, pH often differs between surface soil and the root zone, while it is the root-zone environment that directly governs nutrient uptake. Third, irrigation water quality and localized fertilizer placement can alter soil pH at a micro-scale, potentially distorting preliminary assessments.

3. Why Is Soil Testing an Essential Step Before Fertilization?

When any of the following conditions occur, professional soil testing becomes a necessity rather than an option:

• Fertilization effectiveness declines over multiple years, with increased input failing to improve yield or quality

• Micronutrient deficiency symptoms repeatedly appear, and corrective applications produce only short-lived or negligible effects

• Soil compaction intensifies, water infiltration and aeration slow, and tillage becomes more difficult

• The same crop shows markedly different growth and stress resistance across different fields

Professional soil testing not only provides accurate pH values, but also offers insight into nutrient structure, salinity levels, organic matter content, and ionic balance. These data form a comprehensive scientific basis for pH adjustment, fertilizer formulation, and soil improvement strategies, reducing the risk of soil degradation caused by blind or excessive interventions.

III. How Does Acid–Base Imbalance Develop? Most Problems Are Not Created in a Single Year

Soil pH deviations are typically the result of long-term accumulation rather than short-term events. In regions where natural factors such as rainfall, soil parent material, and climate are relatively stable, human management practices—especially fertilizer structure—become the primary drivers influencing the rate of pH change.

Long-term reliance on simplified or unbalanced fertilization practices can disrupt the original ionic equilibrium within the soil. Examples include prolonged high-intensity application of acidic compound fertilizers without compensatory alkaline inputs, neglect of calcium and magnesium replenishment leading to reduced buffering capacity, and mismatches between fertilizer application rates and crop uptake, allowing excess nutrient ions to accumulate in the soil and gradually alter pH.

These changes may not be immediately apparent, but as planting cycles extend, soil buffering capacity becomes depleted, pH fluctuations intensify, and problems such as unstable fertilization response, reduced crop stress tolerance, and soil structural degradation increasingly emerge—ultimately constraining sustainable agricultural production.

IV. Where Do the Effects of Acid–Base Imbalance Usually Begin?

1. Soil Structure Is Affected First, Followed by Root Restriction

Many farmers observe that fields cultivated for years become progressively more difficult to manage: surface crusting occurs after rainfall, soils harden upon drying, tillage resistance increases, and water infiltration and aeration decline. These symptoms often represent typical manifestations of structural changes associated with soil pH imbalance.

Under acidic conditions, the activity of certain metal ions increases, disrupting soil colloid stability and reducing aggregate cohesion, thereby promoting compaction. Under alkaline conditions—especially when sodium accumulation is involved—soil particles tend to disperse, forming hard crusts upon drying. Once soil structure deteriorates, root growth space becomes severely limited, and the transport efficiency of water and nutrients declines significantly, rendering fertilization less effective even when nutrients are supplied.

2. Nutrients Do Not “Disappear” — They Become “Locked”

This is one of the most misleading consequences of soil pH imbalance and a central reason why fertilization often appears ineffective. Soil tests may indicate sufficient nutrient levels, yet crops still exhibit pronounced deficiency symptoms. In such cases, nutrients are effectively “locked” in forms unavailable to plants.

In acidic soils, high concentrations of active iron (Fe³⁺) and aluminum (Al³⁺) readily react with phosphorus to form insoluble iron and aluminum phosphates, resulting in phosphorus fixation. In alkaline soils, calcium ions (Ca²⁺) dominate, promoting the formation of calcium phosphate precipitates; meanwhile, micronutrients such as iron and zinc tend to form hydroxide precipitates, further reducing their availability. This “nutrient immobilization” not only wastes fertilizer inputs but also exerts long-term negative effects on crop growth.

3. Why Does Repeated Supplementation Become Increasingly Unstable?

When soil pH conditions are unsuitable, nutrients applied in conventional forms rapidly participate in chemical reactions after entering the soil. Before they can be absorbed by crop roots, they may already have transformed into fixed or insoluble forms. This outcome does not indicate poor fertilizer quality, but rather a mismatch between nutrient form and soil environment, resulting in ineffective inputs.

Repeated application of conventional nutrients under such conditions not only fails to resolve deficiency issues but may further exacerbate ionic imbalance and intensify pH deviation. This creates a vicious cycle in which nutrients appear increasingly deficient despite higher application rates, rendering fertilization outcomes progressively more unstable.

V. How Should Management Strategies Be Adjusted for Different Soil Types?

(1) Acidic Soils: Stability Matters More Than Speed

The core challenges of acidic soils include excessive hydrogen ion concentration, deficiencies of calcium and magnesium, reduced buffering capacity, and increased susceptibility to structural degradation.

Consequently, the primary objective of acidic soil management is gradual neutralization and steady conditioning, rather than rapid pH correction that could impose stress on soil microorganisms and crop roots.

An integrated strategy combining neutralization, fertility building, and structural improvement is recommended. Mild neutralizing agents such as limestone or calcium–magnesium phosphate fertilizers can be used to gradually adjust pH while replenishing base cations. The application of well-decomposed organic fertilizers or functional products such as potassium humate can increase organic matter content, enhance buffering capacity, and improve soil aggregate structure. At the same time, long-term reliance on acidic fertilizers should be avoided to reduce the risk of accelerated soil acidification.

(2) Alkaline Soils: The Central Question Is Nutrient Availability

In alkaline soils, the primary limitation lies in nutrient fixation and structural compaction. Simply increasing fertilizer application rates is rarely effective. Instead, management should focus on improving nutrient availability while gently regulating the soil environment, rather than forcibly acidifying the soil.

In practice, a combined approach using active nutrients and functional soil conditioners is effective. Micronutrient products protected by chelating agents such as EDTA—particularly iron, zinc, and manganese—can maintain solubility under alkaline conditions, preventing precipitation and significantly improving utilization efficiency.

Functional fertilizers based on amino acids or seaweed extracts can further support mild pH regulation, stimulate beneficial microbial activity, and enhance soil porosity. In sodic or saline-alkaline soils, gypsum-based compound fertilizers may replace single gypsum applications, achieving both ion exchange and nutrient supplementation; subsequent use of potassium humate can help consolidate improvement effects and reduce the risk of secondary compaction.

VI. From “Applying More” to “Using More Effectively”: The Core Logic of Coordinated Fertilization

The essence of sustainable fertilization lies not in short-term nutrient supplementation, but in maintaining soil pH balance and overall soil health through long-term, moderate management, allowing nutrient inputs to be consistently converted into crop output. This logic can be summarized in four principles:

• Base fertilization and soil improvement strategies on soil test data rather than experience alone

• Avoid rigid, unchanging fertilization patterns by dynamically adjusting fertilizer types and ratios according to soil pH trends and crop growth stages

• Combine conventional compound fertilizers with functional fertilizers and chelated micronutrients to balance nutrient supply and soil conditioning

• Regularly re-test soil pH and nutrient status to refine management strategies and avoid neglect after one-time interventions

Such an approach reduces fertilizer input risks, gradually enhances soil fertility, and enables simultaneous improvements in crop yield, quality, and soil health—fully aligning with the principles of sustainable agriculture.

VII. Conclusion: Understanding Soil Is the True Starting Point of Fertilization

Soil is never a passive container for fertilizers; it is a dynamic system that ultimately determines whether nutrients can function effectively. Within the framework of sustainable agriculture, upgrading fertilization management essentially means upgrading our understanding of soil—from focusing solely on nutrient content to recognizing how soil conditions govern nutrient transformation, and from experience-based practices to precise, science-driven management.

Only by first understanding soil condition, scientifically regulating acid–base balance, and improving soil structure—then pairing these efforts with appropriate fertilizer products—can every unit of nutrient input be translated into tangible production benefits. Protecting soil health is fundamental to achieving long-term land productivity and building a resilient, sustainable agricultural future.