Fertilizer is the “food” for crops and the foundation for increasing crop yields. According to the Food and Agriculture Organization (FAO), fertilizers contribute about 50% to the global increase in food production. China, with only 7% of the world’s arable land, feeds 22% of the global population, a significant part of which is attributed to the use of fertilizers.
Types and Application of Fertilizers
Fertilizers can be divided into two major categories: organic and inorganic. This article mainly discusses inorganic fertilizers, also known as chemical fertilizers, including nitrogen (N), phosphorus (P), potassium (K), micronutrient fertilizers, and compound fertilizers. Chemical fertilizers are characterized by high nutrient content and fast effects, but they also have some limitations, such as potential negative impacts on soil structure. This article will introduces nitrogen fertilizers, phosphorus fertilizers, and potassium fertilizers first.
I. Types, Functions, and Application Strategies of Nitrogen Fertilizers
Nitrogen fertilizers are essential for agricultural production, providing necessary nitrogen nutrition for crops, directly influencing crop growth, yield, and quality. Nitrogen fertilizers come in many types, with different transformation processes in the soil and different application methods. Proper selection and application of nitrogen fertilizers can improve efficiency, reduce waste, and minimize environmental pollution.
1. Types and Nature of Nitrogen Fertilizers
Based on the chemical form of nitrogen, nitrogen fertilizers can be divided into three main categories:
Fertilizer Type | Nature | Form of Nitrogen | Suitability |
Ammonium Nitrogen Fertilizers | Includes ammonium water, ammonium bicarbonate, ammonium sulfate, and ammonium chloride. | NH₄⁺ (Ammonium) | Suitable for various soil types, especially acidic soils. |
Nitrate Nitrogen Fertilizers | Includes ammonium nitrate, sodium nitrate, and calcium nitrate. | NO₃⁻ (Nitrate) | Suitable for dryland farming but prone to leaching in high rainfall areas. |
Amide Nitrogen Fertilizers | Includes urea and calcium cyanamide. Urea is the most used and needs to be converted to NH₄⁺ in the soil. | NH₂ (Amide) | Low-cost and high nitrogen content, suitable for all soil types but requires conversion in the soil. |
2. Transformation of Nitrogen Fertilizers in Soil
The transformation process of nitrogen fertilizers in the soil depends on the type of nitrogen fertilizer:
Ammonium Nitrogen Fertilizers: For example, NH₄⁺ from ammonium sulfate, ammonium bicarbonate, and ammonium chloride can be directly absorbed by plants or converted into NO₃⁻ through nitrification. In acidic soils, these fertilizers increase soil acidity.
Nitrate Nitrogen Fertilizers: After application, NO₃⁻ can be directly absorbed by plants, but nitrate nitrogen can be easily leached, especially in high rainfall areas.
Amide Nitrogen Fertilizers (e.g., urea): After application, urea is broken down by soil microorganisms into ammonium carbonate, which is further converted to ammonium nitrogen. However, when applied to the soil surface, urea can easily volatilize, especially in alkaline soils.
3. Rational Distribution and Application of Nitrogen Fertilizers
To improve nitrogen fertilizer efficiency and reduce losses, proper distribution and application are crucial.
(1) Rational Distribution of Nitrogen Fertilizers
According to Soil Conditions: Different soil types have different nitrogen needs and reactions. Alkaline soils are suitable for acidic or physiologically acidic nitrogen fertilizers, such as ammonium sulfate and ammonium chloride, while acidic soils are more suited to basic fertilizers like sodium nitrate and calcium nitrate. Urea can be applied to all soil types.
According to Crop Requirements: Different crops have different requirements for nitrogen forms. Rice benefits from ammonium nitrogen fertilizers, especially ammonium chloride, while crops like tomatoes and beets require different nitrogen forms at various growth stages (ammonium nitrogen during the seedling stage, nitrate nitrogen during the fruiting stage).
According to Crop Growth Stages: Different crops require nitrogen at different growth stages. Corn requires large amounts of nitrogen during the ear stage, while rice needs nitrogen during the tillering and ear stages. For fruit trees, applying nitrogen fertilizer during basal fertilization can increase yield.
(2) Effective Application of Nitrogen Fertilizers
Deep Application: Deep placement of nitrogen fertilizer reduces nitrogen loss from volatilization, prevents absorption by weeds and algae, and improves utilization. Studies show that deep application can increase nitrogen use efficiency by 20%-30% compared to surface application.
Combined Application: Nitrogen fertilizer should be used in combination with organic fertilizers and phosphorus and potassium fertilizers to ensure balanced crop nutrition and improve nitrogen fertilizer efficiency.
Application of Nitrogen Stabilizers: Nitrogen stabilizers (such as nitrification inhibitors) can suppress the activity of nitrifying bacteria in the soil, delaying the conversion of ammonium nitrogen to nitrate nitrogen, reducing nitrogen loss, and improving fertilizer efficiency.
4. Importance of Proper Nitrogen Fertilizer Use
Nitrogen loss in soil occurs mainly through ammonia volatilization, nitrate leaching, and denitrification. In China, nitrogen fertilizer utilization efficiency is only about 50%, meaning that half of the nitrogen fertilizer is not absorbed by crops, leading to resource waste and environmental pollution.
Through scientific nitrogen management, such as selecting appropriate nitrogen types based on crop nutrient needs, rationally distributing nitrogen application amounts, combining nitrogen with organic and other fertilizers, and promoting nitrogen enhancement technologies, nitrogen fertilizer utilization efficiency can be greatly improved, reducing environmental impact and increasing crop yields and quality.
II. Types, Transformation, and Application Methods of Phosphate Fertilizers.
Phosphate fertilizers are among the most important fertilizers in agricultural production, providing phosphorus to crops, which aids in energy transfer, root development, and flowering/fruiting. There are many types of phosphate fertilizers, their transformation processes in the soil are complex, and their application methods directly affect their efficacy. Effective application strategies can significantly improve the utilization rate of phosphate fertilizers, maximizing their yield-enhancing benefits.
1. Types and Nature of Phosphate Fertilizers
Based on solubility in soil and ease of crop absorption, phosphate fertilizers can be divided into three main categories:
Fertilizer Type | Nature | Solubility | Suitability |
Water-soluble Phosphate Fertilizers | Includes superphosphate and triple superphosphate. These fertilizers dissolve easily in water and are readily absorbed by crops. | Water-soluble | Suitable for immediate crop absorption. |
Weak-acid-soluble Phosphate Fertilizers | Includes calcium magnesium phosphate and slag phosphate. These fertilizers dissolve in weak acidic environments. | Soluble in weak acids | Best for acidic soils. |
Insoluble Phosphate Fertilizers | Includes phosphate rock powder and bone meal. These fertilizers dissolve only in strong acids and release nutrients slowly over time. | Insoluble in water, soluble in strong acids | Suitable for long-term use with slow release. |
2. Transformation of Phosphate Fertilizers in Soil
Once phosphate fertilizers are applied to the soil, a series of chemical reactions occur that influence their effectiveness and the ability of crops to absorb them:
Transformation of superphosphate: After application, superphosphate undergoes hydrolysis, forming a saturated solution of monocalcium phosphate, phosphoric acid, and dicalcium phosphate. The phosphoric acid in this solution diffuses outward and reacts with iron, aluminum, calcium, and other elements in the soil, forming phosphates with different solubility levels, which eventually precipitate as stable calcium phosphate. In acidic soils, phosphorus reacts with iron and aluminum to form iron and aluminum phosphate precipitates, while in calcareous soils, phosphorus reacts with calcium to form dicalcium phosphate.
Transformation of calcium magnesium phosphate: This fertilizer dissolves gradually through acids secreted by plant roots and soil microorganisms, releasing phosphorus for crop absorption.
Transformation of phosphate rock powder: In acidic soils, phosphate rock powder undergoes chemical and biological reactions, gradually releasing phosphorus compounds that crops can absorb.
3. Rational Distribution and Application of Phosphate Fertilizers
The utilization rate of phosphate fertilizers is relatively low, with typically only 10%-25% absorbed by crops during the growing season. The remainder is often fixed in the soil. Therefore, rational distribution and application methods are crucial to improving phosphate fertilizer efficiency.
(1) Application Based on Soil Conditions
Soil phosphorus levels: The effectiveness of phosphate fertilizers depends on the available phosphorus content in the soil. When available phosphorus is less than 10mg/kg, phosphate fertilizer can significantly increase yields. If it exceeds 25mg/kg, the effect of additional phosphate fertilizers diminishes.
Soil pH: Weak-acid-soluble and insoluble phosphate fertilizers are more suitable for acidic soils, while water-soluble phosphate fertilizers work best in neutral and calcareous soils.
Soil organic matter: Soils with low organic matter show more significant yield increases with phosphate fertilizer application, while soils rich in organic matter show less pronounced effects.
(2) Application Based on Crop Phosphorus Requirements
Prioritize phosphorus-loving crops: Crops such as legumes, sugarcane, sugar beet, oilseed rape, corn, and potatoes have high phosphorus demands and should receive priority for phosphate fertilization.
Rotation systems: In crop rotation systems, phosphate fertilizers should be applied to crops with higher phosphorus needs, such as legumes or dryland crops in a rice-dryland rotation system.
(3) Deep and Concentrated Application of Phosphate Fertilizers
Phosphate fertilizers have limited mobility in the soil. Deep and concentrated application reduces contact with the soil and increases contact with crop roots, improving efficiency.
(4) Combined Use of Phosphate and Nitrogen Fertilizers
Combining nitrogen and phosphorus fertilizers can significantly increase crop yields and phosphate fertilizer utilization. For most cereal crops, the nitrogen-to-phosphorus ratio should be between 2:1 and 3:1. Maintaining a balance between nitrogen and phosphorus allows crops to absorb these nutrients more efficiently.
(5) Combined Use of Phosphate and Organic Fertilizers
Organic acids in organic fertilizers can form complexes with iron, aluminum, and calcium in the soil, reducing phosphorus fixation and increasing its availability. Thus, combining phosphate fertilizers with organic fertilizers can improve their efficacy.
4. Residual Effects of Phosphate Fertilizers
Phosphate fertilizers tend to have strong residual effects in the soil. Although only 10%-25% of the phosphorus is absorbed by crops during the growing season, the residual effects of phosphate fertilizers can last 5-10 years. Therefore, after continuous application for several years, phosphate fertilizers can be applied every 2-3 years, utilizing their residual effects to meet crop needs.
Summary
The rational application of phosphate fertilizers requires consideration of soil conditions, crop nutrient needs, and appropriate application methods. Ensuring a balanced supply of nitrogen, phosphorus, and other essential nutrients, along with scientific application techniques, can improve phosphate fertilizer utilization, reduce waste, and enhance agricultural productivity.
III. Types, Transformation, and Application Methods of Potassium Fertilizers
Potassium fertilizers play a crucial role in agricultural production by improving crop yield and quality. They are particularly effective in enhancing crops' drought resistance, disease resistance, and strengthening plant cell walls. Proper application of potassium fertilizers can significantly improve agricultural economic benefits. Below are the main types, transformation processes, and application strategies for potassium fertilizers.
1. Types and Nature of Potassium Fertilizers
Common types of potassium fertilizers include potassium sulfate, potassium chloride, and wood ash. Each type has different characteristics and is suitable for different crops and soil conditions:
Fertilizer Type | Nature | Application | Suitability |
Potassium Sulfate | Contains both potassium and sulfur. Suitable for crops that are sensitive to chlorine. | Used for crops like tobacco, potatoes, and cruciferous plants. | Ideal for crops that prefer potassium but are sensitive to chlorine (chlorine-sensitive crops). |
Potassium Chloride | A cost-effective potassium fertilizer, contains chloride. | Commonly used for fiber crops like cotton and hemp, and some grasses. | Not suitable for chlorine-sensitive crops or poorly drained soils. |
Wood Ash | Residue from burned plant matter. Contains potassium primarily in the form of potassium carbonate, and other nutrients like calcium and phosphorus. | Acts as a fast-acting alkaline fertilizer, used as basal fertilizer, top dressing, or seed fertilizer. | Best suited for acidic soils and crops needing quick potassium availability. Should not be mixed with ammonium fertilizers. |
2. Transformation of Potassium Fertilizers in Soil
When applied to soil, potassium fertilizers dissolve into ions, with some being absorbed by plants and others being adsorbed by soil colloids:
Potassium Sulfate and Potassium Chloride: In neutral or calcareous soils, potassium ions exchange with calcium ions, forming calcium sulfate (CaSO4) or calcium chloride (CaCl2). CaSO4 is slightly soluble and can move with water before depositing in soil pores, causing compaction. CaCl2 leaches easily, resulting in calcium loss and increased soil compaction. Long-term potassium fertilizer use, especially in neutral or calcareous soils, should be combined with organic fertilizers to improve soil structure.
Transformation in Acidic Soil: In acidic soils, potassium fertilizers exchange with hydrogen ions, forming sulfuric or hydrochloric acid, increasing soil acidity. To prevent excessive acidity, potassium fertilizers should be applied alongside lime and organic fertilizers.
3. Rational Distribution and Effective Application of Potassium Fertilizers
The effectiveness of potassium fertilizers depends on soil characteristics, crop types, climate conditions, and application methods. Through proper distribution and application, the efficiency and crop yield can be improved.
Soil Conditions and Potassium Fertilizer Application:
Soil Potassium Supply Levels: The amount of available potassium in the soil determines the effectiveness of potassium fertilizer. Trials show that when the available potassium is less than 90 mg/kg, the effect of potassium fertilizer is significant. Between 91 mg/kg and 150 mg/kg, the effect is less stable, and above 150 mg/kg, the effect is minimal.
Soil Mechanical Composition: Finer soils tend to contain more potassium, while sandy soils have lower potassium levels. Potassium fertilizer is more effective in sandy soils than clay, so priority should be given to sandy soils with potassium deficiency.
Soil Aeration: Soil aeration affects root respiration, which in turn impacts potassium uptake. Improving soil aeration can help enhance potassium fertilizer efficiency.
Crop Conditions and Potassium Fertilizer Application:
High Potassium-Consuming Crops: Crops with high sugar content, such as potatoes, sweet potatoes, sugarcane, fruit trees, and tobacco, require large amounts of potassium. Potassium fertilizer can significantly improve yield and quality for these crops. If potassium supply is limited, prioritize these potassium-loving crops.
Legumes and Oil Crops: Potassium fertilizer also increases the yield of leguminous and oilseed crops.
Deep and Concentrated Application: Potassium has low mobility in the soil and is easily fixed by soil particles. Deep and concentrated application reduces the contact area between potassium and the soil, increasing potassium diffusion and facilitating crop absorption.
Combination with Nitrogen and Phosphorus Fertilizers: Crops require potassium in a certain ratio to nitrogen and phosphorus. When nitrogen and phosphorus levels are low, applying potassium alone is less effective. Potassium must be applied alongside nitrogen and phosphorus fertilizers to maximize its yield-increasing effect.
Application of Wood Ash: Wood ash is an excellent alkaline fertilizer, suitable for acidic soils and leguminous crops. Due to its fast-acting nature and the presence of multiple nutrients, wood ash can be used as a basal, topdressing, or seed fertilizer. However, it should not be mixed with ammonium nitrogen fertilizers or decomposed organic fertilizers, as it may cause ammonia volatilization losses.
4. Application Techniques for Potassium Fertilizers
The application of potassium fertilizers should be planned according to crop potassium requirements and soil potassium content:
Deep and Early Application: Potassium fertilizers should be applied deep into the soil and early in the growing season, especially during the early growth stages. Potassium demand peaks during the critical nutrition periods, such as tillering to jointing in cereals, flowering to boll formation in cotton, and fruit development in fruit trees.
Application Rate: The application rate of potassium fertilizers should be based on soil potassium levels, crop potassium needs, and the balance of other nutrients. The recommended rate for potassium oxide is 6-9 kg per acre for corn and 5-8 kg per acre for rice.
Summary Potassium fertilizers play a vital role in agricultural production. Through rational distribution and application strategies—especially deep, concentrated application and combination with nitrogen and phosphorus fertilizers—potassium fertilizer efficiency can be enhanced, soil fixation reduced, and crop yield and quality improved.
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