Alkaline soil refers to soils with a pH greater than 7.5. It is widely distributed in the arid and semi-arid regions of northwest my country and parts of North China. Its main characteristics include: a high pH that reduces the availability of trace elements such as iron, manganese, and zinc; soil colloids adsorb large amounts of sodium ions, leading to deterioration of soil structure and compaction; and the accumulation of alkaline substances such as calcium carbonate, which impairs normal root respiration and nutrient absorption. Long-term cultivation of crops in alkaline soil often results in stunted plants, yellowing leaves, and reduced yield and quality. Therefore, the appropriate selection and application of fertilizers are key technical measures to improve the physical and chemical properties of alkaline soils and increase crop yields.
A. Suitable Fertilizer Types for Alkaline Soils
a. Acidic Fertilizers
i. Physiologically Acidic Fertilizers
Ammonium nitrogen fertilizers such as ammonium sulfate and ammonium chloride are typical physiologically acidic fertilizers. When this type of fertilizer is applied to the soil, ammonium ions are absorbed by the crops, while anions such as sulfate and chloride remain in the soil. Through the selective absorption mechanism of the crops, this increases soil acidity. Studies have shown that applying ammonium sulfate for three consecutive years can reduce soil pH by 0.3-0.5 units. However, it is important to note that long-term, single application may lead to soil salinization, and alternating with other fertilizers is recommended.
ii. Chemical Acidic Fertilizers
Phosphate fertilizers such as superphosphate and triple superphosphate contain free phosphate, which can directly lower local soil pH when applied to the soil. Furthermore, the phosphorus in phosphate fertilizers promotes root development and enhances crop resistance to adverse environmental conditions. In trials in alkaline cotton fields in Xinjiang, combining superphosphate with organic fertilizers increased cotton seedling emergence by 12% and the number of bolls per plant by 15%.
i. Farmyard Manure
Well-rotted farmyard manure, compost, green manure, and other farmyard manures contain a large amount of humus, which can bind to calcium ions in the soil, improve soil aggregate structure, and reduce soil alkalinity. Applying 30-45 tons of farmyard manure per hectare can reduce soil bulk density by 0.1-0.2 g/cm³ and increase porosity by 8%-12%. Furthermore, the organic acids produced during the decomposition of farmyard manure can neutralize soil alkalinity.
ii. Commercial Organic Fertilizers
Commercial fertilizers such as refined organic fertilizers and bio-organic fertilizers offer comprehensive nutrients and are rich in active substances. The beneficial microbial flora in these fertilizers promotes the conversion of insoluble nutrients in the soil and reduces the effect of calcium carbonate on phosphorus fixation. In experiments in alkaline wheat fields in Gansu, the combination of bio-organic fertilizers with chemical fertilizers increased wheat yield by 18.6% compared to chemical fertilizers alone.
c. Biofertilizers
i. Microbial Agents
Microbial agents, such as rhizobia and phosphate- and potassium-solubilizing bacteria, produce organic acids and enzymes through metabolic activity, dissolving insoluble nutrients in the soil. Furthermore, the polysaccharides secreted by microorganisms during their growth process improve soil structure. Studies have found that the application of silicate bacterial agents can increase the available potassium content in the soil by 20%-30%.
ii. Humic Acid Fertilizers
Humic acid has strong ion exchange and chelating properties, combining with sodium ions in the soil to reduce soil alkalinity. Furthermore, humic acid stimulates root growth and enhances crop resistance. In Ningxia wolfberry cultivation, the application of humic acid compound fertilizers increased the polysaccharide content of wolfberry fruits by 12% and increased the weight of individual fruits by 15%.
d. New Functional Fertilizers
i. Acid Conditioners
Acid conditioners such as gypsum and sulfur powder can directly neutralize soil alkalinity. The calcium ions in gypsum can replace sodium ions in soil colloids, reducing soil alkalinity. Microbial oxidation of sulfur powder to sulfuric acid effectively lowers soil pH. Generally, applying 750-1500 kg of gypsum or 150-300 kg of sulfur powder per hectare can significantly reduce soil pH.
ii. Chelated Trace Element Fertilizers
Chelated fertilizers such as EDTA-iron and EDTA-zinc maintain high stability in alkaline environments, effectively addressing trace element deficiencies in alkaline soils. In a trial in apple orchards in Shaanxi Province, foliar application of chelated iron fertilizer reduced leaf chlorosis by 40% and increased fruit soluble solids content by 1.2 percentage points.
B. Key Fertilizer Application Techniques
Base Fertilizer Application
Use organic fertilizers primarily, supplemented with appropriate amounts of acidic chemical fertilizers. Recommended application rates per hectare include 30-45 tons of well-rotted farmyard manure, 450-600 kg of superphosphate, and 225-300 kg of ammonium sulfate. Use deep plowing to evenly mix fertilizer into the soil layer at a depth of 20-30 cm to ensure full soil-fertilizer contact and maximize soil improvement.
Topdressing
Apply acidic fertilizers at appropriate times based on the crop growth stage and soil test results. For example, during the corn-booming stage, apply 150-225 kg/hectare of ammonium sulfate. During the peak vegetable growth period, apply a water-soluble humic acid fertilizer every 10-15 days. Apply small amounts frequently to avoid concentrated fertilizer application that can cause localized soil salinization.
Foliar Fertilization
Foliar fertilization can be used to supplement trace element deficiencies in alkaline soils. During the critical growth period, spray 0.2%-0.3% chelated iron fertilizer and 0.1%-0.2% zinc sulfate solution every 7-10 days for 2-3 consecutive applications to effectively alleviate nutrient deficiency symptoms.
C. Precautions
Soil Testing: Regularly test soil pH, salinity, and other indicators, and adjust fertilizer application plans based on the results.
Crop Rotation: Rotate grasses with legumes. The nitrogen fixation of legume root nodules can improve soil nutrient availability and reduce soil alkalinity.
Water Management: Practice appropriate irrigation to avoid flooding, which can cause upward movement of soil salt. Using water-saving irrigation methods such as drip irrigation and subsurface irrigation can effectively control soil salinization.
Scientific and appropriate fertilizer application can not only improve the physical and chemical properties of alkaline soils and enhance soil fertility, but also promote crop growth and development, ultimately achieving sustainable agricultural development. In actual production, a personalized fertilization plan should be developed based on local soil conditions and crop needs to achieve optimal improvement results.
