In agricultural production and horticultural cultivation, the acidity and alkalinity of fertilizers have an important impact on the soil environment and crop growth. Acidic fertilizers refer to organic or inorganic fertilizers that can reduce soil pH and increase soil acidity. This type of fertilizer is particularly suitable for the improvement of alkaline or neutral soils and the nutritional needs of acid-loving crops. So, what kind of organic fertilizers are acidic fertilizers? How do they work? How should they be used scientifically? This article will discuss these issues in detail.
- Main types of acidic organic fertilizers
a. Humic acid fertilizers
Humic acid is a natural organic acid extracted from raw materials such as peat and lignite. Its pH value is usually between 3.5-5.5 and is a typical acidic fertilizer. Humic acid can not only reduce soil alkalinity, but also improve soil aggregate structure and promote microbial activity. For example, products such as potassium humate and sodium humate are effective in adjusting the pH value of saline-alkali land.
b. Animal manure
Livestock manure that is not fully decomposed (such as chicken manure and sheep manure) will release a large amount of organic acid and hydrogen sulfide during the decomposition process, which may acidify the soil in the short term. However, it should be noted that fully decomposed manure is usually weakly alkaline, so its acidic properties are closely related to the degree of decomposition.
c. Plant-based compost
Compost made from materials such as pine needles, oak leaves, and tea residues is rich in tannins, organic acids and other ingredients. For example, the pH value of pine needle compost can be as low as 4.0-5.0, which is particularly suitable for acid-loving plants such as blueberries and azaleas. Coffee grounds (pH about 4.6) are also common acid-improving materials for home gardening.
d. Industrial by-products
Some agricultural product processing by-products such as sugar factory filter mud (pH 4.5-5.5) and wine lees contain organic acid residues, which will slowly acidify the soil if used for a long time. However, be wary of pollutants such as heavy metals that may be present.
- Mechanism of action of acidic fertilizers
a. Direct acidification effect
The free organic acids in fertilizers (such as citric acid and oxalic acid) replace the basic ions (calcium, magnesium, etc.) in the soil with hydrogen ions, thereby reducing the pH value. For example, the carboxyl group (-COOH) in humic acid can directly release H⁺ after dissociation.
b. Indirect biological pathway
Organic matter produces metabolic acids such as carbonic acid and nitric acid during microbial decomposition. Studies have shown that every ton of organic matter decomposed can produce an acidification potential equivalent to 2-3 kilograms of sulfuric acid.
c. Chelation
Substances such as humic acid can chelate alkaline metal ions in the soil and reduce its buffering capacity for pH. This property is particularly important when improving calcareous soils (containing calcium carbonate).
- Key points for the scientific use of acidic organic fertilizers
a. Soil testing first
The current pH value and buffering capacity of the soil should be measured before use. For strongly alkaline soils with a pH of more than 7.5, it is recommended to apply 5-8kg of acid compost (such as pine needle compost) per square meter, which can reduce the pH of the top 20cm soil by 0.5-1.0 units.
b. Crop demand matching
Different crops have significant differences in acid tolerance:
- Acid-loving crops (blueberries, tea trees): suitable pH 4.5-5.5
- Neutral crops (rice, wheat): suitable pH 6.0-7.0
- Acid-averse crops (alfalfa, beets): Be cautious when pH is less than 6.0
c. Dynamic monitoring and adjustment
Soil pH should be rechecked after 3 years of continuous use to prevent excessive acidification (pH less than 4.5 will cause aluminum toxicity). It is recommended to use pH test paper or portable acidity meter for regular monitoring.
d. Combined application strategy
The effect can be enhanced when used in combination with sulfur powder (biological acidifier) or ammonium sulfate (chemical acid fertilizer). For example, adding 0.5-1kg of sulfur powder to every 100kg of organic fertilizer can increase the acidification effect by 30%-50%.
- Common misunderstandings and risk prevention
a. Maturity misunderstanding
Some farmers mistakenly believe that all fresh organic fertilizers are acidic. In fact, unripe manure may temporarily increase the pH value due to ammonia volatilization in the early stage, and the acidification potential will only appear after it is fully decomposed. It is recommended to ensure full decomposition through the composting temperature curve (maintaining above 55°C for 15 days).
b. Ignoring regional differences
In the southern red soil area with high rainfall (naturally acidic), excessive use of acidic fertilizers may aggravate soil degradation; while in the northern calcareous soil, the acidification effect may be discounted due to calcium carbonate buffering.
c. Heavy metal risk
Industrial organic fertilizers (such as some urban sludge) may contain pollutants such as cadmium and lead. The national standard “Organic Fertilizer” (NY 525-2021) stipulates that the cadmium content shall not exceed 3mg/kg, and the test report must be checked before use.
- Innovative application cases
a. Precision acidification technology
In modern facility agriculture, the use of humic acid liquid fertilizer combined with a drip irrigation system can achieve precise pH control in the root zone. Test data show that the use of humic acid nutrient solution with a pH of 5.2 in tomato cultivation can increase the iron absorption efficiency by 40%.
b. Waste recycling model
A blueberry planting base uses pine sawdust, pomace and other wastes to make special acid compost, which not only treats agricultural and forestry wastes, but also stabilizes the soil pH in the range of 5.0-5.5, reducing the cost by 60% compared with the traditional sulfur improvement method.
c. Bio-enhancement process
The acidification process of compost is accelerated by inoculating specific fungi (such as Aspergillus niger). Studies have shown that the pH value of compost inoculated with bacterial agents can be 0.8-1.2 lower than that of conventional compost, and the humic acid content is increased by 15%-20%.
The selection and application of acidic organic fertilizers is a comprehensive technology that requires consideration of soil science, plant nutrition and environmental science. In actual operation, we should consider not only the immediate acidification effect of fertilizers, but also its long-term impact on soil ecosystems. With the development of organic agriculture and precision agriculture, the application of acidic fertilizers in the future will pay more attention to the combination with biotechnology, intelligent monitoring and other means to create a more suitable growth environment for crops and maintain the healthy and sustainable development of soil.
