Risks in the Use of Organic Fertilizer and Their Solutions
Preface
Amid widespread concerns over food safety, with the rise of organic farming and ecological agriculture, organic farms and ecological farms have been springing up like bamboo shoots after rain all across the country. Many people have chosen to grow their own vegetables and grains, and many investment institutions have also begun to make major moves into ecological organic agriculture. However, the risks involved are not only financial and market-related, but more importantly technical.
At present, domestic organic farms or ecological farmsteads, regardless of their school of thought, basically all follow the “four no-use” bottom line of not using pesticides, chemical fertilizers, hormones, or antibiotics. Since chemical products are not used, substitutes are needed. Thus organic fertilizer (including farmyard manure) has replaced chemical fertilizer, biological and botanical pesticides have replaced chemical pesticides, natural crop growth has replaced hormones, and Chinese herbal medicines and plant extracts have replaced antibiotics. People are showing their ingenuity in every way in the search for safe production methods, carrying out beneficial exploration and experimentation for the development of safe agriculture.
This article discusses only the risks, problems, and solutions involved in replacing chemical fertilizer with organic fertilizer.
What is organic fertilizer? Organic fertilizer specifically refers to fertilizer made from various animal wastes (including animal manure and animal processing waste) and plant residues (oilseed cake fertilizers, crop straw, fallen leaves, dead branches, peat, etc.), using physical, chemical, biological, or combined treatment technologies, through certain processing techniques (including but not limited to composting, high-temperature treatment, anaerobic treatment, etc.), to eliminate harmful substances (pathogenic bacteria, pest eggs and larvae, weed seeds, etc.) and reach harmless standards, conforming to the relevant national standards (NY 525-2011) and regulations.
Since it has undergone harmless treatment to meet the relevant national standards, how could there still be risks? Please note that this organic fertilizer standard contains no provisions regarding antibiotic residues. Secondly, regarding heavy metal residues, there are only limit values for five heavy metals, and there are no limits on residues of copper-based and zinc-based preparations that are currently used in large quantities at livestock farms.
At present, the raw materials for domestic organic fertilizer production basically all come from the livestock and poultry manure of large-scale farms. The author has said on many occasions: for an “ecological” farm that has long used conventional farm livestock and poultry manure as its organic fertilizer source, its land after ten years is very likely to become an ecological disaster zone. This is not alarmist talk. Let us analyze it layer by layer from source, process, and result.
Current Situation
In 2003, the national livestock and poultry manure discharge reached 3.18 billion tons. It is estimated that by 2010, the national livestock and poultry manure discharge will reach 4.5 billion tons.
Statistics from the Chemical Industry Association and the Pharmaceutical Industry Association in 2005 show that China's annual production of antibiotic raw materials is about 210,000 tons, of which 97,000 tons (accounting for 46.1% of total annual output) is used in the livestock and poultry breeding industry. Research has shown that only 15% of antibiotic drugs can be absorbed and utilized by animals, while 85% is not metabolized and is discharged into the environment through feces. Based on the above data, it is estimated that as much as 82,000 tons of antibiotics are discharged into the environment through livestock and poultry manure each year.
Among 32 pig manure samples from livestock farms in a certain area, the average contents of oxytetracycline, tetracycline, and chlortetracycline were 9.09, 5.22, and 3.57 mg/kg respectively; among 23 chicken manure samples, the average contents of oxytetracycline, tetracycline, and chlortetracycline were 5.97, 2.63, and 1.39 mg/kg respectively. According to research by the Institute of Food Safety Testing of the Chinese Academy of Agricultural Sciences, the residual antibiotic content such as chlortetracycline in some pig manure samples even reached 200 mg/Kg. Surveys of livestock and poultry manure samples and farmland soil fertilized with livestock and poultry manure in the northern part of a certain province also showed that in the surface soil of farmland fertilized with livestock and poultry manure, the detection rates of oxytetracycline, tetracycline, and chlortetracycline were 93%, 88%, and 93% respectively.
According to statistics, the national livestock and poultry breeding industry uses 150,000-180,000 tons of trace element additives each year, of which about 100,000 tons is not utilized by animals and is excreted with feces. Among various trace element additives, copper-based, zinc-based, and arsenic-based preparations are used the most, so the heavy metal pollution caused by their discharge is also the most serious.
A survey of 61 livestock and poultry manure samples from intensive farms in a certain province showed that the average contents of copper, zinc, and arsenic in chicken manure were 107.5, 366.6, and 21.6 mg/kg respectively, and in pig manure were 765.1, 1128.0, and 89.3 mg/kg respectively. According to different fertilizer heavy metal limit standards, heavy metal exceedances in chicken manure and pig manure were mainly copper, zinc, and arsenic, with zinc exceedance being the most widespread.
Risks
Let us look at what risks antibiotic and heavy metal contamination in conventional farm livestock and poultry manure may bring.
Antibiotics are by design meant to inhibit and kill certain bacteria, so after they enter the soil with feces, they inevitably disrupt the balance structure of soil microbial flora and the diversity of functional communities, causing soil microorganisms to show a trend of “directional enrichment,” and having a significant inhibitory effect on the growth and development of microorganisms, animals, and plants in the soil environment.
Antibiotics entering the soil cause some pathogenic bacteria to develop drug-resistant resistance genes, which enter the food chain through pathways such as plants, animals, and water sources.
Heavy metals artificially added to livestock and poultry feed accumulate after entering the soil. Research experiments have shown that in vegetable plots where pig manure has been applied continuously for 10 years, the soil heavy metal content can exceed the national second-level farmland soil standard and become heavily polluted farmland. Ultimately, these excessive heavy metals are either enriched (by plants or herbivorous animals) or migrate through groundwater into the human food chain.
Ultimately, the antibiotics and heavy metals carried in this livestock and poultry manure will all enter our food chain through various pathways, thereby posing potential harm to human health.
Can they decompose naturally in the soil?
Many studies at home and abroad have shown that the decomposition of antibiotics after entering the soil is extremely slow. For example, Danish scientists studied the degradation of chloramphenicol and tylosin in Danish sandy soil and found that the time required for 50% degradation was 25-34 days and 49-67 days respectively. Another study showed that in soil where livestock and poultry manure containing antibiotics was applied, it takes about 44 days for oxytetracycline to degrade by half, about 105 days for tetracycline, and about 160 days for chlortetracycline.
In the environment, some antibiotic metabolites can even revert to the original active drug. It has been reported in the literature that chloramphenicol glucuronide in liquid fertilizer can be converted to chloramphenicol, and N-4-acetylsulfamerazine can be converted to sulfamerazine.
Among 83 commercially sold vegetable samples collected in a certain major city, 96% of the samples were found to contain quinolone antibiotics to varying degrees, with an average of 146.6 micrograms/kg (fresh weight), while the acceptable daily intake (ADI) for children is 60 micrograms. In addition, among the detected antibiotic residues, the levels in pollution-free, green, and organic vegetables were generally higher than in conventional vegetables.
As for the heavy metals discharged with livestock and poultry manure, they remain in the soil for a long time.
Survey results of soil under long-term application of pig manure from large-scale farms and soil without pig manure application showed that the copper, zinc, and arsenic contents in pig manure-applied soil and vegetables (chives, green vegetables, celery, radish) were all significantly higher than in the control group. The highest copper, zinc, and arsenic contents in soil were 11 times, 5 times, and 2 times those of the control group respectively; the highest copper and zinc contents in vegetables were both 4 times those of the control group, and arsenic was 3 times that of the control group. In pig manure-applied soil, the highest copper exceedance reached 133.98%, zinc was 52.05%, and arsenic was 12.04%; in pig manure-applied vegetables, the highest zinc exceedance reached 117%.