3.1. Characteristics of “Organic Fertilizers”
, Figure 2
and Figure 3
show the N, P, and K contents of the collected samples. Table 2
presents the summaries of pH (H2
O), the C: N ratio, and the concentrations of other nutrients.
We found that N and other nutrient contents greatly varied among the domestic products, whereas these were quite similar among the imported products. In the domestic products, the total N, P, and K contents were in the ranges of 4.9–48.5 kg−1, 0.0–12.7 kg−1, and 5.8–26.0 g kg−1, respectively, whereas in the imported products, these were in the ranges of 24.6–40.2 kg−1, 9.7–12.5 kg−1, and 14.2–29.0 g kg−1, respectively. The ratio of inorganic N to total N in most domestic products was high. Two-thirds of domestic products contained approximately 30% of the total N in the inorganic form, and the imported products contained approximately 10%. In contrast, the ratio of Truog P to total P greatly varied among domestic products.
Raw materials, which are the foundation for the quality of organic fertilizers, are varied. They are by-products of vegetable, animal, and human origin that have been popularly used worldwide for over a thousand years. They are organic materials from municipal solid waste, sewage sludge, and waste of agro-industrial origin whose use recently markedly increased in modern agriculture as organic waste-based fertilizers [25
]. These wastes are becoming important recyclable organic materials in developing countries. Composting the wastes has recently begun in Vietnam; however, governance instruments and policies on this recycling activity have not been established. There is no standard for raw materials of organic fertilizers in regulations regarding fertilizer production, distribution, and use [26
]. Varied raw materials and poorly controlled manufacturing could cause a wider range of nutrient content of domestic “organic fertilizers” compared with that of the imported ones.
Since there was no information regarding raw materials on the product packaging of our collected “organic fertilizers”, we guessed their feedstock based on their N content and appearance. The N content of organic fertilizers depends on the raw materials. The percentage of N recorded in poultry manure, dairy manure, municipal solid waste, crop residue, and sewage sludge are in the range of 2.0–4.0, 1.0–2.0, 1.0–1.5, 1.5–2.5, and 3.7–5.0, respectively [16
]. Two-thirds of domestic “organic fertilizers” contained less than 2% N (Figure 1
) and various pieces of litter, branches, nylon, and stones were observed in the fertilizers (Table 3
). To date, the waste has not yet been separated at the source in Vietnam. It appeared that most of the domestic products might have been produced from municipal solid waste.
It must be emphasized that the percentage of inorganic N within the total N in most collected domestic “organic fertilizers” was noticeably high. Many studies show that inorganic N comprised less than 10% of compost N [27
]. The ratio of inorganic N to total N in our collected samples of imported products was approximately 10%. Meanwhile, the ratio for two-thirds of the collected domestic products was over 30%. For example, V6 sold at Hanoi as named Que Lam 01 contained 7.3 kg kg−1
N, but approximately 50% of it was the inorganic form. V1 sold at Thua Thien Hue province and named Song Huong contained 38.6 kg kg−1
N, but inorganic N also accounted for approximately 40% of the total N.
shows the relationship between the total N and P of the collected samples. We categorized them into two groups: the first included four imported and five domestic products (V3, V4, V7, V8, and V9) containing both N and P, and the second included the remaining seven domestic products containing N, but less P. Interestingly, the price of the former group was higher than that of the latter group (Table 3
). It implies that the adjustment of N and P plays an important role in the price of the fertilizers. Thanh and Matsui [14
] reported that the addition of N, P, and K to matured compost is typically the final step in the production process for organic solid waste compost in Vietnam. This supportably explains the common increase in the ratio of inorganic N to total N of domestic “organic fertilizers” in this study. Since the product packaging of the collected samples lacked information regarding raw materials, we could not precisely compare the nutrient content of commercial products with those of their supposed raw materials. To determine the reason for the remarkable proportion of inorganic N in domestic products, it was necessary to investigate the manufacturing processes and changes in nutrient composition during each process of a so-called organic fertilizer made from coffee by-products.
3.2. Production Method of an “Organic Fertilizer” from Coffee By-Products
illustrates the flow of raw materials and manufacturing processes for an “organic fertilizer” made from coffee by-products. After harvesting, coffee cherries were processed by one of two methods: dry or wet. In the wet method, the outer covering of the coffee bean was removed when the cherries were still fresh. This is a popular technique in this area, which generates a large volume of by-products (coffee pulp). For example, a medium-scale processing factory with a working capacity of 150 tons per day generates approximately 100 m3
of coffee pulp. Companies arrange brokers to collect the waste, and the fee is based on the disposal volume (currently 1.3 USD per m3
). The brokers then deposit it on private land or sell it to fertilizer companies (currently at a price of 3.3 USD per m3
The composting companies use aerobic composting over several months, after which extra soil is added to increase the volume and density. Finally, they add chemical substances such as urea and phosphate to enhance the fertilizer effect before packing the product for sale in the markets as “organic fertilizer” at a price of 11 USD per 100 kg (current price). Our investigation results are in accordance with the findings of Thanh and Matsui, as reported above. However, the authors did not provide evidence of changes in the nutrient levels during the manufacturing processes. Our study clarifies this limitation.
shows changes in the nutrient levels during the manufacturing processes of an “organic fertilizer” made from coffee by-products. It indicates that coffee by-products are rich organic material, with nitrogen and potassium. The total C content was high, being up to 423.2 g kg−1
, and the N and K contents were 32.80 g kg−1
and 9.71 g kg−1
, respectively. However, the P content was very low. After composting, the carbon content slightly decreased, but the concentration of total N and K increased. The compost contained were 34.8 gN kg−1
and 12.54 gK kg−1
, respectively. After bulking out the compost with extra soil, the total C, N, and K contents were reduced to 83.20 g kg−1
, 6.40 g kg−1
, and 4.48 g kg−1
, respectively. The concentration of exchangeable K was reduced from 25.68 cmol kg−1
to 4.13 cmol kg−1
. After packing, the total N content nearly doubled from 6.40 g kg−1
to 11.20 g kg−1
concentration increased 34-fold, whereas NO3−
concentration remained unchanged. The total P content tripled from 0.99 g kg−1
to 2.99 g kg−1
, and the Truog P content increased 13-fold from 0.04 g kg−1
to 0.54 g kg−1
3.3. Effects of “Organic Fertilizers” on Plant Growth and N Leaching
The dry weight and N uptake of plants were significantly influenced by the fertilizer type and nutritional supplementation. The combined interaction of these factors had no significant effect on the dry weight and N uptake (Table 5
and Table 6
, respectively). The P uptake was significantly influenced only by the fertilizer type (Table 7
The dry weight and nutrient uptake effects of the fertilizer type and/or nutritional supplementation are presented in Table 8
. Generally, the order of treatments for dry weight and nutrient uptake was as follows: domestic fertilizers ≥ chemical fertilizer ≥ imported fertilizers > control. Conversely, the effect of the domestic V4 treatment was not significantly greater than that of the corresponding control. With a single application (N-fertilizer), there was no significant difference in the dry weight among the domestic V6 and VC treatments and chemical fertilizer. In treatments with additional P and K (N-fertilizer + P, K), the dry weight was significantly greater for half of the domestic treatments (V2, V5, V6, and VC) than that of the corresponding chemical fertilizer. There was no significant difference in dry weight among treatments using the remaining domestic products, imported products, and chemical fertilizers.
The measurement of N uptake by plants and N stored in soil enabled us to estimate N leaching. A single application of chemical fertilizers and most domestic “organic fertilizers” resulted in significantly higher N leaching from soil than that by the application of imported products. The positive correlation between N leaching, and the ratio of inorganic N to total N in the applied fertilizers is illustrated in Figure 6
(r = 0.77, p
It has been reported that the majority of N in manure or compost is in the organic form that must first become mineralized before plants can uptake it, or it becomes susceptible to loss by leaching. Only a small fraction (3.5%) of their total N was mineralized within the growing season, resulting in the lowly met N requirement of crops. Compost is often reported to be less effective in supplying available N to plant during the first year of application compared to inorganic mineral fertilizer [28
]. Organic fertilizers have been commonly applied to the soil to increase soil fertility and minimize N leaching. The application did not increase the loss of N through leaching compared with controls, and the compost provided advantages over mineral fertilizers from a water quality perspective [16
However, the so-called organic fertilizers collected in our study showed the opposite effect. Our study ranked dry weight and nutrient uptake as follows: domestic “organic fertilizers” ≥ chemical fertilizers ≥ imported organic fertilizers > control. In addition, a single application of either chemical fertilizers or most domestic “organic fertilizers” resulted in significantly greater N leaching from the soil than that by the application of imported products. This indicates clearly that in poor-quality sandy soils, the application of chemical fertilizers or “fake” organic fertilizers should be considered a significant threat to groundwater (from excessive N leaching). The high leaching rate can be attributed to the high proportion of inorganic N to total N in the applied fertilizers. Figure 7
illustrates the relationship between dry weight and N leaching under a single application of the fertilizers. The application of chemical fertilizer and domestic “organic fertilizers” V2, V6, and VC resulted in an increase in both dry weight and N leaching, which was probably because of the high ratio of inorganic N to total N in these fertilizers. The application of imported fertilizers (I1 and I2) resulted in a lower dry weight of plants, but reduced N leaching. The poor crop response to the fertilizer, V4, and low level of N leaching from the soil in this treatment indicate N immobilization.
Finally, the effect of domestic “organic fertilizers” on crop yield was not in accordance with their price, which might be decided by the adjustment of the N and P content of the fertilizers. V6, V2, and VC were categorized as lower priced, and had lower concentrations of total N and total P, but their application was effective on plant growth. Meanwhile, V4 was the most expensive domestic “organic fertilizer”, with higher concentrations of total N and total P, but was not effective on plant growth.