Tankage macro‐ and micro‐nutrient content (μg/g), C/N ratio, standard deviation, and CV of samples collected over a 2‐year period.
Soils rarely have sufficient nutrient for crops to reach their potential yield. Applying organic fertilizers without prior knowledge of their properties may cause yield decline under low application or pollute the environment with excessive application. Understanding the nutrient variability and release pattern of organic fertilizers is crucial to supply plants with sufficient nutrients to achieve optimum productivity, while also rebuilding soil fertility and ensuring protection of environmental and natural resources. This chapter presents the authors’ experiences with different organic amendments under Hawaii's tropical conditions, rather than an intensive literature review. For meat and bone meal by‐products (tankage), batch‐to‐batch variability, nutrient content/release pattern and quality, and plant growth response to the liquid fertilizer produced from tankage were evaluated. For animal livestock, dairy manure (DM) and chicken manure (CM) quality, changes in soil properties, and crop biomass production and root distributions were evaluated. For seaweed, an established bio‐security protocol, nutrient, especially potassium (K) variability, and plant growth and yield response were evaluated in different tropical soils.
- organic fertilizers
- tropical soils
- nutrient variability
- plant growth
Sustainable and organic agriculture practices apply management ideals that include a diverse assembly of farming methods, usually with a reduced reliance on purchased inputs , this is especially for new farmers with limited resources . As commercial fertilizer/shipping costs increase, a wide range of food producers in the Hawaii and the Pacific region have realized the need for locally available fertilizers from organic sources to improve soil fertility, crop health, and productivity. In addition to concerns surrounding availability of affordable soil amendments, interest in sustainability and organically produced crops has risen among American consumers in the past few decades. Increased tourism has further amplified the need for fresh local fruits and vegetables, especially “locally grown” labeled goods. Shifting from conventional farming to organic farming has many benefits to the human's well‐being, protecting the environment (soil, water, and air), rebuilding soil fertility through improving its physical, chemical, and biological characteristics, and improving the quality of produced crops . However, producing crops organically may come with higher production costs (i.e., lower yield and higher labor costs). Recycling, composting, and using local inputs may decrease the production cost . In general, soils rarely have sufficient nutrients available for crops to reach their potential yield. Therefore, farmers tend to apply soil amendments (synthetic or organic amendments) that are rich in nutrient, i.e., N, P, and K to enhance soil fertility and increase crop productivity . However, most growers apply fertilizers based on the general recommendations for each crop , without prior knowledge of the soil fertility status and nutrient mineralization and release pattern from the fertilizers . In addition, Hawaii farmers face the continuous challenge of declining soil organic matter (SOM) and fertility  due to the optimum environmental condition (e.g., temperature and rainfall) for SOM decomposition . These losses are more critical with the use of organic amendments, where nutrients have to be converted from organic to inorganic forms in order to be available for plant uptake . Also, rebuilding/restoring soil fertility and improving the physical, chemical, and biological function of soils are critical to support optimal plant growth, yield, and quality . Sustainable health of the soil relies on carbon‐rich amendments that will feed the biological processes that are the core foundation of a healthy soil . Short‐term needs must also be met with fertilizers that rapidly become available to plants, so that nutrients are available in synchrony with plant needs . In Hawaii, there are many locally available resources to meet both long‐ and short‐term crop nutrient and soil function needs when used properly . Improving farmers’ knowledge and their capacity to determine the quality of different fertilizers and soil and crop's needs are essential elements in organic agriculture . This chapter focuses on the authors’ experiences with certain organic fertilizers that are available in Hawaii rather than being an extensive review of them.
2. Meat and bone meal by‐products (tankage)
Tankage is the solid by‐product of animal waste rendering (Figure 1). The nutrient content of tankage varies with feedstock and storage time, but the product available in Hawaii has been fairly consistent on average 9.5, 2.5, and 0.75% of N, P, and K, respectively, With a Carbon/Nitrogen (C/N) ratio of 5:1 [16, 17]. The Hawaii material is derived from fish scraps (∼50%), waste meat, carcasses, and other mixed materials (∼45%) and offal (∼5%). The current and only running plant in Hawaii is producing about 25 tons/month. Often called meat and bone meal, tankage is a valuable agricultural input used as fertilizer in Hawaii for at least 20 years [18, 19]. The material is National Organic Program (NOP) compliant and listed as an approved generic material by OMRI. The primary agricultural use of tankage is as a supplemental N source , especially for, but not limited to, certified organic growers. Nitrogen (N) mineralization rates for tankage have always been assumed to be high given its low C/N ratio and high N content, but actual mineralization rates in Hawaii soils have not been readily available. Other gaps in our knowledge of this material include batch‐to‐batch variability in the material and N loss during storage.
2.1. Nutrient content and nitrogen release pattern
2.1.1. Nutrient content variability among tankage batches
Batch‐to‐batch evaluation was carried out for 2 years by collecting tankage samples (every 3 months) from Island Commodities Co., on Oahu. Initial subsamples were submitted to the University of Hawaii's Agricultural Diagnostic Services Center (ADSC) for total nutrient content analysis. The results showed that tankage can provide fairly good amount of the macro‐ and micro‐nutrient, except potassium (K), which was fairly low. N content in tankage initial samples varied between 8.7% and 12.1% with an average of 9.8%, and C/N ratio varied between 3.5 and 5.3:1 with an average of 4.7:1 (Table 1). Periodical analysis of N content in the stored initial tankage samples under lab condition showed a significant continuous decline in N content from 10% to 30% of the initial N (date not presented). N volatilization in the form of ammonia (NH3) is a major source of N loss to the atmosphere. Soil acidification is caused by an increase in H+ resulting from the deposition of the NH3 into the soil. Under field/farm condition (higher temperature, humidity, rainfall, etc.), N loss is expected to be higher and faster because climate is the major factor leading to increased N loss .
2.1.2. Nitrogen release pattern
To determine the N release pattern from tankage, a leachate column incubation experiment was conducted using tankage applied at four application rates (0, 100, 200, and 400 kg N/ha) with two soils [Wahiawa series (Oxisol) and Waialua series (Mollisol)] with three replicates for each application rate. A total of 24 PVC leachate columns (30 cm long and 10 cm diameter) were used. The columns were set up from top to bottom with 10 cm soil and tankage mixed layer, 15 cm soil layer, 2 cm gravel layer, and plastic fine mesh to prevent soil passing through. Incubation started with adding half‐pore volume of deionized water for each column. At each collection time (weekly), half‐pore volume of deionized water was added, and leachate subsamples were collected with glass beaker up to 3 months. Leachate subsamples were analyzed for nitrate (NO3‐N) and ammonium (NH4‐N) using a Vernier meter and electrodes. Results showed that NO3‐N concentration in the leachate solutions followed the application rate (Figure 2A and 2B), and NH4‐N concentration in the leachate samples was very negligible (0.1–2.7 ppm). The mineralization rate in a 3‐month period was between 50% and 75%. Under field conditions, actual mineralization is expected to be at or above the higher end of this range. The N release pattern under the two soils was the same. However, the NO3‐N values were higher under the Oxisol soil (Wahiawa series), which might be related to the fertility level and structural differences between the two soil types .
2.2. Liquid fertilizer from tankage for fertigation purposes
Fertigation (fertilizer + irrigation) is a practice when both water and nutrient are supplied together through drip irrigation . The practice is very beneficial for long‐term crops, to meet the demand of crops for nutrient, integrated with the use of mulching, and to reduce nutrient losses . Through a Western Sustainable Agriculture Research and Education grant, producing liquid fertilizer with high‐N content from tankage was carried out at the University of Hawaii at Manoa (Figure 3).
2.2.1. Factors studied and final recipe
|Incubation time||0, 4, 8, 24, and 48 hours|
|Incubation temperature||24°C and 35°C (75°F and 95°F)|
|Cover/lid||Covered and open/uncovered|
|Inoculants/accelerators||Baking soda, soil, sugar, and vermicompost|
On the basis of the results from the previous study (Figure 4), different factors in various combinations were evaluated for the N release from tankage. Based on these results, a suggested recipe was developed for greenhouse and on‐farm trials:
Add 1 kg (2.2 lbs) of tankage into 60 l (∼15 gallon) water.
Add about 40 g (1.5 oz) of vermicompost.
Air (brew) for 12–24 hours.
Strain and apply with drip irrigation (fertigation).
We found that the use of fresh tankage and vermicompost resulted in a higher N concentration in the liquid fertilizer. In addition, the use of thick cotton un‐dyed bag to mix the tankage and vermicompost prior to brewing helped significantly reduce the need to strain the liquid fertilizer before fertigation.
2.2.2. On‐farm and field trials
The above recipe was provided to a local farmer in Hawaii. The farmer used the recipe to grow watermelon on a 1‐acre field with a Oxisol soil (Molokai series). The liquid fertilizer recipe was applied weekly till 2 weeks prior to harvest. The experiment was not fully replicated, but the results were consistent throughout the field. Randomly selected watermelon subsamples were taken, and the average weight and total soluble solid (TSS) contents were taken (Table 2). The TSS values were within the excellent range (10.2–13.0) for watermelon . Also, the average weight and watermelon flesh color were representative of the overall crop quality. The yield and high TSS value suggested that the liquid fertilizer provided good amount of nutrient to the watermelon to grow well and accumulate the high‐sugar content. As the on‐farm field trial was not fully replicated, we conducted a field trial on Oahu Island at Poamoho Research Station on an Oxisol (Wahiawa series) soil. The objectives of the trial were to evaluate the effect of two liquid fertilizers (organic and synthetic) on the yield of different vegetable crops. The experiment was conducted on a 21 × 18 m area for three consecutive harvests of lettuce (
3. Livestock manure
Using livestock on small‐scale farms is beneficial for supplying small family needs for milk, eggs, meat, and other goods/products. Also, it can be a good source of organic fertilizer . For example, on average, a 1000‐pound cow may produce about 15 tons of manure annually. This 15‐ton may contain about: 200 lbs of N, 190 lbs of phosphorus (P2O5), and 250 lbs of potassium (K2O). Also, dairy manure (DM) contains the essential micro‐nutrients [calcium (Ca), magnesium, sulfur, manganese, copper, zinc, chlorine, boron, iron, and molybdenum] . Another example is chicken manure (CM), which contains all of the essential nutrients needed for healthy plant growth . These include N, phosphorous, K, Ca, magnesium, sulfur, manganese, copper, zinc, chlorine, boron, iron, and molybdenum. Nutrient content and percentages vary based on the feed, supplement, medications, and water consumed by the animals. CM is known to provide a good portion, if not all of the nutrients required by plants . Livestock manure is commonly applied in irrigated agriculture to improve soil fertility and crop yields [30, 31] and to improve the soil biology . Soil physical properties, for example, bulk density and total soil porosity may change with agricultural management practices . Manure amendments increase SOM, which may decrease soil bulk density and increase porosity of the amended soil . Animal manures need to be well composted before application to benefit both soil and plants . However, NO3‐N leaching can be a problem in organic and conventional farming. Under aerobic soil condition and with heavy application of manure, organically bound N is rapidly converted biologically into NO3‐N and that is highly leachable in soils or runoff and can lead to environmental and health issues. In a field study, we used CM applied to a Mollisol soil (Waialua series) under sweet corn crop, at a high‐application rate (30 ton/ha); the NO3‐N concentration in soil water leached below the root zone of corn was very high and could lead to potential groundwater contamination (Figure 7). Moderate applications (7.5 and 15 ton/ha) and timing of manure application to meet plant needs may reduce the environmental pollution risks .
3.1. Livestock manure effects on soil physical properties and root distribution
3.1.1. Soil physical properties
Most commonly available animal manures in Hawaii are CM and DM. Macro‐ and micro‐nutrient content and C/N ratio for the CM and DM used for field trials on Oahu, Hawaii, are presented in Table 3. Under the Wahiawa series soil using CM and DM applied at four application rates (0, 165, 335, and 670 kg N/ha), soil bulk density (
|Manure type||%||μg/g||C/N ratio|
Changes in soil bulk density and total porosity under chicken manure (CM) and dairy manure (DM) applied at 0 (Con), 165 (L), 335 (M), and 670 (H) kg N/ha. Means followed with different letters are significantly different at 5% probability based on Duncan's multiple test.
3.1.2. Sweet corn root distribution
In the same field study above, sweet corn (
3.2. Livestock manure effects on sweet corn biomass
In a field trial for two consecutive growing seasons, root and shoot biomass of sweet corn were evaluated under the application of CM and DM applied at 0 (Con), 165 (L), 335 (M), and 670 (H) kg N/ha. The analysis of variance showed a highly significant (
4. Algae species
Hawaii imports about 85% of the food consumed in the state, leaving it extremely vulnerable in terms of food safety and global events . High level of goods imported and distributed throughout the state also poses a threat of introduced invasive plants (Figure 11) and animals [41, 42]. Marine non‐native invasive seaweed has proven to be very costly to control in addition to developing a threat to the marine native ecosystem [43, 44]. The non‐native seaweed species that have settled along the reefs of Hawaii grow and propagate more readily than the native seaweeds in Hawaii . This is most likely because these seaweeds have less natural predators and herbivorous grazers since they are non‐native to the area. Below is a description of the most common seaweed species found in Hawaii.
The species that are currently targeted by cleanup efforts on Oahu Island are
4.1. Nutrient variability and bio‐security protocol for algae
4.1.1. Nutrient variability among algae species
Different batches of the four main species (
4.1.2. Viability and bio‐security protocol
Viability and the spread of alien algae species into new shores and beaches across the Hawaiian Islands is a major concern and limitations to the use of these species as a major organic source of K fertilizer in agriculture, especially for direct application (without composting). A lab experiment was conducted to evaluate the effect of time and temperature on four seaweed species (
4.2. Direct application as organic source of potassium
Two field trials were conducted to evaluate the effect of different application rates of K on sweet potato growth and yield. K was applied at four application rates (0, 55, 110, and 220 kg K/ha) under two soil series (Wahiawa and Waialua). The experiment was under RCBD with three replicates. At harvest, the tuber fresh weight was recorded. Harvested tubers were cut down to pieces and dried at 75°C for 72 hours and then dry weight was recorded. The analysis of variance showed a highly (
The authors appreciate the support of the Sustainable and Organic Farming Laboratory, Poamoho and Waimanalo Research Stations, and University's Station Managers and Technicians. This work was funded in part by the following grants: Western Sustainable Agriculture Research and Education (SW11‐055 and SW14‐026), Hawaii Department of Agriculture (HDOA), HATCH (8021H), and the USAID (RAN‐A‐00‐03‐00094‐00).