CRATE (Center for Rural Agricultural Training and Entrepreneurship) Handbook
Chapter 1: Soil Fertility and Nutrient Management
Basic concept of crop nutrient management
Farmers in Hawaii face the continuous challenge of declining soil organic matter (SOM) and fertility due to rapid decomposition of SOM in warm and wet weather. Crop nutrient management is critical to achieve optimum productivity in a production system. In addition to maximizing productivity, rebuilding long term soil fertility and ensuring the protection of environmental and natural resources should also be considered in soil fertility management. Some basic concept of crop nutrient management practices for farming are summarized in the “Managing Crop Nutrition” presentation prepared by CTAHR Cooperative Extension Agents, Sugano, Uyeda and Fukuda. The basic concept of nutrient management is to follow Liebig’s law of minimum, where it was found that increasing the amount of plentiful nutrients did not increase plant growth, but increasing the amount of the limiting nutrient did (Fig. 1-1).
Nutrient Budget: A proactive farming approach is to provide the target amounts of N, P, K and other nutrient elements needed for the entire crop cycle. The nutrient budget for a crop consists of the amount of nutrients available in the soil, plus the amount added by the farmer before and during the crop, minus any nutrient nutrients tied up by the soil or lost to leaching, volatilization, uptake by weeds etc., minus the amount taken off in the harvested crop. Any “balance” will remain in the soil for the next crop. Applying fertilizers solely based on general recommendations for each crop without prior knowledge of the soil fertility and nutrient mineralization (release pattern) from the organic fertilizers is not a sustainable approach of farming. Submitting soil prior to the beginning of a new crop cycle could help farmers to develop their “Nutrient Budget”. Example of a nutrient analysis lab service is:
Agricultural Diagnostic Service Center
University of Hawaiʻi at Mānoa
1910 East-West Road, Sherman Laboratory 134
Honolulu, Hawaii 96822
PH: (808) 956-6706
FAX: (808) 956-2592
email: adsc@ctahr.hawaii.edu
How do you know what your crop needs?
Agriculture Diagnostic Services (ADSC) provide the interpretation of the soil analytical results and made recommendations for fertilization based on crops specified.
After crop planting, farmers can also examine their plant samples in the field to diagnose for nutrient deficiency base on symptoms (Fig. 1-2). ADSC or other nutrient analytical lab can also analyze plant tissue samples to confirm symptom diagnostic.
Fig. 1-2. Images of nutrient deficiency on crop plants. For more deficiency symptoms, visit https://gms.ctahr.hawaii.edu/gs/handler/getmedia.ashx?moid=3708&dt=3&g=12 .(Sugano 2013).
Bear in mind that nutrient requirement of a plant changes over time (Fig. 1-3).
LOCAL FERTILIZERS
To embrace a sustainable low input farming system using local resources, Ahmed and Radovich investigated a series of local organic waste available in Hawaii (meat and bone meal, dairy manure, chicken manure, invasive seaweeds) as organic fertilizers and summarized their findings in “Use of Organic Fertilizers to Enhance Soil Fertility, Plant Growth, and Yield in a Tropical Environment” (Ahmed et al., 2016). While synthetic commercial fertilizer labels provide a clear formulation of the fertilizer, nutrient content of organic fertilizers from recycled local resources are often sold as raw materials without a label. However, farmers can look up publications made available by CTAHR researchers (Ahmed et al., 2016) to estimate nutrient content of different organic fertilizers from local resources in Hawaii.
Tankage
Tankage, also called meat and bone meal, is the solid byproduct of animal waste rendering is valuable fertilizer, primarily used to supplement N source.The nutrient content of tankage varies with feedstock and storage time, but the product in Hawai’i (Baker Commodities, Kapolei, HI) has been analyzed initially at 10 % N, 3 % P, 1 % K, 6 % Ca, and C:N of 5:1 (Radovich et al., 2015). However, mineralization of the nutrient occurred over time. Therefore, it is recommended that tankage be used relatively fresh and not stored over long period.
To determine how much tankage should be applied for a crop, refer to N, P and K made available based on 75% mineralization rate of tankage in Table 1-1 (Ahmad et al., 2006).
However, due to the relatively fast mineralization rate of tankage, it is recommended that tankage fertilizer be applied through split application to avoid nutrient leaching prior to plant uptake. Split application of tankage did not affect sweet corn yield but reduced NO3-N leaching to below the corn root zone by 20% (Fig. 1-5), thus increase fertilizer use efficiency.
Fig. 1-5. NO3-N leaching to below the corn root zone at various tankage application rates through one time application at pre-plant or split application at pre-plant and at 1 month after corn planting (Silva, 2017).
Locally made Compost
Farmers can prepare compost in their farm by recycling crop residues or other local waste. In general, nitrogen availability of compost is depending on N content (%) as well as C: N ratio of the compost (Fig. 1-6).
Fig. 1-6. Plant available nitrogen (NO3-N) released from compost in a Mollisol in Hawaii is pending on N content and C: N of the compost (Ahmed et al., 2016).
Seaweed Compost
Invasive algae is available in large quantities locally and can be an important source of potassium (K) and other plant nutrients. However nutrient content is depended on species (Table 1-1), and potential salinity and other concerns need to be addressed (Radovich and Hue, 2010).
Vermicompost
Vermicompost is the end product of non-thermophilic decomposition of organic materials by certain species of earthworms and their associated microbes. As earthworms consume organic substances, they excrete tiny pellets called worm castings. Vermicompost is a good soil conditioner. Though its nutrient content might not be high, it has been shown to increase plant growth and yield as well as suppressing key pests and pathogens of crops. Vermicompost is generally of finer structure, contains more plant available nutrients, has higher microbial activity, and possesses better plant growth promoting properties.
The most commonly found bacteria in matured vermicompost are Pseudomonas, Bacillus and Microbacterium species. Functions of these bacteria include: 1) producing plant growth promoting agents e.g. auxins, cytokinins and gibberellic acid; 2) producing enzymes that dissolve organic nutrients e.g. phosphatase dissolves organic phosphorus in soil into a plant available form, 3) enhancing enzymes activities that inhibit growth of pathogenic fungi or insect pests e.g. chininase, protease and cellulase that suppress fungal and insect pests, aminocyclopropane carboxylate (ACC) deaminase that has broad spectrum anti-fungal activities, hydrogen cyanide (HCN) which is antagonistic to many root infecting fungi, 4) producing siderophores which have high affinity to iron (Fe) in the soil making siderophore-forming bacteria outcompete other microbes (Pant and Wang, 2014).
Vermicompost tea (VCT) is prepared by soaking freshly harvested or cured vermicompost in water with or without continuous aeration to extract its soluble nutrients, as well as beneficial microbes into water. Generally, 1: 10 ratio of vermicompost to water is used. Higher levels of soluble nutrients and plant growth regulators obtained from applying VCT compared to solid vermicompost. Using VCT allows growers to apply a small amount of material over a larger area through the liquid extract.
For more information about vermicompost, visit:
Drenching plants with VCT was found to suppress pests and pathogens:
High Nitrogen Liquid Fertilizer for Fertigation
Fertigation allows for the application of both water and nutrients simultaneously and uniformly. Benefits of making local organic liquid fertilizer includes: 1) provide plant available nutrients for immediate uptake, 2) suitable for long-term crops where periodic fertilizing after planting is necessary, 3) compatible with weed mat mulching where adding fertilizer at post plant is challenging, and 4) commercial organic liquid fertilizers are expensive.
Ahmad developed a liquid organic fertilizer using local resources available in Hawaii (https://vimeo.com/245473495) using Tankage, vermicompost, in aerated system for 24-48 hours that can maximize the extraction of NO3-N from tankage into water.
Liquid tankage + vermicompost fertigation
- 1.5 lbs of tankage in 10 gal of water
- 1 oz of vermicompost
- Aerated for 12-24 hrs
- Strain prior to injecting into fertigation
Results showed comparable lettuce, pak choi and daikon yield between plants fertilized with same quantity of nitrogen from liquid organic fertilizer (tankage + vermicompost fertigation) and synthetic liquid fertilizer. However, tomato and papaya seedlings were more vigorous in the organic liquid fertilizer than the synthethic fertilizer treatment.
Biochar to correct soil acidity
Soil acidity is a serious constraint for crop production in many crop production areas in Hawaii. Liming is the conventional remedy, yet lime is costly and may not be available in some area. Biochar, a by-product of bio-fuels production, can be a replacement or partial replacement of the use of lime in alleviating soil acidity.
Biochar is a stable form of charcoal derived from burning organic materials such as woody debris, corn stalks, macadamia shell, etc. in a high temperature (400-700
°C) but low oxygen process. Due to its molecular structure, biochar is chemically and biologically more stable than its original carbon form, making it possible to stored the carbon for a long time (carbon sequestration). The surface of biochar contains many chemically reactive groups, such as carboxylate group (COOH-), OH-, ketone, that give biochar a great potential to adsorb toxic substances, such as aluminium (Al), manganese (Mn) in acid soils, and arsenic (As), cadmium (Cd) in heavy metal contaminated soils. Thus, biochar can be used to remediate soil that may be hostile to plant growth (acid soils) or harmful to human health (heavy metal contaminated soils).
For example, Al toxicity on desmodium in an Ultisol was eliminated when soil was amended with 3 tons/acre of lime and 5% rate of biochar. For more information on the use of biochar to correct soil acidity and reduce, please visit: https://www.ctahr.hawaii.edu/huen/nvh/biochar.pdf.