[Editor’s note: During the establishment of our own Small Farm Resource Center here at the ECHO Asia Impact Center in Thailand, we have benefited greatly from the surrounding rice production ‘waste products’ (rice hulls and straw) that are readily available to us. Adding credence to our own favorite motto of ‘use what you have to make what you need,’ this locally available ‘waste’ product has been a tremendous resource, leading us to summarize for our network some of the many benefits and potential uses of this economical resource.]
One of the great challenges of sustainable agriculture is the sourcing of adequate and affordable
organic (carbon based) resources that can be used on-farm for the production of food and feed. Utilizing composts, manures, mulches, and other organic inputs from the farm is a challenge on its own, and the production of each often requires its own input of materials. These are materials that are often in direct competition of each other on the farm and a challenge to supply completely with smaller land holdings or available labor. A mulch for example, may be in direct competition with livestock fodder, thus making it a challenge to feed it out while still producing enough mulch. Using some of that same material to produce a compost or a fuel becomes even more challenging still.
After visiting many diverse small-scale farms in the region, it would seem that the most successful ones appear to share one common approach, and that is the identification of an available organic ‘waste’ product acquired off-farm and used as an input on-farm. One farm in the Philippines acquires fish scraps and waste from the local fish market and turns it into a fish-meal protein source to supplement livestock feed. Another farm in Thailand acquires large quantities of vegetable waste from local markets to supply its vermicompost system. Other farms are benefitting from their proximities to breweries, mills, and processing plants to acquire a wide variety of resources that are being used to create compost, biochar, and feed supplements among other things.
This article will summarize the many potential on-farm uses of one widely available ‘waste’ by-product that is readily available in many parts of the tropics: rice hulls.
Rice Hulls at a Glance
Rice hulls (or husks) are a by-product produced during the milling of rice (Oryza sativa) and are commonly considered a ‘waste product’ in many rice growing regions. Rice hulls are the tough outer layer of the rice grain, biologically referred to as the ‘lemma’, and hold low nutritive value compared to the grain (endosperm) itself. Rice hulls are not to be confused with rice bran; hulls detach from the grain in the first step of milling with the bran being milled separately to produce ‘polished or white rice’ (Figure 2). Rice bran has considerably higher nutritive value than rice hulls overall and are considered a more valuable by-product than that of the hulls, which are often discarded or piled to rot.
10 Potential On-Farm Uses of Rice Hulls
1. Compost Production and Soil Improvement
Perhaps the most obvious and common use of rice hulls is their use as a soil amendment, typically in the form of compost production. The primary advantage of using rice hulls is their availability in large quantities, and their ability to bulk up and increase the volume of compost that can be produced. Rice hulls provide a dense source of carbon which, when composted, can increase the water and nutrient holding capacity of soil, improve soil aggregation, porosity, infiltration, and many other key beneficial soil physical characteristics.
Keep in mind that rice hulls are high in carbon and should be mixed with additional nitrogen-rich materials such as green matter and manure in order to make a well-balanced soil amendment. It is worth noting that the high percentage of carbon provided by rice hulls makes for a good soil conditioner but does not in and of itself provide high amounts of plant essential nutrients needed for uptake. That being said, rice hulls must be mixed with other nutrient rich materials to make high quality compost to be used as a fertilizer. Rice hulls can also be mixed with cow or poultry
manure simply to aid the ease of applications (Figure 3).
*Disclaimer: Applying rice hulls directly (without composting) to the soil repeatedly over time can bind up soil nitrogen. Soils must maintain a healthy balance of carbon and nitrogen, known as the C:N ratio. Rice hulls have a very high C:N ratio; over time as microbes work to break down this carbon, they will consume available nitrogen, potentially limiting its use by plants.
2. Biochar Production
Turning rice hulls into a useful soil amendment can be taken one step further with the production of ‘biochar’. Through the process of ‘pyrolysis’ (burning in a low oxygen environment), any carbon-based material can be transformed into charcoal. Charcoal is a highly porous and stabilized form of carbon that can be used to improve soil fertility when combined with other nutrient sources. This additional step of mixing, or ‘ageing,’ charcoal with nutrient sources such as manures and compost is what makes ‘biochar’ unique (see Dr. Michael Shafer’s article in ECHO Asia Note #35) . On its own, a carbon based material serves little purpose to a plant in terms of providing it with carbon (remember plants take their carbon out of the atmosphere during photosynthesis, not from roots in the soil). However, these highly porous materials act as a sponge and when ‘aged’ are capable of holding and housing numerous other plant essential nutrients, water, and microbes, making it extremely valuable as a soil amendment/conditioner (see Bryan Hugill’s article in ECHO Asia #9).
Different forms of activated charcoal and biochar can also be used in the filtration and purification of water. Several resources exist on the construction of different bio-sand water filters used for household consumption, including systems that are capable of removing synthetic chemicals such as pesticides and pharmaceutical residues (Kearns, 2014).
There are numerous methods and appropriate technologies currently available for producing biochar on the farm, including several designed specifically for the use of rice hulls.
3. Fuel and Briquette Production
Using simple appropriate technologies, rice hulls can be mixed with other carbon-based materials found on the farm and pressed into ‘briquettes’ that can be used as an alternative fuel source to firewood and charcoal (Figure 4). This simple process is highlighted in ECHO Technical Note #85 entitled Briquette Presses for Alternate Fuel Use. Rice hulls can also be used directly as a fuel source and do not necessarily need to be pressed into briquettes to be of use, this process simply adds value and can be used to produce briquettes for income generation. Several gasifier cook-stove models currently exist that are designed to run on rice hulls as a fuel source. Many of these models are designed to burn cleanly and efficiently, and even produce pyrolized rice hulls as a waste product that can be turned into biochar.
4. Supplemental Construction Material
Rice hulls can be used as a supplemental construction material in natural or sustainable building
techniques such as earth-bag (Figure 5) or rammed-earth technology (Tosi, 2017). Rice hulls are useful in that they provide a natural material used for adding volume and bulk, they do not easily break down over time, and typically do not harbor molds or fungus due to their inability to absorb moisture. In addition, rice hulls are good insulators and are an extremely lightweight material.
5. Bedding for Hog/Chicken Deep Litter Systems
One of the more common uses of rice hulls in Asia is in animal husbandry, most commonly as a source of livestock bedding. Deep litter pig systems and deep litter chicken systems are common, and bring several benefits over
conventional systems (see echocommunity.org’s collection of resources). As a bedding material, rice hulls provide excellent drainage and can maintain a mostly dry environment compared to other materials while helping to improve the comfort and living conditions for livestock (Figure 6). Rice hulls can help to absorb smells and foul odors, create limited dust, and can eventually be removed and used as a soil amendment. Regular application of microbial sprays such as EM (Effective Microorganisms) and IMO (Indigenous Microorganisms) will aid in the breakdown of pathogens and nutrients in these systems.
*Disclaimer: Deep litter bedding materials to be used as soil amendments should be given additional time to compost once removed from the pen, as they can be considered ‘hot’ (high salt content) and potentially harmful to seedlings. It should also be noted that these materials can have high pH values due to high sodium content from manure and urine, good for acidic soils but detrimental if pH is already high (>7).
6. Livestock Feed Supplement
In terms of a livestock feed material, rice hulls are considered a low value roughage. Protein content of rice hulls are generally low (~3.8% dry matter), while crude fiber is high (~40%), and are thus used as a ‘filler’ and sometimes for stimulating appetite. Though rice hulls are generally considered a low-value feed material, it is important to note that they can be included in small amounts (up to 15%) in ruminant and hog feed concentrates (AFRIS, 2002). Research has shown that grinding or steaming of rice hulls leads to higher digestible energy intake while ammonia or urea treatment of rice hulls can also aid in the digestibility and nutritive quality of rice hulls (FAO). These are common practices and can make an economic concentration to feeds considering the low cost of rice hulls. It is important to note the considerable differences between rice hulls and rice bran, the latter having higher overall nutritive value.
7. Potting Mix Production
Rice hulls can make an excellent base material for those that seek to produce their own potting mixes on the farm. Rice hulls, when mixed with other materials (compost, manure, soil, etc…) provide volume, drainage, and porosity, characteristics essential for growing sensitive seedlings of vegetables and fruit trees (Figure 8). Rice hulls can provide a cheap and available alternative to perlite, which can be expensive or hard to come by in some regions. Once again, rice hulls are limited in their ability to supply nutrients to the plants, and used in this case as a soil ‘medium’ rather than a source of fertility.
To increase the effectiveness of rice hulls in potting mixes, it is recommended that they be sterilized to prevent the spread of fungal and bacterial pathogens in the nursery setting. Smoked or parched rice hulls, and pyrolized (biochar) rice hulls will bring added benefits when used to make potting mixes (AVDRC, 2000).
8. Mushroom Production Substrate
Mycelium can be propagated by inoculating any number of different substrates, and what is typically used often depends on what is locally available or affordable. Hardwood sawdust and straw are two of the most common substrates used for growing mushrooms worldwide, but a variety of other options exist including, coconut coir, manure, old coffee grounds, and yes… rice hulls. Oyster and Milky mushrooms have been shown to grow on rice hull only substrate, as well as mixed rice hull substrates.
Like most substrates used for mushroom production, rice hulls will need to pasteurized or sterilized before inoculation to avoid competition of other mold and bacterial contaminants. This typically done using a large drum and boiling water, but can also be achieved through solar pasteurization using this innovative solar styrofoam box method (see Dr. Tapani’s article in ECHO Asia Note #33). While certain mushroom varieties may be produced using rice hull only substrate, it is recommended that rice hulls be mixed with other materials, such as sawdust, in order to improve the moisture holding content. It has been pointed out that mushrooms (oyster) will produce well on the first flush using rice hull only substrate, but will not hold moisture long enough for subsequent flushes to do well.
9. Growing Medium in Hydroponics/Aquaponics Systems
Growing plants in a ‘soilless’ system such as in a hydroponics or aquaponics system requires the use of a ‘growing medium’. Different materials can be used depending on the system, and are necessary to support the root system and hold the growing plants upright. Common materials used include coconut coir/fiber, grow rocks, perlite, sand, styrofoam, pebbles, and other various inert materials. The objective of the growing medium is not to supply the plants with nutrients (they are taken up and provided from the water), but rather as a physical aid to the structural growth of the plant itself. Rice hulls are a practical option as they do not absorb water and breakdown very slowly over time. They are also a good option for those looking to use a natural material. It is recommended that parboiled rice hulls be used as they are a sterile material, and will avoid harboring harmful pathogens that could jeopardize the closed hydroponics/aquaponics system.
10. You Tell Us!
ECHO’s strength lies in its network members and the ideas in which you generate from practical experience in the field. We would love to hear from you about the ideas and technologies that you find most practical. Have you found other uses for rice hulls that we might like to hear about? Or other practices and innovations related to this topic? Stay in touch and let us know!
None of these ideas are necessarily novel, and are likely being employed already in many of your contexts. The aim of this article is to remind our readers that ‘using what we have, to make what we need’ can make a significant difference in smallholder farm settings, and using common ‘waste’ products such as rice hulls can be not only practical and productive, but profitable too. Promotion of such techniques may even lead to the valorization of rice hulls in your area, which may offer rice producers (of which there are many), an additional income source, while turning a once-considered ‘waste’ product into something of value to all.
AFRIS. 2002. Rice Hulls. Feedipedia – Animal Feed Resources Information System. http://www.fao.org/Wairdocs/ILRI/x5494E/x5494e07.htm
AVDRC. 2000. Smoked Rice Hulls as a Planting Medium for Seedlings. ECHO Asia Notes. 69:4. Reprinted: Asian Vegetable Research and Development Center. https://www.echocommunity.org/en/resources/5ea0de8f-ab26-4d4b-8dbd-ef35611d2e79
Dahlman, J., C. Forst. 2000. Briquette Presses for Alternate Fuel Use. Edited: C. Bielema. ECHO Technical Notes. 85:1-6. https://www.echocommunity.org/en/resources/66b79f83-17ed-408e-a256-e6ea4cda2f1c
ECHO Collection of Deep Litter Pig Resources. https://www.echocommunity.org/en/resources/846dd289-9b6e-4b60-9a40-0f8cc9788e24
Hough, J. 1956. Possible Uses for Waste Rice Hulls in Building Materials and Other Products. LSU Agricultural Experiment Station Reports. p 1-36. https://digitalcommons.lsu.edu/cgi/viewcontent.cgi?article=1717&context=agexp
Hugill, B. 2011. Biochar – An Organic House for Microbes. ECHO Asia Notes. 9:1-7. https://www.echocommunity.org/en/resources/60196dbc-c62a-4bcf-a33f-251e7195d62c
Kearns, J. 2014. Biochar for Control of Trace Contaminants in Water. ECHO International Conference. North Fort Myers, FL, USA. https://www.echocommunity.org/en/resources/8dff03e9-b819-43b4-a601-2875aa9ac050
Rice Knowledge Bank. Paddy Grain and Its Products after Husking. Accessed January, 2019. http://knowledgebank.irri.org/step-by-step-production/postharvest/rice-by-products/rice-husk
Shafer, M. 2018. Putting Biochar to Use at the Edge: Quality, Soils, and Measurement. ECHO Asia Notes. 30:6-13. https://www.echocommunity.org/en/resources/8ceff8e5-2953-4a8d-bcdf-5cbe39447513
Tapani, H., M. Lyytien. 2017. An Innovative, Inexpensive, and Environmentally Friendly Method to Pasteurize Mushroom Media in the Tropics Using a Styrofoam Box. ECHO Asia Notes. 33:1-3. https://www.echocommunity.org/en/resources/9e3e3a75-aee5-4f37-b43b-9bafb36bb66b
Tosi, M. 2017. An Introduction to Earth Building Techniques. ECHO Asia ‘Improving Lives’ Agriculture and Community Development Conference. https://www.echocommunity.org/en/resources/18100a05-0fe0-4859-99e0-3f9423c917bb