Jay Capasso, UF/IFAS Columbia County. Erin Harlow, UF/IFAS Columbia County.
Organic amendments come from sources that were once alive and are applied to agricultural soils to increase nutrient and/or organic matter content. Increasing organic matter content of the soil has many benefits including improving water holding capacity, infiltration, and nutrient-holding capacity. Many organic amendments are good sources of plant essential nutrients. Organic amendments can also serve as a food source for soil organisms. Common organic amendments used in Florida agriculture include chicken litter, manure, compost, biosolids, and biochar.
Chicken or poultry litter is a common organic amendment used in agricultural production. Chicken litter is defined as chicken manure mixed with bedding material such as straw, rice hulls, and pine shavings. Because it is manure, it is often composted to break down pathogens. The standards that the litter must meet to be used depend on the crop and is regulated by the US Food and Drug Administration.
The pH of chicken litter commonly ranges from neutral to alkaline (6.5 – 8.0). Chicken litter from layer hens tends to contain higher pH compared to broiler hens. Layer hens are fed diets supplemented with calcium to improve eggshell development. Excess calcium present in chicken manure and in broken eggshells can add to the calcium carbonate content of chicken litter, raising pH. Layer hen manure is not always mixed with additional bedding material and can be utilized wet or dry. Broiler chicken litter tends to have a higher nutrient content but varies based on the bedding material used, the diets the chickens are fed, and how frequently chicken litter is emptied out of chicken houses. A local analysis of one source of chicken litter (derived from layer hens) claims a Nitrogen, Phosphorus, and Potassium content of (3%-3%-3%).
Manure is derived from various livestock animals including swine, cattle (beef and dairy), horses, etc. Manure that has not been composted can contain high nitrogen content compared to composted materials. Given the high nitrogen content, plants can be damaged due to ammonium toxicity when applied in excess or not properly incorporated into soils. Raw manure is generally not applied during the growing season to crops grown for human consumption. Like other organic amendments, manure is variable in terms of nutrient content, salinity, and pH depending on source and feedstock of livestock. Manure can contain viable weed seeds.
Compost is a term used to describe a variety of decomposed organic materials. Many compost materials are heated by the decomposition process which can destroy pathogens and weed seeds in the compost. The heating process fueled by microbial decomposition, the thermophilic stage of decomposition, is where temperatures of 104 – 140 degrees Fahrenheit can be reached. Fully decomposed compost looks dark, feels crumbly, and smells earthy. A variety of organic sources can be composted including plant-based sources, manure, food wastes, and even whole livestock. Little if any of the original organic material should be recognizable in a fully decomposed compost.
Vermicompost is a type of compost that uses worms in the decomposition process. Many different sources are used in compost mixes. Therefore, compost blends can differ in their nutrient content. It is always best to test compost blends to better understand their nutrient content and pH. Some compost blends, such as those that derive from food waste, could have high salinity levels which could be problematic for crop production.
Bio-solids are an organic amendment derived from human waste and the treatment of domestic sewage. Like other organic amendments, biosolids should be tested for nutrient content given the variability in nutrient content among different sources. Regulation requires biosolids to be treated to reduce pathogen content. They are also analyzed so as not to exceed heavy metal contaminant concentration thresholds. In Florida, biosolids are separated into three classes based on their degree of treatment of pathogens: Class B (the least treated), Class A (intermediate level of treatment), and Class AA (highest level of treatment). The land application of Class B and A biosolids are restricted to permitted sites, while Class AA biosolids are not restricted and are distributed for use as fertilizer in Florida.
Biochar is an organic amendment that resembles charcoal and is derived from organic amendments. Biochar is usually created through the process of pyrolysis which involves heating organic material to high temperatures around 500 °C in the absence of oxygen. The absence of oxygen prevents the complete combustion of the organic material used. Biochar is a stable form of carbon that resists degradation, potentially releasing nutrients over a prolonged period compared to organic amendments that have not undergone pyrolysis. Various sources are used to create biochar including, but not limited to, manure, chicken litter, wood chips, and sewage sludge. Given the various sources used to create biochar, nutrient content among biochar’s differ.
Apply organic amendments evenly over the soil surface to avoid concentrated deposits. Tillage can assist with the incorporation and distribution of organic amendments across a field. Tillage also speeds up the decomposition process causing nutrients such as nitrogen to become available to plants.
Consider the carbon to nitrogen ratio (C:N ratio) of the organic amendment. Microorganisms present in the soil require both nitrogen and carbon to function. When the C:N ratio is high (high carbon content compared to nitrogen) the organic material decomposes slowly as microorganisms in the soil inefficiently break down the material. In low C:N ratios (low carbon content compared to nitrogen) organic materials break down quickly as microorganisms in the soil uptake nitrogen rapidly. The rate of decomposition will affect when essential nutrients in organic materials become available to plants.
Always consider the moisture content of organic amendments when determining nutrient content. Monitor the pH of the material and the soil at the application site over time. Analyze the nutrient content of the organic amendment and soils to ensure repeated applications do not cause nutrients to build up to excessive levels that negatively impact plant health or the environment. The UF/IFAS Livestock Waste Testing Laboratory provides nutrient analysis for manures. The UF/IFAS Soil Extension Laboratory also provides a container media analysis which may be applicable to organic amendment soil mixes used for potted plants.