By Ruth Vaughan, Msc. Applied Science
Organic fertilizers are derived from animal or plant matter, manures, algal and seaweed products, and organic wastes from processing and food industries. They have a high carbon content and tend to be more diverse in nutrient content, slower acting, longer lasting and benefit agriculture by not only adding important plant nutrients, but also building organic matter content and soil fertility.
Soil organic matter (SOM) is the organic matter component of the soil. It is critical for soil function and soil quality. Soil quality is defined as the capacity of a soil to function, within natural or managed eco-system boundaries, to sustain plant and animal productivity, maintain or enhance soil and water quality and support human health and habitation.
Importance of Soil Organic Matter
Soil organic matter is very important for many soil functions
- Direct supply of plant nutrients through decomposition.
- Stable humus compounds hold and release nutrients – making them more available to plants and reducing volatizing and leaching losses.
- Buffering pH and high salinity effects.
- Providing a nutrient source for soil microbes – increasing microbial activity and biodiversity in the soil – important for plant nutrient uptake and reducing soil born pests and diseases, and natural bio-remediation of pesticide residues.
- Promotion of aggregate formation and soil structure stability – which in turn improves water infiltration, water holding capacity, resistance to compaction and erosion and better root penetration, nutrient uptake, yields and food nutrient density.
- Buffers heavy metals and pesticides, reducing plant uptake, improving food safety.
Soil organic matter levels sit at about 1-8% for mineral soils, and depend a great deal on soil texture, soil management, vegetative cover, moisture levels and temperatures. Low organic matter is seen in desert areas, and sandy soils, higher organic matter can be seen in peaty soils and low-lying flooded valleys. Soil organic matter consists of actively decaying organic material that is the energy source for microbes and is important for the release and recycling of plant nutrients – this breaks down to more stable humus compounds that are important for soil structure.
Natural organic fertilizers may be produced on the farm (composts, manures, green manure crops, vermiculture etc.) or purchased in. Farmers can search http://shambaza.com/organic-fertilizers to source organic manures commercially available in Kenya.
Many farming activities burn off the organic matter of soils resulting in a decline in soil structure and soil fertility – requiring an ever-increasing amount of inorganic farm inputs to maintain yields. This includes but is not limited to mono-cropping, soil cultivation and over use of inorganic nitrogen fertilizers. A responsible GAP farmer will monitor both the organic matter level and the C:N ratio of soil annually and prepare a soil health program that builds up soil organic matter and then maintains it at optimum levels for maximum fertilizer efficiency, plant health and yields.
Organic fertilizers vary a great deal in content and it is important to know the exact chemical composition to be able to balance nutrient applications to the soil. Some organic fertilizers are high in phosphorous and potassium and one can reduce application of these nutrients fertilizers and save money. In addition, one must look at potential risk factors from organic fertilizers, for example: human pathogens, heavy metals, pesticide residues & toxic salts. If there is a risk attached to the organic fertilizer – these can be measured in the laboratory.
Organic matter breaks down to the more stable humus compounds in the soil. The term acid is used to describe humic compounds in the soil because they behave like weak acids. Humus compounds consist of three main substances (these cannot sadly be measured in the laboratory): – Fulvic acids – smaller humus molecules that are light yellow to yellow brown and soluble in water under all pH conditions. These also provide and energy source for microbes. Humic acids – larger molecules, dark brown to black in color, more stable, soluble in water at pH>2. Humins – very large, complex stable molecules that are black, insoluble in water, less of an energy source and more of a soil stabilizer.
Humic & Fulvic acid substances enhance plant growth positively and directly through physiological and nutritional effects. Some act as natural plant hormones (auxins and gibberellins) and are capable of improving seed germination, root initiation and nutrient uptake as well as being direct sources of Nitrogen, Phosphorus and Sulphur. One of the most striking characteristics of humus substances if their ability to interact with metal ions, oxides, hydroxides, mineral and organic compounds including toxic pollutants, to form complexes. By forming these complexes humus can dissolve, mobilize and transport metals and pollutants in the soil and contribute to a reduction in toxicity for example heavy metals and pesticides.
Organic fertilizers do improve soil and plant health and reduce chemical applications in the crop AND are important for reducing waste and for carbon sequestering. Many contain beneficial organisms that are great for promoting microbial activity in the soil and competing with pathogens. I heartily promote the use of organic fertilizers in farming but recommend that the fertilizers are tested for nutrient content and risk factors before use, to be able to assess how much to use and how much to reduce the inorganic fertilizer application by.
Ruth Vaughan is the Technical Director at Crop Nutrition Laboratory Services Ltd. (CROPNUTS). Ruth is also a contributing author to Kenya’s leading horticulture magazines such as the HortFresh Journal, HortiNews and Floriculture. Ruth is a great believer in soil health, organic matter, biochar and carbon sequestration as a way to alleviate climate change and increase food security. Loves visiting farmers and seeing all the different farming methods