Organic fertilisers are made from organic matter or mixtures of organic matter. In addition to organic fertilisers from livestock, also from bird guano, they include green manures, i.e. organic matter of plants grown for ploughing, straw, composts, as well as peat, brown coal, municipal and industrial solid organic waste, sometimes referred to as unconventional organic fertilisers.
Non-conventional organic fertilisers are by-products of human activities, i.e. municipal and industrial wastes and effluents, containing an amount of organic matter and fertilising components that meet agricultural and environmental requirements and restrictions. Depending on their origin and physical, chemical and biological properties, they can be used to fertilise soils in their raw, refined or processed state. Heavy metal and sanitary contamination standards apply to these fertilisers.
Based on the type of pollutants, waste, including wastewater, is divided into those containing mainly organic, organic and mineral or inorganic pollutants. Wastewater from municipalities and the agri-food industry, containing mainly organic pollutants, is of greatest agricultural importance.
7.11.1 Post-harvest residues
Plant residues, post-harvest (harvest) residues, can have very important functions for the soil environment and for future sustainable soil fertility. They are a source of organic matter for the formation of soil humus, contribute nutrients to the soil and increase the activity of soil micro-organisms. An example is the bean plant residues, which are very valuable to the soil, and which form symbiotic atmospheric nitrogen-fixing systems with bacteria of the genera Rhizobium and Bradyrhizobium, as well as Sinorhizobium. These bacteria are one of the most important factors influencing the growth and development of plants and therefore their yields, as they supply these plants and consequently also the soil with nitrogen, the most important element influencing the yield of crops.
Nitrogen fixation by bacteria plays a role in the soil-biological nitrogen cycle. It is estimated that papillary bacteria of the genera Rhizobium and Bradyrhizobium fix 200-500 kg N/ha, and free-living soil bacteria, e.g. Arthrobacter, Azotobacter, Azospirillum, Beijerinckia and Pseudomonas, fix around 5 to 50 kgN/ha per year. Cereal crop residues significantly reduce the need for mineral fertilisers and enrich the soil with humus-forming material in the long term. The ploughing in of crop residues is very important for carbon (humus) and nitrogen accumulation processes in the soil, or at least for limiting humus decomposition processes. In certain situations, post-harvest residues (in the form of mulch) reduce water erosion, wind erosion and soil drying.
Harvesting and selling straw for energy purposes do not fit into a sustainable approach to soil fertility.
7.11.2 Green fertilisers
Green manure is fresh plant matter directly incorporated into the soil to increase soil fertility. Bean crops such as clover, lucerne, beans, vetch, serradella, broad beans, lupins or peas, or brassicas such as mustard, rapeseed and others such as phacelia, sunflower, buckwheat are used as green manure. Green manure is designed to perform functions such as enriching the soil with nitrogen and carbon, improving soil structure, increasing soil microbial activity and reducing weed infestation and soil erosion by covering the soil. The use of green manures is key to maintaining humus levels in the soil and supporting its associated functions (water retention, buffering properties, erosion protection).
7.11.3 Catch crops and intercrops
The preservation of biodiversity on the site and the intensification of humus-forming processes are promoted by the cultivation of catch crops and intercrops. These, as well as the use of green manures, are of great importance for the preservation of soil quality and organic matter levels. Catch crops and intercrops should be used by all farms, but especially by farmers with crops on sandy soils at increased risk of erosion and who do not keep livestock – and therefore do not – or rarely – use manure or slurry. The right choice of intercrop or catch crop can not only prevent soil degradation, but also enrich the soil with nitrogen and improve soil structure. A good catch crop leaves behind a significant amount of plant matter, which helps to rebuild organic matter reserves. Penetration of the soil by the roots of the catch crop leads to soil loosening and improved structure. Mixtures of grass or rye with faba bean plants (broad beans, lupins, field pea, vetch) are particularly valuable. These plants also increase the nitrogen content of the soil through symbiosis with papillary bacteria. Blue phacelia is a valuable species as it grows very quickly and produces significant amounts of biomass. Oilseed radish and mustard have an additional phytosanitary effect on the soil. Cultivating intercrops after an early main crop protects the soil from water and wind erosion and reduces weed growth. In addition, it introduces significant amounts of plant organic matter of species not cultivated in the main crop into the soil. The intercrop improves soil water-air relations, reduces evaporation and water loss, and enhances microbial activity, improves plant health and naturally counteracts weed growth. The plant cover of the soil counteracts wind and water erosion. The intercrop also draws a large amount of nutrients from the lower soil layers, preventing them from leaching into the soil and groundwater. The decomposition of plant residues enriches biodiversity in the surface soil layer. For the cultivation of a stubble catch crop, a crop with a short growing season, fast-growing and producing large amounts of green matter, with low water requirements during germination and growth, should be chosen. The most common catch crops are white mustard, phacelia, radish, lupin, broad bean, serradella, sunflower, fodder peas and rye.
Throughout the year, nutrients are lost from the soil through, among other things, leaching into the soil profile and into groundwater and surface runoff. The intensity of surface runoff depends on the slope of the terrain, the intensity of rainfall, the granulometric composition of the soil, how it is tilled and the type of plant cover. The steeper the slope and the smaller the barriers to water runoff from fields, the more surface runoff occurs. The vegetation cover can be such a barrier. Liquid fertilisers and mineral nitrogen fertilisers may only be applied to fields with a slope greater than 10% (6°), if the fields are under a ground cover. Maintaining crop cover is provided by main crop plants and intercrops, which are plants grown between two crops in the main crop. Intercrops reduce the leaching of fertiliser nutrients by uptake and incorporation into their tissues and by mulching the soil surface with intercrop biomass during the winter.
In practice, the following crops can be grown: intercrops, stubble intercrops and winter intercrops. The species as well as the amounts and sowing dates of the crops that can be grown throughout the country are given in, among others, the 2015 publication: “Good agricultural practices in particularly vulnerable areas (OSN) to nitrates of agricultural origin” available online.
7.11.4 Compost
Farm compost – is one of the best organic fertilisers used in agriculture. Its value is judged by the composition of the raw material used in its production and the composting process. Apart from manure, compost is the main fertiliser used in organic farming as a natural soil fertility maintainer. The application of compost improves the physico-chemical and biological properties of the soil, similar to manure. Systematic application of compost on light soils can increase sorption capacity, while in heavy soils it can improve soil structure. Compost has significantly enriched soil organic matter in many experiments conducted to date around the world. The use of compost also increases microbial activity. Various plant raw materials are used to produce it, as well as manure and waste from the processing of agricultural products. The nutrient content depends on the substrates used and varies in the following ranges: 0.75-1.5% N, 0.25-0.5% P2O5; 0.5-1.0% K2O. In addition to the main macronutrients, it also contains varying amounts of other macro- and micronutrients and, importantly, because of its form, the nitrogen in it is not subject to such rapid losses as that contained in manure. In terms of the stability of the fertilising elements, mainly nitrogen, the rate of their release and the risk to groundwater, compost is the most favourable form of organic fertiliser.
7.11.5 Digestate
Biogas plant digestates are a relatively new material, but one of ever-increasing importance. The digestate (also referred to as digestate pulp, digestion sludge, digestion product or simply: digestate) is the second product of the methane fermentation process after methane. The chemical composition of the digestate depends on the type of substrates used in the methane fermentation process. If typical substrates such as manure, maize, by-products from the agro-food industry (distillery waste, whey, sugar beet pulp) are used in the fermentation process, the digestate is a safe and very valuable fertiliser.