A sufficient content of organic matter (humus) and the availability of plant nutrients in the soil are essential for soil fertility as well as the soil as a production basis for food and feed.

The organic matter and nutrients help to sustain the function of the soil as a plant location and are a prerequisite for the quality and optimal growth of plants. Thus, humus and nutrient content have a significant impact on the yield potential of a site.

The most essential plant nutrients are divided into main nutrients and trace nutrients. The main nutrients are in turn divided into primary nutrients (nitrogen, phosphorus, potassium) and secondary nutrients (calcium, magnesium, sulfur, sodium). Trace nutrients are: Boron, iron, cobalt, copper, zinc, manganese and molybdenum.

All nutrients must be available to the plants in nutritionally adequate amounts, otherwise deficiency symptoms may occur. However, excess can also lead to problems: Decreasing plant quality and yield losses can be the result. Balanced plant nutrition is therefore an essential factor for soil fertility.

Organic matter in the soil also plays an essential role. In addition to physical and biological effects, it has great importance as a nutrient reservoir for plants. Organic matter can bind an oversupply of nutrients and slowly release them later.

Various sources of nutrients are used to ensure plant nutrition. In addition to nutrients from soil reserves, in agriculture these include mineral as well as organic fertilizers such as farm manure (stable manure, liquid manure and slurry), fermentation residues and compost, or straw and other residues from crop production and animal by-products.

Mineral fertilizers

Mineral fertilizers are fertilizers whose nutrients are in inorganic or mineral form, obtained by physical or industrial chemical processes. The raw materials for the production of mineral fertilizers are obtained either by mining from deposits (e.g. potash salts, rock phosphates, etc.) or by chemical processes (e.g. Haber-Bosch process).

Due to their chemical structure, mineral nutrients are referred to as inorganic salts, which dissolve when they come into contact with water. The dissolved nutrients are absorbed by the roots of the plants and serve as their nutrition.

The advantages of mineral fertilizers are their easy handling (weight, storage) and the possibility of fertilizing on time and according to needs.

Fertilizers of organic origin

However, it is impossible to imagine modern agriculture characterized by resource conservation without organic fertilizers. Organic fertilizers are produced exclusively from animal and/or plant source materials. Consequently, organic fertilizers contain mainly nutrients in organic form.

The organically bound nutrients are converted into a plant-available mineral (inorganic) form via the biological process of "mineralization." Soil life, especially microorganisms, is involved in the mineralization or release of nutrients.

The particular benefit of organic fertilizers lies primarily in supplying agricultural soils with organic matter or organic carbon compounds, followed by an improvement in soil structure and its air and water balance.

Another advantage is the continuous release of nutrients throughout the growing season. However, this can be highly dependent on weather conditions. For example, there may be a lack of nitrogen supply in the spring or nitrogen leaching into groundwater in the fall.

Further information

For more information on the various mineral, organic and organo-mineral fertilizers, please see the links below: Fertilizer Regulation 2004 - (RIS ) Regulation (EU) 2019/1009 on fertilizers - (RIS) Guidelines for proper fertilization - (Advisory Council for Soil Protection and Soil Fertility).

Stable manure or solid manure is predominantly understood to be a mixture of manure and urine with bedding. A small proportion of the urine produced is absorbed and bound by the bedding. Composition, storage and processing influence the properties and type of manure, whereby a distinction is made between fresh manure, stacked manure, deep stable manure and red manure. It is important to optimize the process of rotting or mineralization in such a way that nutrient availability is improved and nitrogen losses are reduced to a minimum. The process of rotting (temperature and time of maturity) is influenced by the type and amount of structural material or litter (straw, shrub cuttings, sawdust/flour, etc.), the moisture content (dry, wet), and the degree of compaction (height limit of 2 m for piled manure, compaction of the litter by the own weight of the livestock in the case of deep stable manure, etc.).

Differences in preparation

In principle, warm manure processes (stacked manure, red manure) are to be distinguished from cold manure processes. The latter include a special variant of stacked manure, the cold-stacked manure process, and deep-stacked manure (pedal manure). Here, the air supply is reduced due to the dense storage of the solid manure. This is intended to reduce carbon and nitrogen losses. Temperatures of more than 30 °C should not be exceeded, in contrast to warm manure processes (40 - 50 °C).

Nutrient content and humus

Since the nutrient content of solid manure is influenced on the one hand by the starting material and the storage method, and on the other hand by the intensity of feeding (e.g. choice of concentrate, P-reduced feeding, etc.) and the performance of the livestock, regular analyses are recommended.

Well-composted manure has a favorable humus replenishment potential; likewise, mineralization of nutrients occurs slowly (annual effectiveness, Table 1). For these reasons, manure is beneficial for stabilizing soil structure and subsequently supports balanced plant nutrition.

Tab. 1 Annual effectiveness of farm manure nitrogen in % related to the field-applied nitrogen quantities for arable land and grassland (BML, 2006).

Note that manure or composted manure has a slower "N-effect" compared to liquid manure due to the higher content of crude fiber.

Source: Guidelines for proper fertilization 7th edition.

Annual effectiveness of nitrogen

With the use of different farm fertilizers in agriculture, questions arise regarding the annual effectiveness of nitrogen (N) in farm fertilizers (and composts).

In this context, the annual nitrogen from animal husbandry (per barn site) after deduction of barn, storage and application losses must be distinguished from the remaining field-applied nitrogen. To calculate annual-effect nitrogen, multiply the field-precipitated nitrogen by the values in Table 1. Annual effectiveness refers to the direct effect at the time of application plus subsequent nitrogen mineralization.

If farm fertilizers (and composts) are applied regularly, an after-effect of 3 to 5% can be expected in arable farming (according to BML, 2006). The availability and mineralization of nutrients is influenced on the one hand by the composition of the farm manure, and on the other hand by regional soil conditions and climatic conditions, such as precipitation and temperature.

Slurry is a mixture of manure and urine with small amounts of litter and water as well as their transformation products. Liquid manure, on the other hand, consists mainly of urine, manure leachate, stable cleaning water (possibly rainwater) and small amounts of manure and litter components and their transformation products. With an ammonium content of approx. 90 %, liquid manure corresponds to the effect of mineral fertilizers.

In recent decades, the use of liquid manure has become increasingly common for labor and economic reasons. The increase in litterless farming systems and the lower costs of storing, caring for and spreading the slurry produced compared to manure have probably also contributed to this.

Pollution of surface water and groundwater

The specialization of farms (e.g. in pig and cattle farming) has led to tighter crop rotations with few main crops (e.g. root crops such as corn, millet, etc.), in addition to the predominant use of the easily manageable organic fertilizer slurry. This is to make efficient use of the available nitrogen utilization potential in the main growth phase. In proper fertilization, it is important to consider nutrient loads, such as nitrogen. To counteract contamination of surface water, seepage water and groundwater, it is essential to always take into account the maximum permitted amounts in the application.

Stabilization of humus and nitrogen in the soil

In order to maintain or increase the humus content at a stable level when fertilizing primarily with liquid manure, it is recommended that the straw produced during harvesting be worked in flat. Here, soil-conserving methods such as reduced (non-rotating) tillage (cultivator, disc harrows, etc.) are to be preferred.

In order to avoid nitrogen losses in the nutrient balance, the application rate must be adapted to the natural conditions (development status of the plant stand, precipitation and temperature conditions, relief, etc.), since, for example, too high slurry quantities/ha, unsuitable times of application and unfavorable terrain conditions can lead to increased leaching of nitrogen (nitrate, NO3) into the groundwater. Regulations on application rates and periods can be found in the Nitrate Action Program, the groundwater protection programs and various ÖPUL measures.

Another possibility to reduce nitrogen losses (ammonia, NH3) is the way of spreading liquid manure on agricultural land. In addition to the crop rotation or the crop growth, the location, the relief and the thinness of the soil, the climatic conditions (cool and damp weather, no direct sunlight) must also be taken into account. To avoid losses, slurry should be applied close to the ground (e.g. by using drag hoses). Furthermore, the slurry can also be diluted.

The proper use of biogas slurry and digestate in arable land and grassland

The 2nd edition of the guideline for the proper use of biogas slurry and digestate in arable land and grassland offers users not only a valuable guide to the utilization of a wide variety of substrates in biogas plants, but also important information on the proper and environmentally sound use of the resulting digestate on agricultural land.

Compost is used in agriculture as well as in horticulture and hobby gardening. The quality requirements, the type and the origin of the source materials as well as the end of the waste status of composts from waste are regulated in the Compost Ordinance 2001.

For the in-house compost production of composts, agricultural or horticultural enterprises can use source materials according to the Compost Ordinance 2001. It is equally possible to purchase compost from specialized producers. Composts represent a conceivable alternative to mineral fertilizers, if the operational and technical conditions allow it. In principle, a nitrogen supplementary fertilization (liquid manure, slurry, mineral fertilizer) is recommended, especially in the year of application, due to the low annual effectiveness of compost (approx. 10 %). However, this has to be done under consideration of the legal framework (according to §32 paragraph 2 Water Act - WRG 1990; Federal Law Gazette No. 252/1990).

A large number of composting systems have developed over the past decades. This has to do, among other things, with the use of different technical systems, different heap sizes as well as rotting periods.

The following example gives an overview of the structure and biological processes of/in composts: For the creation of a compost, the accumulating material is laid out in heaps (e.g. triangular heap; max. height 1.5 - 2 m, width 3 - 5 m) and, depending on the quality of the starting material, turned at regular intervals over a period of 8 - 12 weeks 3 - 4 times per week with a compost turning machine or compost composing machine. In total, the heaps remain in place for approx. 20 - 24 weeks.

Biogas slurries and digestate represent an important part of secondary raw materials in agriculture. They enable the establishment of a circular economy that aims at nutrient recycling and resource conservation.

Biogas slurry is a fermented substrate (liquid & solid) from biogas production that can be used agriculturally as fertilizer. The feedstocks used for biogas slurry are mainly from primary agricultural and forestry production (e.g. corn, green rye, clover, etc.), harvest and processing residues of agricultural products are also included.

Digestate is defined according to the Waste Management Act as amended and is classified as waste until it is permissibly recycled (BML, 2007). Fermentation residues include, among other things, source materials from residues from the treatment and processing of agricultural products and/or other biogenic residues from the food, luxury food and animal feed industries or large kitchens and canteens (e.g. food leftovers, grease trap residues, etc.). Origin, quality as well as origin of the source materials are not clearly traceable and possible contamination with heavy metals (lead, chromium, cadmium, nickel, mercury etc.) or organic pollutants (such as polycyclic aromatic hydrocarbons (PAH), dioxins (PCDD)) cannot be excluded.

As in biogas production, the intention is to return e.g. renewable raw materials from agricultural and forestry primary production to agricultural use after fermentation. This enables the establishment of a circular economy with the aim of nutrient recycling and resource conservation. After fermentation in the biogas plant, the nutrients can be returned to agricultural land to the same extent. This prevents on the one hand a discharge of nutrients and on the other hand an "overloading" with nutrients.

In addition, growing media (e.g. "potting soils"), soil additives and plant aids are also relevant for agriculture and hobby horticulture:

• Growing media (e.g., potting soils) serve as a root zone for plants and perform a task similar to that of soil.
• Soil additives can influence the condition of the soil in biotic, chemical or physical terms and also improve the effectiveness of fertilizers.
• Plant auxiliaries are substances or microorganisms that support the natural processes of the plant and thus promote the resistance of the plant to abiotic factors and increase the productivity of plants.

We offer a wide range of testing services to check the quality of mineral and organic fertilizers, growing media, soil additives and plant aids, thus enabling us to ensure sustainable product quality. Further information on our fertilizer testing services can be found here. The quality requirements of fertilizers are laid down in the 2004 Fertilizer Ordinance and in Regulation (EC) No. 2003/2003 on fertil izers.

Federal Ministry of Agriculture, Forestry, Regions and Water Management

Federal Office for Food Safety (BAES)

Arge Compost and Biogas Austria

Info X gen - Farm input evaluation for organic and GMO-free agriculture in Austria

For interlaboratory tests: VDLUFA - Association of German Agricultural Testing and Research Institutes

Nitrate Action Program

ÖPUL - The Austrian Agri-environmental Program

Compost Ordinance BGBl. II No. 292/2001

Regulation (EC) No. 1069/2009 on animal by-products 1069/2009

Regulation (EC) No. 142/2011 implementing Regulation (EU) 1069/2009

Regulation (EC) No. 834/2007 on organic production

Regulation (EC) No. 889/2008 on the implementation of Regulation (EC) No. 834/2007 on organic production

Fertilizer Ordinance 2004 as amended