The humus is formed by biodiversity. A decisive role in the fertility of cultivated soils is played by the so-called organic horizon with well-formed humus proper. In natural, agriculturally unaltered soils, humus substances are extremely durable – the half-life of humic acids is estimated to be several tens to several hundred years, and bitumen even several thousand years.
Soil humus is responsible for the tuberous structure of soils, which ensures adequate air-water relations. This plays an important role in maintaining nutrients in the soil, as it reduces their leaching into groundwater. It also provides better moisture storage and inhibits surface deterioration by wind and water. A 1% reduction in humus reserves in soils results in a drastic decrease in water retention (by up to 30%), also a faster acidification of soils and an increased leaching of fertiliser components amounting to up to 40% of the mineral fertilisers introduced, especially nitrogen fertilisers.
According to the Institute of Soil Science and Plant Cultivation – PIB in Puławy, soils with a low humus content (<1.0%) account for about 7% of the agricultural land in Poland, and with an average (1.1-2.0%) – about 50% of agricultural land. Soils rich in humus (>2.0%) occupy about 33% of the country’s agricultural land and are mostly used by cattle and regular application of manure to the soil. Unfortunately, due to a significant reduction in livestock, the humus content of soils has decreased significantly in the last decades. An increase in humus content is possible and necessary, but this requires several years of consistent action. Only 20-25% of the organic matter introduced into the soil has a chance of being converted into humus. Active humus, which is important for soil fertility, develops within 15 years of e.g. ploughing crop residues, but stabilised humus only develops in 15 to 100 years, and so-called permanent humus develops at least 100 years after the introduction of organic matter into the soil. Therefore, soil fertility must be worked on continuously, knowing that tangible results will come after several years.
Soil organic matter is a mixture of many substances – carbon compounds – with a complex structure and varying properties, depending on the degree of humification. It is formed as a result of biochemical transformations of biological decomposition products of organic compounds included in dead plants and soil organisms. Soil organic matter is a basic indicator of soil quality that determines the physico-chemical properties of soils, such as sorption and buffering capacity, and the biological processes that determine many transformations, referred to as biological activity. The high humus content of soils is a factor in stabilising their structure, reducing their susceptibility to compaction and degradation by water and wind erosion. In addition, soil organic matter can be a source or store of atmospheric carbon dioxide, depending on the use of the soil in question, vegetation cover and water relations.
Intensive soil mixing, especially in combination with monoculture, destroys soil structure, increases oxygenation and causes accelerated mineralisation of humus, resulting in the release of large amounts of carbon dioxide into the atmosphere. Of the human-dependent factors affecting the organic matter (carbon) content of soil, the most important are land use (arable land, permanent grassland, forest), soil movement intensity, crop rotations used and organic fertilisation. A decline in soil organic matter content is a clear indication of soil degradation and declining soil fertility. Irrational agricultural management can lead to a decline in organic matter content as a result of, for example, the use of drainage irrigation or accelerated mineralisation of humus due to over-intensive soil cultivation.
Diverse soil organisms present in organic matter play a major role in humus formation. Soil organisms include micro-organisms – archaea, bacteria, fungi, algae – and animals – annelids, mites, jump mites, nematodes and insect larvae. In soils, micro-organisms are the dominant component of the biomass of soil organisms (they can make up between 1% and 10% of the total organic matter of soils). They are also the main factor responsible for maintaining soil fertility. It is estimated that more than 90% of soil transformations are caused by microorganisms. Dead organic matter is broken down mechanically by soil animals to be transformed biochemically by fungi and bacteria. As plant decomposition progresses, the access surface for fungi and bacteria increases. The plant parts next to the faeces of the organisms shredding the litter create excellent conditions for the growth of micro-organisms. By shredding litter and excreting faeces, soil invertebrates accelerate the decomposition of organic matter, but inhibit its mineralisation and, as a result, promote carbon retention in the soil.
Reducing the supply of organic matter to soils results in a decrease in the proportion of humus-forming fungi, especially in acidified and one-sidedly nitrogen-fertilised soils. Under such conditions, humus-degrading fungal groups take over the dominant role. An important role in soils is played by crop residues, which can provide food for fungi. Soil fauna not only help decompose dead organic matter, but also mix it with the mineral parts of the soil. Earthworms that form underground corridors and labyrinths play a major role in this type of activity. As they dig, they bring small soil mineral particles to the surface. On the other hand, they protect the walls of their corridors by reinforcing them with their secretions. This phenomenon can be seen in the form of darker discolouration on their walls. These corridors have an important function in soil aeration and protection against water erosion. The cavities of earthworms and other soil invertebrates increase the water holding capacity of the soil, which, with a population density of between 50 and 200 per square metre, plays a very important role. Earthworm cavities also have a positive effect on improving the rooting of plants. The roots find the spaces and nutrients they need to grow, which has also been scientifically proven.