Humus content in the soil corresponds to the level of mineral particles smaller than 0.02 mm. The higher the content of these miniscule particles in the soil, the richer it is in humus. Consequently, sandy soils are humus-poor and loamy soils are humus-rich.
Depending on region, humus-poor soils and low-humus soils (< 2% of humus) account for 40–72% of Poland’s arable land. According to humus content they may be divided into: poor soils (0.1–1%), low-humus soils (1.01–2%), medium-humus soils (2.01–4%), high-humus soils (above 4%). The main cause of soil fertility deterioration is a low content of organic matter, which has been depleting for years.
Lower content of organic matter in the soil increases its susceptibility to condensation and decreases its capacity to absorb water, which results in intensified surface flows, water and wind erosion. This is significant, as Poland is a country with relatively low water reserves, which are also utilised in agriculture. Low organic matter content in the soil also reduces its sorption capacity (ability to retain nutrients). A research conducted by the Institute of Soil Science and Plant Cultivation (IUNG) demonstrates that as a result of agricultural use of arable lands the amount of organic matter in soil has been diminishing in Poland on average by nearly 0.53 tons per hectare each year.