Soil is an element of crop cultivation that demands consistent fertilisation to compensate for the nutrients that are removed by the harvest. Changes in soil because of prolonged fertilisation can only be estimated by long-term field trials. Experiments in long-term field trial site Kuusiku (since 1965) include crop rotation of potato, late harvest barley, early harvest barley undersown with forage grasses (red clover + timothy), 1-year forage grasses, 2-year forage grasses, and winter rye. Various combinations of mineral and organic fertilisers were used to investigate the yield, soil humus, phosphorus, and potassium content (available and total) of the top- and subsoil. Fertilisation improved the yield of different crops by 1.3–2.6 times; meteorological conditions caused the yield to vary up to 6.4 times. The concentration of humus decreased 0.2 when not using inorganic and organic fertilisers; use of fertilisers increased the concentration of humus by 0.2–0.6. The humus-rich subsoil (3.5 humus) contained less available phosphorus than humus-poor subsoil (humus 3.0), which had 29 and 63 mg PDL kg~(-1), respectively. Grasses in crop rotation enriched the soil with organic matter and reduced the excess of nutrients remaining from previous fertilisation, thereby decreasing nutrient leakage and eutrophication of bodies of water.
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