Ploughing up ley or green manure crops in late autumn and then sow spring cereals gave less nitrogen and phosphorous leaching than ploughing in early autumn followed by winter wheat. The leakage after faba bean was also studied in the field trial.
Researchers at the Department of Soil and Environment, SLU investigated how leaching can be reduced in organic crop rotations. The trial was conducted at Lanna research station in Västergötland on two adjacent fields with separate draining of each plot on a 47 percent clay soil. The field experiment imitated two rotations representing a dairy farm and a grain farm. On dairy farm field, the crop rotation included winter wheat after ley, field beans, barley, and a two-year clover/grass ley (30/70). The grain farm field included two one-year leys. The dairy farm field was fertilized with liquid manure in amounts that corresponded to the amount of nutrients in feed produced on the field. The grain farm field was only given green manure.
Leaching of nitrogen and phosphorus was overall low from both crop rotations. Most of the losses occurred during the period from 1 October until 31 May, with higher losses from the grain farm of 9.1 kg per hectare and 0.55 kg phosphorus per hectare compared with farm milk by 6.8 kg per hectare and 0.39 kg phosphorus per hectare. The difference was statistically significant only for the leys in growth.
With termination of the ley in September and establishment of winter wheat afterwards much nitrogen leaked during the winter. If the ley instead was ploughed in October/November and the soil was left bare until spring when it was sown with barley, the leakage was less. Phosphorus leaching after termination of one-year ley became as low as from ley during growth. After such clear findings, the researchers recommend that winter wheat should not be grown after ley. Instead it is better to plough late fall and then sow with spring cereals.
Leaching of nitrogen and phosphorus followed a similar pattern in the crop rotations, except that the highest phosphorus losses were measured after faba bean on ploughed soil. The greatest losses of phosphorus were measured during periods of high runoff, which may be due to the taproot of the field bean having caused a lot of macro pores in the clay soil. A possible measures to reduce phosphorus leaching after faba bean can be to cultivate after harvest in order to break the water transport through macro pores. However, this can increase nitrogen mineralization and thus the risk of nitrogen leaching. Establishment of catch crops after faba beans has in other studies, reduced nitrogen leaching, but if it is an effective measure against phosphorus release has not been investigated. However, this study showed that undercropping of ley, which can be likened to a catch crop, clearly decreased phosphorus leaching.