EFFECT OF CATCH CROPS ON THE CONTENT OF SOIL MINERAL NITROGEN BEFORE AND AFTER WINTER LEACHING

Since large amounts of nitrogen may be lost by leaching during the winter period, investigations have been carried out to find suitable crops for catching nitrogen during the autumn. The plant species phacelia, common sunflower and italian ryegrass were sown at three times during the late summer. The soil was analysed for mineral nitrogen at the end of November just before incorporation of the catch crops and in the middle of April the following year. The dry matter production and the uptake of nitrogen decreased as the establishment was postponed and the growth period thereby decreased. When sown in the middle of July or at the beginning of August phacelia and italian ryegrass accumulated 150 kg N per ha in the above ground plant parts. The content of soil mineral nitrogen in November was reduced by growing catch crops during the autumn. The content of soil mineral nitrogen increased as the growth period of the catch crops decreased. The content of mineral nitrogen in the upper 50 cm soil layer in April was, irrespective of plant species, increased by catch crops. When no crop was grown the difference between the content of soil mineral nitrogen measured before and after the winter period indicated a net loss of 144 kg of N per ha in 0-100 cm soil depth. When italian ryegrass was sown in the middle of July the previous year the content of soil mineral nitrogen found in April was 59 kg per ha higher compared to the content found in November.