The nitrate leaching on grassland on drought-sensitive sandy soil decreases if a significant part of the fertilizer application is replaced by slurry. This is apparent from interim results of a two-year field trial conducted by Wageningen Livestock Research and Research Center B-WARE.
The tests showed a 30% to 50% lower nitrate content in leaching water at a depth of 1 meter. The tests were done in the relatively dry year 2020.
The results are relevant, the researchers report, because dairy farmers are allowed to spread a maximum of 230 kilos of nitrogen from animal manure per hectare on drought-sensitive sandy soil. The nitrogen supply may be supplemented with fertilizer.
In a transition to circular agriculture, high utilization of (own) animal manure, with little nitrate leaching, is important. Researcher Herman de Boer of Wageningen Livestock Research emphasizes that these are interim results, after an above-average dry year. "Nevertheless, the results seem to confirm a conclusion from my research literature study from 2017: in grassland on drought-sensitive sandy soils, after dry conditions in the growing season, nitrate leaching from liquid manure can be clearly lower than from artificial fertilizer."
The cause appears to be that drought not only inhibits grass growth and nitrogen uptake of the crop, but also the conversion of organic nitrogen (from liquid manure) to mineral nitrogen, and thus the nitrogen supply. As a result, the soil surplus of mineral nitrogen at the end of the growing season when fertilized with liquid manure can be lower, and thus also the leaching in the winter. This is different when fertilizing with KAS fertilizer, because all nitrogen is then supplied in mineral form, and is therefore not only immediately available for crop uptake, but also for leaching.
Current research must show to what extent the remaining organic nitrogen from liquid manure will still be used in the following year. The final results will be incorporated into a final report in the course of 2022.
Lower leaching is extra relevant during drought, because only in dry years is there a risk of exceeding the nitrate standard under grassland on leaching-sensitive sandy soil. Providing a relatively large portion of nitrogen fertilization in organic form, rather than as fertilizer, can limit nitrate leaching. This principle may also apply to the cultivation of arable crops.