Nitrogen and phosphorus analysis in field cultivation of Pak choi : effect of two different compositions of fertilisers on plant and soil nutrient status

Sammanfattning: This pilot study was implemented to reflect the delivery and the plant availability of nitrogen and phosphorus in response to different compositions of fertilisers approved for organic farming in Sweden. The experimental approach was to compare the concentration of nitrogen and phosphorus in plant sap of Pak choi plants grown in soil treated with different organic waste such as aged cattle manure and a liquid retting digest derived from the biogas industry, with plants grown in soil treated with a blend of pelleted organic fertilisers derived from the Swedish slaughterhouse industry. The organic waste materials and fertilisers in this study were selected with respect to their nutritional properties: concentration of macro elements and the assumed nitrogen accessibility from the organic and inorganic proportion of nitrogen present in these materials. All plant tissues and soil samples were analysed for: the nutritional status in soil (all macro nutrients) prior to fertiliser application as well as the concentration of nitrogen and phosphorus in soil postharvest; plant sap concentration of nitrogen and phosphorus on three occasions along six weeks of field cultivation and the concentration of young leaf tissue total nitrogen concentration on same occasions. In addition, three days prior to final harvest, the treatments were sampled to measure fresh and dry weight of leaves and roots followed by a subsequent analysis of total nitrogen accumulation in the same tissues. Plant sap concentration of inorganic nitrogen was highest in tissues sampled from Pak choi plants grown in soil treated with pelleted slaughterhouse waste in week four, the second occasion ofsampling. This concentration decreased to the lowest relative to the concentration of plants grown in control soil (no fertilisers) and plants grown in soil treated with aged cattle manure and retting digestate in week five, which was the last occasion of sampling. Soil remaining concentration of inorganic nitrogen postharvest was shown to be the highest in soil treated with pelleted fertilisers but the leaf tissue concentration of total nitrogen showed the lowest concentrations in plants sampled for dry weight grown in the pelleted fertiliser treatment. An opposite pattern was found in soil treated with aged cattle manure and retting digestate which is contradictive and further discussed. Plant sap concentration of phosphorus showed the highest values for control plants, surprisingly during all three occasions of sampling. This relationship indicated that the consumption of phosphorus can be limited by a relative low concentration of other macro elements, in planta, for the plants grown in the control soil. Moreover, postharvest soil remaining phosphorus indicated redundancy in soil treated with aged cattle manure and retting digestate, because the plant sap concentration of phosphorus (of plants grown in control soil) where similar to the plant sap concentrations of plants grown in soil treated with the pelleted fertilisers. The initial amount of added phosphorus were three times less in the soil treated with pelleted fertilisers, in comparison to the soil treated with aged cattle manure and retting digestate.