Epsilon Archive for Student Projects

Abstract

The short term effect of phosphorus in sewage sludge was investigated to establish how
much of this a farmer can grant him- or herself, if using sewage sludge as a phosphorus
fertilizer. Possible differences in plant availability between sludge phosphorus precipitated
by iron, aluminum or calcium integrations were studied in a sandy soil at two pH levels.

The sludges were analyzed using pot and incubation trials. As the intention was to investigate
growth limitation due to phosphorus deficiency, a soil with low phosphorus supply
was chosen as well as a low phosphorus application rate. Sludge corresponding to 12
kg phosphorus per hectare was applied. Treatments fertilized by sludge were compared to
treatments where 0, 6 and 12 kg phosphorus per hectare from mineral fertilizer was applied.
Every treatment was replicated six times. Three of those replications had initial pH
6.2 in soil, whereas three were limed to the achieve pH 7.2. In the pots ryegrass (Lolium
perenne) was sown. The pot trial was performed in a greenhouse during 11 weeks.

The yield and uptake of phosphorous was higher in the pots were no lime was applied.
A plausible theory to explain that effect may be that phosphorus strongly bound to calcium
when pH increased. Sewage sludge had under the conditions for this trial a short-term effect
of phosphorus corresponding to 20 – 60 % of that from a mineral fertilizer. Iron and
aluminum precipitated sludge gave in this trial a higher short-term effect than sludge precipitated
by calcium, probably due to the negative effect on phosphorus uptake from liming.
Sewage sludge application elevated ammonium lactate extractable phosphorus (P-AL)
to a varying extent. This variation did not correspond to the observed variations of phosphorus
fertilizer effect between the sludge types that were examined.