Use of sludge from pulp mills in extensive crops: An assessment at a semi‐commercial scale in central South America

Abstract

Abstract Land under agriculture has increased during the last 20 yr in the South American Pampas. Agricultural systems use mostly finite imported sources of P, so the use of local recycled P sources is an alternative. In Uruguay, two current events are relevant to this matter: the increase in the crop area of Brassica carinata (biofuels production) and the presence of large pulp mills. The working hypothesis was that pulp mill sludge applied prior to sowing carinata crops will substitute the use of inorganic fertilizers, and that the combination of sludge and inorganic fertilizers will increase grain yield. Two on‐farm experiments were conducted, in which carinata was sown during the 2019 winter season and followed after harvest by soybean sown at one site and maize at another. At each site, six treatment strips (30 m wide × 400 m long) were established in a control‐plot design. The treatments corresponded to two sludge types (biosludge and mixed sludge), combined with a traditional N, P, K, S chemical fertilizer (29 kg P ha–1), compared with a control treatment (not fertilized). Carinata yields results do not support the proposed hypothesis, because the yields of the sludge treatments were lower than the chemical fertilization treatments. The limiting factor for the crops was associated with N supply, as the nitrogen nutrition index showed low values. Sludge application did not result in changes in summer crop yields and soil chemical properties. Future studies should focus on the sustained use of combined fertilizer sources over time.