Effect of maturation of Grease trap compost on plant growth

Although it is known that immature composts can depress plant growth, few studies have quantified this effect in real-world scenarios with field-grown crops. Glasshouse and field trials were used to investigate the effect of maturation of grease trap compost (GTC) on plant growth. Grease trap waste was composted for 7-14 d in an in-vessel reactor with shredded green waste, sawdust and chicken manure and matured in windrows for up to 6 weeks. In the glasshouse trial, compost samples were mixed with an equal volume of coarse sand and were compared with a standard mix of 3 parts composted pine bark to 1 part coarse sand (v/v). In these trials, fresh GTC suppressed germination and growth of both radish and alyssum compared to the standard mix. Two weeks maturation reduced phytotoxicity of GTC but it was still phytotoxic relative to the standard mix. After 6 weeks maturation, GTC performed as well as the standard mix in all measures of plant growth and germination except for plant height of alyssum. Fresh and mature GTC were also used as soil conditioners in field trials on a commercial vegetable farm in the Werribee irrigation district, Victoria, Australia. Treatments consisted of GTC at 5 or 10 t ha-1, nil compost or a pelletised poultry manure product (Zest®) at 618 kg ha-1 arranged in a factorial design with 100% and 75% of the grower’s standard fertilizer rate (741 kg ha-1 Rustica-Plus®). Head weight of lettuce was significantly reduced (by up to 18%) by the application of 10 t ha-1 of fresh GTC at the grower’s standard fertiliser rate. When the fertiliser rate was reduced to 75%, the negative impact of fresh GTC was less apparent. In contrast to the first crop, the performance of mature GTC compared to the controls was independent of fertilizer rate (soil amendment x fertilizer interaction not significant at p<0.05). Application of 5 and 10 t ha-1 of mature GTC increased head weight of lettuce by 11% and 12% respectively compared to the control plus Zest® treatment. This research highlights the need of a better understanding of the maturity requirements of composts for use as soil amendments and growing media in horticultural production.

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