Research Report: Effect of Compost Extract on Lettuce Yield

Fruit and vegetable farmers are exploring on-farm compost methods to lower their input costs, increase soil microbial diversity, manage waste and improve yields. Following up on a 2014 PFI study with Jason Jones and Siobhan Danreis,  Rob Faux (Bremer County) and Siobhan (Pocohontas County) held on-farm trials investigating the effect of compost extract on yield characteristics of head lettuce. Siobhan also continued with limited Qualitative Soil Analysis at the Living Soils Lab in Fairfield.

Objective: To determine the effect of compost extract application on yield of head lettuce. 

In a Nutshell:

• Compost extract is made by steeping compost in water, then applying the sieved water (with compost extract) as a soil drench with the intention to increase beneficial soil biota, and increased plant health, Brix and yield.
• At two farms, compost extract was applied as a drench treatment to head lettuce. Farms used a randomized block design with treatment and control plots.
• Yield and Brix readings of head lettuce was measured to determine impact of compost extract applications.

Key Findings:

• Drenching soil with compost extract did not affect lettuce yield at either farm.
• Drenching soil with compost extract did not affect Brix levels in lettuce at Danreis’ farm.
• Compost price could be cost prohibitive; at high-intensity application, compost cost was $1.25/row-ft.

After two years of field trials on three farms, the measurable impact of compost extract application on annual vegetable crop yield is scant. In 2015, yield and Brix readings were not significantly different among treatment and control plots at either farm. No significant effect of compost extract application was observed. Furthermore, the application rate of the compost extract was costly. At Siobhan’s, low rate plots use 2.5 lb compost per 10 row-ft (0.25 lb/row-ft), high rate plots used 5 lb per 10 ft (0.5 lb/ row-ft). At a purchase cost of $2.50/lb for compost (not including shipping), compost only for each 10-row-ft plot cost $6.25 ($0.63/ row-ft) for the low-intensity and $12.50 ($1.25/row-ft) for the high-intensity application.

For more detailed results and discussion, view or  download the full research report on the PFI website.