Organic growers rank weeds as the number one obstacle to organic crop production (Walz, 1999). Weeds compete for water, space, light and nutrients at a faster rate than many crop plants, especially in the first 20 to 30 days of crop growth (Dusky et al., 1988) and have a far greater impact on crop yields than later in the season (Teasdale and Cavigelli, 2003). Mechanical cultivation and hand weeding are the most widely used weed control methods in organic vegetable production systems. Hand weeding is labor intensive and time consuming.
Frequent soil cultivation (Stenberg et al, 2000; Holland, 2004) decreases soil health, increases fuel and labor costs (Bernstein et al, 2011; Ryan, 2010), brings buried weed seeds to the soil surface where they are more likely to germinate and compete with crop plants; and reduces yields and growers’ profits. Organic vegetable growers are continually on the lookout for practical and cost-effective technology that can reduce the severity of weed problems and yield losses.
Numerous studies have demonstrated the potential use of compost and compost extracts to stimulate plant growth, suppress soil-borne diseases and improve soil physical properties. However, the use of designed compost extracts has not been demonstrated and verified as an alternative approach for managing weeds for field grown-organic vegetables.
The author has conducted novel laboratory and greenhouse trials to test the effect of using chemically- and microbially-designed compost extracts (CMD-CE) on weed expression and crop germination. Results showed that using the CMD-CE at 1:3 and 1:4 dilutions (V:V), which contained lower nitrate levels and higher nematode-to-protozoa ratios, significantly reduced lambsquarter weed seed germination by 32 percent without affecting crop seed percent germination (Zinati et al, 2015a). On the other hand, compost extracts with lower nematode-to-protozoa ratios (Zinati, 2015b) reduced pigweed seed germination by 20 percent.

Methods

In fall 2016 an experimental trial was established at Aimee and John Good’s Quiet Creek Farm in Kutztown, Pennsylvania to evaluate the use of compost extracts under field conditions on weed suppression and crop yields. Three crops were selected for the fall 2016 field experimental study: Cabbage ‘Melissa’; Turnip ‘Hakurei’; and Lettuce ‘Jericho’. (Additional trial details)
The CMD-CE was prepared from compost made from 30 percent “browns”, 50 percent “greens”, and 20 percent “high nitrogen (N)” feedstocks. The high N was supplied from leguminous sources such as fava beans. Compost extracts were prepared 24 hours before each application at 1:3 dilution (V:V) using deionized water. They were screened before being applied using a backpack sprayer. In a randomized complete block design with four replications, four treatments were tested: 1) Post-planting CMD-CE application; 2) Pre- and post-planting CMD-CE; 3) In-season cultivation or hand-hoeing (standard); and 4) No cultivation or compost application (no treatment). Soil, weed pressure, and crop yields were evaluated.

Results

Weed Expression: Results showed that use of compost extracts significantly reduced weed expression by 43 percent when compared to no treatment and by 11 percent when compared to hand-hoeing (grower’s) treatment in turnip (Figure 1). Similarly, in lettuce, compost extract applications reduced weed biomass by 19 percent and 34 percent, respectively, compared to the grower’s treatment and no treatment (Figure 2). For cabbage, weed biomass in plots that received applications of compost extracts was similar to that in in-season cultivation (grower’s) treatment.
Crop Yield: Mean turnip yield was significantly greater in treatments that received compost extracts than no treatment and not different from the grower’s treatment (Figure 3). Lettuce yield was significantly greater in the grower’s treatment than in no treatment and not different from those treated with compost extract (Figure 4). There was no significant difference in either number of cabbage heads or yield per treatment. Cabbage heads ranged between 30,000 and 35,000 heads per hectare and yield ranged between 11,500 to 13,800 kg per ha.
Soil Compaction: There was no difference in soil bulk density among treatments (0.8-1.07 g/m3) across crops. Mean values of penetrometer readings taken at 300 psi (as a measure of soil compaction) were greater in treatments that received compost extracts as post-planting  (27.58 cm or 10.86 inches) and pre- and post-planting applications (28.75 cm, or 11.32 in) compared to the grower’s treatment (20.04 cm or 7.89 in) in cabbage beds.
Results indicate that the application of compost extracts can be effective in managing weeds and used as an alternative approach to hand-hoeing or mechanical cultivation as well as reducing soil compaction and producing comparable crop yields to the standard grower’s method. The approach of using CMD compost extract can benefit all organic vegetable growers including new and younger growers facing financial, labor, and/or equipment limitations.
Dr. Gladis Zinati is an Associate Research Scientist at the Rodale Institute in Kutztown, Pennsylvania (gladis.zinati@rodaleinstitute.org). This material is based upon work supported by a Northeast SARE Partnership Grant, under Grant Agreement Number ONE 16-278, and by Organic Farming Research Foundation (OFRF), and Frontier Natural Product Co-Op.