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Weed learning module
Do incorporated cover crops kill weed seedlings?
Hypothesis:
We know that tillage stimulates many weed seeds to germinate. We hypothesize that fresh organic matter from incorporated cover crops promotes the growth of disease fungi that kill many of these newly sprouted weed seedlings before they emerge from the soil.
Reasoning:
Several groups of fungi act both as decomposers and as pathogens. Among these are Pythium and Phytophthora that cause damping off and root rot in a wide range of plant species. These fungi proliferate whenever a great mass of organic matter begins to decay, as in a newly built compost pile or after incorporation of a green cover crop. This is one reason why using unfinished compost or planting immediately after incorporating a cover crop are poor management practices. After the organic matter has decayed for about two to three weeks, the pathogens are largely displaced by beneficial fungi. After that, the ratio of beneficial fungi to pathogens may be greater in a cover crop amended soil than in one that receives no cover crop.
Whereas a farmer can delay planting crop seeds, many weed seeds respond to tillage by germinating immediately. The seeds of most weed species are very small. Most are smaller than a lettuce seed. Consequently, newly emerged weed seedlings are weak and compete poorly with well established plants. This has selected individuals that germinate in response to cues associated with soil disturbances like tillage. These cues include light, high soil temperature, large fluctuation in soil temperature between day and night, the presence of nitrate (which forms when organic matter decays), and the absence of volatile substances like ethanol and acetaldehyde that form when aeration is limited. Many weed species, like common lambsquarters, respond to several of these cues, with some seeds germinating if one cue is present and most seeds germinating if several cues occur simultaneously. This is why you often observe a flush of weed seedlings that emerge with the crop.
We hypothesize, however, that when a cover crop is incorporated, the weed seeds are prompted to germinate right at the point when the pathogens are most active and that many die before they ever emerge from the soil.
The experiment:
We will collect soil at 1, 2, 4, 8, 16 and 32 days after we have incorporated a cover crop with tillage. We will also collect soil from adjacent plots or alleys that did not have a cover crop. Half of each soil will be steam sterilized. We will then mix each of the four types of soil with peat/vermiculite potting mix to improve water holding capacity and spread the soil in flats (pure field soil in flats usually becomes caked and cracked, and presents an unrealistic germination environment). Each flat will be planted with known numbers of seeds from several species of weeds. We will count and remove seedlings as they emerge for a period of three weeks.
Interpretation:
Greater emergence in non-cover crop soil or sterilized cover crop soil relative to cover crop soil that hasn't been sterilized would be consistent with the hypothesis. A decline in the difference between soil treatments through time would also be consistent with the hypothesis. If the hypothesis is not rejected, we will attempt to identify the specific pathogen species that are killing the weed seedlings.
The 2005 experiment and results: The cover crops were incorporated early in the season when were too busy to set up a greenhouse experiment. As a substitute, we used soil from Vegetable System 1 after pea vines and a substantial stand of powell amaranth was incorporated in mid-July. This soil was compared with soil from the adjacent alley that had been kept free of plants by regular cultivation. We collected soils the day of incorporation and 1, 2, 4, 8, 16 and 32 days later. For various reasons, soil could not be sterilized in a timely fashion and so the sterilized treatments were not included. 100 seeds each of velvetleaf, powell amaranth, lambsquarters, giant foxtail and barnyardgrass were planted in flats of the soil in a greenhouse and watered daily. Seedlings were counted and removed as they emerged.
All 5 species showed greater emergence from the alley soil than from the soil in which pea and weed material had been incorporated for the first few days after the incorporation date. Generally, the effect was limited to the first 2 days after incorporation of the "green manure". For barnyardgrass, the difference persisted until day 4 and for velvetleaf the difference persisted until day 8.
Thought question:
Does the fact that fewer seedlings emerged from the green manured soil than from the alley soil show that the hypothesis is correct? Think about this and then read the comments.
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