Note to readers, attached is the complete final report for FNE04-533
In terms of treatment effects on soil parameters and on lettuce head weights, Cover Crop seemed to be the least effective of the three active treatments, and the combined treatment (Compost and Crop Crop) appeared to be the most effective. Effects seemed to lessen in the second (October) harvest.
If one assumes that there are approximately 850 lettuce heads to a bed and that there are two harvests, then total pounds harvested under each treatment would be roughly 1680 lbs under the Compost Cover Crop, 1488 lbs under Compost alone, 1248 lbs under Cover Crop, and 1000 lbs without any active treatments (i.e., the Control). Obviously, actual values would depend on the pre-existing soil conditions and other factors; furthermore, these patterns might change if treatments were continued. For example, without any treatments, beds might progressively degrade if repeatedly harvested over time. Nonetheless, we hope that these are ballpark estimates of the actual effects.
The above calculations do not translate directly into farmer income given that lettuce is often sold by the head rather than by the unit weight. Because all plots were planted at equal density, we did not evaluate the potential for differences in the number of heads that could be grown under each treatment.
Lettuce heads under a certain size are not marketable, hence some of the plots which produced the lowest weights would probably have had even lower relative value if our parameter had been total weight of marketable heads. For example, if we assume that any head smaller than half of a pound would not be fit for market, then at the October harvest more than 60% of the heads from “Compost Cover Crop” treatments would be keepers, more than 50% from the Compost treatment, and only around 30-40% of the Cover Crop and Control heads. Heads were generally larger at the July harvest, and similar calculations showed that more than 95% of the heads in any of the actively treated plots were suitable for market, and around 75% of the Control heads were adequate.
Whether these treatment differences are sufficient enough to warrant planting an autumn cover crop atop a composted field is a decision that each farmer must make based on their time availability and marketing.
While more detailed studies would be needed, these relative improvements on lettuce weight seemed to be associated with somewhat similar results for soil macronutrients in 2005 (i.e., Compost Cover Crop and Compost seemed to generally have the biggest effects on soil nutrients). Patterns of soil nutrients in 2006 were less clear, and understanding them is complicated by the fact that they are the results not only of differential treatments, but also of differential harvests given the significant among-treatment differences in lettuce weight, and the fact that open plots had a 2005 lettuce harvest.
Of potential interest would be measurements of active organic matter and nitrogen. These tests measure soil components thought to be more readily accessible by plants and more immediately responsive to management. Furthermore, while it took us three years to complete this study, truly meaningful results would probably require the continued application of these treatments over a longer period of time. After all, one of the most relevant questions is, in some ways, not the immediate results of such practices but rather their long-term consequences. Does one, for example, degrade soils in the long-term by using compost without any cover crop?