1993 Annual Report for LNE93-036
Ecological Management of Potato Cropping Systems (ANE93.018)
1. Determine the effect of green manure, compost and manure use on soil physical properties, nitrate leaching, and potato plant growth, water status and yield.
2. Determine the impact of two microbial pathogens (Bacillus thuringiensis and Beauveria bassiana) and two insect predators (Perillus bioculatus and Coleomegilla maculata), singularly and in combination, on mortality of Colorado potato beetle.
This project compares several alternative management strategies with conventional potato management strategies. In one set of experiments, we compared a conventional soil management system with alternative systems utilizing organic soil amendments - an oat, pea, vetch, green manure crop; and/or a soil amendment of waste potato compost and beef manure. The crop rotation treatments did not affect soil physical properties or yields. The soil amendment treatment improved soil organic matter content, soil structure, and levels of several key soil nutrients. Moisture content of the soil was increased by amendments, particularly under irrigation.
With fertilizer rates held constant, crop growth was enhanced by the soil amendments. The amendment treatments significantly improved potato yields by 8.6, 8.2, and 3.4 tons per hectare (t ha-1) from 1993-95.
Yields responded significantly to soil amendment in both irrigated and non-irrigated systems with the highest yields in each year obtained when both inputs were used, yet short-term economic performance of the amendments was negative.
Economic performance would be enhanced if the amendment rates were optimized, fertility management was adjusted for those applied in the amendments, and if yield benefits occurred several years after application cease. In the related potato cropping systems study, a similar soil amendment system also significantly increased potato yields during 1994 (4.0 t ha-1, 12 percent) and 1995 (5.5 t ha-1, 30 percent), while two-year average N fertilization rates were decreased by 65 percent, P2O5 by 50 percent and K2O by 75 percent.
The pest management studies evaluated three biological control agents for Colorado potato beetle (CPB) singly and in combination.
The results thus far indicate that use of an integrated biological control program for the Colorado potato beetle will not reduce yields of potatoes, and will result in lower densities of the pest. The amount and toxicity of materials used is less in the biological system compared with the conventional pest management program. In 1993, three applications of a broad spectrum insecticide were required to maintain the pest below economic thresholds, while one application of the combination of the pathogens (Bt and B. bassiana) was utilized in the biological management program. Most importantly, the biological control programs tested have carry-over effects on CPB populations and therefore reduce the costs associated with CPB infestations in subsequent growing seasons.
Although there may be concern about the use of insecticides due to regulations regarding food safety tolerances and health effects, the more immediate threat to growers is their decreased effectiveness due to insecticide resistance. Colorado potato beetle has demonstrated a tremendous capacity to develop resistance to all major classes of insecticides. Resistance has been increasing in Maine and the Northeast.
The results indicate that cull potato compost and manure applications to a potato soil can increase yields (10 to 30 percent) relative to a conventional soil management system. This soil management approach increases soil water holding capacity, enhances soil physical properties, and improves yield, but we have shown that it cannot enhance yields during dry growing seasons to the same extent that is possible through use of supplemental irrigation. Our data indicate that yield improvements in response to soil management take place even when supplemental irrigation is utilized.
Short-term economic analysis indicates that the soil amendment program as used in this series of experiments is not economically viable for conventional potato growers in Maine. This is partly because the experimental systems used higher amendment loading rates than would likely be justified on a commercial farm. The amendments are expensive to purchase and apply in northern Maine under current conditions. Also, the data available only allow us to use yield benefit during the year after amendment application as our measure of crop performance in the amended system. It is extremely likely that these positive benefits will be observed for several seasons after soil organic matter has been increased; therefore, the economic viability will be very different when viewed on a more long-term basis.
If adopted, this technology could enhance productivity and stabilize yields for potato growers in the northeast, while also improving soil quality and reducing the risk of soil erosion. Work within a related cropping systems study indicates that the combination of manure, compost, and the legume rotation crop used here can result in these yield increases while allowing average nitrogen fertilization rates to be reduced by 65 percent, phosphate by 50 percent and potash by 75 percent.
We were not able to test the potential for reducing fertilizer rates in the present study. Land spreading of composts and manures has additional value as a waste disposal technique. The Maine potato industry, for example, produces more than 90,000 metric tons of cull potatoes each year that could be composted. Observed changes in soil physical properties suggest that soil erosion may potentially be reduced by the amendment treatments.
Based on the experimental results reported in this study, it appears that supplemental irrigation alone may be a profitable investment for producers of 'Superior' potatoes regardless of market outlet. It is important to note that both of the growing seasons represented within the present study (1994 and 1995) were relatively dry and, therefore, that the observed yield response to irrigation was relatively large.
With current (and reasonably foreseeable) prices for compost and beef manure, amendment alone and amendment coupled with reduced irrigation are not profitable options for Maine potato producers, according to the results obtained in this study. However, only a few years of data may not tell the whole story when it comes to adoption of a technology that could provide increasing, future benefits. The current soil amendment system was designed to rapidly build soil organic matter and was not designed to optimize the rate of amendment application. It is likely that the amendment rates and frequencies of application were much higher than were necessary for obtaining the observed yield increases. Also, we did not lower fertilizer rates in this experiment to compensate for the nutrients present in the amendments. Our other experiments have clearly indicated that fertilizer cost savings can be achieved when these amendments are utilized. Finally, it is extremely likely that the positive effects of the amendments on soil properties and crop yields will last for several seasons after amendment applications have ceased.
The response to the soil amendments should be studied over a much longer time period before any conclusions are drawn as to their true utility for Maine producers.