Macronutrient inputs to annual cropping systems can benefit weeds as well as crops, sometimes decreasing or eliminating the benefits of fertilization. This interaction between fertility management and integrated weed management is becoming increasingly important as these fields focus on efficiency and prevention, respectively. The risk of increased weed competition reflects the fact that weed biomass and height may be highly responsive to nitrogen, phosphorus, and/or potassium. This generalization is supported by monoculture studies of species such as redroot pigweed (Amaranthus retroflexus L.), common lambsquarters (Chenopodium album L.), and barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.], and by ecological theory. However, field studies indicate variation in the effects of macronutrients on weed-crop competition and crop yield, even within species groups. To address challenges in interpreting, comparing, and extrapolating from these diverse reports, we propose a conceptual framework that summarizes the mechanisms underlying observed variation within and between studies. The framework highlights functional traits and trends that help predict yield outcomes in binary weed-crop interactions. Important factors include timing of emergence, maximum heights of the weed and crop, and relative responsiveness to the added nutrient. We also survey recent work on the effects of nutrient source (for example, the composition of organic amendments) on weed-crop competition. Because different sources vary in their nutrient release dynamics and supplied nutrient ratios, they may have dramatically different effects on weed-crop competition and crop yield. Finally, we offer a guide to best practices for studies of fertility effects on weed-crop competition. Although this review highlights several topics requiring further research, including fertility effects on multispecies interactions and interactions with other environmental factors, emerging methods offer considerable promise. Ultimately, an improved understanding of nutrient effects on weed-crop competition will contribute to the efficient and effective management of diverse cropping systems.

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Sufficient fertility is important for crop growth and yield but supplying a balanced amount of N, P, and K with compost and manure is a challenge and nutrient imbalances can benefit weeds more than crops. The goal of this study was to compare the aboveground growth responses of common northeastern U.S. crops and weeds to addition of composted poultry manure (CPM). A secondary goal was to test whether the observed growth responses to CPM could be attributed to the three macronutrients—N, P, and K—supplied in the CPM. Two field experiments were conducted over 2 yr. Species grown were corn, lettuce, kale, Powell amaranth, common lambsquarters, giant foxtail, and velvetleaf. Plants were grown in soil amended with CPM or single-nutrient organic N, P, and K fertility amendments. Single-nutrient P treatments with bone char did not adequately mimic P supply from CPM. In both years, biomass of all weeds studied increased with CPM amendment rate. Powell amaranth was the most responsive to CPM addition, increasing 1,775 and 159% from the control to the highest CPM rate in 2010 and 2011, respectively. Corn biomass increased by 22% with CPM rate in 2010 but did not increase with CPM rate in 2011. Lettuce biomass increased with CPM amendment rate (175% in 2010 and 109% in 2011), but due to the increased weed biomass at high CPM amendment rates, good weed control will be necessary to maintain this yield benefit. The increase in growth of Powell amaranth, common lambsquarters, and giant foxtail with CPM amendment was not due to N or K, and may have been influenced by P or another factor in the CPM. Velvetleaf was the only species that exhibited increased biomass with N addition (as blood meal), similarly to the increase with added CPM, suggesting the velvetleaf growth response to CPM was due to N in the CPM. The results show that nutrient ratios should be considered when applying organic amendments, because application rates of organic amendments that meet the crop's needs for one nutrient may oversupply other nutrients. Overfertilization will not benefit crop yield, but the results of this study show that high organic fertility application rates are likely to increase weed growth.

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