Soil fertility and weed management in long-term conventional and organic crop rotations

Project Overview

Project Type: Graduate Student
Funds awarded in 2008: $10,000.00
Projected End Date: 12/31/2009
Grant Recipient: University of Nebraska-Lincoln
Region: North Central
State: Nebraska
Graduate Student:
Faculty Advisor:
Charles Francis
Grain Place Foundation


  • Agronomic: corn, sorghum (milo), soybeans, wheat, hay


  • Animal Production: feed/forage
  • Crop Production: crop rotation
  • Education and Training: workshop
  • Natural Resources/Environment: biodiversity
  • Pest Management: chemical control, physical control, weed ecology
  • Production Systems: agroecosystems, organic agriculture
  • Soil Management: organic matter, soil analysis, soil chemistry, soil quality/health


    Growing concern for public health, the environment and social issues have stimulated the modern organic agricultural movement. Certified organic production systems eliminate agricultural chemicals and reduce external inputs to minimize environmental impact and increase farm income. The USDA National Organic Program specifies the identity of organic foods and provides consumers with uniform criteria on which to choose food that is produced without synthetic chemicals or transgenic organisms. Organic agriculture is growing rapidly, but many challenges remain.

    The threat of reduced crop yield in organic systems has been a primary argument among critics of organic agriculture. The assertion that conventional methods must be used to feed a growing population has led to numerous studies comparing yields in organic and conventional cropping systems. The results have been mixed, but for most crops conventional systems produce five to ten percent higher grain yields than organic systems. If yields are reduced in organic systems, it is important for researchers to close that gap and to identify other potential benefits of organic agriculture.

    Two potential benefits of organic agroecosystems are the long-term sustainability of soil fertility and the conservation of biodiversity. In this study we compared long-term soil properties, yields and weed communities among two organic and two conventional crop rotations. Improved soil fertility and weed biodiversity have been observed in organic agroecosystems and both can provide substantial benefits to the crop and surrounding ecosystems.

    The results of this study provide valuable production information for both organic and conventional farmers. Comparing yields of organic and conventional grain crops and determining the effects of soil fertility and weed populations on crop yield will assist farmers in making future management decisions. Long-term soil fertility levels also determine the sustainability of fertility building programs in organic and conventional systems. Lastly, the comparison of weed diversity among rotations addresses the possibility and practicality of preserving biodiversity in agroecosystems.


    Spurred by concerns about increasing farm size, environmental pollution, reduced biodiversity across the landscape, and increased consumer demand for organic products, there have been several long-term studies investigating the differences between conventional and organic cropping systems (Lockeretz et al. 1981; Posner et al. 2008; Pimentel et al. 2005; Porter et al. 2003; Cavigelli et al. 2008; Kirchmann et al. 2007). Thus far, the literature has been somewhat contradictory. Reduced yields were reported for most crops in organic systems (Lockeretz et al. 1981; Posner et al. 2008; Porter et al. 2003; Cavigelli et al. 2008), while some organic systems produce equal or greater yields relative to conventional (Pimentel et al. 2005).

    Organic cropping systems rely on complex crop rotations for weed and pest control, and for soil nutrients based on biological nitrogen fixation and the recycling of nutrients (Gosling and Shepherd 2005). Additionally, organic cropping systems often rely on organic soil amendments such as farmyard manure or compost. These organic amendments have been associated with improved soil properties (e.g. increased soil organic matter and water holding capacity, lower bulk density and enhanced pH stabilization, and increased soil levels of Ca, Mg, K and P). The inclusion of a perennial forage such as alfalfa in rotation may further improve soil structure, soil organic matter and nutrient cycling due to its capacity to biologically fix atmospheric nitrogen (Riley et al. 2008).

    Complex organic crop rotations may promote diverse weed communities. When maintained below acceptable economic thresholds, diverse weed communities can provide significant benefits to the crop and surrounding ecosystems. Modern agricultural practices have led to a decline in the diversity of weeds in agroecosystems due to widespread use of herbicides and simplicity of crop rotations (Leeson et al., 2000; Murphy et al., 2006). As a result, many now view organic agriculture as a means of protecting biodiversity within agroecosystems.

    If dominant weed species are not managed, organic agriculture may not be a viable solution for the maintenance of biodiversity in agroecosystems. If dominant weed species are managed properly, one could expect the seedbank and aboveground diversity to be greatest in organic systems due to the complexity of crop rotations (Murphy et al., 2006) and least in conventional systems due to the intensive use of herbicides (Mahn, 1984; Wicks et al., 1988; Moreby and Southway, 1999; Menalled et al., 2001). While the relationship between organic cropping systems and weed diversity has been well established in the aboveground weed community, fewer studies have examined this relationship in the seedbank community of long-term crop rotations.

    Crop rotation, weed density management and nutrient inputs may also have an effect on seedbank diversity and relative quantity of grass and broadleaf species within a system. Grass and broadleaf weed populations are often related to the growth habit, phenology and morphology of the different crops in a rotation (Liebman and Dyck, 1993; Barberi et al., 1997; Menalled et al., 2001). Understanding the factors that influence weed communities in long-term crop rotations will be especially useful to organic farmers, who consistently rank weed management as the most important research priority (Walz, 1999; MNDA, 2007).

    Project objectives:

    Objectives and Hypotheses

    In 1975, the Long-Term Rotation Experiment at the Agricultural Research and Development Center near Mead, NE was initiated to determine the effects of crop rotation and animal manure on grain yield. The first four cycles of the rotation were reported by Lesoing (1992). The experiment was redesigned in 1996 to compare forage- and animal manure-based organic rotations with conventional crop rotations. The objectives of the current study were to:
    1) determine the effects of management system on soil chemical and physical properties and how these factors contribute to grain yield;
    2) and to evaluate seedbank density, species richness, evenness and diversity along with aboveground grass and broadleaf weed abundance and broadleaf weed diversity within and among organic and conventional crop rotations.

    We hypothesized that the application of bovine manure will:
    (i) increase grain yields in the organic animal manure rotation compared to the organic green manure rotation;
    (ii) increase concentration of soil nutrients, including P, Ca, Mg, K, Zn, as well as increase soil pH values; and
    (iii) increase levels of soil organic matter among all rotations.

    We also hypothesized:
    (i) that the conventional rotations (CR and DIR) will have greater grain yields than both organic rotations (OAM and OGM) in all crops;
    (ii) grain yields will be affected by soil P and organic matter levels; and
    (iii) in years of less than average rainfall, yields will be greatest in rotations with high levels of organic matter content.

    Lastly, we hypothesized that weed density, species richness, evenness, diversity, and aboveground weed biomass will each be greatest in the organic crop rotations. The results of this study will help to document the competitiveness and long-term sustainability of organic and conventional crop rotations; results will also help to determine whether or not organic cropping systems provide a solution for increasing biodiversity in agroecosystems and generate insight useful for designing appropriate weed management strategies.

    Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.