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Winter/Spring 2006 - Vol. 6/No. 2


Long-term comparison of yield and nitrogen use in organic,
winter legume cover crop and conventional farming systems

By Kathleen M. Reed, William R. Horwath, Steve Kaffka, R. Ford Denison, Dennis Bryant, and Z. Kabir

Eleven years of data comparing nitrogen at UC Davis’ sustainable ag research site is now available. Researchers (from left to right) Wes Wallender, Sam Prentice, Aaron Ristow and Will Horwath are pictured at the site. (photo by Lyra Halprin)

We have recently analyzed 11-years of data comparing nitrogen use in the organic, winter legume cover crop (WLCC), and conventional farming systems at a field research site for sustainable agriculture at UC Davis, the Center for Integrated Farming Systems (CIFS). This article outlines results from a study at the CIFS site comparing three tomato/corn cropping systems for yield and N uptake from 1994 to 2004, and suggests possible reasons for yield differences among farming systems. CIFS represents the merger of the Sustainable Agriculture Farming Systems (SAFS) project, which began in 1988, and the Long Term Research on Agriculture Systems (LTRAS) project, which began in 1991. The former SAFS site was designed to introduce crop diversity and winter cover crops into rotations, with a specific emphasis on the conversion from conventional to low-input and organic systems. The goals of LTRAS have been to evaluate the effects of differing amounts of fertilizer, organic matter and irrigation on the longterm capacity to sustain or improve crop yields and related environmental properties. Cropping systems in this study are designed to reflect this research purpose rather than as models for commercial farmers.

Yield and N Uptake


Corn yields across all systems ranged from 4,823 to 14,966 lbs/acre (standardized to 15.5% moisture). Conventional yields were highest in 1994 and lowest in 1995 and 1999, when conventional planting dates were delayed and corresponded with the organic and WLCC systems (Fig. 1). Organic corn yield was lowest in 2002 (4,823 lbs/acre). However WLCC corn yields were lowest in 2004 (5325 lbs/acre), while in the same year conventional yields of 14,966 lbs/acre were the highest ever observed in the trial. One reason for low yields that year in the WLCC system may be that the only N input to corn and the prior tomato crop was from the previous winter’s cover crops. Previously, the tomato phase of this cropping system had been fertilized.

Plant N uptake was correlated with corn yields. Conventional yield and N uptake varied from year to year from 1993 to 2004, while organic and WLCC yields and N uptake decreased over the last six years remaining low or declined. In this experiment, cropping systems are compared, so conventional corn tends to be planted several weeks to a month earlier than the other systems, which must accumulate cover-crop biomass in the spring. In the organic and WLCC systems, planting must be delayed until after the incorporation of fresh organic matter. Earlier planting tends to result in higher yields. This may not be the only cause of yield differences. In another study at the CIFS site starting in 2005, fertilizing the WLCC system with the later planting dates has increased yields (data not shown) comparable to the conventional system, showing that N availability or timing also is a critical issue affecting maximum yield potential.


Tomato yields ranged from 21,178 to 70,538 lbs/acre during the study (Fig. 2). Average organic and WLCC tomato yields were, for the most part, comparable to the conventional yields. However, organic yields varied less over the years than conventional and WLCC yields. Tomato N uptake generally mimicked yields over the course of this study.

N balance, N storage in soil, and N loss

The N balance from 1994 to 2004 revealed that the organic system had the greatest cumulative N input and N balance, while the conventional system had the largest N output in harvested crops. Although the organic system had the greatest cumulative N input, it also had the lowest N output of all the systems. Soil N storage was highest in the organic farming system, which was the only system with an increase in soil N. A buildup of soil organic matter is required to increase the potential for N mineralization. Despite that increase in soil N and C, corn yields were consistently lower in the WLCC and organic systems than in the conventional one and have remained low for the last several years.

  Table 1. Cumulative N balance, soil storage (at 0-30 cm soil depth), and loss for the organic, WLCC, and conventional cropping systems at CIFS, 1993-2004.
Farming System N balance
(lbs N/acre)a
Soil N storage
(lbs N/acre)b
N loss
(lbs N/acre)c
Organic 2200a 611a 1589a
WLCC 516c -293b 809b
Conventional 691b -342b 1033b
a N balance = total N input – crop removal
b Soil N storage = soil N in 2004 – soil N in 1993   (based on 2005 and estimated 1993 bulk density measurements)
c N loss = N balance – soil N storage

Comparison among farming systems of total soil N storage at 0-30 cm depth between 1993 and 2004 showed the greatest quantity of N storage in the organic system. While the organic system accumulated 611 lbs N/acre, soil N storage decreased by more than 290 lbs N/acre in both the WLCC and conventional systems (Table 1). The amount of unaccounted N (presumed lost to the atmosphere or groundwater) was greatest in the organic system (1,589 lbs N/acre), however, N input was also highest in this system. There were also large amounts of unaccounted N in the conventional and WLCC systems, 1,033 and 809 lbs N/acre respectively over the duration of the study. The organic system lost 72 percent of its 11-year crop N balance, while storing 28 percent of that N in the soil. However, both the WLCC and conventional systems lost 100 percent of their 11-year crop N balance and showed a depletion of soil N. Though the organic system showed a more positive N balance compared to the other systems, it had an overall greater loss of the cumulative amount of total N applied during the study. Even though lower amounts of C and N were added to the organic system after 1997, amounts may still have been in excess.

The mean balance of soil C over the 11-year study was 8,268 lbs C/acre in the organic system, 345 lbs C/acre in the WLCC system, and -1,144 lbs C/acre in the conventional system (data not shown). The organic system may have reached maximum capacity for N and C storage by 1997, and may have been unable to store additional N and C in subsequent years. The results indicate that despite large amounts of C and N applied as manure and cover crops, these soils have a limited capacity to accumulate organic matter. California’s warm climate, combined with tillage and irrigation during the warm growing season are probably the primary reasons for this limitation. However, the initial accumulation of 8.3 tons of C in the organic system is significant and shows the potential to store C in these intensively managed systems that include cover crops and manure.

At the former SAFS site prior to 2003, there was a four-year crop rotation of tomato, safflower, corn, oats/vetch, and beans in the organic, intermediate (WLCC), and one conventional system, while the other conventional system had a two-year, two-crop rotation of tomato and wheat. The organic and WLCC systems at that site had much lower losses over a 10-year period, 80 and 33 lbs N/acre respectively, compared with the two-year and fouryear conventional systems, which lost 365 and 403 lbs N/acre respectively. Long-term yields for both corn and tomato were comparable among all three farming systems at the former SAFS site, but corn planting in the conventional systems was delayed to correspond to optimum dates for the organic and WLCC systems.

Over the 11-year study at CIFS, the conventional and WLCC systems lost soil organic matter, while there was an increase in soil organic matter in the organic system. The accumulation of N and C in the organic system may improve soil quality over the long-term, while loss of soil N and C in the conventional system may decrease long-term soil quality in that system. Soil quality measured in this way apparently was not correlated with yield over this 11 year period, however. Low crop diversity may have resulted in negligible C and N gains in the conventional system despite this system attaining the greatest overall crop yields. A lower C and N gain in the WLCC systems was most likely attributed to lower corn yields and correspondingly lower stubble dry matter amounts left in the field after grain harvest, combined with cover crops planted only every other year only before corn. In the former SAFS project, N input amounts for some systems were considerably less than in the LTRAS experiment, which may have contributed to the lower amounts of unaccounted N. Furthermore, cropping systems used in the former SAFS project were more diverse than those used in the LTRAS project. These diverse cropping systems may have also contributed to lower amounts of unaccounted N and to more positive long-term soil C and N balances at the former SAFS site. In comparing the SAFS and CIFS studies, the results suggest that future cropping systems should include diverse crop rotations and winter cover crops.

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