The nitrogen supply can be a yield-limiting factor in organic farming, especially when reduced tillage is applied. An organic field experiment was conducted from 2007 to 2013 to analyse the potential of the nitrogen supply through the efficient use of green manure crops in different tillage systems. Three farming systems were compared: a stubble cleaner system (SC) and a plough system (PL), both in a cereal-based crop rotation, and another plough system in a crop rotation that included alfalfa grass ley (PLALF). In the fifth year of the experiment, the experimental design was extended into a split plot design, and seven green manure treatments (Lolium perenne, Phacelia tanacetifolia, Sinapis alba, a mixture of Sinapis alba and Trifolium resupinatum, Trifolium resupinatum, Vicia sativa, and bare fallow as the control) were integrated into each of the three systems. The effects of the three systems and the green manure treatments on N mineralization, the soil microbial biomass and the yield of the main crops of oats and field beans in the sixth and seventh years of the experiment were analysed. The results showed that the choice of green manure species was of minor importance in the PLALF system. This system generally success- fully supplied N to the oats with oat yields from 3.6 to 5.1 t per ha.Vicia sativa was the most promising green manure crop in the SC and PL systems, with the Nmin values and oat yields (4.0 and 4.6 t per ha) being similar to those in the PLALF system. In the subsequent year, the PLALF system again was more successful in most of the Nmin assessments than the PL and SC systems, which often had rather similar results. In addition, a main crop of field beans was able to compensate for the differences in the Nmin content, and the yields were similar in all three systems (3.1 to 3.7 t per ha). The microbial biomass in the top soil was significantly increased in the reduced tillage system compared to the plough systems. In conclusion, reduced tillage in organic farming can promote soil microorganisms and be competitive if the nitrogen supply is improved through the efficient use of green manure or an adequate leguminous main crop.
Methods with reduced tillage, such as shallow ploughing, are recognized as environmentally friendly alternatives to conventional ploughing. Shallow ploughing (approx. 12–20 cm) offers numerous advantages for the structure of the soil and soil life. These advantages include the enrichment of organic material, soil organisms and nutrients on the soil surface [
However, if conventional ploughing is avoided in organic farming, the nitrogen supply for the crops may be reduced due to potentially low or delayed mineralization [
To use reduced tillage in organic farming, the whole system needs to be adapted [
To compare a cropping system with continuous shallow ploughing to two systems with conventional ploughing and two different crop rotations, in 2007, a multi-year field experiment was established at the experimental farm of the University of Kassel. For the shallow ploughing system, an advanced stubble plough, the stubble cleaner produced by the company Zobel/Rot am See, was chosen. As its mouldboards are smaller than those of conventional ploughs, the soil is not only turned but also more thoroughly mixed [
The effects of the three systems and the green manure treatments on N mineralization, the soil microbial biomass and the yield of the main crops of oats and field beans are the subjects of this paper. On the basis of the test results from the sixth and seventh years of the trial, the systems were compared to test the following hypotheses:
• Leguminous green manure crops are able to compensate for the reduced N mineralization in the reduced tillage system, while high N using species will suffer more from reduced tillage.
• The content of the microbial biomass in the top soil increases after six years of exclusive cultivation with the stubble cleaner in comparison to the plough systems.
• Leguminous green manure crops promote the yield of the non-leguminous main crop in the reduced tillage system, while in the plough systems the effect is less pronounced. In the following year, a leguminous main crop is able to compensate for the differences in N
The trial was conducted on the experimental farm of the University of Kassel, the state-owned Frankenhausen farm (51.412 N, 9.439 E; 231 m above sea level), on the “Untere Kiebitzbreite” field. The soil type is Haplic Luvisol. The soil texture in the plough horizon ranges from strong clayey silt (Ut4) to very silty clay (Tu4) [
The temperature profile during the trial period from 2011 to 2013 was similar to the 30-year average (Figure
Precipitation was lower than average in many months. May 2013 stood out due to its above-average total rainfall (data from 2011 to 2013 from the weather stations at Frankenhausen, Kassel Calden airport and Kassel Harleshausen; long-term average data from the German Meteorological Service (DWD), Kassel weather station).
A multi-year trial was carried out in 2007 as a randomized block design with four field replications. Originally the experiment was set up to test if the stubble cleaner is as effective for the control of
Alfalfa grass was sown in 2008 as a second crop rotation. The mixture consisted of 80% alfalfa and 20% grass (based on the seed weight). The cultivation of alfalfa grass lasted three years. The alfalfa grass was mowed two to three times per year, and the harvested biomass was removed. Tillage in this crop rotation took place after 2010 in the same way as the conventional tillage in the cereal-based crop rotation (Figure
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August 24, 2011 | Plough Rotary harrow | Depth 25 cm | Stubble cleaner | Depth 8-10 cm |
August 30/31, 2011 | Sowing green manure, rolling | Row spacing 18.75 cm | Sowing green manure, rolling | Row spacing 18.75 cm |
October 17, 2011 | Flame weeding fallow plots | Flame weeding fallow plots | ||
April 16, 2012 | Mulching the green manure (rotary mower) Cultivator | Depth 7-10 cm | Mulching the green manure (rotary mower) Stubble cleaner | Depth 7-10 cm |
April 17, 2012 | Rotary harrow Sowing (oats) | Row spacing 12 cm | Rotary harrow Sowing (oats) | Row spacing 12 cm |
August 13, 2012 | Harvesting (oats) | Harvesting (oats) | ||
October 01, 2012 | Plough | Depth 25 cm | Stubble cleaner | Depth 8 cm |
October 29, 2012 | - | Stubble cleaner | Depth 10 cm | |
April 18, 2013 | Spring tine cultivator | Depth 8 cm | Spring tine cultivator | Depth 8 cm |
April 22, 2013 | Rotary harrow Sowing (field beans) | Row spacing 15 / 45 cm | Rotary harrow Sowing (field beans) | Row spacing 15 / 45 cm |
June 11, 2013 | Hoeing | Hoeing | ||
August 26, 2013 | Harvesting (field beans) | Harvesting (field beans) |
In autumn 2011, the single factor trial was extended by the factor of green manure in a split plot design. The fourfold replication was maintained. The following legume and non-legume green manure species were cultivated in pure stands and in a mixture of two species (Table
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Lemmos | 40 kg ha |
LP |
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Boratus | 12 kg ha |
PT |
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Asta | 20 kg ha |
SA |
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Asta; Marco Polo | 10 kg ha |
SATR |
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Marco Polo | 20 kg ha |
TR |
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Ereica | 40 kg ha |
VS |
Bare fallow (= control) | BR |
In the trial year 2011/12, all the treatments were sampled or assessed (= 84 plots). In the trial year 2012/13, only selected plots were sampled or assessed in order to check whether the various green manure species still had an effect in the second year of cultivation after growing green manure. The following treatments were selected for this purpose: Bare fallow,
In the trial year 2011/12, growing the green manure crops was followed by sowing the main crop oats (
No fertilization was carried out during the trial period. No weed control was carried out in 2011/12. The area was hoed once in 2012/13.
The above-ground biomass yield of green manure was determined on November 17, 2011. For this purpose, a square area with a side length of 1.5 m per plot was cut off directly above the ground by hand and then weighed, and the dry matter was determined by drying a subsample of the harvested material at 60
Oat was also harvested by hand on August 13, 2012. For this purpose, half a square metre of land was harvested twice per plot. The beans were also harvested by hand on August 26, 2013. For this purpose, five metres of two rows of crops per plot were cut off just above the ground.
To analyse the nitrogen available to plants in the soil, samples up to a depth of 90 cm in 30 cm intervals were taken on three (2011/12) or four (2012/13) dates spread over the trial year. If sampling to a depth of 90 cm was not possible due to the soil conditions, the soil was sampled to a depth of 60 cm. A mixed sample was taken from three cores per plot. The samples were immediately cooled and carried to a freezer as soon as possible. The samples were examined according to DIN ISO 14255 and DIN EN ISO 11732 in the Landesbetrieb Hessisches Landeslabor, Kassel. The entire sample material was tested for NO
In spring 2013, the soil was tested for the content of the microbial biomass in addition to the N
The mean values and standard errors were calculated to describe the distribution of N
The statistical analyses were performed with SPSS-21 [
The analysis of variance of green manure crop and tillage/crop rotation system for green manure above ground biomass yield, green manure above ground biomass N content and green manure C/N ratio showed a significant influence of the green manure crops, the tillage/crop rotation systems and a significant interaction at the 0.001 probability level each (Table
The stubble cleaner system (SC) led to the lowest green manure yield for each green manure species, with the exception of
The green manure in the PLALF system had the highest N content, and green manure in the SC system had the lowest, with the exception of
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Green manure (GRM) | *** | *** | *** |
Tillage/crop rotation system (TCS) | *** | *** | *** |
GRM x TCS | *** | *** | *** |
*** Significant at the 0.001 probability level |
Of the green manure species studied,
Green manure |
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16.1 | ±0.35 |
19.8 | ±1.34 |
21.3 | ±0.75 |
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12.8 | ±0.53 |
15.9 | ±0.42 |
15.9 | ±0.59 |
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15.3 | ±0.63 |
16.1 | ±0.91 |
16.7 | ±0.27 |
Mixture of |
12.8 | ±0.56 |
16.1 | ±0.60 |
17.1 | ±0.54 |
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10.6 | ±0.38 |
9.4 | ±0.20 |
10.1 | ±0.23 |
The analysis of variance of N
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Source of variation | ||||||||
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*** | *** | *** | ** | n.s. | n.s. | n.s. | n.s. |
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*** | *** | *** | *** | *** | *** | *** | *** |
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*** | *** | *** | ** | n.s. | n.s. | * | ** |
* Significant at the 0.05 probability level | ||||||||
** Significant at the 0.01 probability level | ||||||||
*** Significant at the 0.001 probability level | ||||||||
n.s. not significant |
On March 15, 2012,
On May 15, 2012, the PLALF system had the significantly highest N
On September 05, 2012, the N
The analysis of variance of oats yield showed a significant influence of the green manure crops, the tillage/crop rotation systems and a significant interaction each on the 0.001 probability level, both for grain yield and straw yield.
The SC system had the lowest grain yield, the PLALF system had the highest grain yield, and the PL system had a medium grain yield. Bare fallow,
The SC and PL systems had significantly lower straw yields than did the PLALF system.
The analysis of variance of N
On April 08, 2013, and May 23, 2013, the N
On September 08, 2013, there were no significant differences in the N
The analysis of variance of N
In terms of the bean and straw yield of the field beans, the analysis of variance showed no significant influence neither of the tillage/crop rotation system nor of the green manure treatments in the second year after their cultivation. The bean yield at 86% DM varied between 3.1 t ha
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n.s. | n.s. | n.s. | n.s. | n.s. | n.s. | n.s. | n.s. | n.s. | n.s. | n.s. | n.s. |
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*** | *** | *** | ** | *** | *** | * | *** | *** | n.s. | * | *** |
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n.s. | n.s. | n.s. | n.s. | n.s. | n.s. | n.s. | n.s. | n.s. | n.s. | n.s. | n.s. |
* Significant at the 0.05 probability level | ||||||||||||
** Significant at the 0.01 probability level | ||||||||||||
*** Significant at the 0.001 probability level | ||||||||||||
n.s. not significant |
The C/N ratio of
The mixture of
The bare fallow was overgrown with weeds in every system despite flame weeding. This result means that N was also retained by the vegetation in these treatments. N mineralisation in all bare fallow treatments was comparable or higher in spring than N mineralisation in the non-legume green manure treatments. This result is in line with results from Baggs
The biomass yield of
In the second year after the green manure crops were grown, no effects of the green manure crops on the N content of the soil were detected. In contrast, the influence of crop rotation was measurable throughout the entire trial year. The cereal based crop rotation either with conventional tillage or with reduced tillage had similar N
The SC system had significantly more microbial N and C in the top soil (0–10 cm) than did the other two systems, while there were no significant differences in the underlying layers. This result is consistent with that from studies by Berner
The yield of oats in the PLALF system was statistically the same for all green manure treatments. In the PL and SC systems,
The late sowing date (April 22, 2013) could be an explanation for the relatively low field bean yield of less than 4 t ha
The field bean as a legume was insensitive to the various systems.
In the cereal-based crop rotation, the PL system was only partially superior to the SC system in terms of the stimulation of mineralisation and oat yield. The negative impact of unilateral crop rotation overshadowed the effects of tillage. A diverse crop rotation including forage growing would create better starting conditions for the use of a device for reduced tillage, and even ploughing the alfalfa or clover grass would not necessarily have to be done with a conventional plough. However, there is still a need for further research and testing in this area.
This study confirms that organic farming depends on a combination of different practices for successful arable farming (such as stubble breaking, primary tillage, mechanical weed control, green manure cultivation, crop rotation, and perennial forage growing) and that a reduction in one area (
This work was part of the FP7 ERA-Net (CORE Organic II) - project “Reduced tillage and green manures for sustainable organic cropping systems” (TILMAN ORG, www.tilman-org.net). It was funded by grants from the Federal Program for Organic and Sustainable Farming supported by the German Federal Ministry of Food and Agriculture.
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