The performance of ecological, integrated and conventional nutrient management systems in cereal cropping in Finland

ELSEVIER

Field Crops Research

Field Crops Research 37 (1994) 3-10

The performance of ecological, integrated and conventional nutrient management systems in cereal cropping in Finland R.T. Poutala a'*, O. Kuoppamaki b, J. Korva c, E.

Varis a

aDepartment of Plant Production, PL 27, Helsinki University, 00014 Helsinki, Finland bAgricultural Economics Research Institute Finland, PL 3, 00411 Helsinki, Finland CThe International Potato Center (CIP), P.O. 17-16-129-CEQ, Quito, Ecuador

(Received 13 May 1993; accepted 9 February 1994)

Abstract A field trial was conducted during 1989-1992 in order to study the performance of cereal crops under three different nutrient management systems. The systems were divided into conventional, integrated, and ecological groups according to the fertilizer/ organic manure application ratio and crop rotation. Each group was further divided into the crop-production- and animalproduction-oriented rotations, the latter including also leys. Average cereal grain yield of the crop-production-oriented rotation was 5% higher than that in the animal-production-oriented one. However, this difference was not significant. Conventional nutrient management systems produced 6% and 32% more cereal grain than the integrated and ecological nutrient management systems, respectively. Difference to the ecological nutrient management system was significant. The 2-year leys in the integrated nutrient management system produced 8% more dry matter than both the conventionally and the ecologically managed leys. This difference was not significant. Nitrogen yield of the integrated leys exceeded that of the conventional leys by 30% and the ecological leys by 10%. The economic result of the systems indicated greater annual variation in the ecologically managed systems. However, by taking into account 30% surcharge on the ecologically produced products, the greatest gross margin/crop was achieved under ecological nutrient management. Gross margin/crop in the conventional and the integrated nutrient management systems did not differ significantly. A significant part of the mineral fertilizers can be replaced with animal manures, legume undercropping or green fallowing. Grass-legume leys also enabled successful reduction in mineral fertilization. Key words: Cereal; Crop rotation; Integrated farming; Legume; Organic farming; Undercropping

I. Introduction The use of agrochemicals and the application levels of fertilizers are still increasing worldwide (Vereijken, 1992). However, as it has become evident that high application rates of mineral fertilizers, characteristic of conventional nutrient management, account for a significant part of environmental pollution, more diverse *Corresponding author. 0378-4290/94/$07.00 © 1994 Elsevier ScienceB.V. All rights reserved SSD10378-4290(94)00013-3

nutrient management practices are under investigation (Vereijken, 1989, 1990). In ecological nutrient management, as it is practiced in Scandinavia, the supply of nutrients is dependent on slow-release fertilizers and organic manures. However, having nitrogen as the most limiting factor in cereal production in the agro-ecological conditions of Finland, a 50% reduction in mineral nitrogen fertilization has led to 30% yield reduction, and total exclusion of mineral nitrogen fertilizers combined with organic manuring to 50% yield reduction

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R.T. Poutala et al./Field Crops Research 37 (1994) 3-10

compared with the yields achieved under conventional fertilizer regimes (Korva and Varis, 1990; J. Peltonen, pers. commun., 1993). It has been suggested that the use of fertilizers can be reduced significantly in conventional nutrient management without a risk of drastic yield reductions by adopting selected methods of ecological nutrient management. This so-called integrated nutrient management (INM) proposed by Vereijken (1990) features maintenance of soil fertility, nutrient supply adjusted to achieve maximum utilization by the crop plant and thus minimum environmental stress and partial substitution of mineral fertilizers by nutrients from organic sources. It has, however, been questioned whether it is feasible to carry out reduced-input nutrient management systems on farms engaged only in crop production, as the utilization of cattle manure and biological nitrogen fixation on these farms is limited. The objective of the present experiment was to study the performance of six cropping systems differing in their nutrient management regimes and crop rotations. More specifically, the hypothesis tested was that reduction in mineral nitrogen application levels combined with efficient organic manuring and diverse crop rotations does not affect the present yield levels of cereal production in Finland.

2. Materials and methods

The trial was initiated in 1989 and conducted for three successive seasons. The performance of six cropping systems differing in their nutrient management regimes and crop rotations were studied. Systems were divided into three groups: ecological, integrated and conventional. Each group was further divided into crop-production- and animal-production-oriented rotations. The experiment was carried out as a duplicate in two parallel blocks (a and b) having different rotation start points. Nutrient management history prior to the experiment and soil characteristics were similar for the blocks. Crop rotations are described in more detail in Table 1. Due to a failure in mineral fertilization in 1989, integrated systems in rye (Secale cereale L.) production were carried out only in 1991. The experiment was conducted at the Suitia Experimental Farm (60°11 'N, 24°10'E) of Helsinki University and was laid out in a field that had been managed

either conventionally or ecologically since 1982 (Korva and Vails, 1990). The soil at the site was classified as silty clay. As the success of reduced external input cropping is strongly based on long-term improvement in the agro-ecology (Dabbert and Madden, 1986), testing of the integrated and ecological nutrient management systems in the field with an ecological cropping history was chosen. However, it has to be noted that ecological cropping (1982-1988) had not had any major impact on soil fertility (Heinonen-Tanski, 1990) nor had the yield level of ecologically grown crops improved during 1982-.1988 (Korva and Vails, 1990). Also, a uniform barley (Hordeum vulgare L.) crop was established in the experimental field prior to the experiment to minimize direct precrop effects. The plot size of each system was 20 m X 60 m, allowing the crop management practices to be carried out by normal farm-scale machinery. No grazing was allowed. Mineral fertilizers were drilled under the seed simultaneously with sowing (spring cereals and fall application of rye) or were spread on the soil surface (spring application of rye and leys). The NPK-fertilizers applied for spring cereals contained 20% nitrogen, 4% phosphorus and 8% potassium. In fall, rye received NPK-fertilizer containing 14% nitrogen, 6% phosphorus and 16% potassium. Single nutrient fertilizer (27.5% N) was broadcast for rye in the spring and as a split application for leys. Application rates are shown in Table 1. A MCPA-based herbicide (Diklo, Kemira) application (3 l/ha) took place mid-June and propiconazol (TILT, Ciba-Geigy) fungicide application (0.5 1/ha) in late June. Pesticides were sprayed, when necessary, to the crops in the conventional and the integrated systems. When red clover was sown under the cereal crop, application rate of Diklo was reduced to 1 1/ha. Nutrient management and pest control practices of the different nutrient management systems are described in Table 1. Winter cereal (rye, cv. Voima) was sown from the 28th to the 31 th of August and spring cereals (wheat, Triticum aestivum L., cv. Tapio; barley, cv. Pohto) from the 30th of April to the 15th of May at densities of 500 seeds/m 2 (rye, barley) and 650 seeds/m 2 (wheat). The rye crop was combine harvested from the 1st to the 22nd of September and the spring cereals (barley, wheat) from the 8th to the 30th of August. The species mixture of the leys in the integrated and ecological systems consisted of red clover (Trifoliurn

R.T. Poutala et al. / Field Crops Research 37 (1994) 3-10

Table 1 Descriptionof the nutrient managementsystemsstudied 1989-1992 System I A. Croppinghistory 1982-1988

conventional conventionalIcy/ cereal rotation cerealrotation

B. Nutrient management C. Rotationtype

D. NPK-fertilizing~ -barley, wheat -rye -leys

SystemII

System III

System IV

ecologicalcereal/ ecologicalIcy/ root crop/ legume/ legume rotation cerealrotation

conventional ( CNM)

integrated ( INM)

SystemV

System VI

ecologicalcereal/ ecologicalIcy/ root crop/ legume/ legume rotation cereal rotation ecological ( ENM)

crop production animalproduction cropproduction animalproduction crop production animalproduction oriented (CP) oriented(AP) oriented(CP) oriented(AP) oriented(CP) oriented(AP)

90 kg N/ha 50+ 70 kg N/ha 120+ 80 kg N/ha

45 kg N/ha 25 + 35 kg N/ha -

45 kg N/ha 25 + 35 kg N/ha 60+40 kg N/ha

E. Crop protection -herbicide application annual -fungicide application 1989

annual -

1991 + 1992 -

1991 + 1992 -

F. Rotation (a/b) 1989/ 1991 -1990/1992 - 1991/ 1989 -1992/1990

ley ley rye barley

spring wheat green fallow rye + clover barley+ clover

ley Icy rye barley

spring wheat green fallow rye + clover barley+ clover

ley ley rye barley

1988 clover 1990 vetch 1991 clover

1988 slurry 1990 compost 1992 slurry

1988 clover 1990 vetch 1991 clover

1988 compost 1990 compost 1992 slurry

-

G. Manuringb

90 kg N/ha 50 + 70 kg N/ha

spring wheat pea rye barley

aThe rate of fertilization/growthperiod. bApplieationrates of earle manure compost and slurry were 30 t/ha ( 130 kg N, 35 kg P, 90 kg K) and 80 t/ha (130 kg N, 40 kg P, 160 kg K), respectively.

pratense L., cv. B j 6 m ) , timothy (Phleum pratense L., cv. A l m a ) and meadow rescue (Festuca pratensis Huds., cv. Kalevi) at sowing rates of 9 k g / h a , 16 k g / ha and 5 k g / h a , respectively. The ley in the conventional system consisted o f timothy and meadow fescue at sowing rates of 18 k g / h a and 12 k g / h a , respectively. Undercropping was carried out by sowing red clover (6 k g / h a ) in rows perpendicular to the main crop. Sowing took place in the spring before the emergence of the main crop (except for winter rye). The growth of the undercrop was observed by cutting the red clover stand at six (0.25-m 2) locations/plot immediatelyafter the harvest o f the main crop. For the determination of dry matter ( D M ) accumulation and tissue nitrogen content, six 0.25-m 2 samp i e s / p l o t were collected at late tillering, at early

heading and at maturity (Feekes stages 5, 10 and 11.4; Large, 1954). All of the samples were oven-dried at 103°C to constant mass. Nitrogen content of the leaf tissue was analyzed by the Kjeldahl procedure ( A A C C , 1983). For the determination of simple grain mass, hectoliter mass, falling number, grain protein content and grain moisture content 2 kg of grain was randomly sampled from the plot yield. Simple grain mass was calculated as the mean of three random samples of 100 grains. Hectoliter mass and grain moisture content were analyzed in duplicate by a Grain Analyses Computer G A C II (Dickey-John Corp.). For the determination of grain protein content and falling number, 500 g of grain was milled and duplicate samples of flour were subsequently analyzed by the Kjeldahl procedure ( A A C C , 198 3 ) and by Falling Number Analyser (Fall-

6

R.T. Poutala et al./Field Crops Research 37 (1994) 3-10

ing Number AB). All grain yields were adjusted to 15% moisture. The experiment was a randomized complete block design with three replicates. Cereal production data were also subjected to gross margin analyses using prevailing product and input prices. The gross income consisted of the sales of the grain and the variable costs included seed, fertilizers, pesticides, labor and interest of invested capital (60% winter rye, 30% spring cereals). Analyses of variance were performed using the general linear model (GLM) procedure of the SAS program (SAS Institute Inc., 1987) and the mean separation according to least significant difference procedure (LSD). In all of the analyses significance level used was 5%.

and INM. On average, the reduction was 1065 k g / h a and 855 k g / h a compared to the CNM and the INM, respectively. Grain yields of individual crops are presented in Table 2. Rainfall was exceptionally low during the period from May to July 1992 ( 60 mm vs. 19611991 average of 148 mm) and drought stress had a major influence on barley yields, levels being generally only 1 / 8 - 1 / 5 of the normal. Leys were included only in the AP systems. The total DM yield consisted of two harvests. Across management systems the DM yield of the first year ley exceeded that of the second year by 25% (2.4 t/ha). In 1989 the DM yield of the ENM ( 10.8 t/ha) and INM ( 11.4 t/ha) exceeded the DM yield of the CNM (7.8 t/ha). Contrary to this, in 1990 the DM yield of CNM (9.6 t/ha) exceeded both that of the INM (7.4 t/ha) and the ENM (6.5 t/ha) (Table 2).

3. R e s u l t s

3.2. Hectoliter mass and simple grain mass 3.1. Yields

Rotation type (crop production oriented, hereafter CP and animal production oriented, hereafter AP) did not significantly affect grain yield of the cereal crops. Grain yield from the conventional nutrient management systems (hereafter CNM) did not differ significantly from the grain yield from the integrated nutrient management systems (hereafter INM) in any cereal crops. In 1989 and 1990 the grain yield from the ecological nutrient management system (hereafter ENM) remained significantly lower than that from the CNM

Rotation type did not significantly affect the hectoliter mass or the simple grain mass of the cereals. Nutrient management system influenced the hectoliter mass of the cereals in 1989 and 1990 when hectoliter mass of barley and wheat remained lower in the ENM (Table 3). Simple grain mass of barley was affected by rotation type in 1990 as the AP systems produced heavier seeds than the CP systems. Among the nutrient management systems there were no consistent differences in simple grain mass (Table 3).

Table 2 Grain yields of the cereal crops and dry matter and nitrogen yields of the leys (t/ha) by crop, year, nutrient management system and rotation type Wheat

Barley

Rye 1991

1990

1992

Ley, dry matter

Ley, nitrogen

1989

1990

1989

1990

AP

AP

AP

AP

1989

1991

1989

CP

CP

CP

AP

CP

AP

CP

AP

CP

AP

mean

3.31

3.96

2.66

2.72

3.96

3.57

3.73

3.28

0.62

0 . 7 4 10.03a

7.61b

0.237a 0.192b

CNM INM ENM

4.15a 3.92 3 . 6 2 a 4.47 2.17b 3 . 4 8

1.03 0.71 0.47

9.59a 7.42b 6.46c

0.149b 0.198a 0.290a 0.208a 0.273a 0.172b

3.29a 3.49a 4 . 0 7 3 . 5 6 a 4 . 6 8 a 4.24a 0.63 4.02 3.83ab 3.80a 3.66ab 0.66 2 . 0 2 b ! . 9 5 b 3 . 7 9 3 . 3 1 b 2 . 7 1 b 1.94b 0.57

7.82b 11.46a 10.84a

CP refers to crop-production-orientedrotation, AP animal-production-orientedrotadon. CNM refers to conventionalnutrient management,INM to integrated nutrient managementand ENM to ecologicalnutrient management. The mean values are calculated over the nutrient management systems within a rotation type.

R.T. Poutala et al. / Field Crops Research 37 (1994) 3-10 Table 3 Simple grain mass and hectoliter mass of grain by crop, year, rotation type and nutrient management Hectoliter mass, kg

Simple grain mass, mg Wheat

Rye

Barley

Wheat

Rye

Barley

1989

1989

1990

1989

1989

1990

CP

CP

AP

CP

AP

CP

AP

CP

CP

AP

CP

AP

CP

AP

mean

35.1

28.1

27.9

37.6a

40.3b

40.1

39.0

80.4

72.9

72.1

59.8

60.1

61.3

61.3

CNM INM ENM

35.9 35.6 33.9

28.5 27.7

28.6a 27.1b

38.7 38.5 35.8

41.8a 39.9a 38.8b

38.8b 41.1a 40.3ab

37.2 40.6 39.3

81.5 81.1 78.5

73.1 72.7

72.2 71.9

59.9a 60.4a 58.7b

61.5a 60.1b 58.0c

60.6 62.3 61.1

61.0 61.2 61.6

1992

1992

The mean values are calculated over the nutrient management systems within a rotation type. See Table 2 for abbreviations.

Table 4 Protein content (%) of grain by crop, year, rotation type and nutrient management system Wheat

Rye

1989

1991

1989

CP

CP

CP

mean

10.5

10.2

9.9

CNM INM ENM

12.1a 9.6b 9.9b

10.7a 10.1b 9.8b

9.9 9.8

Barley 1991 AP

1990

CP

AP

9.8

11.9

11.6

10.4 9.3

10.8 12.3 12.4

11.9 11.8 11.0

CP

1992 AP

CP

AP

9.5

10.5

11.5

11.3

10.4 9.4 9. I

11.2 9.9 10.1

12.1a I 1.5ab 10.8b

11.9 11.2 10.9

Mean values are calculated over the nutrient management systems within a rotation type. See Table 2 for abbreviations.

DM/ha

a

/

"

."

4

...

sampling I

•

°

sampling II

sampling III

Conventional ~ Integrated )< Ecological

Fig. 1. Dry matter accumulation by the nutrient management system. Mean values across the cereal crops and the experimental years at three developmental stages: late tillering (I), early heading (II) and maturity (HI).

R.T. Poutala et al. / FieM CropsResearch 37 (1994) 3-10 %N of the DM 3.5 3 2.5

a

1.5p 1~-

b

~

b b

0,5i--

I

0

sampling I

sampling II

Conventional

Integrated

sampling Ill ~, Ecological

Fig. 2. Nitrogencontentof the leaf tissue by the nutrientmanagementsystem. Mean values across the cereal crops and the experimentalyears at three developmentalstages: late tillering(I), early heading (II) and maturity (III). Table 5 Gross margin/ha FIM (FinnishMark)" of the cereal crops in the crop-production-orientedrotations (CP) by the nutrientmanagementsystem and year

Returns Variable costs fertilizers pesticides others Grossmargin

Conventional

Integrated

Ecologicalb

Rye Wheat Barley 1989/1991 1989/1991 1990/1992

Rye Wheat Barley 1989/1991 1989/1991 1990/1992

Rye Wheat Barley 1989/1991 1989/1991 1990/1992

9947/84659553/8566 7622/

1418

1077/1077 889/ 889 889/ 889 111/ 45 319/ 45 83/ 45 1190/12751557/1517 1247/ 808 7569/6068 6787/6115 5403/ -324

-/8712 8073/9791 6359/

1179 7274/11917 6035/9605 4291/

962

- / 539 445/ 445 445/ 445 0/ 0 0/ 0 0/ 0 -/ 0 0/ 45 0/ 47 0/ 0 0/ 0 0/ 0 -/1240 1450/1914 1448/ 1138 1024/ 1233 1355/1867 1319/ 1120 -/6933 6178/7387 4466/ -451 6250/10684 4680/7738 2972/-158

"IFIM = 0.2 US$. b30% surchargeadded on the products.

3.3. Grain protein and falling number Protein content or falling number of the cereals did not differ among rotation types. As the mean of 1989 and 1991, the protein content of the wheat grain of the CNM, exceeded that of the INM by 1.6% and the ENM by 1.5%. In 1992 the protein content of barley was 1.3% higher in the CNM than in the ENM (Table 4). The falling number of wheat and rye did not differ between the rotation types nor between the nutrient management systems (data not shown).

3.4. Nitrogen yield of the leys On average, the nitrogen yield of the leys was higher in 1989 than in 1990. In 1989 the leys of the INM and the ENM produced more nitrogen than those of the CNM. Nitrogen yield of the second year ley was lower in the ENM than in the INM and CNM (Table 2).

3.5. Accumulation of biomass and nitrogen Crop stand The accumulation of DM and nitrogen across the cereal crop species and experimental years was affected

R.T. Poutala et al. /Field Crops Research 37 (1994) 3-10

by nutrient management system. At the early heading stage, the DM yields of CNM (4.3 t/ha) and INM (3.9 t/ha) was higher than those of ENM (2.7 t/ha) (Fig. 1). At the late tillering stage, the nitrogen content of the crop canopy of CNM and INM exceeded that of the ENM. At the early heading stage and at maturity the nitrogen concentration of'plant tissue was higher in CNM than in INM and ENM (Fig. 2). Undercrops

Undercrop was included only in the integrated and the ecological nutrient management systems. Mean red clover dry matter over the four experimental years and three cereal crop species was 135 kg/ha in INM and 751 kg/ha in ENM. 3.6. E c o n o m i c results

Gross margin analyses of cereal crops in the different nutrient management systems are presented in Table 5. 4. Discussion Compared to CNM, average yield reduction of cereals grown in the INM and ENM was 6.4% and 32.2%, respectively. Identical trends have been observed by Vereijken (1989). In the work of van Faassen and Lebbink (1990) and Varis and Pietila ( 1991 ), 0-15% yield reduction was recorded in the integrated crop production system. In this study, cereal yields were affected little by the rotation type (CP, AP). However, there was an indication of yield reduction in the AP type rotations. Poor yield performance of ecologically managed cereals has been reported in similar conditions by Korva and Varis (1990). In their study the reduced cereal yield was explained by poor nitrogen supply in the soil, which was mainly due to the short growth period characteristic of the high latitudes. During the short growing season, release of nitrogen through mineralization of organic matter is too slow and does not always coincide with the nitrogen requirements of the crop (Varis and Lehtiniemi, 1984). Also in this study, there was an indication of nitrogen deficiency at the onset of growth in the crops in ecological nutrient management as the nitrogen content of the leaf tissue was 30% lower than that of the crops grown under conven-

9

tional nutrient management. According to our results, this could be overcome by low-dose mineral nitrogen application. In a nutrient management system which features reduced mineral fertilization, the role of legumes as a source of nitrogen supply is pronounced as has been suggested by Nanta (1987). In addition to the positive yield effect of green fallowing (Breland, 1989; Varis and Kauppila, 1992), undercropping can significantly contribute to the nitrogen supply of the subsequent cereal crop. In this study, DM production of grass-legume leys seemed to be quite good under reduced mineral fertilization. Total DM production did not differ between the nutrient management systems and the total N-yield was even greater under reduced fertilization. The observed general reduction in DM production of the second year leys was greater in the integrated and ecologically managed leys than in the conventionally managed ley. This was probably caused by poor overwintering of the legume species as has earlier been reported by Pulli (1980). However, N-yield of the second year leys did not differ across nutrient management systems. In the cereal crops, associated with the reduction of mineral fertilization, there seemed to be a decline in quality characteristics such as protein content, hectoliter mass and simple grain mass. A similar trend was observed in our previous study (Poutala et al., 1993). According to our results, establishment of a productive undersown crop stand can meet with some difficulties. It has been demonstrated that the intensity of shading from the main crop largely determines the growth rate of the undercrop (Breland, 1989; Varis and Kauppila, 1992). This was confirmed in this study as the DM yield of the undersown red clover was only one fifth in the more dense integrated cereal stand of that in the less shading ecological cereal stand (visual observation). However, as most of the growth of the undercrop takes place after the harvest of the yield crop (Schjcrring et al., 1988; Breland, 1989; Varis and Kauppila, 1992), the differences in the DM yield of the undercrop grown in the different nutrient management systems could have been diminished at a later harvest. As the sustainability of the reduced external input crop production relies heavily on successful legume cropping, increased focus on improvement of management practices for undercropping would prob-

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R.T. Poutala et al. / Field Crops Research 37 (1994) 3-10

ably increase the stability and productivity of such a system, as has been shown by Varis and Kauppila (1992). Intensive production practices of the last few decades in Europe have largely led to soil degradation ( A r d e n Clarke and Hodges, 1987) and the observed good performance o f integrated nutrient m a n a g e m e n t systems could probably be explained by i m p r o v e d soil structure and subsequent nutrient cycling. O n e can also hypothesize that the good performance of the integrated nutrient m a n a g e m e n t system could be further p r o n o u n c e d w h e n cropping the structurally sensitive heavy clay soils. The gross m a r g i n calculations indicated greater a n n u a l and crop-dependent variation in the ecological than in the integrated or c o n v e n t i o n a l m a n a g e m e n t systems. However, in this study it was demonstrated that in ecological cereal cropping it is possible to achieve gross margins well above the average obtained in conventional cereal cropping at the prevailing F i n n i s h product prices. As fertilizers make up a significant part of variable costs in F i n n i s h cereal cropping, a m i n o r return reduction under the integrated nutrient management did not lead to significantly different gross marg i n s / c r o p w h e n compared to the conventional nutrient management.

Acknowledgments W e are grateful to Dr. Eija Pehu and Dr. John Coulter for their critical review of the manuscript and to agronomist Antti H a n n u k k a l a for his contribution to this study. Technical assistance of the staff of the department o f Plant Production is also gratefully acknowledged. The work was funded by a grant from the University of Helsinki.

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Breland, T.A., 1989. Soil organic carbon and nitrogen dynamics in grain nutrient management: Effects of undersown catch crops and green manuring. PhD. thesis, Dept. of Microbiology, Agricultural University of Norway. Dabbert, S. and Madden, P., 1986. The transition to organic agriculture: A multi-year simulation model of a Pennsylvania farm. Am. J. Alternative Agric., I: 99-107. Heinonen-Tanski, H., 1990. Conventional and organic cropping systems at Suitia. III. Microbial activity in soils. J. Agric. Sci. Finland, 62: 321-330. Korva, J. and Varis, E., 1990. Conventional and organic cropping systems at Suitia. II. Crop growth and yield. J. Agric. Sci. Finland, 62: 309-319. Large, E.C., 1954. Growth stages in cereals. Illustrations of the Feekes scale. Plant Pathol., 3:128-129. Nauta, R.S., 1987. Agricultural production in the Netherlands by natural supply of nitrogen. Biol. Agric. Hortic., 4: 181-201. Poutala, R.T., Korva, J. and Varis, E., 1993. Spring wheat cultivar performance in ecological and conventional cropping systems. J. Sust. Agric., 3: 63-84. Pulli, S., 1980. Growth factors and management technique used in relation to the development rhythm and yield formation pattern of a clover-grass stand. J. Sci. Agric. Soc. Finland, 52:215-280. SAS Institute Inc., 1987. SAS Users guide. SAS Institute Inc., Cary, NC. Schj~rring, J.K., Gottschau, A., NCrlund,T., Nielsen, N.E. and Jensen, H.E., 1988. The dynamics of nitrogen in the root zone of field-grown spring barley as affected by nitrogen application, irrigation and undersown catch crops. Experimental results 1984, Report nr. 1208. Department of Soil and Water and Plant Nutrition, The Royal Veterinary and Agricultural University, Copenhagen, 166 pp. van Faassen, H.G. and Lebbink, G., 1990. Nitrogen cycling in highinput versus reduced-input arable fanning. Neth. J. Agric. Sci., 38: 265-282. Varis, E. and Kauppila, R. (Editors), 1992. Viherlannoituskokeen Tuloksia Vuosilta 1979-1987. Publication no. 30. Dept. of Plant Production, University of Helsinki (in Finnish). Varis, E. and Lehtiniemi, K., 1984. Reaction of some spring barley cultivars to mineral nitrogen and farmyard manure. In: H. Vogtmann, E. Boehncke and I. Fricke (Editors), Proc. 5th IFOAM International Scientific Conference at the University of Kassel, Germany. Verlagsgruppe Witzenhausen, Germany, pp. 98-t09. Vails, E. and Pietila, L., 1991. J~imf6resleav olika potatisodlingsmetoder. In: Minimerad Anviinding av Kemiska Productionsmedel vid Matpotatisodling. Rapport fSr den Finl~indskaDelen av det Samnordiska Projektet, Publ. No. 27. Helsingfors Universitet, Institutionen fSr V~ixtodlingslara,pp. 1-28. Vereijken, P., 1989. Experimental system of integrated and organic wheat production. Agric. Syst., 30: 187-197. Vereijken, P., 1990. Integrated nutrient management for arable farms. Rech. Agron. Suisse, 29: 359-365. Vereijken, P., 1992. A methodic way to more sustainable farming systems. Neth. J. Agric. Sci., 40: 209-223.