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Managing nitrogen fertilization for year-round vegetable production in paddy rice fields

Managing nitrogen fertilization for year-round vegetable production in paddy rice fields
Kleinhenz, V.; Schnitzler, W. H.; Midmore, D. J., 1996
Gartenbauwissenschaft, 61, 25-32


Managing Nitrogen Fertilization for Year-Round Vegetable Production in Paddy Rice Fields

Gartenbauwissenschaft, 61 (1). S. 25-32, 1996, ISSN 0016-478X. © Verlag Eugen Ulmer GmbH & Co., Stuttgart

 

Managing Nitrogen Fertilization for Year-Round Vegetable Production in Paddy Rice Fields

 

 

Stickstoffversorgung von Gemüse bei Ganzjahresproduktion in Naßreisfeldern

 

V. Kleinhenz, W.H. Schnitzler and D.J. Midmore

(Asian Vegetable Research and Development Center, Taiwan; Institute for Vegetable Science, Technical University of Munich, Germany)

 

 

Introduction

 

Permanent high bed cultivation is known to exist in many peri-urban lowland production zones in Asia ranging from China (Chiu 1987, Plucknett and  Beemer 1981) to India (Singh and Gangwar 1989).

At present, heavy applications of N fertilizer are common practice in many of these and other intensive vegeta­ble production areas in the Tropics and Subtropics. Increasing concern for the negative consequences of over-fertilization, particularly regarding nitrate contamination of ground water and potentially harmful high nitrate contents in vegetables, has led to the demand for the develop­ment of innovative N management strategies to improve fertilizer use effi­ciency. One ap­proach to improve N management is to fine-tune the amount of N fertilizer to better syn­chronize soil N availability with plant requirements.

Widely introduced to Central Europe is the Nmin-method that essentially depends on the measure­ment of soil N and takes into account crop needs.

The limited use of Nmin in commercial fields is attributed more to the immense requirement for time and labour to sample and analyse the soil than the doubt that the soil analysis data would not reliably represent plant nutritional status (Matthäus et al. 1994). The first part of this study deals with the experience of the Nmin-method in a 14-month, 5-crop continuous year-round vegetable crop­ping sequence in the subtropical environment of Southern Taiwan, where crops were either grown on traditional flat beds, or on permanent high beds in previous paddy rice fields.

If the Nmin-method is too slow and expensive, what other method may be suitable and accurate enough to improve N fertilizer manage­ment sufficiently (Hartz 1994). Plant tissue analysis is regarded helpful in indi­cating plant nutrition status and forecasting crop yields, but conventional tissue tests, as for the Nmin-method, create significant costs and time lag between sampling and result.

A new promising analytical procedure is to measure NO3-N concentration in fresh petiole sap (SN-test; sap nitrate test), a method that has proved to be highly correlated with dry tissue NO3-N concentrations in several vegetable species (Hartz et al. 1993). Since nutri­ent con­centrations de­cline most quickly in rapidly expanding new tissues (Burns 1991), and the petiole acts as a storage and transport organ for nitrate-N, the petiole of re­cently matured leaves is a sensitive indica­tor of plant N status (Vitosh and Silva 1994) and N nutrition (Prasad and Spiers 1984). The goal is to determine the minimum level of petiole sap NO3-N associated with maximum yield (Coltman 1989), a "critical petiole NO3-level" above which a crop would be adequately supplied with N and no additional N fertilizer would be needed, and below which the crop would be deficient in N nutrition and would require additional N fertilizer to ensure maximum yield.

At present, the calibration of the method poses the most significant hindrance to practical ap­plication since diagnostic standards for critical NO3-N concentrations in plant sap are still lack­ing (Beverly 1994). Vegetable extension institutions in Europe (Matthäus et al. 1994), and the US (Hochmuth 1992) are, however, beginning to use this new technology and are presenting first guidelines to the commercial grower. New, portable nitrate-selective elec­trodes and quantitative reflectometric analysis procedures for test strips make it much easier to achieve reliable results.

The objec­tive of the second part of this study is to describe a model that integrates (1) soil N status (soil NO3-N), (2) crop N status (petiole sap NO3-N), and (3) crop yield response to provide a theo­retical background for further studies of its kind, and to apply this model to two field-grown vegetable crops, vegetable soybean and Chi­nese cabbage, cultivated either on flat beds, or on permanent high beds.

 

Materials and Methods

 

Nmin-study

In spring 1993, an integrated permanent high bed - deep furrow system was laid out and con­structed in a randomized 3-factor split-split block design with 4 replications. Treatments in­cluded: (1) high bed width, (2) legume green manure living mulch, and (3) N fertilization method. Results on the first two ex­perimental factors will not be resported here. The N fer­tilization method con­sisted of 2 levels, (1) traditional (standard) N fertilizer input, and (2) Nmin-method. The two levels of this treatment were additionally randomized in 4 replications on 1.5 m wide flat beds to represent a control to the permanent high beds.

In a 14-month continuous year-round vegetable cropping sequence, summer rainy season crops consisted of Chinese cabbage (Brassica pekinensis (Lour.) Rupr.; variety "ASVEG No. 1", AVRDC), and chili (Capsicum annuum L.; variety "Hot Beauty", Known You Seed Co.), whereas carrot (Daucus carota L. ssp. sativus (Hoffm.) Arcang.; "Red Judy", Known You Seed Co.) and vegetable soybean (Glycine max. (L.) Merr; variety "AGS 292", AVRDC) were grown during the dry subtropical winter months. Aquatic crops cul­tivated in the continuously flooded furrows were Taro (Colocasia esculenta (L.) Schott) and rice (Oryza sativa L.).

In the place of single rows for direct sown vegetable soybean and the pre-nursed and trans­planted crops Chinese cabbage and chili, carrot was sown in paired rows. Dimensions of culti­vation systems and plant rectangularity are presented in figure 1, and other cultural details are summarized in table 1.

 

Fig. 1.  Dimensions of cultivation systems and plant rectangularity

Pflanzsysteme auf verschiedenen Hoch- und Flachbeetformationen

 

Table 1. Summary of cultural details

Übersicht über den Versuchsaufbau

Crop

Chinese cabbage

Chili

Carrot

Vegetable soybean

Chinese cabbage

Cultivation period

May-Jun '93

Jun-Nov '93

Dec-Feb '93/4

Mar-May '94

Jun-Jul '94

Plant distance (cm·cm)

50·60

50·60

25·05

50·05

50·40

Plant density (pla./m2)

 

 

 

 

 

flat bed system

3.33

3.33

80.00

40.00

3.33

high bed system

3.33

3.33

80.00

40.00

5.00

N fertilization

 

 

 

 

 

WAS/WAT

01

2

4

01

4

8

12

0

7

0

2

4

0

2

4

standard rate (kg N/ha)

60

30

30

50

50

50

50

60

60

20

20

20

60

30

30

Nmin-method (kg N/ha)

0

30

30

20

50

50

50

0

0

0

0

0

20

0

0

1Nmin-method only for basal fertilizer application (WAS/WAT = weeks after seeding/weeks after planting)

 

All nitrogen was applied as Ammonium sulphate ((NH4)2SO4), an inexpensive and readily available N fertilizer source.

Soil Nmin was measured before each N fertilizer appli­cation by sampling soil 30 cm deep in every Nmin-treatment plot. Extracted 1:2 by volume in 0.8 % KCl water solution, sam­ples were ana­lyzed for NO3 by use of Merck's RQflex re­flectometer and Reflectoquant ni­trate analytical test strips. N application rate for the Nmin-treatment was calculated by sub­tracting the aver­age Nmin-value from the traditional N appli­cation rate. All other cultural practices were standard.

Since not all experimental treatments are reported in this study, statistical comparisons (mean comparisons within and between main factors "N fertilization method", and "cultivation sys­tem") were performed using orthogonal contrasts.

 

Integrated study of soil N, plant N, and crop yield

During the 1994 crops of vegeta­ble soybean and Chinese cabbage, soil NO3 and plant nitrate data were also collected at weekly intervals. Soil was sampled for 0-30 cm and 30-60 cm lay­ers in 4 flat bed plots and 12 high bed plots. Petioles were collected from about 8 newly ex­panded leaves per plot for vegetable soybean, and 5 midribs of recently matured leaves per plot for Chinese cabbage. Petiole sap was extracted by use of a small hand garlic press, and the sap di­luted with deion­ized water to fit the range of the Reflectoquant test strips (5-225 ppm), for NO3-N analysis by the RQflex.

To integrate soil N status, crop N status, and crop yield response, a functional rela­tionship model was chosen which makes sense from theoretical and biological points of view and which was sufficiently well correlated with the data. The most widely used model in scientific publi­cations is the polynomial approach (V = a+bS+cS2 where V = crop yield, S = soil or plant N status; Apolinares and Recel 1994, Fang et al. 1994, Chaiwanakupt et al. 1994). In con­trast, the Michaelis-Menten model of saturation kinetics describes the velocity of an enzyme-mediated reaction as a function of substrate concentration (Geissler et al. 1981; figure 2). Analogous to the Michaelis-Menten model, the relationships (1) plant sap ni­trate = f(soil nitrate), (2) crop yield = f(plant sap nitrate), and (3) crop yield = f(soil nitrate) were fit­ted to the hyperbolic model:

 

V = (Vmax S) / (Km + S)

 

where: V = plant sap nitrate concentration or crop yield, Vmax = (constant) upper limit for V, S = soil nitrate concentration or plant sap nitrate concentration, and Km = (Michaelis con­stant) affinity for S.

 

Fig.  2. The Michaelis-Menten curve

Kurve des Michaelis-Menten Models

 

In an earlier study by (Westcott and Knox 1994), the relationship plant sap nitrate = f(soil nitrate) was well established for potato and peppermint. For the present analysis, however, since field experiments were not specially laid out for this purpose (N fertilizer rates were not systematically varied), it was expected that the regression equations would not provide very accurate predictions.

Yield as a function of soil N is usually poorly correlated. Integration of (1) yield = (A · plant NO3-N)/(B + plant NO3-N) and (2) plant NO3-N = (a · soil NO3-N)/(b + soil NO3) results in: ((Aa/B+a) · soil NO3-N)/((Bb/B+a) + soil NO3-N). Comparison of this estimated function with the regression equation of yield = f(soil NO3-N) can function as a control to predict whether the mutual relationship of hyperbolic dependencies is sufficiently determined. Using the polynomial approach instead would result in a 4th-order polynomial function, im­plausible and unsuitable from a theoretical and practi­cal standpoint.


Results

 

Nmin-study

The cultivation system was much more influential on productivity than the N fertilization regime throughout the 14-month cropping sequence (table 2). This tendency was at the same time much more pronounced for marketable yields than for total biomass production. Yields of the standard fertilization treatment on high beds surpassed those on flat beds by 58 %, 240 %, and 161 % for the summer crops of Chinese cabbage and chili 1993, and Chinese cabbage 1994. Winter season crop yields of carrot and vegetable soybean were, however, only slightly (8 % and 14 %) reduced compared to flat bed cultivation.

 

Table 2. Influence of Nmin fertilization method and cultivation system on vegetable yields

Der Einfluß von Stickstoffdüngung über Nmin-Steuerung und zwei Kultursystemen auf Gemüseerträge während der Regen- und Tockenzeit

Crop

Chinese cabbage

Chili

Carrot

Vegetable soybean

Chinese cabbage

 

Cultivation period

May-June 1993

June-November 1993

December-February 1993/94

March-May 1994

June-July 1994

 

Nmin

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Flat bed system

431

kg N / ha

 

301

kg N / ha

 

345

kg N / ha

 

214

kg N / ha

 

135

kg N / ha

 

 

High bed system

601

kg N / ha

 

341

kg N / ha

 

177

kg N / ha

 

137

kg N / ha

 

131

kg N / ha

 

 

Fertilization

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Traditional fertilization

120

kg N / ha

 

200

kg N / ha

 

120

kg N / ha

 

60

kg N / ha

 

120

kg N / ha

 

 

Nmin method

60

kg N / ha

 

180

kg N / ha

 

0

kg N / ha

 

0

kg N / ha

 

20

kg N / ha

 

 

Biomass (kg/m2)

Flat bed system

High bed system

P level

Flat bed system

High bed system

P level

Flat bed system

High bed system

P level

Flat bed system

High bed system

P level

Flat bed system

High bed system

P level

 

Traditional fertilization

2.97

3.37

0.07

0.98

1.62

< 0.01

2.41

2.31

0.83

2.16

1.80

0.01

2.76

4.46

< 0.01

 

Nmin method

3.06

3.42

0.10

0.80

1.46

< 0.01

2.52

2.18

0.41

1.96

1.69

0.04

1.80

3.86

< 0.01

 

P level

0.72

0.77

 

0.27

0.19

 

0.81

0.70

 

0.15

0.29

 

0.04

0.08

 

 

P level input

0.66

 

 

0.09

 

 

0.86

 

 

0.09

 

 

0.01

 

 

 

P level system

0.02

 

 

< 0.01

 

 

0.46

 

 

< 0.01

 

 

< 0.01

 

 

 

Marketable yield

(kg/m2)

Flat bed system

High bed system

P level

Flat bed system

High bed system

P level

Flat bed system

High bed system

P level

Flat bed system

High bed system

P level

Flat bed system

High bed system

P level

 

Traditional fertilization

1.37

2.16

< 0.01

0.22

0.75

< 0.01

1.29

1.20

0.70

1.26

1.11

0.09

0.75

1.96

< 0.01

 

Nmin method

1.49

2.22

< 0.01

0.20

0.63

< 0.01

1.40

1.17

0.33

1.20

1.06

0.12

0.19

1.22

< 0.01

 

P level

0.44

0.57

 

0.88

0.16

 

0.68

0.88

 

0.47

0.42

 

0.10

< 0.01

 

 

P level input

0.36

 

 

0.22

 

 

0.91

 

 

0.28

 

 

< 0.01

 

 

 

P level system

< 0.01

 

 

< 0.01

 

 

0.33

 

 

0.02

 

 

< 0.01

 

 

 

1Nmin method only for basal fertilizer application

 

 

Soil Nmin contents were very similar in flat bed and high bed systems during the rainy season summer crops (table 2) In the dry season, however, high soil nitrification rates in both culti­vation systems resulted in Nmin-concentrations that in the most part largely exceeded require­ments of both crops of carrot and vegetable soybean. Very obvious is the accelerated mineraliza­tion potential on the flat beds during that season. Consequently, no additional N fertilizer was applied in the Nmin-treatment. No significant yield differences were finally re­corded between N fertilization regimes for these cool season vegetables.

The tendency for reduced N fertilization to become a more critical factor for crop production when yields reach higher levels can be summarized from summer crops of chili (1993) and Chinese cabbage (1994): With chili yields around 0.21 kg/m2 on flat beds the orthogonal com­parison between fertilizer treatments is far from significant (P=0.88). Higher yields of 0.70 kg/m2 on high beds come closer to significance (P=0.16). For Chinese cabbage in 1994, low yields of 0.75 and 0.19 kg/m2 did not differ on flat beds (P=0.10) but did on high beds (1.96 and 1.22 kg/m2; P<0.01).

No interactions were found between the main factors cultivation system and fertilization method for biomass production and yield. In other words, higher N fertilizer application did not overcome the disadvantageous crop environment of the commonly used flat planting beds on summer vegetable production.

Over all, N-fertilization for individual crops was reduced by up to 100 % and a total of 360 kg N (42 % of the traditional rate) was saved in a 14-month continuos vegetable cropping se­quence by use of the Nmin-method. Significant loss of yield due to the reduced N fertilization was only observed in the 1994 Chinese cabbage crop when grown on high beds. The yield re­duction oc­curred in the second harvest, after a first harvest that showed no yield  differences between traditional N fertilization and Nmin-method (P=0.79). Heavy typhoon rain between harvests may have diluted or leached out plant available nitro­gen essential for head formation. This thesis is supported by less pro­nounced differences in total biomass at the second harvest (P=0.08).

 

Integrated study of soil N, plant N, and crop yield

 

Vegetable soybean

The most satisfactory fit for yield and nitrate was obtained at 4 WAS (weeks after seeding) when plants set flowers and the first side dressing is applied. While the fit was good for the regression plant sap NO3 = f(soil NO3), crop yield was only poorly related to soil nitrate con­tent (0-60 cm depth; table 3). Scatter plots in figure 3 illustrate significant differences between cultural systems for ability to uptake available soil nitrate and to reduce this absorbed NO3 efficiently to amino acids. Although plant sap nitrate accumulation was significantly higher for vegetable soybean grown on high beds, these higher concentrations resulted only in relatively low yields compared to the crop grown on flat beds.

 

Table 3. Regression equations for the hyperbolic relation of soil NO3 (0-60 cm), plant sap NO3, and yield of vegetable soybean and Chinese cabbage grown on flat beds and permanent high beds

Ergebnisse der Regressionsanalysen für hyperbolische Relation von Boden- und Pflanzensaft-Nitratwerten, sowie Erträge von Gemüsesojabohne und Chinakohl auf Flach- und Hochbeeten.

Function type

Flat bed system

High bed system

 

Equation1

r2

Equation2

r2

 

Vegetable soybean

 

 

 

 

 

plant NO3= f(soil NO3)

V = (2751*·S)/(115n.s.+S)

0.89n.s.

V = (4142**·S)/(62**+S)

0.53**

 

yield = f(plant NO3)

V = (1.46*·S)/(299n.s.+S)

0.50n.s.

V = (2.12*·S)/(3667*+S)

0.45*

 

yield = f(soil NO3)

V = (1.30*·S)/(115n.s.+S)

0.33n.s.

V = (1.10**·S)/(24n.s.+S)

0.17n.s.

 

estimated function

 

 

 

 

 

yield= f(soil NO3)

V = (1.32·S)/(11+S)

 

V = (1.12·S)/(29+S)

 

 

Chinese cabbage

 

 

 

 

 

plant NO3= f(soil NO3)

V = (7910n.s.·S)/(173n.s.+S)

0.85n.s.

V = (4584n.s.·S)/(70**.+S)

0.54**

 

yield = f(plant NO3)

V = (7.65n.s.·S)/(5978n.s.+S)

0.82n.s.

V = (9.89n.s.·S)/(2721**+S)

0.51**

 

yield = f(soil NO3)

V = (5.51**·S)/(112**+S)

0.99**

V = (4.63**·S)/(8 n.s.+S)

0.04n.s.

 

estimated function

 

 

 

 

 

yield= f(soil NO3)

V = (4.36·S)/(75+S)

 

V = (6.21·S)/(26+S)

 

 

13 df; 211 df

 

** sign. at 1 % level; *sign. at 5 % level; n.s. not sign.

 

 

Fig.  3. Hyperbolic regression curves for functional relationships between soil NO3, plant petiole sap NO3, and crop yield (left: vegetable soybean; right: Chinese cabbage; thick line: permanent high beds; thin line: flat beds; fad dotted line: regressed function; fine dotted line: estimated function)

Hyperbolische Regressionskurven der funktionalen Relationen zwischen Boden- und Pflanzensaftnitrat, sowie Erträgen. (links: Gemüsesojabohne; rechts: Chinakohl; dicke Linie: permanentes Hochbeet; dünne Linie: Flachbeet; dickgepunktete Linie: Regressionsfunktion; feingepunktete Linie: geschätzte Funktion)

 

Regression equations for marketable yield as a function of plant NO3-N overestimated yield potentials. Although low in r2 and P, estimated functions of yield versus soil NO3 predicted upper limits of productivity that were almost precisely realized by the crop.

Small differences between yield and calculated yield potential suggest that soil N was not a factor limiting growth of this crop, irrespective of cultivation system. This is confirmed by a lack of significant differences in yield between Nmin- and standard fertilization method (table 2). Differences can be explainted by excessive root growth par­ticularly in the early growth stages on the expense of assimilatory leaf area (as indicated by crop cover and root distri­bution, data not shown), leading to a delay in crop maturity.

 

Chinese cabbage

Soil nitrate and plant sap nitrate fitted best to gross yield data when collected 4 WAT (weeks after transplanting), before application of the second side dressing. Marketable yield of Chinese cabbage during the rainy season is largely affected by soft rot (Erwinia carotovora) and poor head formation. Soil and plant analysis cannot account for these losses.

In contrast to the vegetable soybean crop, scatter plots of plant sap nitrate versus soil nitrate (figure 3) show that soil nitrate uptake of crops grown on either flat beds or high beds was not different, whereas efficiency of trans­formation of absorbed NO3 to biomass (yield = f(plant NO3)) was. With similar NO3-con­centrations in plant sap, Chinese cabbage yields were much better on high beds than on flat beds. The regressions plant sap nitrate = f(soil NO3) produced upper limits for plant sap nitrate concentrations that were distinctly higher than those meas­ured in the crop. The same is true for the relationship yield = f(plant NO3). Regressions indi­cate yield potentials in case of higher plant sap nitrate concentrations. Soil nitrate content at the end of the cultivation pe­riod resulted in sub-optimum plant sap nitrate levels that were possibly too little to support maximum yields. If the higher plant sap nitrate concentrations as recorded 2 WAT (4473 ppm in high beds; 5293 ppm in flat beds) were maintained, it may have been possible to achieve much better yields.

 

Discussion

 

Level of soil Nmin-concentration depended largely on season. High nitrification potential was evident particularly for the flat bed system during the dry winter season. Much less plant bio­mass was produced on flat beds compared to high beds during the preceding rainy season. Thus, since N fertilizer application rates were the same, much less N was removed from the soil system during that season when the soil is wet and partially anaerobic during most of the time. Nitrification of ammonium fertilizer is largely inhib­ited under anaerobic conditions, under which NH4 is easily immobi­lized by soil microbes or fixed to clay minerals (Drury & Beauchamp 1991). Following drying of the soil in winter and intensive aeration through management (e.g. bed construction), this immobilized, or fixed N obviously reappeared in the oxidized N form, leading to immense NO3 accumulation. It maybe assumed from this that ex­cessive leaching of nitrates during the rainy season is not as important as always thought.

Since soil water content in the top soil averaged less in the high than low beds, plant available N was pro­duced more readily, and less nitrogen for current plant biomass accumulation was lost through immobilization and fixation. Permanent high beds in combination with use of the Nmin-method was the most resource-efficient option for rainy season summer vegetable pro­duction in this experiment.

For the summer season crop of Chinese cabbage, the limited NO3 supply from the soil system most likely restricted yield. Since reduced N fertilization in the Nmin-treatment did not significantly reduce gross yields compared to the traditional N application rate, both nitrogen application rates were too low given the reduced nitrification potential (= low fertilizer efficiency) of the soils under rainy season conditions.

The differences between cultural systems for reducing this absorbed NO3 efficiently to amino acids for accumulation of biomass and yield can possibly be attributed to soil - plant water relations, indicating higher water stress (reduced water intake through anaerobic conditions in the root zone) of Chinese cabbage on flat beds and other physiological disorders.

Under presented experimental conditions the integrated analysis of soil N, plant sap N, and crop yield using the hyperbolic approach of the Michaelis-Menten model of satura­tion kinetics resulted in a sufficiently well-determined mutual functional relationship for vege­table soybean as expressed by the similarities of parameters of regressed and estimated func­tions for yield = f(soil NO3-N). Besides highlighting deficiencies in plant nutrition, the signifi­cance of petiole sap NO3 analysis lies also in using the dependency of plant NO3 = f(soil NO3), which is usually highly determined to estimate the relationship yield = f(soil NO3), which is commonly poorly correlated.

For vegetable soybean, the analysis was able to show that no limitations in plant nutrition oc­curred and that other factors were responsible for the somewhat poorer performance of the crop on permanent high beds. For Chinese cabbage, however, large differences between calcu­lated upper limits for plant NO3 and measured concentrations revealed sub-optimum nitrogen nutrition of this high-N demanding leafy vegetable during the rainy hot summer season. High soil water contents inhibited nitrification of ammonium fertilizer, even when applied in large doses.

 

Summary

 

Permanent high bed cultivation systems are primarily used to overcome flood damage in vege­table crops cultivated in many lowland peri-urban production zones throughout Asia.

Over-doses of nitrogenous fertilizers in intensive tropical and subtropical vegetable production have created concern about impact on environment and product quality, and have led to a de­mand for better N management practices.

The Nmin-fertilization method has been tested in a 14-month continuous vegetable cropping sequence with 5 crops in two cultivation systems: flat planting beds, and permanent high beds. Permanent high bed cul­tivation and Nmin-fertilization method were compared with standard practices to test their potential for more resource-effi­cient productivity. An integrated study of soil NO3, plant sap NO3, and crop yield was undertaken for a dry season crops of carrots and vegetable soybean and rainy season crops of Chinese cabbage and chili.

The influence of cultivation system (flat beds versus high beds) on productivity was much more conspicuous than the effect of fertilization regimens. Summer crops of Chinese cabbage and chili pepper in 1993, and Chinese cabbage '94 on high beds outyielded those on flat beds by 58 %, 240 %, and 161 % for the standard N application rates.

Plant available N concentrations in soil (Nmin) followed a seasonal pattern of fixation and/or immobilization of ammonium N fertilizer during the rainy summer season, when water satu­ration inhibited nitrification most of the time. During the winter season this fixed nitrogen re­appeared as soils dried out. This was particularly true for the more flood-prone flat beds.

The accelerated mineralization potential during the dry season led to Nmin-concen­trations obviating the need for N fertilization for carrot and vegetable soybean crops. No yield reductions were observed compared to the standard N application rate. Al­though soil N supply was more limited during the rainy season, only yields of Chinese cab­bage during 1994 on high beds were significantly reduced by the Nmin-method. A total of 360 kg N/ha (42 %) was saved by the Nmin-method during 14 months and 5 vegetable crops.

The combination of hyperbolic relations of plant NO3-N as a function of soil nitrate, and yield as a function of plant NO3 resulted in a good estimation of yield = f(soil NO3) for vegetable soybean. Small differences between measured sap NO3-levels and calculated upper limits demon­strated that soil N was not a limiting growth factor.

Permanent high bed cultivation in combination with use of the Nmin-fertilization method was the most resource-efficient combination for out-of-season summer vegetable production.

 

Zusammenfassung

 

Bodenverdichtung und stagnierende Nässe verursachen häufig starke Wachstumsschäden bei Gemüsekulturen im tropischen Tiefland während der Regenzeit. Permanente Hochbeete verbessern häufig diese Situation.

In Intensivgemüsekulturen der Tropen und Subtropen sind hohe Stickstoffgaben die Regel. Aber auch hier beginnt man umzudenken zum Schutze der Umwelt und für gesteigerte Produktqualität. Daher wird ein besseres Verständnis für eine optimale Stickstoffdüngung unter diesen besonderen Umweltbdingungen dringend gefordert.

In der vorliegenden Arbeit wurde bei fünf fortlaufenden Gemüsekulturen über 14 Monate auf permanenten Hochbeeten und auf Flachbeeten die bis dahin unbekannte Nmin-Methode mit der Standartstickstoffdüngung verglichen. Als Maßstäbe dienten Effizienz und Produktivität, da besonders während der regenreichen Sommermonate eine erfolgreiche Gemüseproduktion sehr problematisch ist. In der Studie wurden fortlaufend Nitratwerte im Boden und im frischen Pflanzensaft (mit Merck RQflex) gemessen, sowie die Erträge von Gemüsesojabohnen und Karotten während der Trockenzeit und von Chinakohl und Chili-Paprika während der heißen Regenzeit.

Der Einfluß der beiden Kutursysteme (Hoch- und Flachbeet) war sehr viel ausgeprägter als die Steuerung der N-Düngung. Erträge von Chinakohl und Chili-Paprika während der Regenmonate 1993 und von Chinakohl in 1994 waren auf Hochbeeten gewachsen um 58 %, 240 % bzw. 161 % höher als auf Flachbeeten bei Standardstickstoffdüngung.

Wassersättigung des Bodens während der Regenzeit verursachte durch Sauerstoffmangel über längere Zeit eine Nitrifikationshemmung. Dadurch wurde der applizierte Ammoniumstickstoff im Boden immobilisiert bzw. fixiert, was ein typisch jahreszeitlich bedingtes Erscheinungsbild der Stickstoffverfügbarkeit und N-Konzentration im Boden (Nmin) ergab. Während der kühleren Jahreszeit und sobald der Boden austrocknete, wurde dieser Stickstoff pflanzenverfügbar. Dies war besonders in den sehr viel nasseren Flachbeeten zu beobachten.

Das beschleunigte Mineralizationspotential während der Trockenzeit führte zu Nmin-Konzentrationen, die eine Stickstoffdüngung bei Karotten und Gemüsesojabohnen erübrigte, was sich durch die Nmin-Testmethode sehr gut zeigte. Keine Ertragsreduzierungen wurden gegenüber der Standardstickstoffdüngung beobachtet. Obwohl die N-Verfügbarkeit während der Regenzeit limitiert war, äußerte sich dies signifikant bei Chinakohlertrag nur 1994 auf Hochbeeten unter Nmin-Stickstoffsteuerung. Wahrscheinlich wurde dies aber durch sehr starke Auswaschungen während eines Taifuns in der Hauptwachstumszeit verursacht.

Die Kombination der hyperbolischen Relation von Nitrat in der Pflanze zu Bodennitrat, und Ertrag als eine Funktion des Pflanzennitrates ergab eine gute Schätzung des Ertrags für Gemüsesojabohne. Kleine Differenzen zwischen gemessenem NO3 im Pflanzensaft und kalkulierten Höchstmengen demonstrierte, daß Boden-NO3 keinen begrenzenden Wachstumsfaktor darstellt.

Permanente Hochbeetkulturen in Kombination mit Nmin-gesteuerter Sticksoffdüngung erwies sich als das beste Produktionssystem für Gemüsekulturen während der sonst problematischen sommerlichen Regenzeit im tropischen Tiefland.

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Sincere thanks to BMZ/GTZ for sponsoring this research project.

 

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Eingegangen: 3.4.1995.

 

Anschrift der Verfasser: Volker Kleinhenz und Dr. David Midmore, Asian Vegetable Research and Development Center (AVRDC), Shanhua, Taiwan, ROC, Prof. Dr. W. Schnitzler, Lehrstuhl für Gemüsebau, Technische Universität München, 85350 Freising.