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.