effects of different organic fertilization practices and mineral fertilization on potato quality

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Effects of DifferentOrganic FertilizationPractices and MineralFertilization on PotatoQualityPirkko Roinila a , Jaana Väisänen a , ArturGranstedt b & Susanna Kunttu ca MTT Agri food Research Finland, EcologicalProduction , Huttulantie 1, FIN-51900 , Juva ,Finlandb Biodynamiska Forskningsinstitutet , SkillebyGård, S-15391 , Järna , Finlandc VTT, Industrial Systems , P.O. Box 1306,FIN-33101 , Tampere , FinlandPublished online: 24 Apr 2012.

To cite this article: Pirkko Roinila , Jaana Väisänen , Artur Granstedt &Susanna Kunttu (2003) Effects of Different Organic Fertilization Practices andMineral Fertilization on Potato Quality, Biological Agriculture & Horticulture:An International Journal for Sustainable Production Systems, 21:2, 165-194,DOI: 10.1080/01448765.2003.9755260

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Biological Agriculture and Horticulture, 2003, Vol. 21, pp. 165-194 0144-8765/03 $10 © 2003 A B Academic Publishers Printed in Great Britain

Effects of Different Organic Fertilization Practices and Mineral Fertilization on Potato Quality

Pirkko Roinila\ Jaana Vaisanen1·*, Artur Granstedt2 and Susanna Kunttu3

1 MTT Agrifood Research Finland, Ecological Production, Huttulantie 1, FIN-51900 Juva, Finland. 2Biodynamiska Forskningsinstitutet, Skilleby Gard, 5-15391 Jarna, Finland. 3 VTT, Industrial Systems. P.O. Box 1306, FIN-33101 Tampere, Finland

ABSTRACT

The short-term effects of different fertilization practices were the target of the three year series of experiments. The effects of organic fertilizers and a mineral fertilizer were investigated on selected quality characteristics of potato (Solanum tuberosum cv. Matilda). Chemical composition, new contemporary quality parameters, 'change of electrical conductivity' and 'darkening of potato extract', and a parameter of environmental quality were analysed to discover the qualitative effects of increasing rates of fertilizers (fresh and composted farmyard manure, aerated slurry and mineral fertilization). In contrast to farmyard manure, the high application rate of the mineral fertilizer raised the concentration of nitrogenous compounds (nitrate and free amino acids) in potato. The slurry fertilization caused a similar but smaller reaction. The potato dry matter content differed more strongly than starch between the organic and the mineral fertilizer and between the application rates. The minerally-fertilized potatoes showed a lower dry matter content than the organically-fertilized. Thus, the dry matter and starch yield showed no significant difference between the mineral and organic treatments, even though the fresh matter yield was higher with the mineral fertilization. The 'change of electrical conductivity of potato extract' was highest with the mineral fertilization and the difference between organic fertilizers widened with increasing fertilization levels. The concentrations of mineral compounds and vitamin C, and the 'darkening of potato extract', due to a strong variation between replications, showed only a few statistically significant differences between the treatments.

INTRODUCTION

The research of product quality in organic agriculture has concentrated on

*Corresponding au thor -jaana. vaisanen@ mtt. fi

165

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166 PIRKKO ROINILA AND OTHERS

comparing products of conventional and organic agricultural systems. Woese et al. ( 1995) have reviewed results of this type of research. The interpretation of the results is complicated because of insufficient duration of the study, lack of replication, differences of cultivars, inadequate experimental design or difficulties in comparing mineral and organic fertilizers containing different nutrient ratios and quantities (Hornick, 1992). According to Worthington ( 1998), the data comparing the nutrient contents in organic vs. conventional products are insufficient for most individual nutrients to draw any conclusions, with the exceptions of nitrate and vitamin C concentrations, and protein quality. For these parameters, there is evidence for organically cultivated products being nutritionally better.

The concept of fertilization in organic farming is greater than merely the use of manure. The effects of the preceding crop and the whole crop rotation, as well as the techniques of soil tillage and green manuring, can be considered as a part of an organic fertilization system. The qualitative comparison of fertilization systems is not simple; even cultivars selected for optimal growth in one agricultural system may show different responses in another (Knorr & Vogtmann, 1983). Thus, the influence of cultivation techniques on the quality of plant products has to be considered in the genetical framework of the crop and the cultivar.

Kolbe et al. (1995a) differentiate the long-term and short-term effects of organic fertilizers. Higher soil organic matter contents and availability of nutrients, better soil structure, increased yield potential and nutrient uptake efficiency are the long-term effects of organic fertilization. On the other hand, quality properties of potato achieved by organic manure fertilization, such as higher contents of dry matter and starch, higher vitamin C and lower nitrate concentrations than with mineral fertilization are mostly due to different nutrient supply, and could only be observed in the same growing period when the fertilization was applied. Kolbe et al. ( 1995b) concluded that these effects can be considered as the short-term effects of organic fertilizers.

The goal of the study was to compare the short-term effects of organic and mineral fertilizers on potato quality. This basic information is needed for the development of optimal organic fertilization practices under North European growing conditions. The background and parallels to this study can be found in the long-term fertilization and farming system experiments in Sweden initiated by Bo Pettersson in 1958 (Granstedt & Kjellenberg, 1996), which continued for 32 years; the K-experiment comparing organic, mineral and biodynamic fertilization (Pettersson & von Wistinghausen, 1979), and the UJ-experiment (Dlouhy, 1977; Pettersson, 1982) comparing conventional and biodynamic farming at two locations, as well as a recent fertilization experiment parallel to our study (Granstedt, 2000). Potato (Solanum tuberosum) was chosen as the experimental crop because of its importance in the Finnish organic market and its sensitivity to fertilization practices. Similar experiments studying the effects of

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EFFECT OF FERTILIZERS ON POTATO QUALITY 167

fertilization systems on potato quality have also been conducted by Raupp (1997), Schulz & Kopke (1997) and Warman & Havard (1998).

Depending on the point of view, quality can be described as technological (suitability for processing and storage), sensory (appearance, taste), nutritional (safety and health), ecological (sustainability of the food chain), and even social, psychological and political (Woese et al., 1995; Bylund et al., 1997). However, the food quality parameters commonly discussed are based on analyses of the product for single beneficial or harmful chemical components. The idea is that food contributes to the human metabolism by supplying chemical, physical and biochemical substances (Raupp 1998). From this viewpoint, little attention has been paid to the balance of the different substances and the health impact of food as a whole. For example, the beneficial effects of antioxidants (carotenoids, vitamin C, vitamin E) in vegetables, which decrease the risk of cancer, could not be found with isolated antioxidants in the form of food supplements (Sandstrom et al., 1994). Too little is known about the form and amount of antioxidants needed for optimum health. The complexity of the effects of food on health is also shown in animal feeding experiments, in which the beneficial effects of organic feed on health and reproduction of the test animals compared with conventional feed could not be explained by differences in chemical substances (Staiger, 1986).

Nitrate concentration in vegetables is one of the most commonly studied nutritional quality parameters when different farming systems or fertilization regimes are compared. Based on numerous investigations, Woese et al. (1995) state that organically-fertilized crops tend to accumulate less nitrate than minerally-fertilized crops. The concentration of free amino acids is used as a quality parameter of organic vegetables (Dlouhy 1977; Pettersson 1982; Bohm & Dewes 1997), because the level of free amino acids in the plant describes its state of development and nutrition. These compounds act as intermediates of protein and secondary metabolite synthesis, and transport forms of nitrogen, representing 27.7-48.3% of the total N (Wtinsh 1975). Pettersson (1982) stated that the increasing mineral N fertilization raised the concentration of free amino acids in potatoes more steeply than the organic fertilization. However, in a Norwegian study, increased mineral N fertilization from 50 to 150 kg ha-1 raised the contribution of free amino acid N to the total N only by 3%. The cultivars and years showed large variation as well (Baerug et al., 1979).

Supplementary approaches to food quality, such as storage and degradation tests (Pettersson, 1982; Kjellenberg & Granstedt,1998; Raupp,1998), feeding experiments (Neudecker, 1987; Staiger, 1986), picture creating methods (Balzer-Graf & Balzer, 1991; Andersson, 2001; Tingstad, 2001), biophoton measurement (Popp, 1988) and quality indexes (Schulz et al., 1997) have been developed and introduced, especially by researchers associated with organic farming. The common feature of these methods is the approach of describing the "vitality" of the product. However, the methods of new complementary

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approaches need standardization and a deeper understanding of their physiological and microbiological backgrounds in order to be used as routine analyses of quality.

Two of the new approaches to product quality parameters, the so called degradation tests, were included in the programme of analysis of the present study. The objective was to test if the methods 'darkening of potato extract' and the 'change of electrical conductivity' of potato extract (Pettersson, 1982) are able to distinguish mineral from organic fertilization regimes.

MATERIALS AND METHODS

Field experiments

The effect of different organic fertilizers compared with mineral fertilization on the quality of potato (Solanum tuberosum cv. Matilda) was studied at three application rates of fertilizer in a field experiment for three years (1995-1997). Since no fungicides were used, a cultivar with good fungal resistance to Phytophtora infestans was chosen in order to minimize the effects of disease on the yield and quality of tubers. The effects of organic fertilizers were compared with those of mineral fertilization (M) at comparable levels and with control plots without fertilization (0). The organic fertilizers were compos ted (C) and fresh (F) farmyard manure and aerated slurry (S), each one at three different rates. The fertility levels were chosen according to the following principle: the medium level corresponded to the application rate recommended or commonly used in each type of farming practice. The aim was to reach comparability closer to practical farming conditions than, for example, by using identical levels of total N or mineral N in each fertilizer type. The other levels were half and double the chosen medium level. The fertilization levels and their nutrient contents are shown in Table 1.

The experimental design was split plot consisting of four replications with treatments fertilizer type and fertilizer application rate. The sub plot measured 4.5 x 4.5 m. The seed tubers were sprouted 3 weeks before planting, which occurred in the last week of May with a row spacing of 7 5 em and a tuber distance of 25 em. Mechanical weed control and hilling was done twice in the early stages of development in all treatments. No pesticides were used in any of the treatments of the experiment. The tubers were harvested by hand-digging in the middle of September.

The experiments were conducted at Partala Research Station in Juva (61 °55'N, 27°53'E) on a sandy soil (Table 2). The experiment was on a different field each year; thus, the soil parameters differed from year to year. This can be observed in the yield differences of unfertilized plots between years. The preceding crop for the experimental years 1995 and 1996 was oats (A vena sativa) from which the seed crop and straw were harvested. In 1995, clover-grass ley was undersown in

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TABLE 1

Fertilizer and N P K rates, and the amount of applied mineral N (Nmin) in different types and rates of fertilizer: C = compost, F = fresh FYM, S = slurry and M = mineral fertilizer.

Fertilizer rates N P K rates in organic fertilizers

1995-97 1995-97 1995 1996 1997 (t ha-1) (kg ha-1) (kg ha-1) (kg ha-1) (kg ha-1)

c F s M c F s c F s c F s

Without fertilizer 0 0 0 0 0 0 0 0 0 0 0 0 0 Half 15 15 15 N 40 74 81 30 61 83 39 87 88 30

Nmin 40 2 8 18 2 15 21 4 II 17 p 30 27 30 5 24 23 7 29 21 6 K 50 72 71 44 60 56 64 71 66 35

Medium 30 30 30 N 80 147 162 60 122 166 77 174 176 60 Nmin 80 3 15 35 4 31 41 7 21 33

p 60 54 60 10 48 47 14 58 42 12 K 100 144 141 88 121 113 128 142 132 70

Double 60 60 60 N 160 294 324 120 244 332 154 348 352 120 Nmin 160 6 30 70 8 62 82 14 42 67

p 120 108 120 20 96 94 28 116 84 24 K 200 288 282 176 241 226 257 284 264 140

TABLE 2

Pre-crops and soil characteristics before fertilization in the potato experiments 1995, 1996 and 1997.

Pre-crop

Mineral N (kg ha- 1)

pH Calcium (mg 1-1)

Phosphorus (mg 1-1)

Potassium (mg 1-1)

Oats + undersown grass-clover

1995

mean S.d.

15.3 2.4 6.1 0.1

851 142 19 3

110 25

mean

29.0 5.3 545

11 57

Oats Green manure: oat-vetch-phacelia

1996 1997

s.d. mean s.d.

4.5 16.7 1.2 <0.1 6.4 0.3

39 1387 301 0.9 12 4

8 108 14

the oat and ploughed down in spring before the planting of potato, which affected the nutrient status of the soil. In 1997, the preceding crop was a green manure mixture with oats (Avena sativa), fodder vetch (Vicia sativa) and phacelia (Phacelia tanacetifolia). The fields used for the experiments were quite recently converted to organic farming; the conversion period was finished in 1994 for the first 2 years, and finished in 1991 for the last year of the experiment; thus, the soil conditions differ from a situation in which organic farming, with its typical crop rotations, has been practised for a long time.

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During the first experiment in 1995, the precipitation was quite evenly scattered throughout the growing season, which was warmer than average. In 1996, most of the precipitation occurred in the first half of the growing season but the end of the season was cool and dry. The warm season (1997) started with a cool dry period and ended with a rainy autumn (Figure 1). On average, from May to September, the precipitation and the efficient degree-days recorded at St. Michel airport, 45 km distant from the experimental farm, are 318 mm and 1169 Celsius degrees, respectively, and the growing season lasts 160 days (Hakkinen, 1995).

Analytical methods

The tuber quality parameters studied can be divided into physical and chemical characteristics, parameters of product resistance and maturity and environmental quality. The amount of mineral nitrogen (N) remaining in the soil after harvest was considered as a parameter for describing environmental quality.

Tuber nitrate concentration was measured in a water extract with an Orion nitrate ion-selective electrode. Concentration of minerals (Ca, Mg, Na, P, K) were determined by ICP after wet digestion in nitric acid (Tahvonen & Kumpulainen, 1991). The free amino acid concentration was determined according to the Sorensen formol titration method (CEN/TC 174 N 57 E). The proportion of first class yield was sorted out according to the Finnish quality standards given in Statute 356/84. The starch content was measured by the under water method (lSI, 1986). Vitamin C was determined according to Speek et al. ( 1984) as a sum of ascorbic and dehydroascorbic acid by HP' s 1090 Series HPLC equipped with fluorescent detector. Analytical column was Spherisorb (125 x 4.0 mm, 5 f.lm) operated at 35°C. !socratic mobile phase consisted of methanol and 0.08 M phosphate buffer (pH 7.8). The analysis of the 'electrical conductivity change' in potato extract and the 'darkening of potato extract', by spectrophotometer at 530 nm, were made according to Pettersson (1982).

For the organic fertilizers, total nitrogen, phosphorus, potassium and mineral nitrogen (NH/-N + N03-N), were determined before fertilization in May. For each fertilizer type, the sample comprised ten subsamples. Total P and K were analysed using ICP after wet digestion in nitric acid, total N by the Kjeldahl method and mineral N according to Kemppainen (1989). Soil samples for analyses of pH, easily soluble phosphorus, exchangeable potassium, magnesium and calcium were taken before the field experiments and analysed by acid ammonium acetate extracts according to Vuorinen et al. (1955). Mineral N was analysed colorimetrically (Linden, 1981) from the same samples.

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E 400 E c: 0

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100

EFFECT OF FERTILIZERS ON POTATO QUALITY

1997

1200

1000 ~ a.

800 t2l

600 ~ 400 ~

200

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600 b..

400 ~

200

1200

1000 ~ a. (I)

800 <C

m 600 b..

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May June July August September

171

FIGURE I. The efficient degree-days and the weekly precipitation during the growing seasons 1995, 1996 and 1997.

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Statistical methods

The effect of four fertilizer types at three application rates was studied in a split plot design, in which the application rate of fertilization was the main plot and fertilizer type was the sub-plot. All two-factor L interactions of fixed effects were also included in the model. Because each year the trial was situated on a different site and in a different phase of the crop rotation, the years were analysed separately. If the three years were analysed in the same analysis the prerequisities of the statistical analysis would not have been met The control treatments with no fertilization were not included in the statistical model but the results were still utilized as a reference in the evaluation of the results of the fertilizer treatments. The structure of the model is presented below:

Model components: Yijk: measured value 11: grand mean Bi: effect of block i (random effect) Li: main effect of fertilizer level (fixed effect) (B*L\i: interaction of block and fertilizer level (random effect) Fk: main effect of fertilizer type (fixed effect) (L *F)jk: interaction of fertilizer level and fertilizer type (fixed effect) Eijk: random error

The random variables (Bi and (B*L\j) are all assumed independent and normally distributed with zero means and variances (02B, cr2BL, cr2£) respectively.

Parameter estimation in the model was made by a restricted maximum likelihood estimation method (REML). Appropriateness of the models was studied by residual plots. The box plot (Tukey, 1977) was used to check the normality of residuals. Differences between parameter means of mineral fertilizer and organic fertilizers were tested by contrasts and the t -test. Degrees of freedom were determined by Satterwaite's approximation. The fitting of the model was made with the MIXED procedure of SAS software (SAS Institute, 1996). In the figures, the results of individual plots (four replications) are shown as single points and the mean value forms a line from one application rate of fertilization to another.

RESULTS

Yield and quality of tubers

The tuber yield correlated positively with the increasing fertilization rate with all fertilizer types. In the first experiment, the difference between the mineral

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fertilizer and the organic fertilizers became wider as the fertilization rate increased, but in the following years the difference remained rather constant (Table 3). The highest yields were reached with the double rate of mineral fertilizer (Figure 2). The tuber yield from the composted and the fresh manure correlated highly with the amount of total N added in the manure (r = 0.9983, p = 0.037), independent of the amount of inorganic nitrogen contained in the manure.

The yield response of potato to fertilization depended on the year. In other words, the intensity of fertilizer effect was determined by the conditions in the soil and by the season. The unfertilized treatment produced 36.9 t ha-1 in the first year ( 1995), and the average yield response to compost N fertilization was 2.0%. In 1996, the yield of the unfertilized treatment was 29.8 t ha-1 and the response 5.1 %, but in 1997, 28.8 t and 6.3%, respectively.

External quality and classification

The proportion of small tubers was greater with organic fertilizers than with mineral fertilizers. The effect of the mineral fertilization rate on tuber size was very small. No differences between the fertilizer treatments were found in greening or darkening of tubers or in the proportion of first class tubers (data not shown). The contribution of first class tubers to total yield varied between 85 and 95% in the years 1996 and 1997, and depended mostly on tuber size.

TABLE 3

Potato yield. The effect of fertilizer rate, type and the contrasts between fertilizer types; M: mineral fertilizer, C: composted and F: fresh cattle manure, S: aerated cattle slurry;

*p < 0.05; **p < 0.01; ***p < 0.001.

1995 1996 1997

Fertilizer Difference p Difference p Difference p (t ha-1) (t ha- 1) (t ha- 1)

Rate 0.027 0.032 0.017 Type <0.001 <0.001 <0.001

M vs C 14.58*** 6.78*** M vsF 10.63*** 5.74*** M vs S 8.23*** 6.01 ***

Rate*Type 0.023 0.430 0.339 Half M vs C 5.29*

M vs F 0.55 M VS s 5.24*

Medium M vs C 5.75* M vsF 9.31 *** M vs S 6.42*

Double M VS c 13.46*** M vsF 5.95* M vs S 10.14***

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Dry matter and starch content

175

Potatoes fertilized with organic fertilizers had a higher dry matter content than the potatoes from the mineral fertilization (Table 4). The unfertilized treatment and the half rate of all fertilizers produced equal dry matter contents. In the two first experiments the difference between the mineral and the organic fertilizations widened with increasing fertilizer application rate, but in the last experiment in 1997, the dry matter content decreased proportionally in all fertilizations as the fertilizer rate increased (Figure 3).

The potato starch content differed significantly between the mineral and organic fertilizations in the first two experiments but not in the third (Figure 4 and Table 5). The minerally-fertilized potatoes had a lower starch content than the organically-fertilized. The smallest differences in starch content were observed between the mineral and the slurry fertilization. With organic fertilizers, the starch yield was mostly connected to the tuber yield, whereas with mineral fertilizer, the starch yield remained nearly constant at each fertilizer application rate owing to the strong reduction of starch content with increasing mineral fertilization. As a consequence, there were no significant differences in the starch yield between the mineral and the organic treatments.

TABLE 4

Potato dry matter content. The effect of fertilizer rate, type and the contrasts between fertilizer types; M; mineral fertilizer, C: composted and F: fresh cattle manure, S: aerated cattle slurry;

*p < 0.05; **p < 0.01; ***p < 0.001.

1995 1996 1997

Fertilizer Difference p Difference p Difference p (%units) (%units) (%units)

Rate 0.002 0.013 0.022 Type <0.001 <0.001 <0.019

MvsC -0.69 Mvs F -0.85 M vs S -1.11**

Rate*Type 0.003 0.030 0.739 Half MvsC -0.85

M vsF 0.04 M vs S -0.71

Medium M vsC -2.13*** M vsF -1.72*** M vs S -2.16***

Double MvsC -3.17*** M vsF -2.33*** M vs S -2.80***

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TABLE 5

177

Starch content in potato dry matter. The effect of fertilizer rate, type and the contrasts between fertilizer types; M: mineral fertilizer, C: composted and F: fresh cattle manure, S: aerated cattle

slurry; *p < 0.05; **p < 0.01; ***p < 0.001.

1995 1996 1997


Rate 0.014 0.413 0.069 Type 0.017 0.005 0.753

M vsC -2.83** -2.21 ** -0.14 Mvs F -2.26* -2.87*** -0.72 M VS s -1.66 -1.15 -0.55

Rate*Type 0.278 0.345 0.890

TABLE 6

Nitrate concentration in potato. The effect of fertilizer rate, type and the contrasts between fertilization types; M: mineral fertilizer, C: composted and F: fresh cattle manure, S: aerated

cattle slurry; *p < 0.05; **p < 0.01; ***p < 0.001.

1995 1996 1997

Fertilizer Difference p Difference p Difference p (%units)) (%units) (%units)

Rate 0.096 0.006 <0.001 Type <0.001 0.298 0.049 Rate*Type <0.001 <0.001 0.008 Half MvsC 3.81 -1.82 -2.40

Mvs F 2.60 -2.61 -2.17 M vs S 2.65 -3.83* -0.93

Medium M vs C 8.53** -2.97 1.14 M vs F 8.31 ** -1.15 3.83 M VS s 10.09*** 0.21 0.93

Double M VS c 17.61 *** 9.04*** 11.24*** M vsF 18.51*** 8.81 *** 9.39*** M vs S 17.30*** 8.06*** 8.15**

Nitrogenous compounds

The minerally-fertilized potato tubers contained more nitrate at the double fertilizer rate each year, as did the medium rate in 1995, than the organicallyfertilized tubers (Figure 5). The mean concentration of nitrate never exceeded 50 mg kg-1 fresh weight. Increasing rates of organic manures did not raise the nitrate concentration as strongly as the mineral fertilizer (Table 6). The concentration of free amino acids in the potato was strongly related to the type, but not so much to the rate, of fertilization applied (Table 7 and Figure 6); it was highest in the

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minerally-fertilized potato in each experiment. No difference between the organic fertilization treatments at the half fertilization rate was found. The unfertilized treatment also showed equal concentrations of free amino acids with the fresh and composted manure fertilization treatments. The difference between the mineral fertilizer and slurry were the smallest at the medium and the double application rate.

Vitamin C and mineral compounds

The concentration of vitamin C in potato tubers varied between field replications (Figure 7) but not between treatments. In the last experiment ( 1997), potatoes fertilized with the slurry or the mineral fertilizer contained somewhat higher concentrations of vitamin C than the tubers from the fresh and com posted manure fertilization (Table 8). The potato tuber K concentration from the unfertilized plots varied each year (18.5-19.9 g kg-1) with no significant positive correlation to the exchangeable K content in the soil. In two of the three experiments, the K concentration increased in a similar manner in all fertilizer treatments as the fertilization rate increased (Table 9). However, there was a clear tendency for a higher K concentration in the tubers fertilized with solid manure. In 1995, the K concentration was highest with composted manure and in 1997 fresh and com posted solid manure led to significantly higher K concentrations than mineral fertilizer (Figure 8). The P, Mg, Ca and Na concentrations did not show any significant difference between the treatments from year to year (data not shown).

TABLE 7

Free amino acid concentration in potato. The effect of fertilizer rate, type and the contrasts between fertilizer types; M: mineral fertilizer, C: composted and F: fresh cattle manure,

S: aerated cattle slurry; *p < 0.05; **p < 0.01; ***p < 0.001.

1995 1996 1997

Fertilizer Difference p Difference p Difference p (t ha- 1) (t ha- 1) (t ha- 1)

Rate 0.157 0.743 0.127 Type <0.001 <0.001 <0.001

M VS c 1235** M vs F 1338*** M VS s 1137**

Rate*Type 0.008 0.196 0.019 Half M vsC 944 356

M VS F 1396** 501 M VS s 1229* 708

Medium MvsC 1922*** 1942*** MvsF 1467** 2509*** M VS s 1387** 1079***

Double M vs C 3991*** 2575*** M VS F 3253*** 1578** M vsS 2433*** 1502**

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TABLE 8

Vitamin C concentration in potato. The effect of fertilizer rate, type and the contrasts between fertilizer types; M: mineral fertilizer, C: composted and F: fresh cattle manure,


1995 1996 1997


Rate 0.038 0.337 0.328 Type 0.899 0.781 0.003

M vsC 1.668 M vs F 2.063 M VS s -2.113(*)

Rate*Type 0.903 0.129 0.380

TABLE9

Potassium concentration in potato. The effect of fertilizer rate, type and the contrasts between fertilizer types; M: mineral fertilizer, C: composted and F: fresh cattle manure,


1995 1996 1997


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Rate*Type 0.140 0.047 0.684 Half M vs C -2.65***

M vs F -2.35*** M VS s -1.41 *

Medium M VS c -0.38 M vsF -1.13 M vs S -0.57

Double M VS c -0.15 M VS F 0.52 M VS s 0.07

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TABLE10

Change of electrical conductivity in potato extract. The effect of fertilizer rate, type and the contrasts between fertilizer types; M: mineral fertilizer, C: composted and F: fresh cattle manure,


1995 1996 1997


Rate 0.557 0.362 0.758 Type <0.001 <0.001 <0.001

MvsC Mvs F M vs S

Rate*Type 0.022 0.010 0.017 Half MvsC 6.57* 6.58** 0.79

M vsF 5.24 6.63** 0.79 Mvs S 5.72* 2.19 1.84

Medium M vs C 12.67*** 6.81 ** 2.65 M vsF I 1 .88*** 6.88** 3.45* MvsS 12.02*** 8.41 ** 0.55

Double MvsC 20.31 *** 15.47*** 7.62*** MvsF 19.14*** 16.44*** 6.51 *** M vs S 14.32*** 14.23*** 3.75*

rates of slurry fertilization caused an increase in the change of conductivity. In contrast to the study of Kjellenber & Granstedt ( 1998), there were no differences in the 'tuber extract darkening' between treatments (data not shown).

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TABLE II

Soil mineral nitrogen after harvest. The effect of fertilizer rate, type and the contrasts between fertilizer types; M: mineral fertilizer, C: composted and F: fresh cattle manure,


1995 1996 1997


Rate 0.027 <0.001 0.055 Type 0.015 <0.001 0.288 Rate*Type <0.001 <0.001 0.071 Half MvsC 7.31 3.12 -0.77

M vs F 5.61 0.15 -1.05 M VS s 5.66 1.79 -0.53

Medium MvsC -1.02 0.51 -1.72 M vsF -1.69 0.06 -0.90 M vsS -2.54 -0.38 -0.49

Double MvsC -0.60 19.46*** 5.59** M vsF 0.16 16.26*** 2.95 M vs S -1.54 17 .35*** 7.04***

DISCUSSION

The short-term effects of organic and mineral fertilization on potato yield and quality were quite small in these experiments. Potatoes reacted differently each year to fertilization due to the variation of the growing season and the soil conditions. The variation is revealed by the yields of the unfertilized plots. The unfertilized control treatment in 1995 produced a high yield after oats with undersown clover-grass mixture as a pre-crop, but in the next experiment, the after-effect of the oats was low. In 1997, the yield in the control treatment was the same as the previous year, although the pre-crop had been a green manure with a high amount of forage legumes. The low after-effect is partly explained by the very dry first half of the growing season, which might have retarded nutrient mobilization from the crop residues. The effect of organic manures on potato yield was more prominent during 1996 and 1997 than in 1995 by promoting the crop growth in the more unfavourable soil and climatic conditions. In the last two experiments the yield increased proportionally in organic and mineral fertilization when the fertilizer dosage was raised from half to double. In the first experiment, the influence of organic fertilizers was partly suppressed by the good after-effect of the pre-crop together with the good climatic conditions. These factors guaranteed a high potato yield but could not hide the influence of the double rate of the mineral fertilizer.

Varis et al. (1996) reported that nitrogen release was the main determinant of the yield differences between organic and mineral fertilization systems. The same

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conclusion must be drawn from the results of these experiments. With the mineral fertilization, the amount of soil mineral N at the first quarter of the growing season was much higher than with the organic fertilizers. During this period, within 4 weeks after emergence, the potato plant takes 80-90% of its nitrogen (Fischer et al., 1992). The amount of mineralized nitrogen from the organic fertilizers at a certain fertilization ;ate seemed to remain much lower than that delivered to the soil by the mineral fertilizer at the corresponding rate. Within the organic solid fertilization treatments the amount of added total N and the yield had a close positive correlation, which means that mineralization of organic N from the composted and the fresh manure must have been similar.

The amount of soil mineral N remaining at harvest was more related to the climatic conditions than to the fertilization type. Warman & Havard (1998) also concluded from their three-year experiments that the climatic conditions affected plant production more than the type of fertilization. In the first experiment, the residual mineral N content from the organically-fertilized soils exceeded the minerally-fertilized soils at the double fertilization rate. The whole growing season was characterized by humid and warm weather. Two weeks before the harvest there were heavy rainfalls and after that light showers now and then, and the daily mean temperature remained high to the second week of September (Figure 1 ). Thus, the conditions enhanced nutrient mineralization from the organic fertilizers. In 1996, the mineral fertilization left more mineral N in the soil and the difference between the fertilizer types widened as the application rate increased. The rate of mineralization from the organic N sources seemed to be slower after the long dry period in August, and the light showers and cold weather in September could not activate it. In the third experiment, the remaining soil mineral N was at the same level in all fertilizer types, varying slightly between the fertilization rates. The dry and cold beginning of the growing season had retarded the start of crop residue decomposition. Two weeks before the harvest, in the middle of September, the warm temperatures and daily rainfalls enhanced nutrient mineralization from the organic fertilizers. According to Griffm & Hestermann ( 1991 ), the recovery of N from mineral fertilization in dry conditions is higher than from organic fertilizers, but in the humid seasons, when the release from the soil organic pool is higher, the N recovery from mineral fertilizer decreases. In the Swedish K-experiment, the potato yields were higher in the organic treatments in the years with high precipitation when the conditions for N mineralization were better (Kjellenberg & Granstedt, 1998). Honeycutt (1997) also assumed that the magnitude of the crop rotation effect on potato growth, referring to the rate of N mineralization and potato N uptake, depended on the amount of precipitation.

Kjellenberg & Granstedt (1998) argued that in different fertilizer types the potato quality parameters were correlated differently with tuber yield. With mineral fertilization, high yield was connected to poorer quality, which did not occur with the organic fertilizer treatments. Schulz et al. (1997) reported that

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tubers from minerally-fertilized plots showed the strongest response to intensification of fertilization; for example strongest decrease in dry matter content or sensoric quality, and concluded that the reason was a higher availability of nutrients from mineral fertilizers. In the present experiments, the same tendency could be observed for the concentration of nitrogenous compounds and dry matter.

The concentrations of nitrate and free amino acids in the tuber, commonly used as negative quality parameters, were clearly connected to the solubility ofN in the fertilizer. The positive correlation of fertilizer application rate with tuber nitrate and free amino acid concentrations was obvious with mineral fertilizer, noticeable with slurry and insignificant in composted or fresh solid manure (Table 7). In the present experiments, the plant uptake of mineral N from the solid manures never seems to have become excessive and, thus, the content of intermediate products of the nitrogen metabolism remained low. Our results are in accordance with Bohm & Dewes ( 1997), who observed a small increase in the free amino acid concentration (41-43 mg g-1 d.w.) and somewhat larger in nitrate (26-45 mg g-1) as the manure fertilization increased from 0 to 30 t ha-1.

The high amount of potassium applied with the fresh and compos ted manure was reflected in the high potassium concentration of the potato. Accordingly, there was a low concentration in the potatoes from the mineral fertilizer and slurry treatments. In the Swiss experiments, the potatoes from the organic farming systems had a lower K content than those from the conventional (Spiess & Besson, 1995). In the present experiments, the potato K concentration was relatively high (> 1.8%) with all fertilizer types and even in the unfertilized control plots, which indicates a high availability of K from the soil.

The results of various experiments showing a higher concentration of vitamin C in organically fertilized potato tubers (Pettersson, 1982; Kolbe et al., 1995b; Varis et al., 1996) could not be corroborated in the present experiment. Rautavaara (1973) observed in his trial comparing different farming systems that the vitamin C concentration varied more between single tubers than between treatments and depended on the physiological age of the tuber. This kind of variation is typical for the North European conditions with a short vegetation period, in which a proportion of the tubers always stay immature.

The 'increase of electrical conductivity in the potato extract' correlated highly with the amount of mineral N applied in the fertilizer. The present results, as well as those from the Swedish experiments (Kjellenberg & Granstedt, 1998), showed a significantly stronger increase of conductivity in the extract of minerallyfertilized tubers. Also the slurry fertilization resulted in a larger increase than the other organic fertilizers. The larger change of conductivity, according to Pettersson (1982), indicates that the potato tissue has a lower resistance to enzymatic and microbial decomposition. The 'potato extract darkening' was greater in minerally-fertilized tubers in the Swedish experiment, but the present experiment could not confirm those results. The physiological background of

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'extract darkening' is based on enzymatic processes involving phenolic acids and oxygen that occur more easily in destroyed tissue. Potato tuber can be regarded as more vital if it stays less coloured after being damaged, wrote Raupp ( 1998), but concluded that no correlation between darkening and other durability or vitality parameters was found.

CONCLUSIONS

In this study, which concentrated on the short-term effects of organic and mineral fertilizers, the differences in the quality parameters of potato were quite small. The low rate of fertilizer application had hardly any effect on the potato yield or quality because the soil characteristics and the cultivation history were the focal factors for the crop growth within the prevailing weather conditions. However, the interaction between the high rates of organic and mineral fertilizer application and the climate-dependent nutrient mineralization from the pre-crop residues were reflected in the quality of potato. To proceed further in the research work considering the question of fertilization in organic farming, it is necessary to carefully describe the soil cultivation history and fertility as well as the climate at the site of the experiments.

The long-term effects of manure in the whole crop rotation may have a greater influence on potato quality than the short-term effects owing to the slow release of nutrients and the beneficial effects of manure on soil biological activity. This aspect can only be studied in long-term experiments in which the crop rotation can be kept constant.

Some of the quality parameters related to human nutrition; concentrations of nitrate and free amino acids, varied between the fertilization regimes. The 'change of electrical conductivity' of potato extract, a parameter describing product immaturity and lack of resistance to microbial and enzymatic decomposition, was clearly greater with mineral fertilization. Neither the concentration of vitamin C nor minerals showed any clear differences between fertilizer types.

The new biological tests, 'extract darkening' and 'change of electrical conductivity', seem to require more theoretical background for proper interpretation. However, this kind of biological assay may give important knowledge about essential quality parameters for human nutrition and product sustainability in storage.

As the concept of food quality includes more than the composition of the product itself, the environmental quality, represented by the residual inorganic N in the soil after harvest, was also evaluated in the experiment series. All fertilization treatments in the experiment series appeared to be environmentally safe although the residual inorganic N was clearly higher in minerally-fertilized soil than in the organically fertilized soils.

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References

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(Received 5 April 2002; accepted 31 December 2002)

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effects of different organic fertilization practices and mineral fertilization on potato quality

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