Agricultural and Food Science in Finland, Vol.11 (2002):59 –74 59 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 11 (2002): 59–74. Efficiency of split nitrogen fertilization with adjusted irrigation on potato Paavo Kuisma Potato Research Institute, Ruosuontie 156, FIN-16900 Lammi, Finland, e-mail: paavo.kuisma@petla.fi The split application of fertilizer nitrogen and adjusted irrigation were studied at Potato Research Institute in Lammi in 1993–1995. Independent of year, variety or irrigation, the split application of nitrogen had no benefit on yield or yield quality compared to a single dose at planting on the levels of recommended nitrogen fertilization. The highest yields were received with the highest dose 110 kg ha–1 N, given all at planting, followed by 80, 50 + 30 + 30, 50 + 30 and 50 kg ha–1 N. The yields without fertilizer nitrogen averaged 24.8 t ha–1. The adjusted irrigation increased tuber yields 17%. The utilization of fertilizer and organic soil nitrogen was good on potato crop if water supply was taken care of. Some values of apparent fertilizer nitrogen recovery exceeded 1.0 thus suggesting that the moderate fertilization improved the ability of potato crop to utilize the natural nitrogen sources in soils. Key words: irrigation, nitrogen, potatoes, Solanum tuberosum, fertilizers © Agricultural and Food Science in Finland Manuscript received May 2001 Introduction A sufficient level of soil nitrogen is needed for an abundant yield growth during the tuber bulk- ing (Burton 1989). However, high amounts of fertilizer nitrogen in planting may cause too abundant growth of the foliage in potato crop during early phases of development, and conse- quently delay tuber initiation and senescence of leaves (Harris 1978, Beukema and van der Zaag 1979, Allen and Scott 1980, Linnér 1988). Ex- cessive nitrogen availability during the entire growing season, particularly near the end of tu- ber bulking, delays tuber maturity (Ojala et al. 1990). The length of the growing season in Finnish potato production areas is only 140–180 days, and delayed tuber formation and late senescence are a risk for the yield and its quality. Therefore the application of fertilizer nitrogen must always be in a balance between potential yield and crop senescence (Varis 1972, Mustonen 1997). High amount of fertilizer nitrogen applied as a single dose at planting is an environmental risk. The nitrate nitrogen easily leaches into ground wa- ter in case of high rainfall or excessive irriga- tion on light, sandy soils (Harris 1978, Burton mailto:paavo.kuisma@petla.fi 60 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Kuisma, P. Split nitrogen fertilization with irrigation on potato 1989, Goffart and Guiot 1996) which are typi- cal in potato production in Finland. The splitting of fertilization is suggested to avoid the unfavourable effects of excessive ni- trogen (Harris 1978, Burton 1989). In a split application, only a part of the total fertilizer ni- trogen is given at planting, and the rest is ap- plied later in one or several doses during the growing period. Many studies in Western Europe have report- ed significant yield increase and better quality of potatoes owing to split nitrogen dressing (e.g. van Loon et al. 1987, Nitsch and Varis 1991), especially if early summers are rainy and risks to leaching are high (Linnér 1988, Goffart and Guiot 1996). Svensson et al. (1973) state that the benefits of nitrogen splitting are most obvi- ous under the conditions where the prerequisites to growth are good and high yields are expect- ed. The profits of split application however, are minimal if leaching is no risk (Harris 1978). With a split application the availability of nitrogen in soil can be better matched to the crop uptake of nitrogen throughout the season. In this way, late application could be adjusted to take account of reductions in nitrogen requirement as a result of drought, frost, etc. Also higher rates of mineralization than expected can be dealt without raising the amount of residual nitrogen after harvest (Vos and Marshall 1993). Accord- ing to Beukema and van der Zaag (1979) in the split fertilization the additional nitrogen should be applied not later than three weeks after the emergence. The aim of this study was to test the effect of split application of nitrogen with adjusted irri- gation on potato yield in Finnish conditions where the recommendations of nitrogen fertili- zation are rather low, 40–100 kg ha–1. A partial aim was also to study if split fertilization en- sured a high yield on potato in the case of rapid early development. Material and methods In the three year field experiment at Potato Re- search Institute in Lammi (61°06' N, 23°02' E, 97 m above sea level) the irrigation and the split application of fertilizer nitrogen on potato were studied in 1993–1995. In 1993–1994, the study was done with two cultivars: Fambo, early table potato, and Tanu, early starch potato. In 1995, only a middle late crisp variety Lady Rosetta was used. The trials were located on sandy soils (Ta- ble 1) typical for potato production in Finland. Irrigation and nitrogen fertilization were ar- ranged in a split-plot design with three complete- ly randomised blocks. In 1993–1994 the varie- ties located on two neighbouring trials. Irriga- tion was placed in the main plots: A0 = unirrigated A1 = irrigated The irrigation was applied with nozzle boom when the soil moisture in top soil decreased to 50% of plant available water capacity (Elonen et al. 1967), determined by the gypsum block techniques (Boyoucos 1950). Gypsum blocks were installed at the depth of 30 cm to the mid- Table 1. Soil properties (0–30 cm) on trial fields. Year Soil type pH Ca P K Mg Soil inorganic N mg l–1 kg ha–1 at planting 1993 coarse sandy loam 5.9 1050 12.0 182 083 1994 fine sandy loam 6.1 1210 07.3 185 111 32 1995 fine sandy loam 5.2 0888 13.0 162 063 25 61 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 11 (2002): 59–74. dle of bed on plots of nitrogen fertilization 80 kg ha–1. Dates and amount of irrigation are pre- sented in Table 2. The nitrogen fertilization in subplots was done with six combinations: B0 = no nitrogen B1 = single dose 50 kg ha –1 N at planting B2 = single dose 80 kg ha –1 N at planting B3 = single dose 110 kg ha –1 N at planting B4 = split fertilization; 50 kg ha –1 N at planting, 30 kg ha–1 N at hilling B5 = split fertilization; 50 kg ha –1 N at planting, 30 kg ha–1 N at hilling, 30 kg ha–1 N before blooming At hilling, about 40 days after planting, the potato crop reached the developmental stage of 33–35 (the end of leaf expansion) (Hack et al. 1993). Second application was about 20 days later, just before blooming. In both applications fertilizers were given as top dressings. The base nitrogen 50 kg ha–1 N on split ferti- lization and all rates of single fertilization were placed in two rows about 5 cm beneath and 10 cm aside of seed tubers at planting with ammonium nitrate limestone (Oulunsalpietari, Kemira Agro Oy, Finland, N 27.5%). Split applications after planting were top dressed with ammonium ni- trate limestone in 1993 and with calcium nitrate (Kemira Agro Oy, Finland, N 15.5%) in 1994– 1995. Phosphorus and potassium were broadcast before planting as superphosphate (P 9%) and potassium sulphate (K 42%), respectively, ac- cording to information of soil analysis and re- quirements of varieties. At planting an individual plot consisted of four rows each 10.0 m long and 80 cm apart. Potato yields were quantified by harvesting two middle rows of 8.0 m length from each plot. A trial field was ploughed to the depth of 25 cm in the previous autumn after the harvest of spring barley. A day before planting, the seed bed was tilled twice with a spring-tined harrow to the depth of 17 cm. Seed presprouted for three weeks in light, was planted to the depth of 5 cm with a semiautomatic two-row planter on 12 May, 18 May and 5 June in 1993, 1994 and 1995, respec- tively. The planting distance was 31 cm on both varieties in 1993. In 1994 Fambo was planted to 31 cm and Tanu to 26 cm. In 1995 Lady Rosetta was planted to 28 cm. The size of certified seed (class A) was 30–45 mm, 35–50 mm and 30–40 mm in 1993, 1994 and 1995, respectively. Trials were earthed about 40 days after planting with a two-row mouldboard ridger. Plant protection was done according to the local agricultural practice. Trials were harvested with a tractor carried one- row harvester on 28 August, 6–7 September and 13 September in 1993, 1994 and 1995, respec- tively. Crop analysis during growing season includ- ed observations of emergence time, occurrence of crop diseases and determinations of growth stage on 45, 75 and 90 days after planting (DAP) using a modification of the growth stage identi- fication keys described by Hack et al. (1993). After the harvest, yields were graded into tuber sizes on 10 mm differences which were summa- rized as size distribution 30–40 mm, 40–70 mm, and > 70 mm. Before grading a sample of about Table 2. Dates and rates of irrigation. 1993 1994 1995 Date mm Date mm Date mm 10 June 19 12–13 July 25 30 June 15 16 June 15 15–16 July 30 18 July 10 30 June 16 19 July 12 02 August 22 09 July 22 21–22 July 26 13 July 20 29 July 17 Total 92 1100 47 62 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Kuisma, P. Split nitrogen fertilization with irrigation on potato 5 kg tubers per plot was taken for specific grav- ity determinations and for analysis of external quality. Specific gravity was converted to starch content according to the table used on official variety trials (Kangas 1998). After the determi- nation of specific gravity samples, treatments were joined over replicates for analysis of ex- ternal quality and in 1994–1995 also for analy- sis of total nitrogen content in tubers. Thus the differences between means of these analyses were not studied statistically. Classification of tuber defects described by Kangas (1998) was used in analysis of external quality. Marketable yield on cultivars Fambo and Lady Rosetta was determined as a yield of 40– 70 mm sized healthy tubers. The total nitrogen content of tubers at final harvest was analysed in a commercial laboratory (Novalab Oy) with the Kjeldahl method, the sample being 20 medi- um size tubers. The nitrate nitrogen content was analysed from 20 middle size tubers at Potato Research Institute using ion specific electrodes according to VTT Method 4207-84 described detailed by Ruippo and Alikärri (1989). In 1994–1995 the soil inorganic nitrogen (NO3 –-N and NH4 +-N) before planting, weekly during the growing period between 40–75 DAP and after harvest were determined using the “soil baggage”, developed by Kemira Agro Oy (1994). Before the planting one sample consisted of twenty sub samples from the depth of 0–30 cm, taken randomly on the trial field before the till- age. After the harvest, samples consisted of five sub samples on every plot from the depth of 0– 30 cm. At 40, 47, 54, 61, 68 and 75 DAP the samples were collected as two whole bed pro- files on every plot. Weather in May–September (Table 3) was measured at Potato Research Institute. The long term averages were taken from the Finnish Me- teorological Institute’s weather station at Hel- sinki University, Lammi Biological Station, lo- cated 4 km from the experiments. The data were collected and calculated on a MS®Excel-Worksheet. The results were further analyzed with a SPSS 10.01 -statistical package using the analysis of variance by split-plot de- sign on GLM-procedure where years, and varie- ties in trials 1993–1994 were analyzed as fixed factors in series of experiments: Yijklm = µ + Tj + Vk + TVjk + eij(1) + Il + TIjl + VIkl + TVIjkl + eijkl (2) + Nm + TNjm + VNkm + TVNjkm + INlm + TINjlm + VINklm + TVINjklm + eijklm (3) where Yijklm is the response for the block i, trial year j, variety k, irrigation l and nitrogen fertili- zation m; µ is the overall mean; B is the random block effect; T, V, I and N are the fixed effects of trial year, variety, irrigation and nitrogen fer- tilization; TV,…,TVIN interactions of fixed ef- fects; and the random error terms eijk (1) = Table 3. Weather conditions at Potato Research Institute in 1993–1995 and 30-year averages at Helsinki University, Lammi Biological Research Station. Mean temperature (°C) Precipitation (mm) Month 1993 1994 1995 1961–1990 1993 1994 1995 1961–1990 May 12.6 07.6 07.2 08.8 003 041 098 038 June 11.0 12.2 15.6 14.0 058 060 028 047 July 14.8 19.0 13.5 16.7 118 004 056 074 August 12.0 14.3 13.3 14.8 129 070 063 087 September 04.8 09.4 07.8 09.7 032 105 043 068 Mean/Sum 11.1 12.5 11.5 12.8 340 280 288 314 DD °C1) .967 1160. .914 1) DD °C = effective temperature sum (> + 5°C) 63 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 11 (2002): 59–74. T ( V ( B ) ) , e i j k l ( 2 ) = T ( V ( I B ) ) a n d e i j k l m ( 3 ) = T(V(I(NB))) (Snedecor 1956). In 1995 the analysis of variance was based on the following mixed model for split-plot de- sign: Yilm = µ + Bi + Il + eil(1) + Nm + INlm + eilm(2) where Yilm is the response for the block i, irriga- tion l and nitrogen fertilization m; µ is the over- all mean; B is the random block effect; I and N are the fixed effects of irrigation and nitrogen fertilization; IN is the two-factor interaction of fixed effects; and the random error terms eil (1) = BI and eilm (2) = I(NB). Summaries of variance analysis are presented in Tables 4 and 5. The sta- tistical differences (P < 0.05) between group means were further studied by Tukey’s HSD (Honestly Significant Difference) test. Results and discussion Crop development On the emergence, the greatest differences were between years and varieties, as expected. May 1993 was exceptionally warm, and potatoes planted in the middle of May emerged in two Table 4. Summaries of variance analysis on varieties Fambo and Tanu in 1993–1994. Source of Emergence DAP45 DAP75 DAP90 Tuber Starch-% Starch < 40 mm 40–70 mm > 70 mm variation yield yield Year (Y) *** *** *** *** * Ns *** *** *** *** Variety (V) * Ns Ns *** *** *** Ns *** *** Ns YxV * * * Ns Ns ° Ns *** *** * Irrigation (I) * Ns ° * *** ° *** *** *** Ns YxI * * ° *** *** ° *** *** *** Ns VxI Ns Ns ** * Ns Ns Ns ** *** ° YxVxI ° Ns Ns * Ns Ns Ns ** ** Ns Nitrogen (N) *** *** *** *** *** *** *** *** *** *** YxN *** *** * Ns *** * *** ** *** Ns VxN Ns * Ns ** ° * ** ** Ns Ns IxN * Ns Ns * Ns *** Ns * * Ns YxVxN * Ns Ns Ns ** Ns * *** ** * YxIxN Ns Ns Ns Ns *** Ns ** * ** Ns VxIxN Ns Ns Ns Ns Ns Ns Ns * ** Ns YxVxIxN ° Ns Ns Ns Ns Ns Ns * Ns Ns DAP = days after planting; Ns = no significant difference; ° = 0.1 < P < 0.05; * = 0.05 < P < 0.01; ** = 0.01 < P < 0.001; *** = P < 0.001 Table 5. Summaries of variance analysis on variety Lady Rosetta in 1995. Source of Emergence DAP45 DAP75 DAP90 Tuber Starch-% Starch < 40 mm 40–70 mm > 70 mm variation yield yield Irrigation (I) Ns Ns Ns Ns Ns Ns Ns Ns Ns ** Nitrogen (N) Ns Ns Ns * *** *** *** Ns ° Ns IxN Ns Ns Ns Ns Ns Ns Ns Ns Ns Ns DAP = days after planting; Ns = no significant difference; ° = 0.1 < P < 0.05; * = 0.05 < P < 0.01; ** = 0.01 < P < 0.001; *** = P < 0.0 64 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Kuisma, P. Split nitrogen fertilization with irrigation on potato and a half weeks. May 1994 was cooler than the average, and the emergence required about one month. In 1995 the trial was planted at the be- ginning of June, and variety Lady Rosetta emerged about five days faster than Fambo and Tanu in 1993. The development of crop followed the order of emergence in the early growth (45 DAP). In 1995, the faster growth of Lady Ro- setta continued to the harvest (Table 6). In 1993 the main plots of irrigation emerged about 0.9 days faster than unirrigated plots. The difference was statistically significant (P < 0.05) although it had no practical meaning. Nitrogen fertilisation, interacted with year (P < 0.001), irrigation (P < 0.05) and year x variety (P < 0.05), also had highly significant influence (P < 0.001) on the emergence, the influence, however, hav- ing again no practical meaning. Table 6. Crop development in 1993–1994. Year Variety Fertilization 45 DAP 75 DAP 90 DAP kg /ha N U I U I U I 1993 Fambo 0 36a 36a 77a 75abc 81a 83ab 50 36a 36a 81a 81a 83a 84a 80 37a 36a 78a 78ab 82a 84a 110 36a 36a 78a 71c 82a 81b 50 + 30 35a 36a 81a 81a 83a 84a 50 + 30 + 30 36a 35a 81a 74bc 82a 82ab Mean 36A 36A 79A 77B 82A 83A Tanu 0 34a 34a 74a 74ab 78b 78c 50 33a 34a 74a 78a 82a 84a 80 34a 35a 71a 78a 81a 83ab 110 35a 36a 71a 71b 81a 81b 50 + 30 34a 34a 74a 78a 82a 83ab 50 + 30 + 30 33a 35a 74a 77ab 81a 81b Mean 34A 35A 73B 76A 81A 82A 1994 Fambo 0 25c 26b 72a 71a 82a 81a 50 30a 29a 74a 72a 84a 81a 80 27bc 27ab 73a 72a 82a 81a 110 28ab 28ab 72a 72a 82a 81a 50 + 30 28ab 29a 72a 72a 84a 82a 50 + 30 + 30 30a 29a 73a 72a 83a 81a Mean 28A 28A 73A 72A 82A 81A Tanu 0 26b 25c 74a 72a 82a 77bc 50 30a 30ab 76a 73a 84a 81a 80 30a 28b 75a 72a 82a 77bc 110 30a 30ab 75a 71a 82a 74d 50 + 30 30a 31a 75a 72a 84a 79ab 50 + 30 + 30 31a 31a 75a 72a 83a 75cd Mean 30A 29A 75A 72B 83A 77B DAP = days after planting; U = unirrigated; I = irrigated Within a single column group means followed by the same letter (with capital letters between irrigation treatments in each DAP) are not statistically different at level P < 0.05 in Tukey’s HSD-test. 65 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 11 (2002): 59–74. The effect of irrigation on the development of the canopy was seen in 1994 when the irriga- tion significantly (P < 0.001) delayed the growth from 75 DAP, especially on cultivar Tanu. In 1994 the last irrigation was given near the end of July. Ojala et al. (1990) reported that irriga- tion can slightly delay the maturity of potato crop if applied abundantly in the latter part of the growing season. The effects of nitrogen fertilization varied during the growing period and were dependent on year and variety. On Lady Rosetta in 1995, the only significant difference (P < 0.05), al- though practically meaningless, was detected at 90 DAP on the development of potato canopy due to nitrogen fertilization. On cultivars Fam- bo and Tanu in 1993–1994 at early growth (45 DAP), when the only differences between nitro- gen fertilization were from planting and no irri- gation was yet given, unfertilized potatoes grew significantly (P < 0.001) slower, other nitrogen rates being similar. The slow development of unfertilized potato continued to the harvest. The unfavourable effect of high nitrogen rate in plant- ing and the late split application came out dur- ing the summer, and was most visible at 75 DAP in 1993. Similar results were published by e.g. Harris (1978). The interaction between irriga- tion and nitrogen fertilization on the develop- ment of potato canopy was weak as Harris (1990) reported. Yield The results of this study showed that the split application of nitrogen compared to a single dose at planting did not give any additional advan- tages on yield, size distribution, starch content or external quality in Finnish conditions when moderate nitrogen rates were used (Figs. 1 and 2). The highest yields were produced when the highest rate 110 kg ha–1 N was given at planting. The same amount of nitrogen split into three applications resulted to about the same yield as 80 kg ha–1 N in a single dose, and 80 kg ha–1 N split into two applications gave the yield com- parable to a single dose of 50 kg ha–1 N. Howev- er, the yield decrease due to split fertilization was clearly smaller than noticed in earlier stud- ies in 1980s at Potato Research Institute (Peru- nantutkimuslaitos 1985). In Michigan, USA, Joern and Vitosh (1995a) made the same con- clusion when studying the similar levels of ni- trogen fertilization for Russet Burbank potatoes under the similar length of growing season as in Finland. Neither in conditions of Alaska, did the split fertilization influence the potato yield (Gavlak et al. 1993). Similarly e.g. Svensson et al. (1973), Carlsson (1977, 1988, 1995) and Goffart and Guiot (1996) report that the split ap- plication of fertilizer nitrogen does not differ from the fertilization where all nitrogen is given at planting when moderate rates of nitrogen are used. In Middle Sweden, Carlsson (1995) noticed yield decrease even up to 30% due to split ap- plication of nitrogen fertilization compared to a placement of all fertilizers at planting. Van Loon et al. (1987) in Holland noticed at the end of eighties that a split application was advantageous only when the total rates of ferti- lizer nitrogen were high (> 200 kg ha–1 N). Sieczka et al. (1993) reported that there was no response to supplemental nitrogen during the growing period when ≥ 168 kg ha–1 N was given at planting. MacLean (1984) in Atlantic Canada also noticed that no yield increase was reached if the nitrogen rate exceeded 135 kg ha–1, and the split application didn’t change the situation. In this study the highest rate of fertilizer nitro- gen was clearly smaller, only 110 kg ha–1 N, than in the studies mentioned. The irrigation affected significantly (P < 0.001) the tuber yield in 1993–1994 on cultivars Fambo and Tanu (Table 4). The influence was, however, dependent on the year. In 1993, July and August were rainy and cool, but in 1994 July was extremely dry and hot. Consequently all yield increase due to irrigation was acquired in 1994 when the increase of tuber yield on irrigat- ed plots was 42% (Table 7). In 1995, June and July were slightly drier, but clearly cooler than normally and irrigation had no effect on the tu- ber yield of variety Lady Rosetta. The signifi- 66 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Kuisma, P. Split nitrogen fertilization with irrigation on potato Fig. 1. Effects of irrigation and nitrogen fertilization on yields and starch content of varieties Fambo in 1993–1994 and Lady Rosetta in 1995. 67 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 11 (2002): 59–74. Fig. 2. Effects of irrigation and nitrogen fertilization on starch production of variety Tanu in 1993–1994. cant interaction of irrigation and nitrogen ferti- lization was seen in the way that differences be- tween fertilized plots were smaller compared to the unirrigated crop (Figs. 1 and 2). Especially yields on split fertilization approached yields produced with the same amount of nitrogen as in a single dose at planting. Similar results are reported by e.g. Guarda et al. (1990, 1993, 1996). Compared to the tuber yields the variation was much smaller on starch content or external quality of tubers. Consequently starch yields and marketable 40–70 mm yields were most related to total tuber yield (Figs. 1 and 2). As expected the highest rates of fertilizer nitrogen produced lower starch content. The effect was stronger on irrigated potato and nearly same wether a single dose or a split application was used. Carlsson (1977) and Walther (1984) reported that the split application of nitrogen had influence on neither the starch nor dry matter content of tubers. The irrigation had no influence on starch con- tent. Ojala et al. (1990) also summarized that 68 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Kuisma, P. Split nitrogen fertilization with irrigation on potato irrigation didn’t necessary decrease the starch content of potato. At Potato Research Institute in 1989–1991, the irrigation even increased the starch content of potato in years with dry early summers and early start of irrigation (Kuisma 1998). Due to small and irregular, although statisti- cally significant differences caused by a larger portion of small tubers (< 40 mm) on unferti- lized potatoes, tuber size distribution and exter- nal quality are worth examining only in relation to irrigation. The effect of irrigation on tuber size distribution was strongly affected by the trial year. In 1993 and 1995 the irrigation had no ef- fect on tuber size. In the dry summer of 1994, the irrigation increased the proportion of tubers 40–70 mm at the expense of those of a small size < 40 mm on variety Tanu. On variety Fambo the influence of irrigation on tuber size was similar but clearly smaller than on variety Tanu (Ta- ble 7). On the average, the variation on quality de- fects due to fertilization was small and uneven for any conclusions. Thus they are ignored in further examination. The studies in Norway have shown that split application of nitrogen has no effect on the external quality of tubers (Hveem forsøksgard 1991). Hunnius et al. (1972) also noticed that the split application of fertilizer ni- trogen didn’t affect tuber defects. The clearest influence of irrigation was seen on common scab. Irrigation decreased the pro- portion of scabby tubers by 60% in 1993 when June was very dry. Tuber shape was also slight- ly better on irrigated crop (Table 7). Nitrogen balance Soil nitrogen mineralisation is approximately 50 kg ha–1 annually in southern Finland (Linden et al. 1992). In this study soil inorganic nitrogen in 0–30 cm layer before planting was 25 kg ha–1 N and 32 kg ha–1 N in 1994 and 1995, respec- tively. Values are on the level reported in Fin- land by Leppänen and Esala (1995). In the dry summer of 1994, soil inorganic nitrogen fluctuated independently of variety, water supply or even of supply of fertilizer ni- trogen up to middle of July (Fig. 3). The highest values were unreliable because the analysis method was no more accurate when soil inor- ganic nitrogen exceeded ca. 150 kg ha–1 N. From the latter part of July the effects of drought could Table 7. Effects of irrigation on tuber yield, size distribution and external quality in 1993–1995. Tuber yield Tuber size (mm) % of tubers Year Variety Irrigation t ha–1 < 40 40–70 > 70 Healthy Scab Shape Green defects 1993 Fambo Unirrigated 38.2a 11a 87a 2a 64 16 04 08 Irrigated 37.4a 13a 84a 3a 65 08 02 11 Tanu Unirrigated 31.4a 18a 78a 4a 48 29 11 06 Irrigated 32.2a 19a 77a 3a 63 08 06 05 1994 Fambo Unirrigated 34.0b 13a 85b 2a 81 00 00 02 Irrigated 48.9a 06b 91a 3a 82 00 00 02 Tanu Unirrigated 28.2b 61a 39b 0a 89 00 00 01 Irrigated 42.2a 35b 65a 0a 85 02 00 02 1995 Lady Rosetta Unirrigated 41.5a 09a 90a 1a 42 04 02 02 Irrigated 43.8a 08a 90a 2a 54 02 01 02 Within a single column group, means followed by the same letter are not statistically different at level P < 0.05 in Tukey’s HSD-test 69 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 11 (2002): 59–74. be seen, and the soil inorganic nitrogen stayed high on an unirrigated field up to the harvest. On irrigated plots the soil inorganic nitrogen decreased below 20 kg ha–1 N at the beginning of August. At the harvest, the residual nitrogen in soil was about 10 kg ha–1 N, independently of nitrogen fertilization. In 1995 on cultivar Lady Rosetta, the changes of soil nitrogen were much more even than on varieties Fambo and Tanu in 1994. The effects of irrigation also were mini- mal, and the soil inorganic nitrogen was only slightly over 10 kg ha–1 N on the unirrigated plots. The level of tuber nitrate nitrogen content was in general very low, only 5 and 12 mg kg–1 tuber fresh weight on Fambo in 1994 and on Fig. 3. Effect of irrigation and nitrogen fertilization on soil inorganic nitrogen in 1994–1995. 70 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Kuisma, P. Split nitrogen fertilization with irrigation on potato Lady Rosetta in 1995, respectively. However, higher values of residual soil nitrogen at harvest on unirrigated treatments resulted in a tuber ni- trate content which was about triple compared to irrigated potatoes, and clearly higher due to high rates of fertilizer nitrogen or split applica- tion (Table 8). Johnson et al. (1996) also report- ed that on suboptimal nitrogen application a full irrigation resulted to lower nitrate concentration on tubers after harvest. On the irrigated field, the low residual nitrogen in soil after harvest shows that the potato crop can utilize nitrogen effectively in good soil moisture conditions (Fig. 4) as e.g. Carlsson (1977) and Johnson et al. (1996) reported. Independently of cultivar or irrigation, the nitrogen uptake of tubers was 46–62 kg ha–1 on unfertilized plots in 1994–1995. On fertilized treatments nitrogen uptake of tuber yield was related to given fertilization. Nitrogen uptake of tubers was clearly higher than the rate of ferti- lizer nitrogen (Fig. 4). One kilogramme of nitrogen in tuber yield gave 301 kg ha–1 tubers on fertilized plots when irrigation was not used, and 347 kg ha–1 on irri- gated potatoes. These were rather independent on used nitrogen fertilization and on the similar level as Joern and Vitosh (1995a) reported in Michigan. On the unfertilized plots, efficiency of nitrogen was clearly higher; 1 kg of tuber ni- trogen produced a tuber yield, which was 412 kg ha–1 on irrigated plots and 353 kg ha–1 when irrigation was not used. The apparent nitrogen recovery (ANR = (N-yieldN-fertilized–N-yieldN-0)/fertilizer-N) (Novoa and Loomis 1981) of tuber yield in 1994 was on the average 0.68 and 0.58 on unirrigated Fambo and Tanu, respectively. When irrigation was used ANR was 0.99 and 0.79. On Lady Rosetta in 1995, ANR was independent of irrigation and slightly higher than on irrigated Fambo in 1994 (Table 9). On unirrigated potato in 1994, ANR was about on the same level as Guarda et al. Table 8. Effects of irrigation and nitrogen fertilization on nitrate content of tubers (NO 3 -N mg kg–1 tuber fresh weight) in 1994–1995. Fertilization Fambo Lady Rosetta Irrigation kg N/ha 1994 1995 Unirrigated 0 2 09 50 6 07 80 8 02 110 190 23 50 + 30 5 16 50 + 30 + 30 9 62 Irrigated 0 1 02 50 1 04 80 2 12 110 1 10 50 + 30 1 15 50 + 30 + 30 8 10 Table 9. Apparent nitrogen recovery in 1994 and 1995. 1994 1995 Irrigation N kg ha–1 Fambo Tanu Lady Rosetta Unirrigated 50 0.83 0.58 1.14 80 0.88 0.64 1.02 110 0.46 0.61 1.04 50 + 30 0.72 0.57 0.85 50 + 30 + 30 0.50 0.50 0.94 Irrigated 50 1.48 0.97 1.20 80 1.01 0.74 1.22 110 0.89 0.73 1.00 50 + 30 0.78 0.72 0.88 50 + 30 + 30 0.77 0.81 0.92 71 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 11 (2002): 59–74. Fig. 4. The balance sheet of nitrogen in 1994. 72 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Kuisma, P. Split nitrogen fertilization with irrigation on potato (1993) noticed in Italy, but slightly higher than Molfetta et al. (1993) reported. In Michigan, USA, Joern and Vitosh (1995b) received a con- siderably lower recovery of fertilizer nitrogen. In their studies the tuber yield contained only 37% and the whole crop altogether 52% of ap- plied nitrogen, independently of the nitrogen rate or application time. They also found 27% of fer- tilizer nitrogen to be in the soil 0–120 cm after harvest. Vos (1997) reported ANR-values 0.4– 0.8 on whole plant basis. Guarda et al. (1996) reported that irrigation improved remarkably the efficiency of nitrogen fertilization in early pota- to. In this study the split application of nitrogen didn’t improve the efficiency of fertilizer nitro- gen as Westermann et al. (1988) reported. Conclusions The study showed that in general the split appli- cation of nitrogen fertilization isn’t beneficial on potato in Finnish conditions when the recom- mended levels of fertilizer nitrogen are used. The most interesting result was to discover how very efficiently the potato crop utilises nitrogen sources in soil. From an environmental aspect the study proved that the use of fertilizer nitro- gen according to general recommendations with irrigation, is not only a yield maximizing and most cost-effective but also a environmentally safe way to grow potato. References Alaska. American Potato Journal 70: 571–578. Goffart, J.-P. & Guiot, J. 1996. Influence of timing and type of N-fertilizer on N-uptake and yield of potato and soil mineral nitrogen status in Belgian loam soils. Abstracts of Conference Papers, Posters and Dem- onstrations of the 13th Triennial Conference of the EAPR, Veldhoven 14–19 July 1996. p. 391–392. Guarda, G., Giovanardi, R. & Tassoni F. 1990. 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Kes- kimäärin parhaat sadot saatiin, kun kaikki lannoite- typpi annettiin istutuksen yhteydessä. Tutkimus toi myös esille, että peruna käyttää tehokkaasti hyväk- seen kasvukauden aikana maasta mobilisoituvan ty- pen, jos vesitaloudesta pidetään hyvää huolta esimer- kiksi sadetuksella. Tästä osoituksena olivat kuivan heinäkuun 1994 noston jälkeen sadettamattomassa maassa olleet yli 100 kg/ha typpimäärät. Sadetetus- sa maassa ja kosteampana kesänä 1995 maasta löy- tyi liukoista typpeä noston jälkeen noin kolmannes niistä määristä, mitä keväällä oli istutusvaiheessa. Parhaimmillaan typpilannoituksen näennäinen hyväksikäyttö sadossa oli yli yhden osoittaen, että lannoitetun perunakasvuston kyky ottaa käyttöönsä maan luontaisia typpivaroja oli itse asiassa parempi kuin ilman typpeä viljellyn perunan. Ilman typpilan- noitusta perunasadossa pellosta poistuvat typpimää- rät olivat 46–62 kg N/ha, ja lannoitetussa perunassa keskimäärin 76–176 kg N/ha. Suurten typpimäärien antaminen lannoituksessa yh- dellä kertaa on riskialtista. Runsas lannoitetyppi kas- vukauden alussa voi viivästyttää perunan kehitystä niin, että kasvustot eivät ehdi tuleentua riittävästi en- nen sadonkorjuuta. Osa keväällä istutuksessa anne- tusta lannoitetypestä saattaa myös huuhtoutua poh- javesiin runsaan sateen tai sadetuksen seurauksena, sillä perunaa viljellään yleensä keveillä, helposti lä- päisevillä maalajeilla. Keskieurooppalaisissa ja myös ruotsalaisissa tutkimuksissa suurten lannoitetyppi- määrien jakamisesta useampaan lannoituskertaan on saatu hyviä tuloksia. Jaettua typpilannoitusta käyte- täänkin jossain määrin käytännön viljelyssä. Perunantutkimuslaitoksella tutkittiin vuosina 1993–1995 kenttäkokeessa sadetukseen liitettynä pe- runan typpilannoitustarvetta ja suurimpien lannoite- määrien jakomahdollisuuksia suomalaisessa perunan- tuotannossa. Muuten viljelytekniikassa pyrittiin hyö- dyntämään mahdollisimman hyvin kaikki perunan kasvua aikaistavat toimet. Tutkimus osoitti, että kasvuoloissamme typpilan- Title Introduction Material and methods Results and discussion Conclusions References SELOSTUS