Potassium, magnesium and nitrogen nutrition of Estima

Transcription

1 Project Report Potassium, magnesium and nitrogen nutrition of Estima Ref: 807/182 First Year Report, April 1998 M F Allison, J H Fowler & E J Allen Cambridge University Farm 1999 Project Report 1999/7

2 Any reproduction of information from this report requires the prior permission of the British Potato Council. Where permission is granted, acknowledgement that the work arose from a British Potato Council supported research commission should be clearly visible. While this report has been prepared with the best available information, neither the authors nor the British Potato Council can accept any responsibility for inaccuracy or liability for loss, damage or injury from the application of any concept or procedure discussed. Additional copies of this report and a list of other publications can be obtained from: Publications Tel: British Potato Council Fax: Nash Court publications@potato.org.uk John Smith Drive Oxford Business Park South Oxford OX4 2RT Some of our reports, and a list of publications, are also available on the internet at

3 CONTENTS Page Summary 7 Introduction 9 Objectives 9 Methods 9 Results and Discussion 9 Effect of fertilizer treatments on tuber yield, DM concentration and size distribution 10 Ground covers and radiation interception/conversation 10 Effect of fertilizers on the efficiency of conversion of intercepted radiation to DM 11 The relationship between leaf area index and N uptake 13 Effect of fertilizer treatments on Mg concentration and uptake 23 Effect of K fertilizers on soil exchangeable K, K uptake, tuber K and 24 DM concentration Effect of nitrogen fertilizers on soil ph 26 Soil and crop calcium dynamics 28 Conclusions 31 References 32 Appendix 1. Field plan for effect of N, K and Mg on Estima, Cambridge University Farm 1997 Appendix 2. Field diary for Estima crop grown in Huntingdon Rd Pasture, Cambridge Universty Farm

4 LIST OF FIGURES Figure Title Page 1 Effect of N and K fertilizer on ground cover development by Estima 2 Relationship between total radiation intercepted and tuber DM yield at final harvest (158 DAP) for all fertilizer treatments 3 The relationship between N uptake by leaves and total N uptake and leaf area index for an Estima crop LIST OF TABLES Table Title Page 1a 1b 1c 1d 2a 2b 2c Effects of N application rate on number of stems and tubers; tuber fresh weight (FW); tuber, stem, leaf and total dry weight (DW) (63 days after planting) Effects of N application rate on the concentration of nitrogen and phosphorus in leaf, steam and tubers and leaf, stem, tuber and total uptakes of nitrogen and phosphorus.(63 days after planting) Effect of N application rate on the concentration of potassium and magnesium in leaf, stem and tubers and leaf, stem, tuber and total uptakes of potassium and magnesium.(63 days after planting) Effects of N application rate on the concentration of calcium and sodium in leaf, stem and tubers and leaf, stem, tuber and total uptakes of calcium and sodium.(63 days after planting) Effects of N application rate on number of stems and tubers; tuber fresh weight (FW); tuber, stem, leaf and total dry weight (DW) (76 days after planting) Effect of N application rate on the concentration of nitrogen and phosphorus in leaf, stem and tubers and leaf, stem, tuber and total uptakes of nitrogen and phosphorus.(76 days after planting) Effects of N application rate on the concentration of potassium and magnesium in leaf, stem and tubers and leaf, stem, tuber and total uptakes of potassium and magnesium.(76 days after planting)

5 2d 3a 3b 3c 3d 3e 4a 4b Effects of N application rate on the concentration of potassium and magnesium in leaf, stem and tubers and leaf, stem, tuber and total uptakes of potassium and magnesium.(76 days after planting) Effects of N application rate on number of stems and tubers; tuber fresh weight yield (FW) and dry weight yield (DW) and Travis parameters.(94 days after planting) Effects of N application rate on leaf, stem and total dry matter yields; harvest index (tuber : total DM); dry matter content of tubers; leaf area index and the quantity of N contained within one unit of LAI.(94 days after planting) Effects of N application rate on the concentration of nitrogen and phosphorus in leaf, stem and tubers and leaf, stem, tuber and total uptakes of nitrogen and phosphorus.(94 days after planting) Effects of N application rate on the concentration of potassium and magnesium in leaf, stem and tubers and leaf, stem, tuber and total uptakes of potassium and magnesium.(94 days after planting) Effects of N application rate on the concentration of calcium and sodium in leaf, stem and tubers and leaf, stem, tuber and total uptakes of calcium and sodium.(94 days after planting) Effects of N application rate on number of tubers; tuber fresh weight yield (FW) and dry weight yield (DW); tuber dry matter contents and Travis parametres.(158 days after planting) Effects of N application rate on the concentration and uptake of nitrogen, phosphorus, potassium, magnesium, calcium and sodium in tubers.(158 days after planting) Effects of N application rate on total dry matter (DM) yield, estimated total radiation intercepted and on the estimated efficiency of DM production, measured of four occasions. Intercepted radiation was estimated from ground covers 6 Factors known to effect the association between N uptake and leaf area index 7 Effect of N, K and Mg fertilizer on the Mg concentration of Estima leaf, stem and tuber. A + shows an increase in concentration, - a decrease in concentration and n.s. shows that there was no significant effect Effect of N and K fertilizer on the Mg content (%) of Estima leaves 24 5

6 9 Effect of applying K fertilizer on top-soil exchangeable K(kg K/ha). Results have been averaged over other treatments. The efficiency is the difference in exchangeable K divided by K applied 10 Effect of applying K fertilizer on total K uptake (kg K/ha). Results have been averaged over other treatments. The efficiency is the difference in K uptake divided by K applied 11 Effect of applying K fertilizer on tuber K and DM concentration. Results have been averaged over other treatments 12 Effect of nitrogen fertilizers on soil ph measured on six occasions. The standards errors are based on 22 degrees of freedom. F is the probability of differences occurring by chance 13 Effects of N fertilizer on availble N, P, K, Mg and Ca measured on six occasions during the season. The standard errors are based upon 22 degrees of freedom. F is the probability of the differences occurring by chance 14 Change in tuber concentration (%) and the increase in tuber uptake (kg/ha) of N, K, Mg and Ca for crops given 300 N/ha

7 The Magnesium, Potassium and Nitrogen Nutrition of Estima Summary In 1997, an experiment at Cambridge University Farm tested the effects of applying N (0-300 kg/ha), K (0-400 kg K 2 O/ha) and Mg (0-150 kg MgO/ha) on an Estima crop. It is suspected that Estima has a larger Mg requirement than other cultivars and that N and K may influence crop Mg uptake. There were no treatment effects on emergence no number of stems and tubers. Only N fertilizer had any effect on tuber yield, size distribution and tuber dy matter (DM) concentration. There was some evidence that when no N but large amounts of K were applied ground covers and the Mg concentration of the leaves were reduced. The relationship between ground cover and N uptake was also studied in this experiment. Up to 94 days after planting each unit of LAI was associated with 17 kg leaf-n/ha or 45 kg total- N/ha. These values are similar to other published values and indicate that this association may be stable and of practical use. The experiment was also used to study soil/crop K and Ca dynamics. Applying K fertilizer had only a small effect on soil exchangeable-k, crop K uptake and had no effect on tuber DM concentration. Calcium uptake did not appear to be limited by soil supply but, once in the crop, Ca was not efficiently remobilized from haulm to tubers. Work will continue at CUF to refine Mg recommendations for Estima and other sensitive cultivars and to investigate the relationship between LAI and N. In addition, further work will be done to investigate Ca and K dynamics in soils and crops. 7

8 8

9 The Magnesium, Potassium and Nitrogen Nutrition of Estima Introduction There is evidence to suggest that Estima (and some other Dutch, determinate varieties such as Wilja and Marfona) are prone to magnesium deficiency even on soils known to contain moderate amounts of plant-available Mg. Magnesium deficiency appears to result in premature senescence of the foliage and loss of yield potential. In addition, Mg deficient crops may be more likely to produce tubers prone to bruising or disease. It is also known that potassium and nitrogen interact with soil and crop processes to affect Mg nutrition. Objectives To compare the performance of Estima on a soil with moderate reserves of available Mg when given different amounts of Mg, K and N fertilizer and to use the data produced by this experiment to investigate other areas of potato crop nutrition. Methods The experiment was done at Cambridge University Farm using an experimental design of three randomized blocks. Each block contained all combinations of three Mg application rates (0, 75 and 150 kg MgO/ha, as magnesium sulphate), three rates of N (0, 150 and 300 kg N/ha, as ammonium nitrate) and two rates of K (0, 400 kg K 2 O/ha, as potassium sulphate). The fertilizers were broadcast, by hand, on 2 April. Seed (Average weight = 28.8 g, Count (50 kg) = 1738) was planted at 25 cm spacing in 76 cm ridges on 4 April. The effect of treatments on emergence and percentage ground cover was measured. Four harvests were taken; 6 June (63 days after planting, DAP); 19 June (76 DAP); 7 July (94 DAP) and 8 September (158 DAP). The crop received a total of 149 mm irrigation. Weed, blight and aphid control were satisfactory throughout. Results and Discussion Initial soil analysis, done before any fertilizer was applied, showed that the nutritional status of the top soil (0-30 cm) was: ph 6.7; Mg 60 mg/l (ADAS Index 2); K 168 mg/l (Index 2); P 88 mg/l (Index 5) and available-n 39 kg N/ha. The date of 50% emergence was 10 May

10 (36 DAP). There was no effect of fertilizer treatment on the rate of emergence or total emergence. Effect of fertilizer treatments on tuber yield, DM concentration and size distribution Total fresh weight and dry matter yields were only increased by N fertilizer (Tables 1, 2, 3 & 4). Nitrogen fertilizer increased leaf, stem and tuber yields in all three early harvests and increased tuber yields at the final harvest. The harvest index (ratio of tuber yield : total yield) was decreased by N fertilizer. Due to uncertainty in the shape of the response curve the optimum N application rate cannot be accurately defined (Allison et al. 1998). However, the optimum N application rate was between 150 and 300 kg N/ha and was likely to be between 200 and 300 kg N/ha. This optimum is larger than would normally be expected for CUF and may be due to some loss of N by leaching during the very wet June. The increase in yields was in accordance with the effect of N on ground cover. At the smallest N rate, K fertilizer significantly reduced ground cover and reduced total intercepted radiation from 926 to 862 MJ/m 2 but this did not correspond to a significant decrease in yield. Assuming a conversion efficiency of 0.96 g tuber DM/MJ the reduction in radiation interception corresponds to a yield reduction of c. 60 g/m 2. The least significant difference for tuber DM yield is c. 100 g DM/m 2 and this may explain why the decrease in yield was not detected. For the second and third harvests, N fertilizer significantly reduced tuber DM concentrations although there was no significant effect of N at final harvest (Table 4). At the third harvest (94 DAP), total tuber DM yields were only increased by the first increment (150 kg N/ha) of fertilizer. At the final harvest (158 DAP) there were significant differences in yield between all N treatments. Since, none of the treatments had any effect on number of tubers the yield increases were solely due to increases in tuber size. At the third harvest, 150 kg N/ha increased µ (the tuber size grade with the largest yield) from 44 to 48 mm. At the final harvest µ was increased from 51 to 60 mm by applying 300 kg N/ha. (Tables 3 & 4). Ground covers and radiation interception/conversion Ground covers were measured from 15 May (41 DAP) until the end of the experiment (Figure 1). There were no treatment effects until 3 June 1997 (60 DAP) when the N0 treatment had produced less ground cover. The N150 and N300 treatments reached 95% ground cover on 18 June (75 DAP), whilst the N0 treatments had 77% cover. There were no differences between the N150 and N300 treatments. Ground covers started to senesce towards the end of July (c. 10

11 110 DAP) with the N0 declining faster than the other N treatments. Generally there were no treatment effects due to K or Mg. However, between 75 and 117 DAP there were often significant N and K interactions - at the smallest N rate, K significantly reduce ground cover, but at the middle and largest N rates, K had no effect on ground cover. The mechanism by which K fertilizer reduced ground cover is not known although induced Mg deficiency may be implicated. Figure 1. Effect of N and K fertilizer on ground cover development by Estima Ground cover (%) N0K0 N150K0 N300K0 N0K400 N150K400 N300K Days after planting Effect of fertilizers on the efficiency of conversion of intercepted radiation to DM If it is assumed that there is a linear relationship between ground cover and radiation interception then values of ground cover may be used in estimating the efficiency of conversion of radiation to DM. The relationship between total tuber DM yield at final harvest and intercepted radiation for all fertilizer treatments is shown in Figure 2. The regression shows that total DM production was related to intercepted radiation and has an efficiency of c. 1.1g DM/MJ. There is, however, considerable scatter of treatment means about the regression 11

12 line. Analysis of variance of yield, total intercepted radiation and efficiency of conversion for all four harvests are shown in Table 5 and these data suggest that conversion efficiency was increased by the N fertilizer treatments. However, other workers (Millard & Marshall; Harris 1992) showed that, under field conditions, the conversion efficiency was independent of N for typical application rates. This discrepancy is most likely due to overestimates of radiation interception on the N0 treatments and this is an artefact of the methodology: 100% ground cover measured using a grid does not necessarily correspond to 100% radiation interception. This problem may also be compounded by the canopy architecture of Estima which is relatively sparse allowing considerable wastage of incident radiation even though ground covers are a nominal 100%. Using the relationship published by Firman & Allen 1989, it is Figure 2. Relationship between total radiation intercepted and tuber DM yield at final harvest (158 DAP) for all fertilizer treatments. Tuebr DM yield (g/m^2) y = 1.12x R 2 = Total intercepted radiation (MJ/m^2) possible to estimate the amount of radiation intercepted from the leaf area indices shown in Tables 1, 2 & 3. Doing this reduces the amount of radiation intercepted and increases the conversion efficiency of the N0 crop and in turn removes some of the effect of N fertilizer. Another potential cause of the discrepancy may be due to the effect of N on Mg uptake. Magnesium is the central co-ordinating atom within chlorophyll and is also involved in electron transport. This experiment has shown that N fertilizer increases the Mg concentration of leaves (Tables 1, 2, 3 & 7). Therefore, it is possible that in N deficient 12

13 crops, Mg may be sufficiently limiting to decrease the leaves photosynthetic capacity and conversion efficiency. This area will be studied further at Cambridge in 1998 using infra red gas analysis to study the effect of nutrients on net photosynthetic activity. The relationship between leaf area index and N uptake The addition of 150 kg N/ha significantly increased the leaf area index (LAI) compared with the control, however the LAI was not significantly increased by further additions of N (Tables 1, 2 & 3). In a recent BPC review (Allison et al. 1998) the idea of using LAI to monitor crop nitrogen status was discussed. The concept of canopy management is not new and protocols have been developed for cereals and oil seed rape. To use LAI as a means to help improve N fertilizer recommendations a series of linked relationships need to be described. First, the most efficient LAI needs to be defined for any particular cultivar through the season. Second, the relationship between LAI and N uptake needs to be described. It is still debatable whether N uptake controls production of leaves or whether N uptake is a consequence of transpiration and photoassimilate production. However, the current experiment shows that reasonably robust relationships exist between both leaf and whole crop N uptake and LAI for the first 94 days after planting. (Figure 3). The data are derived from three sampling dates and show that each unit of LAI need c. 17 kg N/ha of leaf uptake which corresponds to c. 45 kg/ha of total uptake. For a Desirée crop, Allen and Scott (1992) found that each unit of LAI needed c. 30 kg N/ha of total uptake. Whilst Allison et al (1998) using the Estima data together with data from crops of Hermes and FL1833 showed that each unit of LAI needed a total uptake of 49 kg N/ha. Fowler (1994 & 1995) showed that whilst LAI and N uptake were related, many factors could affect the nature of the relationship and these are summarised in Table 6. The final stage would be to relate development of LAI with available nitrogen at planting and the response to fertilizer N added as a top dressing. Using such a system it may be possible to fine tune existing N recommendation systems to make them more cultivar and season specific. Work will continue at CUF in 1998 to investigate the usefulness of a canopy management approach to N nutrition of potatoes. 13

14 Table 1a. Effects of N application rate on number of stems and tubers; tuber fresh weight (FW); tuber, stem, leaf and total dry weight (DW); harvest index (tuber DW : total DW), dry matter content of the tubers, leaf area index (LAI) and the quantity of N contained within one unit of leaf area (LAIN). The crop was sampled on 6 June (63 days after planting). Unless stated otherwise the standard errors are based upon 34 degrees of freedom and F is the probability of the differences occurring by chance. N applied Stems Tubers Tuber FW Tuber DW Leaf DW Stem DW Total DW Harvest index Tuber DM LAI LAIN kg N/ha 000 s/ha 000 s/ha t/ha t/ha t/ha t/ha t/ha % m 2 /m 2 kg N/ha Mean s.e F Table 1b. Effects of N application rate on the concentration of nitrogen and phosphorus in leaf, stem and tubers and leaf, stem, tuber and total uptakes of nitrogen and phosphorus. The crop was sampled on 6 June (63 days after planting). N applied Nitrogen Phosphorus Leaf Stem Tuber Leaf Stem Tuber Total Leaf Stem Tuber Leaf Stem Tuber Total kg N/ha % % % kg N/ha kg N/ha kg N/ha kg N/ha % % % kg P/ha kg P/ha kg P/ha kg P/ha Mean s.e F

15 Table 1c. Effect of N application rate on the concentration of potassium and magnesium in leaf, stem and tubers and leaf, stem, tuber and total uptakes of potassium and magnesium. The crop was sampled on 6 June (63 days after planting). N applied Potassium Magnesium Leaf Stem Tuber Leaf Stem Tuber Total Leaf Stem Tuber Leaf Stem Tuber Total kg N/ha % % % kg K/ha kg K/ha kg K/ha kg K/ha % % % kg Mg/ha kg Mg/ha kg Mg/ha kg Mg/ha Mean s.e F Table 1d. Effects of N application rate on the concentration of calcium and sodium in leaf, stem and tubers and leaf, stem, tuber and total uptakes of calcium and sodium. The crop were sampled on 6 June (63 days after planting). N applied Calcium Sodium Leaf Stem Tuber Leaf Stem Tuber Total Leaf Stem Tuber Leaf Stem Tuber Total kg N/ha % % % kg Ca/ha kg Ca/ha kg Ca/ha kg Ca/ha mg/kg mg/kg mg/kg g Na/ha g Na/ha g Na/ha g Na/ha Mean s.e F

16 Table 2a Effects of N application rate on number of stems and tubers; tuber fresh weight (FW); tuber, stem, leaf and total dry weight (DW); harvest index (tuber DW : total DW), dry matter content of the tubers, leaf area index (LAI) and the quantity of N contained within one unit of leaf area (LAIN). The crop was sampled on 19 June (76 days after planting). N applied Stems Tubers Tuber FW Tuber DW Leaf DW Stem DW Total DW Harvest index Tuber DM LAI LAIN kg N/ha 000 s/ha 000 s/ha t/ha t/ha t/ha t/ha t/ha % m 2 /m 2 kg N/ha Mean s.e F Table 2b. Effect of N application rate on the concentration of nitrogen and phosphorus in leaf, stem and tubers and leaf, stem, tuber and total uptakes of nitrogen and phosphorus. The crops were sampled on 19 June (76 days after planting). N applied Nitrogen Phosphorus Leaf Stem Tuber Leaf Stem Tuber Total Leaf Stem Tuber Leaf Stem Tuber Total kg N/ha % % % kg N/ha kg N/ha kg N/ha kg N/ha % % % kg P/ha kg P/ha kg P/ha kg P/ha Mean s.e F

17 Table 2c. Effects of N application rate on the concentration of potassium and magnesium in leaf, stem and tubers and leaf, stem, tuber and total uptakes of potassium and magnesium. The crop was sampled on 19 June (76 days after planting). N applied Potassium Magnesium Leaf Stem Tuber Leaf Stem Tuber Total Leaf Stem Tuber Leaf Stem Tuber Total kg N/ha % % % kg K/ha kg K/ha kg K/ha kg K/ha % % % kg Mg/ha kg Mg/ha kg Mg/ha kg Mg/ha Mean s.e F Table 2d. Effects of N application rate on the concentration of calcium and sodium in leaf, stem and tubers and leaf, stem, tuber and total uptakes of calcium and sodium. The crop was sampled on 19 June (76 days after planting). N applied Calcium Sodium Leaf Stem Tuber Leaf Stem Tuber Total Leaf Stem Tuber Leaf Stem Tuber Total kg N/ha % % % kg Ca/ha kg Ca/ha kg Ca/ha kg Ca/ha mg/kg mg/kg mg/kg g Na/ha g Na/ha g Na/ha g Na/ha Mean s.e F

18 Table 3a. Effects of N application rate on number of stems and tubers; tuber fresh weight yield (FW) and dry weight yield (DW) and Travis parameters. The crop was sampled on 7 July (94 days after planting). N applied Stems Tubers Tubers Tuber FW Tuber FW Tuber DW Tuber DW Travis parameters >10 mm >40 mm >10 mm >40 mm >10 mm >40 mm mu sigma cov kg N/ha 000 s/ha 000 s/ha 000 s/ha t/ha t/ha t/ha t/ha mm mm mm Mean s.e F Table 3b Effects of N application rate on leaf, stem and total dry matter yields; harvest index (tuber : total DM); dry matter content of tubers; leaf area index and the quantity of N contained within one unit of LAI. The crop was sampled on 7 July (94 days after planting). N applied Leaf DW Stem DW Total DW Harvest index Tuber DM LAI LAIN kg N/ha t/ha t/ha t/ha % m 2 /m 2 kg N Mean s.e F

19 Table 3c. Effects of N application rate on the concentration of nitrogen and phosphorus in leaf, stem and tubers and leaf, stem, tuber and total uptakes of nitrogen and phosphorus. The crop was sampled on 7 July (94 days after planting). N applied Nitrogen Phosphorus Leaf Stem Tuber Leaf Stem Tuber Total Leaf Stem Tuber Leaf Stem Tuber Total kg N/ha % % % kg N/ha kg N/ha kg N/ha kg N/ha % % % kg P/ha kg P/ha kg P/ha kg P/ha Mean s.e F Table 3d. Effects of N application rate on the concentration of potassium and magnesium in leaf, stem and tubers and leaf, stem, tuber and total uptakes of potassium and magnesium. The crop was sampled on 7 July (94 days after planting). N applied Potassium Magnesium Leaf Stem Tuber Leaf Stem Tuber Total Leaf Stem Tuber Leaf Stem Tuber Total kg N/ha % % % kg K/ha kg K/ha kg K/ha kg K/ha % % mg/kg kg Mg/ha kg Mg/ha kg Mg/ha kg Mg/ha Mean s.e F

20 Table 3e. Effects of N application rate on the concentration of calcium and sodium in leaf, stem and tubers and leaf, stem, tuber and total uptakes of calcium and sodium. The crop was sampled on 7 July (94 days after planting). N applied Calcium Sodium Leaf Stem Tuber Leaf Stem Tuber Total Leaf Stem Tuber Leaf Stem Tuber Total kg N/ha % % mg/kg kg Ca/ha kg Ca/ha kg Ca/ha kg Ca/ha mg/kg mg/kg mg/kg g Na/ha g Na/ha g Na/ha g Na/ha Mean s.e F

21 Table 4a. Effects of N application rate on number of tubers; tuber fresh weight yield (FW) and dry weight yield (DW); tuber dry matter content and Travis parameters. The crop was sampled on 8 September (158 days after planting). N applied Tubers Tubers Tuber FW Tuber FW Tuber DW Tuber DW Tuber DM Travis parameters >10 mm >40 mm >10 mm >40 mm >10 mm >40 mm mu sigma cov kg N/ha 000 s/ha 000 s/ha t/ha t/ha t/ha t/ha % mm mm mm Mean s.e F Table 4b. Effects of N application rate on the concentration and uptake of nitrogen, phosphorus, potassium, magnesium, calcium and sodium in tubers. The crop was sampled on 8 September (158 days after planting). N applied Nitrogen Phosphorus Potassium Magnesium Calcium Sodium kg N/ha % kg N/ha % kg P/ha % kg K/ha % kg Mg/ha mg/kg g Ca/ha mg/kg g Na/ha Mean s.e F

22 Table 5 Effects of N application rate on total dry matter (DM) yield, estimated total radiation intercepted and on the estimated efficiency of DM production, measured on four occasions. Intercepted radiation was estimated from ground covers see text for a discussion of the limitations of this approach. 1st harvest 6/6/97 (63 DAP) 2nd harvest 19/6/97 (76 DAP) 3rd harvest 7/7/97 94 DAP Final harvest 8/9/97 (158 DAP) N applied Total Total Int. Efficiency Total Total Int. Efficiency Total Total Int. Efficiency Total Total Int. Efficiency yield radiation yield radiation yield radiation yield radiation kg N/ha g/m 2 MJ/m 2 g/mj g/m 2 MJ/m 2 g/mj g/m 2 MJ/m 2 g/mj g/m 2 MJ/m 2 g/mj Mean s.e F

23 Figure 3. The relationship between N uptake by leaves and total N uptake and leaf area index for an Estima crop. 250 Leaf N uptake Total N uptake N uptake (kg N/ha) y = 17.7x R 2 = 0.82 y = 45.3x 2 R = Leaf area index Table 6. Factors known to effect the association between N uptake and leaf area index. Factor Effect on N uptake/lai Increasing amount of N fertilizer Increases N required for each unit of LAI Increasing crop age Decreases N required for each unit of LAI Increasing stem population Increases N required for each unit of LAI Increasing determinacy Increases N required for each unit of LAI Effect of fertilizer treatments on Mg concentration and uptake The effects of the N, K and Mg fertilizer treatments on the Mg concentration of leaves, stems and tubers are summarised in Table 7. Nitrogen and Mg fertilizers tended to increase tissue Mg concentrations whereas potassium fertilizer tended to decrease it. No fertilizer treatment, at any harvest had any effect on tuber Mg concentration. There were also interactions between N and K fertilizer in leaf dry matter concentration (Table 8). These data show that at the final harvest when no nitrogen was applied, addition of K had no effect on leaf Mg, but in the second and third harvests the addition of 400 kg K 2 O/ha significantly reduced the concentration of Mg in leaves. 23

24 Table 7. Effect of N, K and Mg fertilizer on the Mg concentration of Estima leaf, stem and tuber. A + shows an increase in concentration, - a decrease in concentration and n.s. shows that there was no significant effect. Nitrogen Potassium Magnesium Harvest Leaf Stem Tuber Leaf Stem Tuber Leaf Stem Tuber 1 (63 DAP) n.s. + n.s. n.s. n.s. n.s. + + n.s. 2 (76 DAP) + + n.s. - - n.s. + n.s. n.s. 3 (94 DAP) + + n.s. - - n.s. + + n.s. 4 (158 DAP) * * n.s. * * n.s. * * n.s. Other workers have shown that N fertilizer can cause increases in tissue Mg concentration (James et al. 1994; Mulder 1956), although Mulder demonstrated that ammonium-n inhibited Mg uptake whilst nitrate-n increased it. The inhibitory effect of K fertilizer on Mg uptake of potatoes has also been demonstrated by several workers (Giroux 1986; James et al. 1994). Compared to other nutrients, the binding energy between the highly hydrated Mg ion and exchange sites on cell walls is small and other cations can compete with it (Marschner 1995). Table 8. Effect of N and K fertilizer on the Mg content (%) of Estima leaves. N K Harvest 1 Harvest 2 Harvest 3 (kg N/ha) (kg K/ha) 63 DAP 76 DAP 94 DAP F (N x K) s.e Hossner and Doll (1970) showed that the yield response to Mg fertilizer was related to both soil Mg and Mg/K ratio. Above 0.3 meq Mg/100g soil (c. 50 mg Mg/l, Index 1+) yield was not affected by either the amount of soil Mg or Mg/K ratio. However, below 0.3 meq/100g, soil yields decreased sharply if the soil Mg ratio fell below 0.8 equivalent basis (c on a mg basis). Effect of K fertilizers on soil exchangeable K, K uptake, tuber K and DM concentration It is the view of some UK processors that adding large amounts of K fertilizer will help to minimise internal damage to tubers. The logic of this process is that adding K fertilizer will increase soil exchangeable K, and K uptake by the crop. The increased K uptake will then 24

25 result in reduced tuber dry matter concentrations and thereby reduce risk of bruising. This experiment applied K fertilizers in large excess of current MAFF recommendations and can be used to test some of these assumptions. It should, however, be remembered that K sulphate was used in this experiment whereas K chloride should be more effective at reducing tuber dry matter concentrations (Cowie 1943). The effect of adding K fertilizer on top-soil exchangeable K is shown in Table 9. The fertilizer was added between the first and second samplings. The increases in soil exchangeable K never amounted to more than 22% of the amount applied and due to the large standard errors, the increases were never statistically significant. Even though the crop removed c. 250 kg K/ha from the soil, the exchangeable K decreased by a maximum of only 150 kg K/ha. These results are not surprising since exchangeable K exists in dynamic equilibrium with other pools of K (i.e. fixed K). Fertilizer K added to the soil will dissolve and enter the soil solution, however due to the equilibria, most of the soil solution-k will become exchangeable K on soil colloids and then become fixed into non-exchangeable forms. Table 9. Effect of applying K fertilizer on top-soil exchangeable K (kg K/ha). Results have been averaged over other treatments. The efficiency is the difference in exchangeable K divided by K applied. kg K 2 O/ha Efficiency Harvest s.e. F (%) -10 DAP DAP DAP DAP DAP DAP Likewise, as K is removed from the soil solution by crop uptake or by leaching this will be replaced by exchangeable K and this is replaced in turn by fixed K. Thus, adding large amounts of K fertilizer has a relatively small effect on soil K supply. The small effect on K supply corresponds to the small effect of K fertilizer on total K uptake by the crop (Table 10). The addition of K fertilizer increased K uptake on two occasions but the efficiency of uptake was always less than 10%. 25

26 Table 10. Effect of applying K fertilizer on total K uptake (kg K/ha). Results have been averaged over other treatments. The efficiency is the difference in K uptake divided by K applied. kg K 2 O/ha Efficiency Harvest s.e.. F (%) 1 (63 DAP) (76 DAP) (94 DAP) (158 DAP) The effect of K fertilizer on tuber K and DM concentration are shown in Table 11. Adding K fertilizer significantly increased tuber K concentrations at each harvest. However, these increases in K concentration did not correspond to decreases in DM concentration. The effect of K nutrition on crop K uptake and tuber dry matter concentration will be studied further in 1998 so that more robust recommendations can be given to growers and processors. Table 11. Effect of applying K fertilizer on tuber K and DM concentration. Results have been averaged over other treatments. Tuber K concentration (%) Tuber DM concentration (%) kg K 2 O/ha kg K 2 O/ha Harvest s.e... F s.e.. F 1 (63 DAP) (76 DAP) (94 DAP) (158 DAP) Effect of nitrogen fertilizers on soil ph Due to nitrification, fertilizers containing ammonium have the potential to acidify soils: NH 4 NO 3 + 2O 2 2 NO H 2 O + 2H + On neutral or moderately alkaline soil this decrease in soil ph is often claimed to reduce the incidence of common scab. However, on soils with moderate calcium contents and cation exchange capacities the decrease in soil ph is likely to be quite small: the acidifying effect of 300 kg N as ammonium nitrate will neutralise c. 150 kg of calcium carbonate. However, acidification will only occur if the nitrate produced by the nitrification is lost from the soil by leaching (causing co-leaching of calcium as a balancing ion). If all the fertilizer derived nitrate is taken up by the crop then there will be no acidification. In this experiment the fertilizer treatments were applied on 2 April, and soil ph was measured at regular intervals through the season until harvest (Table 12) and N fertilizer had no significant effect on soil ph. The absence of any effect, even at the largest N application rates, is probably due to a 26

27 combination of factors including: the field having a large amount of exchangeable Ca little N was leached during the course of the season. Experiments will continue at CUF in 1998 to investigate the effect of ammonium sulphate (which has c. twice the acidifying power of ammonium nitrate) on soil ph and skin blemishing diseases. Table 12. Effect of nitrogen fertilizers on soil ph measured on six occasions. The standard errors are based on 22 degrees of freedom. F is the probability of differences occurring by chance. 0 kg N/ha 150 kg N/ha 300 kg N/ha e.s.e F 25/03/ /06/ /06/ /07/ /08/ /09/

28 Soil and crop calcium dynamics Internal rust spot (IRS) is a common physiological disorder. It is characterised by rust coloured lesions in the medullary tissue on the inside of the vascular ring. It appears to be induced by several environmental factors including restriction of water supply, rapid tuber growth and high temperatures. Calcium supply and metabolism are also known to be important factors. Collier et al. (1978) showed that the incidence of IRS in pot-grown Maris Piper was related to the calcium concentration of the nutrient solution. At solution concentrations above 3 mm Ca there was no internal rust spot and this corresponded to a tuber concentration of c g Ca/100g DM. This work was then repeated with ten potato cultivars (Collier et al. 1980) which showed that whilst incidence of IRS was inversely related to tuber calcium concentration, the actual incidence was cultivar specific. Whilst Estima is not particularly prone to IRS problems, this experiment does provide useful data to study the dynamics of Ca movement between soil and crop. The field at Cambridge University Farm contains a moderate amount of exchangeable Ca (Table 13), and whilst it was not measured is likely to contain typical amounts of Ca in the soil solution. Values for arable-soil solution Ca are typically in the range of 0.4 to 2.1 mm Ca (Adams et al. 1980; Campbell et al. 1989). These are smaller than the 3 mm found by Collier et al (1978), however these workers grew the potatoes in vermiculite filled pots and supplied the Ca in nutrient solution. The vermiculite is unlikely to contain any significant quantities of exchangeable Ca and in consequence the soil solution Ca concentration will not be buffered with respect to crop uptake. 28

29 Table 13. Effects of N fertilizer on available N, P, K, Mg and Ca measured on six occasions during the season. The standard errors are based upon 22 degrees of freedom. F is the probability of the differences occurring by chance. N applied Available nutrients (mg/l) Available nutrients (kg/ha) (kg N/ha) P K Mg Ca N P K Mg Ca 25/03/ s.e F /06/ s.e F /06/ s.e F /07/ s.e F /08/ s.e F /09/ s.e F

30 It is therefore likely that between additions of nutrient solution the concentration of Ca fell below 3 mm and it is this that caused an increase in IRS. Studies of the nutrient inflows of field-grown winter wheat (Barraclough 1986), showed that, in a moist soil, its Ca requirement could be met solely by diffusion if the difference in Ca concentration between the bulk soil solution and root surface was 0.01 mm. However, making the soil drier increased the minimum concentration of soil solution Ca to 0.15 mm. Other studies (Brewster & Tinker 1972) showed that in most circumstances mass flow of Ca in the transpiration stream would supply the crops Ca requirements and diffusion was probably a minor pathway. Simple calculations of water use would support this view: a 70 t potato crop would use c. 300 mm water, assuming average Ca concentration of 1 mm, this would result in an uptake of 40 kg Ca. Whilst this calculation ignores the fact that Ca is taken up by root tips, Evans (1982) found a good relationship between Ca and water uptake. In conclusion, it is likely that the soil supply of Ca is not limiting and is unlikely to be the factor controlling IRS incidence. Once in the crop, most Ca is allocated to the leaves and stems and little to the tubers (Tables 1, 2 & 3). As the tubers grow, nutrients, derived from fresh uptake or remobilization from leaves and stems are allocated to the tubers. In the current experiment, the concentration of most nutrients in tubers decreased with time (Table 14). The decrease in this concentration is most noticeable with Ca: the tuber concentration halved between 63 and 157 days after planting. However, the concentration of the other divalent cation Mg remained more or less constant. In this experiment the final concentration of tuber Ca was similar to the threshold for Maris Piper suggested by Collier et al (1978). In this experiment the application of N fertilizer increased the Ca concentration of tubers at all four samplings. It is not known whether the N facilitates uptake of Ca (i.e. by producing larger fibrous roots systems) or aids its redistribution from haulm to tubers, although the data shown in Table 14 show that N and Ca distribution are not closely linked. There are few data which indicate whether certain physiological stages of potato crop development are more sensitive to Ca stress than others. Detailed nutritional/physiological experiments would be needed to understand this. Other nutritional factors may also be important in the development of IRS. In 1937, Dennis and O Brien reported that applications of Borax could prevent IRS. This finding is not surprising since B and Ca have similar roles in maintaining cellular integrity. 30

31 Table 14. Change in tuber concentration (%) and the increase in tuber uptake (kg/ha) of N, K, Mg and Ca for crops given 300 kg N/ha. Days after planting N K Mg Ca (%) (kg/ha) (%) (kg/ha) (%) (kg/ha) (%) (kg/ha) Conclusions Whilst it is important to remember that the above results are for only one crop year the following preliminary conclusions may be made: Only N fertilizer had an effect on yield, tuber size distribution and tuber dry matter concentration. The uptake of Mg was reduced by K fertilizer, but this effect only occurred when no N was applied and did not result in yield loss. Adding N fertilizer increased the tissue concentrations of Mg and K. Adding large amounts of K fertilizer had little effect on soil exchangeable K and total K uptake. Whilst K fertilizer did increase tuber K concentrations there was no corresponding decrease in tuber DM concentration. The soil supply of Ca appears to be adequate for crop needs. However, translocation of Ca within the plant is limited. Calcium mobilisation in the plant needs to be studied in detail. There appears to be a relatively robust relationship between leaf area index and N uptake. Further work will concentrate on investigating the utility of this relationship to improve fertilizer recommendations. There was no effect of N fertilizer on soil ph. 31

32 References ALLEN, E.J. & SCOTT, R.K. (1992). Principles of agronomy and their application in the potato industry. In, The Potato Crop - The Scientific Basis For Improvement. Second Edition. Edited by P. M. Harris. Chapman & Hall: London. ALLISON, M.F., ALLEN, E.J. & FOWLER, J.H. (1998). The nutrition of the potato crop - a review for the British Potato Council, March BPC: Oxford. pp ADAMS, F., BURMESTER, C., HUE, N.V. & LONG, F.L. (1980). A comparison of column displacement and centrifuge methods for obtaining soil solutions. Journal of the Soil Science Society of America 44, BARRACLOUGH, P.B. (1986). The growth and activity of winter wheat roots in the field: nutrient inflows of high yielding crops. Journal of Agricultural Science, Cambridge 106, BREWSTER, J. L. & TINKER, P.B. (1972). Nutrient flow rates into roots. Soils and Fertilizers 35, CAMPBELL, D.J., KINNIBURGH, D.G. & BECKETT, P.H.T. (1989). The soil solution chemistry of some Oxfordshire soils: temporal and spatial variability. Journal of Soil Science 40, COLLIER, G.F., WURR, D.C.E. & HUNTINGDON, V.C. (1978). The effect of calcium nutrition on the incidence of internal rust spot in the potato. Journal of Agricultural Science, Cambridge 91, COLLIER, G.F., WURR, D.C.E & HUNTINGDON, V.C. (1980). The susceptibility of potato varieties to internal rust spot. Journal of Agricultural Science, Cambridge 94, COWIE, G.A. (1943). The relative responses of the potato crop to different potash fertilizers. Empire Journal of Experimental Agriculture 11, DENNIS, R.W.G. & O BRIEN, D.G. (1937). Boron in Agriculture. Research Bulletin of the West of Scotland Agricultural College Plant Husbandry Department. No 5, EVANS, K. (1982). Water use, calcium uptake and tolerance of cyst nematode attack in potatoes. Potato Research 25, FIRMAN, D.M. & ALLEN, E.J. (1989). Relationship between light interception, ground cover and leaf area index in potatoes. Journal of Agricultural Science Cambridge 113, FOWLER, J.H. (1994). Report on progress Nitrogen and water project Year 3. Cambridge University Farms/Potato Marketing Board. FOWLER, J.H. (1995). Report on progress Nitrogen and water project Year 4. Cambridge University Farms/Potato Marketing Board 32