Potash for Silage. Contents

Transcription

1 Potash for Silage No. 9 Contents Introduction 2 Methods 3 Sites 4 Measurements 5 Results 5 Potash balance 6 Soil fertility 7 % herbage K content 9 N:K ratio 10 Mineral balance 11 Timing 12 Yield response 13 Potash recommendations 14 Conclusions 15 Acknowledgement 16 More information 16 (C) Potash Development Association 1

2 Introduction Potash is an important nutrient for grass and especially for silage where the whole crop is removed. Between 1986 and ADAS trials examined the response of first and second cut silage to different potash rates and timing. Results indicated that previous recommendations for potash on silage needed revision. A large number of silage fields have low soil K status which also indicates the need for review of potash use in practise. A further series of trials to examine the optimum long-term strategy for potash manuring of silage was started in 1988 by ADAS on 5 sites. Funding from the Potash Development Association allowed this study to be continued in 1990 and 1991 on 2 sites. A scientific paper on this work is published in:- Gross & Forage Science Vol 47 pages , "The effect of timing & rate of potash application on the yield & herbage composition of gross grown for silage". Author P M R Dampney, ADAS. This leaflet provides details of the silage work and draws conclusions with practical recommendations. 2

3 Methods Five rates of potash were examined at five sites in 1988 & 1989 and continued on two of the sites in 1990 and The K treatment rates were increased in the last year. Potash treatments Per cut Per annum 1991 Per cut Per annum K 2 O Units/acre Potash was applied before each of three cuts of silage. Where possible a fourth cut was taken but no potash was applied. No manure or slurry was used in the year before the trial or during the experiment. An additional treatment examined the same five rates for first cut but applied in November instead of March. ADAS recommended rates of nitrogen and phosphate were used for all plots. 3

4 Sites Soil level Autumn 1987 mg/l (Index) Location Soil Series Soil type K Mg Dyfed Denbigh Clay loam 118 (1) 69 (2) Yorkshire Kexby Sandy loam 44 (0) 105 (3) 4

5 Measurements Dry matter yields and % content of N, K, Mg and Na were measured for each cut and soil analyses were undertaken in early spring and after the second and the final cuts each year. Results Whilst there was considerable yield variation according to the weather pattern each year, the results confirmed that silage removes large quantities of potash. For instance in 1988 four cuts removed over 500 units/acre of potash in a single season. Over all four years, average removal in first cut (at optimum K for yield) ranged between units/acre. For three cuts on the sandy loam average removal, was 290 units/acre whilst four cuts on the clay loam soil removed 350 units/acre. These figures are somewhat higher than the standards currently used for potash offtake in silage which are:- K 2 O Units/acre 1st cut 120 ) 2nd cut 75 ) 195 = 2 cut total 3rd cut 50 ) 245 = 3 cut total 4th cut 50 ) 295 = 4 cut total 5

6 Potash Balance Total units/acre K 2 O over 4 seasons Annual Potash rate unit/ac Sandy Soil Clay Loam Crop K Crop K Fert input Fert input offtake balance offtake balance Increasing rates of potash fertilizer increased potash offtake in the crop. On the clay loam, the four-year balance at all fertilizer rates was negative ie. more potash was removed in the crop than was applied as fertilizer. Large negative balances occurred at the lower rates in the earlier years when large crops and large offtakes were sustained by soil reserves, As these reserves were depleted, yields and offtake reduced leading to smaller negative balances. A positive balance occurred at the highest fertilizer rate in 1990 owing to yield restriction by summer drought and in 1991 when this treatment was increased from 300 to 480 units/acre for the season. Potash offtake was generally lower at the lighter site because of the lower yield potential. K balance for the four years was negative for all but the highest fertilizer treatment. Potash balance has a direct bearing on soil reserves and these results confirm the need for large amounts of potash to balance crop removal and maintain soil fertility 6

7 Soil Fertility Sandy loam On the sandy soil there was little change in the very low K level for any potash rate in the first three seasons. The highest treatment (which was increased to 480 units/acre per year in the final season) resulted in a large increase in soil level at the end of the fourth year. Where no potash was applied, the ability of this soil to provide K to succeeding crops diminished markedly and this was reflected in falling yields. Sandy soil K 2 O offtake (units/ac) where no fertilizer K Yield of dry matter t/acre

8 Soil Fertility Clay loam On the clay loam, soil K levels fell over the four years under all treatments except in the final year under the highest rate of 480 units/acre which increased soil level to 168 mg/1 (Index 2). This soil continued to provide higher levels of potash to the plant than the lighter soil, but this ability again markedly reduced over the four years, as did yields. Clay loam K 2 O offtake (unit/ac) where no fertilizer K Yield of dry matter t/acre If soil fertility is allowed to fall to low levels, potential yield will fall drastically 8

9 % Herbage K Content Whilst % K content tended to increase with higher potash rates there was little evidence of "luxury" levels of K on these sites. The highest value recorded for first cut was 2.77% on the clay loam in 1991 when 200 units/acre was applied in a single dressing in March. Levels in excess of 3% occurred in a few cases at high potash rates in 1990 and 1991 where second and third cut yields were particularly low as a result of drought. Lower K treatments frequently produced concentrations in the plant which would suggest inadequacy. % K in the dry matter is not regarded as a meaningful measurement as it alters according to growth stage and is difficult to interpret in practise. 9

10 N : K Ratio The N:K ratio provided a better indication of optimum potash rate for yield. The following guide is suggested:- Less than 0.9: 1 High :1 Satisfactory More than 1.3: 1 Deficient Grass requires equal levels of nitrogen & potash in the plant for optimum yield. 10

11 Mineral Balance Soil magnesium levels were 69 mg/1 (Index 2) in the clay loam and 105 mg/1 (Index 3) in the sandy soil. Herbage magnesium reflected this difference with higher values on the lighter site. First cut levels on this site were generally around the suggested critical threshold of 0.2% Mg whilst on the clay loom site averaged only 0.15%. On both sites herbage magnesium rose through the season. Potash treatment had no significant effect on herbage magnesium level on the clay loam even in 1991 when 200 units/acre were applied in March. On the lighter soil, additions of potash had more influence on herbage Mg level. % Mg in 1st cut herbage Clay loam Sandy soil Sandy soil 1991 K 2 O treatment for first cut K 2 O treatment for first cut There was no significant difference between autumn or spring applied K in respect of herbage magnesium. The use of sub-optimum potash rates for silage will not necessarily avoid mineral problems and are likely to result in reduced yields. Potash requirements for grazing are much lower anyway because of dung & urine return and applications avoiding the critical periods in spring and early autumn can minimise problems. In practise for both grazing and cutting, optimum potash rates should be used and mineral balance in the animal should be ensured by appropriate feeding. Sodium levels were also measured and tended to decrease with higher potash rates. No values were recorded below the critical threshold of 0.15% and the target K:Na ratio of 20:1 was not exceeded even at the highest K rates. The third cut on the lighter soil approached these values at which sodium benefits may occur. 11

12 Timing Autumn applied K produced similar first cut yields to spring applied K on the sandy soil and slightly higher first cut yields on the clay loam soil. This trend persisted through the full season yields and clearly endorses other direct studies of leaching which indicate that losses of potash beyond rooting depth are very small. Better distribution of nutrient from autumn application on heavier soils may provide additional benefit to conventional spring timing. First cut yields t/acre Sandy loam Clay loam Autumn Spring Autumn Spring Some of the potash for first cut may be applied in the autumn. This may offer particular benefits where optimum rates are large. 12

13 Yield Response Apart from the first cut in the first year, yield responses from potash occurred at all cuts in all years. Optimum K rate and the yield response tended to increase each year. Larger responses resulted on the lighter soil with a lower soil K reserve. Optimum potash rate for each cut in each of the four years is shown for the two sites in the following table. K 2 O units/acre Clay loam Sandyloam st cut 2nd cut 3rd cut st cut 2nd cut 3rd cut st cut 2nd cut 3rd cut st cut 2nd cut 3rd cut The levels of response to the optimum rate of potash indicated a very worth while economic return. If grass is valued at 80/t dry matter and potash at 10p/unit the extra profit after fertilizer cost is shown in column Clay loam Sandy Loam Yield response Profit Yield response Profit t/acre % of control /acre t/acre % of control /acre On the basis of these results and the earlier series of trials investigating potash requirements, a new set of recommendations for potash have been produced by ADAS. 13

14 Potash Recommendations Potash Recommendations (units/acre) Silage Soil K index over 3 One Cut System -before cutting nil -after cut or in autumn nil nil Multi Cut System -before first cut nil -before 2nd cut nil -before each further cut nil -after last cut or in autumn nil nil These recommendations assume "average" yields and a more accurate approach if actual yields are known is to adjust potash quantities to the actual removal. Allowance should be made to these total potash requirements where organic manures are used. 14

15 Conclusions 1. Silage removes large quantities of potash which should be replaced in order to maintain the fertility of soils and to sustain optimum yields. 2. Potash provides large economic responses from silage on soils in Index 0 and N:K balance is important and nitrogen & potash manuring should seek to achieve a ratio in the plant of around 1:1 4. Autumn application of potash for first cut is as effective as spring application except on very light sandy soils which may be at risk to potash leaching. 5. Lighter soils generally retain less K than soils with more clay and potash reserves will be run-down more quickly on sandy soils. 6. Mineral balance is more easily altered on light soils. Responses to K are large on such soils and sub-optimum potash rates are not a solution to mineral problems which should be dealt with by appropriate supplementation of animal diet. 7. Mineral balance is less likely to be a problem on clay loam soils and magnesium depression from potash application will be small. 15

16 Acknowledgement This work was carried out by ADAS and particular thanks are given to Mark Aitken, the Project Leader, and Peter Dampney for overseeing the work. The Potash Development Association is an independent technical organisation formed to support the efficient use of potassium fertilizer in the UK. More Information For more information on Potash Contact: Potash Development Association Brixtarw, laugharne, Carmarthen SA33 4QP Tel & Fax