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Seasonal variation in soil tests andnutrient content of pasture at twosites in TaranakiA. H. C. Roberts aa Taranaki Agricultural Research Station, Ministry of Agricultureand Fisheries , P.O. Box 8, Normanby , New ZealandPublished online: 16 Jan 2012.

To cite this article: A. H. C. Roberts (1987) Seasonal variation in soil tests and nutrient contentof pasture at two sites in Taranaki, New Zealand Journal of Experimental Agriculture, 15:3,283-294, DOI: 10.1080/03015521.1987.10425573

To link to this article: http://dx.doi.org/10.1080/03015521.1987.10425573

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New Zealand Journal of Experimental Agriculture 1987, Vol. 15 " 283 - 294 0301-5521/87/1503-0283$2.5010 © Crown copyright 1987

283

Seasonal variation in soil tests and nutrient content of pasture at two sites in Taranaki

A.H.C.ROBERTS Taranaki Agricultural Research Station, Ministry of Agriculture and Fisheries P.O. Box 8, Normanby, New Zealand

Abstract Two sites in Taranaki on well developed, grazed pasture with generous fertiliser histories were selected for monitoring changes over time in Ministry of Agriculture and Fisheries soil Quick Test values and nutrient levels in mixed pasture. Soil samples were taken at 14-day intervals, and plant samples every 28 days, over a 3-year period. Large temporal variations in soil Quick Test values occurred but few seasonal trends were evident. Obvious seasonal trends occurred with plant N, S, P, and Ca, with less well defined seasonal variations in K and Mg contents. There was no correlation between soil test values and plant mineral composition for Ca, K, P, and Mg except at Stratford in the summer period. Plant mineral composition in relation to animal requirements is briefly discussed.

Keywords soil analysis; Quick Test; plant analysis; seasonal variation; temporal variation; optimum range; sampling time

INTRODUCTION

Significant temporal differences in soil tests occur (Mountier & During 1966; Edmeades et al. 1985) although no seasonal trends have as yet been demonstrated. In contrast, plant mineral composition, including nitrogen (N), sulphur (S), phosphorus (P), magnesium (Mg), potassium (K), and calcium (Ca), but not sodium (Na), shows marked seasonal trends (NcNaught & Dorofaeff 1968; McNaught et al. 1968; Saunders & Metson 1971; Metson & Saunders 1978a).

Concern at the variability in soil and plant mineral analyses from samples taken at different

Received 18 February 1987; revision 2 June 1987

times of the year has been expressed by farm advis­ory officers in Taranaki. In an attempt to determine the extent of seasonal changes in soil and plant analyses and to determine a time of the year at which soil and plant mineral analyses reflect the general state of fertility, two sites on grazed dairy pasture in Taranaki were monitored over a 3-year period.

SITE DESCRIPTIONS

Site 1 was a grazed dairy pasture at the Waimate West Demonstration Farm in South Taranaki. The soil, Egmont brown loam, is classified in the New Zealand system as a yellow-brown loam (Campbell & Wilde 1970) and in the United States Taxonomy as an Entic Dystrandept (Soil Survey Staff 1975). The farm lies at 90 m a.s.!., and the mean annual rainfall (25-year mean) is 1244 mm (New Zealand Meteorological Service 1984). Heaviest mean monthly rainfall occurs from May to August (Table 1). This pattern was not evident over the 3 years of the study with May, July, and August being drier than the long-term mean. June, July, and August of 1983 were particularly dry. The pasture was more than 30 years old and predominantly perennial ryegrass (Lotium perenne L.) and white clover (Trifolium repens L.), with a small proportion of prairie grass (Bromus willdenowii Kunth.), cocksfoot (Dactylis glomerata L.), and Poa species. No fertiliser was applied during the 3 years of the study. Before the study, no P fertiliser had been applied to the site since 1977. However, for 10 years before 1977, an average of 60 kg P Iha was applied annually to the site. Since 1977, K fertiliser was applied at a rate of 50 kg K/ha, only when soil K levels fell below 8 on the Ministry of Agriculture and Fisheries (MAF) Quick Test scale. This did not occur during the study period.

Site 2 was located on a grazed dairy pasture at the Stratford Demonstration Farm in Central Taranaki. The soil, Stratford coarse sandy loam, is classified in the New Zealand system as a yellow­brown loam (Aitken et al. 1978) and in the United States Taxonomy as a Typic Dystrandept (Soil Survey Staff 1975). The farm lies at 311 m a.s.l., and has a mean annual rainfall (25-year mean) of 2051 mm (New Zealand Meteorological Service

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Table 1 Rainfall data for Waimate West Demonstration Farm.

Long-term Month 1981 1982 1983 Average (3-year) average (25-year)

January 65 58 59 61 83 February 33 63 34 43 80 March 123 97 69 96 86 April 85 47 147 93 99 May 55 142 108 102 137 June 159 159 66 128 129 July 129 90 56 92 126 August 85 76 54 72 117 September 111 95 115 107 99 October 135 118 90 114 87 November 90 116 44 83 85 December 52 123 84 86 96 Total 1122 1184 926 1077 1224

Table 2 Rainfall data for Stratford Demonstration Farm.

Month 1981 1982 1983

January 42 NA 121 February 31 73 20 March 154 94 36 April 158 74 254 May 87 175 227 June 428 244 100 July 236 124 120 August 169 84 143 September NA 221 254 October 149 123 218 November 141 127 213 December 138 285 152 Total 1733 1624 1858

NA = data not available.

1984). Mean monthly rainfall is well distributed throughout the year, although the wettest period is from May to August (Table 2). Over the 3-year study period, both the late summer (January, February) and late winter (July, August) periods were drier than normal (Table 2). June, July, and August 1983 were considerably drier than the long­term mean. The long-standing pastures, more than 30 years old, were predominantly perennial ryegrass (L. perenne) and white clover (T. repens) , with other grass species including brown top (Agrostis capillaris L.), Yorkshire fog (Holcus lanatus L.), and Poa species. Fertiliser applications over the study period totalled 500 kg superphosphate/ha and 250 kg KClIha per year, applied in three dressings each year in late March, late August, and late December. This fertiliser policy had been in force at the site for at least 8 years before the present study commenced.

Long-term Average (3-year) average (25-year)

82 129 41 136

161 130 162 155 163 202 257 202 160 218 132 193 238 168 163 185 160 171 192 162

1911 2051

EXPERIMENTAL PROCEDURES

Soil samples

A sampling area 40 x 50 m was selected in a flat paddock at each site. These paddocks were subjected to the normal rotational grazing management employed on the farms. Hay and silage were not conserved in the trial paddocks. Triplicate soil samples were collected at 14-day intervals from January 1981 to December 1983. Each sample consisted of 30 cores (19 mm diameter) taken to a depth of 75 mm. The soil samples were transported in insulated containers from the field and chilled to 2°C until sent to the testing laboratory as soon as practicable after removal from the field. These precautions were taken to minimise storage and transport effects on test results.

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Roberts - Seasonal variation in soil tests

At the testing laboratory, soil samples were air dried and sieved to 2 mm, then analysed for pH, Ca, Mg, K, and Olsen P using the MAF Quick Test procedures (Cornforth & Sinclair 1984). Soil sample test results are reported as MAF soil test units (Cornforth & Sinclair 1984).

Plant samples Mixed pasture samples were collected every 28 days from January 1981 to December 1983 from pasture exclosure cages (3.3 x 1.5 m) used for pasture growth measurements (Roberts & Thomson 1984a, b). These cages were located in the same paddock and closely associated with the pegged area used for soil sampling described above. At each sampling, mixed pasture was collected from each of two cages using hand shears cutting at mower height (50 mm above ground level). The duplicate samples from each site were combined before oven drying at 60°C for 24 h.

Nitrogen and phosphorus were determined after Kjeldahl digestion by the method of Basson (1976). Sulphur, after wet ashing of the plant material (nitric/perchloric acids), was determined turbidimetrically (as BaS04) using autoanalysis equipment (Garrido 1964; Blanchar et al. 1965; Mottershead 1971). The major cations (Mg, K, Ca, and Na) were determined, after wet ashing, using an automated four-channel flame photometer (Clinton 1979).

All plant material analyses are reported as a percentage of dry matter.

RESULTS AND DISCUSSION

Soil Quick Test and plant mineral analyses for major nutrients (meaned over 3 years) are shown in Fig. 1 and 4, respectively, for Waimate West and Fig. 2 and 5, respectively, for Stratford. The soil Quick Test results given in Fig. 1 and 2 are reported in empirical units. The reasons for this are simplicity and to prevent users assuming the tests extract particular chemical fractions of soil nutrients. Relevant factors for converting Quick Test results into other units may be found in Cornforth & Sinclair (1984).

Soil analyses

The MAF Quick Test results for both Waimate West (Fig. 1) and Stratford (Fig. 2) exhibited marked temporal variations between 14-day values, as has previously been reported by Mountier & During (1966). The intensity of core samplings in this study should have minimised spatial variation

285

as a component of the temporal variation recorded. Previously, laboratory variation has been suggested as making a significant contribution to temporal variation of soil tests, as well as the effect of dry weather on pH values and of urine spotting on K values on some soils (Mountier & During 1966\ Little obvious seasonal pattern is evident from the data, although K and P soil tests showed lowest values in mid -late summer (January - February) and mid -late winter (June - August) at both sites.

Over the June - August period at Waimate West, and to a lesser extent at Stratford, there was a consistent, marked decrease in soil test K followed by a marked rise. Although these changes in soil test K occurred in all years, they did not occur at exactly the same sampling time each year. For example, at Waimate West the lowest K tests ranged from early to late July and at Stratford from mid July to mid August over the 3 years of the study (Fig. 3). Although the declines in soil test K shown in Fig. 3 occur in the months of highest average rainfall (Tables 1 and 2), there is no clear relationship between the actual monthly rainfall and soil test K recorded in the individual years of the present study. Over the 3 years of the study, May, July, and August rainfall was considerably less than the average rainfall (Tables 1 and 2). Therefore, although leaching of K is likely to be greater in winter than in other seasons because of high rainfall on moist soils, this is not clearly evident from the data in the present study. A partial explanation for the K soil test decline recorded over the July and August period may be that the rotation length of the grazing dairy animals is much slower over this period, i.e., 80-toO days compared with 20 - 30 days for rotation lengths over the lactation period of August - May. Long winter rotation lengths mean that dairy animals grazed the sampling site only once between drying off (May) and calving (August). Therefore, less opportunity for return of urinary K, coupled with the increased opportunity for leaching of soil K, may explain the marked decline in soil test K, at both Waimate West and Stratford, shown in the data (Fig. 1,2, and 3).

Magnesium soil tests peaked in summer (November - December), but dropped by winter (June - August).

Olsen P at the Waimate West site (Fig. 1) showed two distinct peaks, in April (autumn) and in September - October (spring), even though no P fertiliser had been applied at this site since 1977. Similar, though less well defined, peaks occurred in April (autumn) and October - December (spring - early summer) at Stratford (Fig. 2), despite P fertiliser being applied three times each year. These autumn and spring peaks in Olsen P recorded at both sites could be explained by a significant

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42

40 ! 38 36

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32 30 28 42 40 38 36 34 32

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New Zealand Journal of Experimental Agriculture, 1987, Vol. 15

l ~SEM MEAN SEM

~~

J FMAMJJ ASOND Fig. 1 Seasonal variations (mean of 3 years) in soil test values at Waimate West.

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Roberts - Seasonal variation in soil tests

18 16

Mg 14

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Fig_ 2 Seasonal variations (mean of 3 years) in soil test values at Stratford. (Arrows indicate times of fertiliser application.)

287

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(a) 16

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contribution to Olsen P values from microbial biomass P. Sparling (1985) stated that air drying soil samples before analysis killed soil micro­organisms, with consequent release of biomass P which was measured as an increase in the bicarbonate-extractable P pool (i.e., Olsen P). The soil microbial biomass is likely to be highest in autumn and spring and, because the two sites in the present study are under well developed pasture on soils which do not dry to any great extent in the field in most years, two of the three conditions necessary for a large microbial P input to Olsen P values are fulfilled (Sparling 1985).

The autumn and spring peaks in Olsen P found in the present study conflict with the summer peak in soil P levels reported by Saunders & Metson (1971). However, these authors measured soil P by anion exchange and calcium chloride extraction, and hence it is possible that identical soil pools were

not being measured in the two studies. Furthermore, the earlier work did not include a central yellow-brown loam soil.

The standard errors of the means (shown as shaded areas on Fig. 1 and 2) did not markedly vary with sampling time throughout the year or with soil test value.

Plant analyses

In general, at both Waimate West (Fig. 4) and Stratford (Fig. 5), plant concentrations of N, S, and P, declined from late summer - autumn (February - May) to spring - early summer (September - December). Previous studies (McNaught & Dorofaeff 1968; Saunders & Metson 1971; Metson & Saunders 1978b) of seasonal variations in plant mineral composition have reported minimum N in November and maximum

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290 New Zealand Journal of Experimental Agriculture, 1987, Vol. 15

N in winter (June-August), and maximum P concentrations in mixed pasture in late winter - early spring (July - September) and minimum levels in summer - autumn (December - March).

Although maximum values for P in mixed pasture did not coincide with those in previous studies (McNaught & Dorofaeff 1968; Saunders & Metson 1971), the present data shows near­maximum values for P in the June and July period which is consistent with the peak P values shown in the earlier studies.

Plant Mg content showed less variation throughout the year, with lowest levels measured in the winter months (June, July, and August) and early spring (September), as has previously been reported (McNaught et al. 1968; Metson & Saunders 1978a). Highest Mg levels were recorded in late spring - early summer (October - December), when the ryegrass component attains the maximum growth rate at these sites (Roberts & Thomson 1984a, b). This result is consistent with the finding that the Mg level of the grass component exceeds that of clover (Metson & Saunders 1978a). Highest plant Mg levels in previous studies have been reported in December and January (McNaught et al. 1968; Metson & Saunders 1978a) which, at Waimate West and Stratford, coincides with the lowest grass : clover ratios in the swards (Roberts & Thomson 1984a, b). Differences in peak Mg levels between the studies could be a reflection of the differences in sward composition between sites.

Plant Ca content showed a different trend from the other major elements (except Mg in terms of maximum and minimum periods) in that levels were low through summer and autumn (January - May) and rapidly increased over late winter - early spring (August - September) to attain maximum levels in mid -late spring (October - November). The maxima and minima for plant Ca in the present study occurred some 2 months earlier than those reported by McNaught et al. (1968) and Metson & Saunders (1978a). In the present study, the highest plant Ca content coincided with the maximum rate of pasture growth (Roberts & Thomson 1984a, b). This period is associated with the reproductive phase of pasture growth and the accumulation of Ca in the plant tissue may be related to the physiological change from vegetative to reproductive growth. However, in a study of annual plants, including legumes and ryegrass, Loneragan et al. (1968) found that plant Ca content decreased with increasing plant maturity. A contributing factor to the rise in plant Ca levels over late winter and early spring would be the increasing legume content over this period, because clover Ca content

has been found to greatly exceed grass Ca content (Loneragan et al. 1968; Metson & Saunders 1978).

The standard errors of the means (shown shaded on Fig. 4 and 5) for N, S, P, Mg, and Ca were generally small compared with those for Na and K. Plant Na levels showed marked temporal fluctuations but no evidence of seasonality, as has previously been reported (McNaught & Dorofaeff 1968; Metson & Saunders 1978a). There is no obvious reason for the marked decline in Na levels at Stratford recorded in April- May.

At Waimate West (Fig. 4), where no K fertiliser was applied over the 3-year study period, plant K content was highest in June, July, and August and lowest in spring (September - November). Metson & Saunders (1978a) reported highest plant K levels in early spring, and McNaught & Dorofaeff (1968) recorded maximum K levels in late autumn - early winter. These authors both reported minimum plant K levels in late spring - early summer (November - January). Plant K levels at Waimate West were highest and changed little from mid April to August; hence, the results of the present study do not differ greatly from previous work. At Stratford (Fig. 5), K levels were highest in late summer and declined to lowest levels in late spring - summer, a similar trend to that shown by N, S, and P.

Relationship between soil test values and plant mineral composition At both Waimate West and Stratford, there was no correlation between soil test value and plant nutrient level for any of the nutrients Ca, K, P, and Mg except at Stratford over the summer period (Table 3). The soil cations K, Ca, and Mg were correlated to plant content (at the 5%, 10,10, and 5% probability level, respectively).

Even though Olsen P levels and plant P levels both appear to peak in autumn (Fig. 1, 2, 4, and 5), especially at Waimate West, the data gave no significant correlation between Olsen P and plant P in any season. The practical significance of this finding is, for example, that even though Olsen P levels showed a peak in September - October at Waimate West (Fig. I) and in November - December at Stratford (Fig. 2), plant P content was lowest (0.25 - 0.30%) between October and December (Fig. 4 and 5).

Quick Test Mg at Waimate West was approximately double Quick Test Mg at Stratford (Fig. 1 and 2), and yet plant Mg content was very similar at the two sites (0.21-0.33% at Waimate West and 0.20-0.27% at Stratford). Although there was little seasonal trend in Quick Test Mg content, plant Mg content was lowest over the

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Table 3 Correlation coefficients (r) between soil tests and plant nutrient levels (n = 9 for autumn - winter; n = 10 for spring - summer; n = 39 for annual values) for P, K, Ca, Mg.

Nutrient Spring Summer Autumn Winter Annual

Waimate West P 0.15 -0.01 -0.49 -0.07 0.13 K -0.08 -0.44 -0.31 0.35 -0.10 Ca 0.03 0.43 -0.39 0.05 0.23 Mg -0.09 -0.36 -0.28 -0.53 -0.07

Stratford P -0.09 0.28 -0.07 0.07 0.03 K 0.27 0.72 * -0.11 -0.07 0.30 Ca -0.49 0.86 ** -0.22 -0.64 -0.16 Mg 0.43 0.68 * 0.21 -0.07 0.07

winter months. Soil test P was higher at Waimate West (32-40) than Stratford (22-30) yet herbage P concentrations were similar (0.27 - 0.46070). Al­though Olsen P values for both sites were well above those considered to indicate deficient soil levels for pasture growth, plant P content fell below the opti­mum level in spring and early summer at both sites.

Optimum ranges for plant growth Optimum ranges for pasture mineral composition (Cornforth & Sinclair 1984) are shown in Fig. 4 and 5. At Waimate West (Fig. 4) Ca, Mg, and K remained above or within the optimum range throughout the year, whereas S fell below the optimum range briefly in mid -late spring (October - November). Nitrogen and P levels in herbage fell below the reported optimum levels for a longer period of the year. Phosphorus levels were below optimum over spring (September - mid December), whereas N was below optimum from July to December. Although trial work has confirmed that responses to N application are greatest in the July- December period (Thomson & Roberts 1982; Roberts 1985), current work on withholding P fertiliser at Waimate West (Roberts unpublished data) has shown no response to applied P, despite pasture P levels falling below the apparent optimum in spring.

At Stratford, pasture Ca, Mg, K, and S remained above or within the optimum range throughout the year whereas P fell below the optimum in late spring - early summer (October - December). Pasture N levels only reached the optimum range briefly in early - mid autumn (March - April).

At both sites, K levels were well in excess of the optimum range required for plant growth, a finding similar to that reported by Smith & Middleton (1978) and indicative of the high degree of soil and pasture development and intensive grazing management at both sites.

Pasture mineral content in relation to animal requirements

Recent North American and British standards for mineral nutrition of lactating cows are given as 0.18% S, 0.32% P, 0.19% Mg, 0.44% Ca, and 0.58% K (Cornforth & Sinclair 1984). Russian standards are higher: Kal'nitskii et al. (1981) have reported that the diet of high yielding dairy cows should contain 0.23 - 0.26% S, 0.38 - 0.52% P, 0.20 - 0.26% Mg, 0.49 - 0.69% Ca, and 1.0 - 1.4% K. Data from the present study suggest that Sand K requirements should be well supplied by mixed pasture. However, plant P contents were lowest (0.25-0.30%) between October and December at both sites (Fig. 4 and 5) at a time when high producing, lactating, dairy cows require between 0.32 and 0.52% P in their diet (Kal'nitskii et al. 1981; Cornforth & Sinclair 1984). Early work in Taranaki (N. A. Thomson unpublished data) and more recent work in Northland (Betteridge 1985) have indicated that plant P levels may be limiting dairy production over the spring and summer period.

Throughout most of the year, Mg and Ca levels were adequate for animal requirements, except through the winter months of June - August, leading up to calving, when plant Mg and Ca were at or near the lowest levels. The low dietary intake of Ca and Mg would predispose dairy cows to hypomagnesaemia or milk fever if nutritional and! or climatic stress occurred either side of calving. However, it has recently been suggested that total Mg in pasture is an insensitive estimate because there is no indication of Mg availability to the grazing animal. This follows from the finding that plant Mg content was not significantly related to the average herd Mg status (Feyter et al. 1986).

At Waimate West the K : Na ratio for mixed herbage averaged 10 (range 7 -12), whereas at Stratford the average ratio was 12 (range 8 -18) over the three years of the trial. The higher ratio at Stratford probably reflects both the higher usage of K fertiliser at this site, and the lower input from atmospheric deposition of marine-derived Na. The K : Na ratio has been suggested as important in the bloat syndrome of dairy cattle, and the recommended K : Na ratio for dairy pastures is less than or equal to 10 (Turner 1985). Although the K : Na ratios at Waimate West and Stratford are at or above the recommended ratio, a recent survey of more than 300 farms in the main North Island dairying areas found no relationship between pasture K : Na ratios and the bloat susceptibility of the properties (Carruthers et al. 1987).

Previous survey data of plant mineral concentrations showed that, for Taranaki, 16% of sites sampled were deficient in plant P and 20% in

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Roberts - Seasonal variation in soil tests

plant Mg for lactating dairy cows (Smith & Cornforth 1982). Calcium and S were well supplied.

CONCLUSION

The occurrence of large temporal variations in soil test levels, but little definitive seasonal pattern, together with no large difference in standard errors between different times of the year, makes choice of a single optimum time for soil sampling difficult. However, if the object of soil testing is to take samples when soil test levels tend to be lowest then the indications from the data are that samples should be taken either in January or from June to August. These sampling times fit in well with normal farmer practice of applying fertilisers in autumn or spring, or both.

Unlike soil analysis, plant analysis showed distinct seasonal trends and much less marked temporal variations, except for K and Na. Plant samples for mineral analysis should probably be taken in spring (September - October), the time when N, S, P, Mg, and K levels were at or near minimum values.

ACKNOWLEDGMENTS The author would like to express his thanks to Mr Fordham and Mr Hobbs for assistance with field work and the Ruakura Plant and Soil Testing Laboratories for sample analyses.

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