Paired paddock trial, 24 paddocks, five samplings

Soil science research poster example

Four years of soil organic carbon under cover crops across 24 paired paddocks, laid out as an A0 portrait board built on a before and after grid. The top half sets up the paddocks and the question, the bottom half reports what four years changed, and the figure sits between them.

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RU
Nineteen hundredths of a point in four years: soil organic carbon under cover crops in 24 paddocks
A. Whitcombe ¹, J. Mkhize ¹, R. Donnelly ¹ ²
1 School of Agriculture, Ridgemount University · 2 Soil Carbon Monitoring Group, Ridgemount University
BACKGROUND
Twenty four paddocks on six mixed farms were paired within farm on soil type, slope and cropping history, and one of each pair was sown to a multi species cover crop in the summer fallow for four consecutive years while the other stayed in the farm usual bare fallow. At the start, soil organic carbon in the top 10 centimetres averaged 1.42% in the paddocks that would be cover cropped and 1.40% in the controls, a difference well inside the variation between cores in a single paddock. The question is narrow and it is the one a grower asks: after four years of doing this, is there more carbon in the top 10 centimetres, and is the difference large enough to see through the noise of sampling. It is not a question about deeper carbon, about permanence, or about whether the change would survive a return to bare fallow, and this design cannot answer any of those. Four years is also short. Most published soil carbon responses take five to ten years to separate from the sampling error, so a measurable difference at four years is the interesting part of the result rather than its size. The design that makes it visible is not the laboratory method but the sampling: six cores per paddock returned to the same GPS points every year, which removes the largest single source of noise in a field soil carbon series, namely sampling a slightly different patch of ground each time.
METHODS
Design. Paired paddocks within farm, twelve pairs across six mixed farms, matched on soil type, slope and the cropping history of the previous five years.
Treatment. A multi species summer cover crop sown for four consecutive years against the farm own bare fallow, with seed supplied but no other input from the study.
Sampling. Six cores per paddock to 10 cm, returned to the same GPS points every year rather than randomised afresh, giving 720 cores over the five samplings.
Analysis. Dry combustion for total organic carbon, with bulk density measured on two cores per paddock so the concentrations can be converted to mass.
Statistics. A paired comparison on the within pair difference, so farm, season and operator effects cancel by construction rather than by adjustment.
End
Start
CoverControl00.450.91.351.8
Figure 1. Soil organic carbon in the top 10 cm, start and end of the four year period, by treatment.
Source: 720 cores, dry combustion, 24 paddocks on six farms.
RESULTS
Cover cropped paddocks moved from 1.42% to 1.61% organic carbon in the top 10 centimetres, a gain of 0.19 percentage points. Control paddocks moved from 1.40% to 1.38%, a change of 0.02 points in the other direction that its own interval crosses. The paired difference is 0.21 points, 95% CI 0.12 to 0.30, and it is positive in 10 of the 12 pairs. The two pairs that are not are on the same farm, the only one in the set that grazed its cover crop rather than leaving it standing. Bulk density did not change measurably in either treatment, at 1.31 and 1.32 grams per cubic centimetre, so the carbon figures can be read as concentrations without a mass correction changing the conclusion. Year by year the cover cropped mean runs 1.42, 1.45, 1.51, 1.57 and 1.61, so the gain accumulated steadily rather than arriving in one season. Controls over the same five samplings run 1.40, 1.41, 1.39, 1.39 and 1.38, flat within the sampling error throughout. Across the 12 pairs the within pair difference at year four ranges from a loss of 0.04 points to a gain of 0.41, and the two negative pairs are both on the farm that grazed its cover crop rather than leaving it standing.
REFERENCES
1.Mkhize, J. and Whitcombe, A. (2025) Fixed point resampling and the detection of small soil carbon change. Soil Research Practice, 63(2), 130 to 155.
2.Donnelly, R. (2024) Bulk density correction in shallow soil carbon comparisons. Journal of Agronomy Methods, 22(4), 301 to 318.
3.Enriquez, V. (2026) Grazed and standing cover crops: a paddock scale comparison. Ridgemount Agriculture Reports 2026-01.
0.19 points
of soil organic carbon gained in four years
From 1.42% to 1.61% in the top 10 cm; paired difference 0.21, 95% CI 0.12 to 0.30
DISCUSSION
Converted to mass with the measured bulk density, 0.19 percentage points over the top 10 centimetres is about 2.5 tonnes of carbon a hectare, or roughly 9 tonnes of carbon dioxide equivalent, accumulated over four years. That is a real number and a modest one, and three things should be said beside it. The first is depth. This study sampled to 10 centimetres because that is where a cover crop changes things fastest and where a grower can afford to sample, but carbon below 10 centimetres is where permanence lives, and nothing here speaks to it. The second is the grazed pair. The one farm that grazed its cover crop showed no gain in either of its two pairs, which is a sample of two and therefore not a finding, but it is the single most useful thing in this dataset for deciding what to test next. The third is what happens afterwards. A gain that took four years of cover cropping to build could be lost in one or two seasons of bare fallow, and this design stopped measuring at the point where that question begins. Sampling design mattered more than any of the laboratory choices. Returning to the same GPS points each year cut the year to year noise by roughly half compared with fresh random points, which is the difference between seeing a 0.19 point change at four years and waiting until year seven.
CONCLUSIONS
Four years of summer cover cropping raised soil organic carbon in the top 10 centimetres from 1.42% to 1.61%, against a control that moved from 1.40% to 1.38%. The paired difference is 0.21 points, 95% CI 0.12 to 0.30, positive in 10 of 12 pairs, and worth about 2.5 tonnes of carbon a hectare in the top 10 centimetres. Nothing here measures carbon below 10 centimetres, and nothing here measures what happens if the cover cropping stops.
Cover cropped carbon 1.42% to 1.61% in four years
Controls moved 1.40% to 1.38% over the same period
Paired difference 0.21 points, 95% CI 0.12 to 0.30
About 2.5 tonnes of carbon a hectare in the top 10 cm

Block by block

What each block on the board is for, in the order a reader walks it.

Title band and three authors
The title gives the size of the effect and the sample: nineteen hundredths of a point in four years, soil organic carbon under cover crops in 24 paddocks. Three authors carry superscripts over a school and a monitoring group.
Before: the paddocks and the question
Twenty four paddocks on six mixed farms paired within farm, one of each pair sown to a multi species summer cover crop for four years. Starting carbon was 1.42% and 1.40% in the top 10 centimetres, a difference inside the between core variation. The block is headed Background, since this is a finished trial rather than a proposal.
Methods as a labelled protocol
Five entries under the heading Methods: twelve pairs across six farms matched on soil type and slope, a multi species summer cover crop against the farm bare fallow, six cores per paddock to 10 centimetres at the same GPS points each year, dry combustion with bulk density on two cores, and a paired comparison. The sampling entry gives 720 cores over five samplings, which is 24 paddocks x 6 cores x 5 samplings, one at the start and one after each of the four years.
Figure
Four bars: cover cropped and control paddocks at the start and end of the four year period, at 1.42 and 1.61 against 1.40 and 1.38. The source line names the dry combustion method and the 24 paddocks across six farms, and it gives the same core count as the methods entry, 720 cores from 24 paddocks with 6 cores each at every one of the 5 samplings.
After: what four years changed
The gain of 0.19 points under cover crops, the 0.02 point drift in controls, the paired difference of 0.21 with its interval, the ten of twelve positive pairs, unchanged bulk density at 1.31 and 1.32, and the year by year series for both treatments.
The headline number
A key number block reads 0.19 points of soil organic carbon gained in four years, with the detail line giving the 1.42% to 1.61% move in the top 10 centimetres and the paired difference of 0.21 with its 95% CI of 0.12 to 0.30.
What the number is worth, and references
About 2.5 tonnes of carbon a hectare, with three cautions: nothing below 10 centimetres, the grazed pair with no gain, and no measurement of what happens if cover cropping stops. Four checked points and three numbered references close the board.

What makes this board work

The before and after grid matches the study

The top block sets up the paddocks and the starting carbon, the bottom block reports the change. A layout whose shape matches the design means a reader never has to work out which number belongs to which year.

The paired difference is the headline, not the means

Two group means can move for a dozen reasons. The paired difference of 0.21 points with a 95% CI of 0.12 to 0.30, positive in 10 of 12 pairs, is the number the design actually supports, and it is the one in the key number block.

The two negative pairs are explained

Both are on the one farm that grazed its cover crop rather than leaving it standing. That is a sample of two and the board says so, while still naming it as the most useful thing in the dataset for deciding what to test next.

Questions people ask

What goes on a soil science research poster?

The pairing or blocking, the treatment and its duration, the sampling depth and the number of cores, the laboratory method, bulk density, and the paired difference with its interval. A carbon result without a depth and a bulk density statement cannot be compared with anything.

Why report a paired difference rather than two means?

Because the design is paired. Two group means can drift with season, farm or sampling position; the within pair difference cancels the farm effect by construction. This board gives both, with the means in the figure and the paired difference in the key number block.

How do I show a four year change on one figure?

Plot the endpoints by treatment and put the year by year series in the text. This board shows start and end for cover cropped and control paddocks as four bars, and lists the five annual means for both treatments in the results block underneath.

What text size does an A0 portrait poster need?

Body text here is near 24 pixels on a 1656 by 2340 canvas, about 12 millimetres printed, readable at a metre. The two large text blocks on this layout hold roughly 1,600 characters each, so resist the urge to shrink the type to fit a third one in.

Why convert a soil carbon percentage to tonnes per hectare on a poster?

Because growers and carbon schemes trade in mass, not concentration. The conversion needs bulk density and depth: 0.19 points over the top 10 centimetres at about 1.31 grams per cubic centimetre is roughly 2.5 tonnes of carbon a hectare, or about 9 tonnes of carbon dioxide equivalent. This board measures bulk density on two cores per paddock, so the conversion is stated, not assumed.

How do I adapt this to my own trial?

Retype the title, the two long text blocks and the five method entries, then edit the figure series and categories. The layout labels its two text blocks Current practice and Proposed protocol by default; on a finished trial retitle them Background and Methods, as this board does.

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Sources

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