Cover crops and humic inputs: what the research shows

We sell cover crop seed and humic soil inputs, so take what follows with that in mind. This page collects what the published research actually reports about legume cover crops and humic substances in tropical plantations, including the trials that found nothing, the numbers that disagree with each other, and the trade-offs worth planning around. Every figure below is attributed to a named source you can go and read.

Established Mucuna bracteata cover crop between oil palm rows at Kota Tinggi, Johor
Established Mucuna bracteata ground cover in an oil palm block at Kota Tinggi, Johor.

On this page

  1. How to read the numbers
  2. Nitrogen fixation: what has been measured
  3. Ground cover and weed suppression
  4. Choosing species: shade, persistence, mixtures
  5. Erosion and slope
  6. Where the evidence is weakest
  7. Trade-offs to plan for
  8. Humic substances: what the meta-analyses show
  9. What this means for SoilBoost EA
  10. References

1. How to read the numbers

Published estimates of how much nitrogen a legume cover fixes in an oil palm plantation differ widely, and they are often not the same kind of number: some are annual figures for a whole legume cover, while others are the nitrogen found in shoots and litter at one sampling, with no time basis.8 That is not because the science is bad. It is because the answer depends on the measurement method, on whether roots and litter turnover are counted at all, and above all on how much ground the legume actually covers.

Pardon and colleagues, reviewing the nitrogen budget for oil palm and citing Giller and Fairhurst (2003), report that most published fixation estimates are probably under-estimates, because they were based on harvested plants and ignored the nitrogen continuously returned to the litter layer.8 So when a supplier quotes you a single confident figure for nitrogen fixation, treat it as marketing. A range with the method attached is the honest form.

2. Nitrogen fixation: what has been measured

Proportion of nitrogen derived from the atmosphere

The cleanest measure is %Ndfa: the share of the plant’s nitrogen that came from fixation rather than from the soil.

Published %Ndfa values for legume cover crops in tropical plantation systems.
Species %Ndfa reported System and method Source
Mucuna bracteata 67 to 84% (mean 79%) Oil palm, Malaysia; 15N isotopic dilution, 60-day microplot 1
Pueraria phaseoloides 85 to 93%, varying by year Young rubber, NE Thailand; 4-year field trial 6
Calopogonium caeruleum 49% mean (range 28 to 75%) Oil palm, Papua New Guinea; ureide technique, a survey of oil palm plantations 4
Pueraria phaseoloides 32% mean, significantly lower than C. caeruleum Oil palm, Papua New Guinea; ureide technique 4

The two Pueraria figures come from very different systems: a four-year rubber trial in Thailand, and a survey of oil palm plantations in Papua New Guinea where legume ground cover was at most 44% (as little as 0.6% in the oldest plantation sampled) and where the share of nitrogen from fixation fell as soil nitrate rose.4 Ground cover sets how much nitrogen a cover fixes in total; soil nitrate affects how much of each plant’s nitrogen comes from fixation.

The finding most growers have not heard

Across 13 plantations, %Ndfa was strongly and negatively correlated with soil nitrate (R² = 0.55), and declined markedly as soil nitrate-N rose above 5 mg/kg.4 In practical terms: where soil nitrate is high, for example after heavy nitrogen fertilisation, the legume takes more of its nitrogen from the soil and fixes less. Heavy fertilisation and a legume cover work against each other.

Nitrogen accumulated in biomass

  • In a four-year trial under young rubber in northeast Thailand, P. phaseoloides produced about 8 Mg/ha/yr of above-ground biomass and accumulated about 250 kg N/ha/yr, with 85 to 93% of its nitrogen from fixation, depending on the year. Between 39% and 46% of the rubber trees’ leaf nitrogen was traced back to legume-fixed nitrogen.6
  • A legume mixture of P. phaseoloides, Centrosema pubescens and Calopogonium mucunoides supplied 151 kg N, 10 kg P and 20 kg K per hectare per year more than a grass cover in Malaysian rubber trials reviewed by Broughton (1977). It is a result for that mixture, not a rate for any one species, and it traces to work from the 1950s to the 1970s.7
  • Legume intercropping increased rubber tree girth by 11 to 29% in 4- to 6-year-old plantations against natural cover, and shortened the immature period by five months to a year.7
  • Annualised for young Papua New Guinea plantations: 48 to 102 kg N/ha/yr, the authors’ own estimate, made by treating the fixed nitrogen standing in the shoots as four months’ production and multiplying by three, for the legume cover as a whole rather than any one species.4
  • Pipai’s 2014 work, as reported by Pardon et al. (2016), measured above-ground biomass only and found just 0.3 to 34.2 kg N/ha, without counting root turnover; older Malaysian estimates run several times higher, which is why a figure means little without its method.8

Biomass production

In a 24-month trial under 10-year-old oil palm, the more often M. bracteata was cut, the more biomass was collected, while the standing biomass and leaf area of the cover fell.2 That is biomass removed under an experimental cutting regime, not standing biomass, and we do not quote a tonnage from it.

3. Ground cover and weed suppression

Dense Mucuna bracteata canopy suppressing weeds beneath oil palm at Kota Tinggi
Closed M. bracteata canopy. Weed suppression comes from shading and litter, and only after the cover closes.

In a trial at Universiti Putra Malaysia in Selangor, on mineral soil, Mucuna bracteata planted at 680 seedlings per hectare reached about 100% ground cover at 6 months.3

Against unweeded plots, 9 to 24 months after planting, the cover treatments together (the legumes and the grass Axonopus compressus) cut weed dry weight by 97.3 to 99.9% and weed density by 94.77 to 99.73%.3

Two honest qualifications

The establishment phase is the hard part. Until the cover closes over the ground, weeds compete with it. Budget for weeding in the establishment phase.

Soil type and planting density change the establishment curve. On deep peat, with 320 seedlings per hectare, an MPOB extension bulletin recorded only 31% cover at 6 months, 58% at 12 months, 75% at 18 months and 78% at 24 months, against about 100% at 6 months on mineral soil at 680 seedlings per hectare in the trial above.5 Both are Malaysian. Anyone quoting a single “covers ground in X months” figure is not telling you the whole story.

4. Choosing species: shade, persistence, mixtures

Species choice is mostly a question of what survives once the palm or rubber canopy closes. The field evidence here is unusually clear.

Species characteristics as reported in the literature. Nitrogen figures are given only where a source reports them for that species specifically.
Species Shade and persistence Reported role
Mucuna bracteata seed
Mucuna bracteata 		Vigorous in open conditions; a review of the Mucuna genus notes that shade affects its growth and nodulation10 Fast cover on mineral soil (about 100% cover at 6 months3); the standard choice for immature blocks
Pueraria javanica seed
Pueraria javanica
(P. phaseoloides)		 Described as very shade tolerant, but in Papua New Guinea it was found only in plantations under 5 years old4 Highest %Ndfa recorded of any species here (85 to 93% in Thai rubber6); one of the most used covers in rubber, oil palm and coconut across humid tropical Asia13
Calopogonium caeruleum seed
Calopogonium caeruleum 		The only species still present in plantations over 5 years old in the PNG survey4 Persistence under closed canopy, not tonnage. Still yields about 1 to 1.5 t DM/ha/yr under mature oil palm at about 10% light14. Slow to establish; typically sown as a minor component of a mixture
Calopogonium mucunoides seed
Calopogonium mucunoides 		Fast early cover, short-lived; declines as canopy closes Appears in the literature almost only as a mixture component. We have found no peer-reviewed figure for its fixation rate in isolation, so we do not quote one
Centrosema pubescens seed
Centrosema pubescens 		Moderately shade tolerant; persists longer than C. mucunoides Also a mixture component. Same caveat: no isolated fixation figure we are willing to publish

This is the practical case for a mixture rather than a monoculture. Fast species close the ground early, and a shade-tolerant species such as C. caeruleum is what is still there in year six.

5. Erosion and slope

An MPOB review synthesising Malaysian erosion studies from 1977 to 2014 gives a sense of the scale of the problem and of what dominates it.9

  • Oil palm on Oxisols: 13 to 78 t/ha/yr; on Ultisols: 1 to 28 t/ha/yr.9
  • On Munchong soil under legume cover, years 2 to 4 after planting, soil loss rose with slope: 8.8 t/ha/yr at 2°, 24.0 at 5°, 35.4 at 9°, 50.0 at 15°.9
  • Runoff was 30.8% with fronds stacked down the slope, 17.9% with contour frond stacking, and 10.7% with silt pits added.9
  • In one of the earlier studies the bulletin reviews, legume cover produced 5,370 kg/ha of dry matter against 1,930 kg/ha for natural weed cover, contributing 113 kg N, 11 kg P and 106 kg K per hectare.9

Do not read the slope figures as a cover-crop comparison

Those 8.8 to 50 t/ha/yr values are all from legume-covered plots at different slopes, drawn from different studies. They show how strongly slope drives erosion. They are not a controlled legume-versus-bare comparison, and presenting them as one would be misleading. On the evidence here, slope and frond management dominate; ground cover is one contributor among several.

6. Where the evidence is weakest

The claim that cover crops measurably improve soil properties is the weakest link in the whole case, and we would rather say so than pretend otherwise.

  • The 24-month trial under mature oil palm found that harvesting M. bracteata did not affect any of the soil physicochemical properties measured, but it compared how often the cover was harvested, so every plot had cover. It does not test cover against bare ground, so it cannot show whether a cover improves soil properties.2
  • A 2025 systematic review of cover crops, drawn largely from temperate studies, reported gains in soil organic carbon on average, with yield effects that varied between studies. Tropical and plantation data are scarce in that literature.11

7. Trade-offs to plan for

Drought competition on shallow soils

The Thai rubber trial that produced the strongest nitrogen-transfer result also produced the most important warning in this literature. At the upper positions of the slope, where soil water storage was low, the legume cover improved tree nutrition and growth but reduced the trees’ ability to survive intense drought.6 On shallow or low water-holding soils, a vigorous cover competes for water. Plan the cover to the soil, not to the block.

Establishment is slow and the seed needs treating

Germination trays used in seed testing at Chemiseed
Germination testing. Hard-seeded tropical legumes need scarification before the numbers mean anything in the field.

M. bracteata has a hard seed coat and pronounced dormancy. A laboratory germination test reported in 2024 recorded 88 ± 9.1% germination from shell nicking combined with a fungicide treatment, with mean germination time of 4.25 days.12 Untreated seed will disappoint you regardless of its quality.

Vigour cuts both ways

A 2024 review of Mucuna cover cropping notes that Mucuna requires “proper maintenance and pruning… due to its significant and aggressive vegetative growth” and that it can compete with young seedlings of the main crop.10 A cover that establishes well is a cover you will need to manage.

8. Humic substances: what the meta-analyses show

Humic inputs are the part of this field with the widest gap between marketing claims and published evidence. Two meta-analyses give the honest range.

Meta-analytic effect sizes for humic substances applied to crops.
Study Scope Effect reported
Rose et al. (2014)15 81 studies, over 700 observations Shoot dry weight +22 ± 4%; root dry weight +21 ± 6%
Ma, Cheng and Zhang (2024)16 120 articles, 479 observation pairs Yield +12% (95% CI 10 to 13%); nitrogen use efficiency +27%; nitrogen uptake +17%

The caveat that belongs next to every one of those numbers

In the Rose meta-analysis, roughly half of the shoot dry weight studies and a third of the root studies failed to improve growth by more than 5%, the level the authors take as agronomically significant. Their own summary is that responses “varied considerably” and are “relatively unpredictable when compared to inorganic fertilisers.”15 An average of roughly +20% sits on top of a large number of trials where nothing much happened.

Feedstock matters more than dose

The same meta-analysis broke response down by where the humic material came from: compost-derived 28 to 29%, soil-derived 25%, brown coal-derived 12%, and peat-derived just 4%, not statistically significant.15 Two products can both be honestly labelled “humic acid” and behave completely differently. Ask what the source material is.

More is not better

Every dose-response study we retrieved contradicts the idea that raising the rate raises the benefit.

  • In a 35-day pot study on three tropical Brazilian soils, Brachiaria biomass increased by about 30%, with total biomass rising up to about 25 mg/kg of humic acid carbon, while the highest concentration tested (60 mg/kg) inhibited nutrient accumulation in the shoots. Response depended on soil type, and in one of the three soils shoot dry matter was not affected at all.17
  • In a laboratory study on three Italian soils, low rates of coal-derived humic acid (0.05 to 0.10 g/kg) gave a 40 to 120% improvement in aggregate stability. Available water capacity rose by 10 to 30% at the highest rate (1.00 g/kg), but beyond 0.05 g/kg the differences in soil water properties were not significant.18
  • For brown coal-derived material, Rose et al. describe a non-linear response best fitted by two quadratic functions, with an initial peak around 20 mg/kg.15

Cation exchange capacity

In a study summarised in a 2022 review, all 26 humic acids tested (from peat and coal) increased the CEC of amended soil, by anywhere from 1% to 58%, with no linear relationship between a sample’s initial CEC and the increase it produced.19 That review’s central conclusion is inconsistency: effects depend on environmental and soil conditions in ways that make prediction difficult, and some studies report no effect at all.

Mechanism

The mechanistic literature is more settled than the field literature. A 2021 review by Nardi and colleagues names nutrient transporters, plasma membrane H+-ATPases, hormone pathways and the genes and enzymes of nitrogen assimilation as the main targets of humic substances in plants.22 Low-molecular-size humic fractions activate the plasma membrane H+-ATPase, driving secondary active transport, with nitrate uptake showing the most prominent stimulation; they also show auxin-like activity, stimulating root growth more than shoot growth.20 In maize, humic acid stimulated plasma membrane H+-ATPase activity and promoted lateral root emergence through a nitric-oxide-mediated pathway.21 This is controlled laboratory work. It explains why a response happens; it does not predict whether it will happen in your block.

One claim we will not repeat

You will find it widely stated online that humic acid holds 80 to 90% of its own weight in water. We traced that claim to a low-rigour book chapter where it appears without a citation. We have found no primary measurement supporting it, so it does not appear on our pages or in our documents.

9. What this means for SoilBoost EA

SoilBoost EA is a humic soil conditioner, manufactured exclusively for Chemiseed Sdn. Bhd. and Kudzu Seeds Trading. On the product’s Certificate of Analysis, humic acid is 96.55% by the testing laboratory’s proprietary method, or 60.6% by the CDFA method: the same sample measured two ways, so the figure only means something alongside the method used to produce it. Batches vary: the certificate for a second batch gave 92.59% by the laboratory method and 35.71% by the CDFA method. These are typical analyses, and a Certificate of Analysis for the batch supplied is available on request. Read against the evidence above, three things follow, and we would rather set the expectation correctly than win an order on an overclaim.

Neither Rose et al. (2014) nor Ma, Cheng and Zhang (2024) tested SoilBoost EA. Both tested humic acid products as a class, and Ma, Cheng and Zhang found the effect strongest at moderate soil pH (above 6 and up to 8).

  • It is a soil conditioner, not a fertiliser. It is not a substitute for a fertiliser programme and we do not position it as one. The meta-analytic evidence on nitrogen use efficiency16 points toward using it alongside nutrition, not instead of it.
  • Trial it before you scale it. Response is soil-dependent and dose is not monotonic. Run a limited area first, keep an untreated comparison strip, and judge it on your own ground.
  • Start from the label rate, not above it. The label rates are 50 to 100 kg/ha broadcast and 10 to 15 kg/ha as a drench or through fertigation. Overdosing is documented to reduce response in the studies above, so more is not better. Talk to us or to your own agronomist about the rate for your soil before a large-scale application.

Certificates of Analysis, Safety Data Sheets and technical data sheets are available on request for the specific lot supplied. Where a document exists for a lot, the figures on that document govern for that lot.

Ask us the awkward questions

If something on this page contradicts what you have been told by a supplier, including us, we would rather have that conversation than not. Tell us your crop, hectarage, soil type and slope, and what you are currently applying.

Ask on WhatsApp Contact form

Common questions

How much nitrogen does a legume cover crop actually fix in an oil palm plantation?

Published estimates differ widely and are often not the same kind of number: some are annual figures for a whole legume cover, others are the nitrogen standing in the plants at one sampling. The spread is driven by measurement method, whether root and litter turnover are counted, and how much ground the legume actually covers. Pipai’s 2014 work, as reported by Pardon et al. (2016), measured above-ground biomass alone and found 0.3 to 34.2 kg N/ha, with no time basis. Any single confident figure should be treated with suspicion.

How long does Mucuna bracteata take to cover the ground?

About 100% cover at six months on mineral soil in a Selangor field trial, at 680 seedlings per hectare. On deep peat, an MPOB bulletin with 320 seedlings per hectare recorded only 78% cover after 24 months. Soil type, planting density and maintenance change the answer substantially, so a single figure is not reliable.

Which cover crop species survives once the palm canopy closes?

Calopogonium caeruleum. In a survey of 13 Papua New Guinea plantations it was the only species still present in plantations over five years old, and Tropical Forages records it still yielding about 1 to 1.5 t of dry matter per hectare a year under mature oil palm. Pueraria phaseoloides was found only in younger plantations. This is the main argument for sowing a mixture rather than a single species.

Do legume cover crops improve soil properties?

The evidence is weaker than commonly claimed. Few plantation trials compare cover with no cover over enough seasons to settle it; a 24-month trial under mature oil palm compared harvest frequencies of Mucuna bracteata, so every plot had cover. A 2025 systematic review, drawn largely from temperate studies, reported gains in soil organic carbon on average, with yield effects that varied between studies. The better-supported benefits are nitrogen contribution, weed suppression after canopy closure, and surface protection.

Does humic acid work?

On average yes, but unreliably. Meta-analyses report roughly +12% crop yield and +19 to +22% biomass. However, in the largest meta-analysis about half of the shoot-growth studies improved growth by less than 5%, the level the authors take as agronomically significant. Source material matters greatly: compost-derived humic substances averaged 28 to 29% response while peat-derived averaged a non-significant 4%. Dose response is not linear, and higher rates can reduce the benefit.

Does applying nitrogen fertiliser affect how much a legume cover fixes?

Yes, negatively. Across 13 plantations, the proportion of nitrogen derived from fixation was strongly negatively correlated with soil nitrate, declining markedly as soil nitrate-N rose above 5 mg/kg. Where heavy mineral nitrogen use keeps soil nitrate high, the legume relies less on its own fixation.

References

  1. Cheah, S.S., Zaharah, A.R. & Aminuddin, H. (2010). Biological nitrogen fixation by Mucuna bracteata under oil palm field environments. Oil Palm Bulletin 60: 22-27. MPOB. Institutional research bulletin.
  2. Mohd Noor, M.A.Z., Sulaiman, M.F., Wan Abdul Karim Ghani, W.A. & Teh, C.B.S. (2021). Effects of harvesting Mucuna bracteata on the legume biomass and soil properties under mature oil palm. Pertanika Journal of Tropical Agricultural Science 44(1): 139-155. doi:10.47836/pjtas.44.1.08
  3. Samedani, B., Juraimi, A.S., Rafii, M.Y., Sheikh Awadz, S.A., Anwar, M.P. & Anuar, A.R. (2015). Effect of cover crops on weed suppression in oil palm plantation. International Journal of Agriculture & Biology 17(2): 251-260.
  4. Pipai, R., McNeill, A., Unkovich, M., Banabas, M. & Nelson, P.N. (2023). Biological nitrogen fixation by legume cover plants in oil palm plantations. Plant and Soil 491: 665-680. doi:10.1007/s11104-023-06147-8
  5. Hasnol Othman, Farawahida Mohamad Darus & Zulkifli Hashim (2012). Best management practices for oil palm cultivation on peat: Mucuna bracteata as ground cover crop. MPOB Information Series, TT No. 501. PDF. Extension bulletin, not peer-reviewed; peat soil only.
  6. Clermont-Dauphin, C., Suvannang, N., Pongwichian, P., Cheylan, V., Hammecker, C. & Harmand, J.-M. (2016). Dinitrogen fixation by the legume cover crop Pueraria phaseoloides and transfer of fixed N to Hevea brasiliensis: impact on tree growth and vulnerability to drought. Agriculture, Ecosystems and Environment 217: 79-88. doi:10.1016/j.agee.2015.11.002
  7. Vrignon-Brenas, S., Gay, F., Ricard, S., Snoeck, D., Perron, T., Mareschal, L., Laclau, J.-P., Gohet, É. & Malagoli, P. (2019). Nutrient management of immature rubber plantations. A review. Agronomy for Sustainable Development 39: 11. doi:10.1007/s13593-019-0554-6. The girth and mixture figures trace to Broughton (1977) and Watson et al. (1964).
  8. Pardon, L., Bessou, C., Nelson, P.N., Dubos, B., Ollivier, J., Marichal, R., Caliman, J.P. & Gabrielle, B. (2016). Key unknowns in nitrogen budget for oil palm plantations. A review. Agronomy for Sustainable Development 36: 20. doi:10.1007/s13593-016-0353-2
  9. Afandi, A.M., Zuraidah, Y., Nurzuhaili, H.A.Z.A., Zulkifli, H. & Yaqin, M. (2017). Managing soil deterioration and erosion under oil palm. Oil Palm Bulletin 75: 1-10. MPOB
  10. Dissanayaka, D.M.N.S., Udumann, S.S., Nuwarapaksha, T.D. & Atapattu, A.J. (2024). Harnessing the potential of Mucuna cover cropping. Circular Agricultural Systems 4: e003. doi:10.48130/cas-0024-0001. A review of the Mucuna genus, mostly M. pruriens; its figures are compiled from other studies.
  11. Salisu, M.A., Ampim, P.A.Y., Oyebamiji, Y.O., Kotochi, A.B.A. & Imoro, M.M. (2025). Cover crops enhance soil organic carbon and soil quality for sustainable crop yield: a systematic review. Agronomy 15(12): 2865. doi:10.3390/agronomy15122865. Predominantly temperate data.
  12. Hastuti, P.B. et al. (2024). Seed dormancy breaking and germination rate improvement in Mucuna bracteata seeds. BIO Web of Conferences 94: 06002. doi:10.1051/bioconf/20249406002. Laboratory germination test; conference paper.
  13. Heuzé, V., Tran, G., Hassoun, P., Bastianelli, D. & Lebas, F. (2017). Tropical kudzu (Pueraria phaseoloides). Feedipedia, INRAE/CIRAD/AFZ/FAO. feedipedia.org. Institutional reference work.
  14. Cook, B.G. et al. (2020). Calopogonium caeruleum. In: Tropical Forages: an interactive selection tool, 2nd revised edition. CIAT, Cali, and ILRI, Nairobi. tropicalforages.info. Institutional reference work.
  15. Rose, M.T., Patti, A.F., Little, K.R., Brown, A.L., Jackson, W.R. & Cavagnaro, T.R. (2014). A meta-analysis and review of plant-growth response to humic substances. Advances in Agronomy 124: 37-89. doi:10.1016/B978-0-12-800138-7.00002-4
  16. Ma, Y., Cheng, X. & Zhang, Y. (2024). The impact of humic acid fertilizers on crop yield and nitrogen use efficiency: a meta-analysis. Agronomy 14(12): 2763. doi:10.3390/agronomy14122763. It pooled published trials, tested humic acid products as a class and did not test SoilBoost EA.
  17. Valenciano, M.N., de Morais, E.G., Rosa, S.D. & Silva, C.A. (2024). Soil solution properties of tropical soils and Brachiaria growth as affected by humic acid concentration. Soil Systems 8(3): 86. doi:10.3390/soilsystems8030086. A 35-day pot study.
  18. Piccolo, A., Pietramellara, G. & Mbagwu, J.S.C. (1996). Effects of coal derived humic substances on water retention and structural stability of Mediterranean soils. Soil Use and Management 12(4): 209-213. Laboratory study on temperate soils; the authors state field validation is still needed.
  19. Ampong, K., Thilakaranthna, M.S. & Gorim, L.Y. (2022). Understanding the role of humic acids on crop performance and soil health. Frontiers in Agronomy 4: 848621. doi:10.3389/fagro.2022.848621. A review; the CEC figures it reports are from Giannouli et al. (2009).
  20. Nardi, S., Pizzeghello, D., Muscolo, A. & Vianello, A. (2002). Physiological effects of humic substances on higher plants. Soil Biology & Biochemistry 34: 1527-1536. doi:10.1016/S0038-0717(02)00174-8
  21. Zandonadi, D.B. et al. (2010). Nitric oxide mediates humic acids-induced root development and plasma membrane H+-ATPase activation. Planta 231(5): 1025-1036. doi:10.1007/s00425-010-1106-0
  22. Nardi, S., Schiavon, M. & Francioso, O. (2021). Chemical structure and biological activity of humic substances define their role as plant growth promoters. Molecules 26(8): 2256. doi:10.3390/molecules26082256. A review.

Note on sources. Where a source is an institutional bulletin, a conference paper, a review compiling other people’s figures, or a laboratory rather than field study, we have said so at the point of use. Figures we could not verify in the original are not published here. This page was last reviewed in October 2026.