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 range from 0.3 to over 150 kg N per hectare per year — a spread of roughly 500-fold.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, concluded 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 67–84% (mean 79%) Oil palm, Malaysia; 15N isotopic dilution, 60-day microplot 1
Pueraria phaseoloides 85–93%, varying by year Young rubber, NE Thailand; 4-year field trial 6
Calopogonium caeruleum 49% mean (range 25–75%) Oil palm, Papua New Guinea; ureide technique, 13 plantations 4
Pueraria phaseoloides 32% mean (range 14–27%) Oil palm, Papua New Guinea; ureide technique 4

The two Pueraria figures look contradictory until you look at ground cover. The Papua New Guinea plantations carried only 17–44% legume ground cover, against close to 100% in the Malaysian studies.4 A sparse cover fixes less in total, not less per plant.

The finding most growers have not heard

Across 13 plantations, %Ndfa was strongly and negatively correlated with soil nitrate (R² = 0.55), and fell significantly wherever soil nitrate-N exceeded 5 mg/kg.4 In plain terms: the more mineral nitrogen you apply, the less nitrogen your legume bothers to fix. Heavy fertilisation and a legume cover work against each other.

Nitrogen accumulated in biomass

  • 250 kg N/ha/yr accumulated by P. phaseoloides over 4 years in young rubber in northeast Thailand, from 8 Mg/ha/yr of biomass. 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 — the best-supported mixture figure available, though it traces to trials from 1977.7
  • Legume intercropping increased rubber tree girth by 11–29% 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: 48–102 kg N/ha/yr, the authors’ own extrapolation from a four-month production window.4
  • Historical Malaysian estimates averaged 150 kg N/ha/yr over the first five years; a 2014 study measuring above-ground biomass only found just 0.3–34.2 kg N/ha, without counting root turnover.8

Biomass production

In the most rigorous recent Malaysian trial — 24 months under 10-year-old oil palm at Rembau, Negeri Sembilan — harvested M. bracteata biomass ranged from 7.7 to 24.5 t/ha/yr depending on cutting frequency, with tissue nitrogen of 2.3–3.2%.2 That is biomass removed under an experimental cutting regime, not standing biomass, and an El Niño drought ran through part of the trial, so it is likely conservative.

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.

A 24-month trial at Universiti Putra Malaysia in Selangor recorded time to complete ground cover on mineral soil:3

  • Axonopus compressus (a grass, for comparison) — 3 months
  • Mucuna bracteata — 6 months
  • Legume mixtures (P. javanica + C. pubescens 4:1, or C. caeruleum + C. pubescens 1:1) — 9 months

At the 24-month mark, weed dry weight in covered plots was 97.3–99.9% lower than unweeded control, and weed density 94.8–99.7% lower. The unweeded control carried 734.7 g/m² of weed biomass against 1.8–5.9 g/m² under cover.3

Two honest qualifications

The first year is worse, not better. In the same trial, weed densities at 9 to 12 months were higher in the legume plots than in the comparison, before the canopy closed. The 97–99% figures are a 24-month endpoint, not what you should expect in year one.3

Soil type changes the establishment curve entirely. On peat, an MPOB extension bulletin recorded only 31% cover at 6 months, 58% at 12 months and 78% at 24 months — against 100% at 6 months on mineral soil 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; nodulation and growth are reduced by shading10 Fast cover on mineral soil (6 months to full cover3); deep taproot reported at 7–10 m10; 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 (85–93% in Thai rubber6); one of the most used covers in rubber, oil palm and coconut across humid tropical Asia14
Calopogonium caeruleum seed
Calopogonium caeruleum 		The only species still present in plantations over 5 years old in the PNG survey4; produces under 0–25% sunlight15 Persistence under closed canopy, not tonnage. Yields about 0.6 t DM/ha/yr under heavy shade15. 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

  • Natural forest baseline: under 1 t/ha/yr of soil loss.16
  • Oil palm on Oxisols: 13–78 t/ha/yr; on Ultisols: 1–28 t/ha/yr.9
  • On Munchong soil under legume cover, years 2–4 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
  • Contour frond stacking reduced runoff from 30.8% to 17.9%, and to 10.7% with silt pits added; contour stacking alone brought erosion below 5 t/ha/yr.9
  • 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 Rembau trial — the most rigorous Malaysian study in this set — found that harvesting regime did not significantly affect any soil physicochemical property measured: not pH, organic carbon, nitrogen, phosphorus, potassium, calcium, magnesium, CEC, bulk density, aggregate stability, porosity or soil water. The authors attribute the stability partly to the conventional fertiliser programme running throughout.2
  • A 2025 North Sumatra study comparing M. bracteata against sweet potato found no significant differences in microbial population, organic carbon, soil nitrogen or soil phosphorus. It also had no bare-soil control, so it says nothing about cover versus no cover.11
  • A 2025 systematic review of cover crops across all systems found 34% of studies reported no significant yield effect and 11% reported decreases, against 55% reporting increases — and noted that tropical and plantation data are scarce in that literature.12

Against this, a 2024 review reports substantially higher soil organic matter under M. bracteata — 2.58% against 1.98% on flat terrain, 2.22% against 1.44% on slopes — but those figures are compiled from other papers rather than measured by its authors.10 We flag that rather than quote it as settled.

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 2024 review reports untreated germination of 53% against 93% after scarification.10 A field study in Riau, Indonesia recorded 88 ± 9.1% germination from shell nicking combined with a fungicide treatment, with mean germination time of 4.25 days.13 Untreated seed will disappoint you regardless of its quality.

Vigour cuts both ways

The same review notes that Mucuna requires “proper maintenance and pruning… due to its significant and aggressive vegetative growth”, that it can compete with young seedlings of the main crop, and that it is susceptible to Cercospora, Rhizoctonia and Phytophthora among others.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)17 81 studies, over 700 observations Shoot dry weight +22 ± 4%; root dry weight +21 ± 6%
Ma et al. (2024)18 120 articles, 479 field observation pairs Yield +12% (95% CI 10–13%); 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% — which the authors describe as agronomically insignificant. Their own summary is that responses “varied considerably” and are “relatively unpredictable when compared to inorganic fertilizers.”17 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 28–29%, soil-derived 25%, brown coal-derived 12%, and peat-derived just 4%, not statistically significant.17 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.

  • On three tropical Brazilian soils, shoot biomass increased about 30% peaking around 25 mg/kg, but at 60 mg/kg biomass and nutrient accumulation decreased across all parameters, suggesting phytotoxicity. Response was strongly soil-type dependent, and one of the three soils showed no significant shoot response at all.19
  • In a laboratory study on Mediterranean soils, aggregate stability rose 40–120% at optimum rates of 0.05–0.10 g/kg, and available water capacity rose 10–30% — but stability declined above 0.50 g/kg, leading the authors to describe humic materials as both aggregating and disaggregating agents.20
  • Rose et al. found a non-linear dual-quadratic response with an initial optimum around 20 mg/kg.17

Cation exchange capacity

A soil incubation study using 26 different humic acids from peat and coal increased amended soil CEC by anywhere from 1% to 58% — with no linear relationship between a sample’s initial CEC and the increase it produced.21 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. 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.22 In maize, humic acid stimulated H+-ATPase activity threefold and promoted lateral root emergence through a nitric-oxide-mediated pathway.23 Both are controlled laboratory work. They explain why a response happens; they do 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 80–90% 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. Humic acid is typically 96.55% by the supplying laboratory's proprietary humic acid 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. Analysis is run per batch, on request, and batches genuinely vary: a second batch assayed 92.59% by the laboratory method and 35.71% by the CDFA method. Typical analysis. 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 diminishes outside roughly pH 6 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 efficiency18 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.
  • Rates are a conversation, not a label. The published optima cluster far lower than intuition suggests, and overdosing is documented to reduce response. Talk to us or to your own agronomist about 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 range from 0.3 to over 150 kg N per hectare per year. The spread is driven by measurement method, whether root and litter turnover are counted, and how much ground the legume actually covers. Historical Malaysian estimates average around 150 kg N/ha/yr over the first five years; a study measuring above-ground biomass alone found 0.3 to 34.2 kg N/ha. Any single confident figure should be treated with suspicion.

How long does Mucuna bracteata take to cover the ground?

About six months to full cover on mineral soil in a Selangor field trial. On peat, an MPOB bulletin recorded only 78% cover after 24 months. Soil type 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 it is documented as producing under 0 to 25% sunlight. 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. The most rigorous 24-month Malaysian trial in this review found no significant effect on any soil physicochemical property measured. A 2025 systematic review found 34% of cover crop studies reported no significant yield effect. 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, about half of the trials in the largest meta-analysis produced improvements of less than 5%, which the authors call agronomically insignificant. 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 overdosing reduces or reverses 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, falling significantly where soil nitrate-N exceeded 5 mg/kg. Heavy mineral nitrogen application suppresses the legume’s 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).
  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. Agriculture, Ecosystems and Environment 217: 79–88. CIRAD Agritrop
  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 — 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; its figures are compiled from other studies.
  11. Sakiah et al. (2025). Soil health under cover crops Mucuna bracteata and sweet potato in oil palm intercropping systems. Malaysian Journal of Soil Science 29: 393–402.
  12. 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: 2865. doi:10.3390/agronomy15122865 — predominantly temperate data.
  13. 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
  14. 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.
  15. Heuzé, V., Tran, G., Hassoun, P. & Lebas, F. (2017). Caeruleum calopo (Calopogonium caeruleum). Feedipedia, INRAE/CIRAD/AFZ/FAO. feedipedia.org
  16. Mohsen, B., Christopher, T.B.S., Husni, M.H.A. & Zaharah, A.R. (2014). Soil, nutrients and water conservation practices in oil palm plantations on sloping and steep lands in Malaysia. International Agriculture Congress 2014, Putrajaya. — conference paper.
  17. 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
  18. 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 — corpus heavily weighted to China and to lower-rainfall systems than Malaysia; it tested humic acid products as a class and did not test SoilBoost EA.
  19. Silva, C.A., Valenciano, M.N., de Morais, E.G. & Rosa, S.D. (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 — 35-day pot study.
  20. 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: 209–213. — laboratory study on temperate soils; the authors state field validation is still needed.
  21. 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
  22. 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
  23. 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

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 on 30 August 2026.