Farm Science: Cool soil and plant research worth sharing

Some research to show that just compost, carbon or manures might not be building the most carbon they can in the soil and the role of S and P and others may be key to holding and keeping that carbon in the soil for generations to come! It is a wonder where all that carbon goes.

Inorganic Nutrients Increase Humification Efficiency and C-Sequestration in an Annually Cropped Soil
Clive A. Kirkby , Alan E. Richardson, Len J. Wade, Mark Conyers, John A. Kirkegaard
Published: May 4, 2016


Removing carbon dioxide (CO2) from the atmosphere and storing the carbon © in resistant soil organic matter (SOM) is a global priority to restore soil fertility and help mitigate climate change. Although it is widely assumed that retaining rather than removing or burning crop residues will increase SOM levels, many studies have failed to demonstrate this. We hypothesised that the microbial nature of resistant SOM provides a predictable nutrient stoichiometry (C:nitrogen, C:phosphorus and C:sulphur–C:N:P:S) to target using supplementary nutrients when incorporating C-rich crop residues into soil. An improvement in the humification efficiency of the soil microbiome as a whole, and thereby C-sequestration, was predicted. In a field study over 5 years, soil organic-C (SOC) stocks to 1.6 m soil depth were increased by 5.5 t C ha-1 where supplementary nutrients were applied with incorporated crop residues, but were reduced by 3.2 t C ha-1 without nutrient addition, with 2.9 t C ha-1 being lost from the 0–10 cm layer. A net difference of 8.7 t C ha-1 was thus achieved in a cropping soil over a 5 year period, despite the same level of C addition. Despite shallow incorporation (0.15 m), more than 50% of the SOC increase occurred below 0.3 m, and as predicted by the stoichiometry, increases in resistant SOC were accompanied by increases in soil NPS at all depths. Interestingly the C:N, C:P and C:S ratios decreased significantly with depth possibly as a consequence of differences in fungi to bacteria ratio. Our results demonstrate that irrespective of the C-input, it is essential to balance the nutrient stoichiometry of added C to better match that of resistant SOM to increase SOC sequestration. This has implications for global practices and policies aimed at increasing SOC sequestration and specifically highlight the need to consider the hidden cost and availability of associated nutrients in building soil-C.

Wow, I haven’t read such pretentious diction in a long time :slight_smile:. In English, I think the key findings are:

  • When amending your soil with compost or manure, you should augment with nitrogen (N), phosphorus § and sulphur (S) in order to have the right proportions.
  • The impact of getting it right is 5.5 metric tons of carbon sequestered per hectare (“5.5 t C ha-1”) in the top 1.6 metres in addition to what would be sequestered by not optimizing the C:N, C:P and C:S ratios.
  • The overall impact of assiduously screwing up your C:N, C:P and C:S ratios is reduced carbon sequestration of 3.2 metric tons of carbon sequestered per hectare in the top 1.6 metres. In other words, just applying the compost or manure they used for the research, they still managed to sequester 3.2 t/ha [of course our manure is not necessarily like their manure so this number can not be trusted as a guideline].
  • [news to me] Most of the carbon was sequestered more than 0.3m down.

To apply this finding, you will need a lab test your compost or manure so you can calculate the N, P and S to amend. In order to save the time and money of the lab test, you might make an educated guess based on the compost or manure they used in the research.

You will need to read the article to determine the target ratios of C:N, C:P and C:S.

I hope the community finds my “translation” helpful. Thanks for sharing, Harris!