We are analyzing the soil organic matter (SOM) in cultivated soils in the Provence area under Mediterranean climate (500-800 mm/year) with my students in agroecology. Soils in the area are usually shallow soils Xerepts soils (USDA classifc.) sometimes with calcic / argillic horizons (Calcic/Alfs ).
I frequently find diagnosis by soil scientists or agronomists after soil sampling analysis of these soils saying 1%; 1,5% or 2,5% of organic matter is too low and it should be increased. However, in soils with a high turnover and low microbial activity during long and dry summer seasons (>4 months), it seems unclear if the soil can potentially increase their total SOM a lot more than their actual content. Frequently, these conclusions come along with recommendations to farmers, promoting the application of huge amounts of organic matter applications to soils, in order to reach a target threshold of minimum SOM percentage, ranging from 1 to 4 % of SOM depending of the cases.
Therefore, in your opinion, do you know references of acceptable thresholds of SOM% under these conditions? Or do you have any criteria to determine it?
Is 1% of SOM a good universal minimum threshold that any soil should reach to prevent desertification and sustain crop production ?
Recommendations by agronomist may vary depending on this threshold and may create confusion in the farming sector. In France, some agronomists tend to consider 2,5% of SOM threshold to sustain crop production. This recommendation is mainly done in continental and atlantic contexts and sometimes in the Mediterranean, however, other Mediterranean regions use lower thresholds, such as 1 or 1,5% of SOM.
Maybe see thse two recent papers:
Article Global meta-analysis of the relationship between soil organi...
Chapter The Role of Soil Organic Matter for Maintaining Crop Yields:...
Because of the low organic matter your water and aeration might be compromised. If you have access to leaves straw and manure the preparation of compost could targeted in areas you planting. If a tree in the planting hole if row planting along side the row . If you can get soil tests this would be very helpful. Your soil may be compromised by alkalinity and salts and sulfur might be useful to get your soil activated along with compost.
With mean annual rainfall from 500-800 mm, if soils contain 1% OC (0-30 cm) indicates a good storage for sustainable crop production.
I am not soil scientist or agronomist, and cannot give an OM number. Our latter work to restore severely eroded, gullied lands suggested to us that considering the possibility of reestablishing and maintaining a component of native plants can be helpful. I do not know the OM number of saprolitic, C horizon soils, but expect it to be low. The difficulty for us was that most of the native grasses were gone from severely eroded landscapes. Some areas were galled lands with essentially no cover. Frequent to regular cropping was the primary cause of converting once suitable forest and crop lands to non-productive, and one can imagine how quickly cropping puts marginal or damaged soils under stress.
We read in literature that native plants were often adjusted to drought periods that affect many crops. On one of our gullies, we used switchgrass as a primary component of mix, and we were surprised in the organic accumulation in one year. Reestablishing forest and grassland was our primary goal however, and tried many things over many decades. Before looking into native plants, we often selected mix of plants that worked, and found later that some like Serecea lespedeza were great at fixing nitrogen needed, but their persistence as a non-native plant eventually was assessed by botanists as invasive.
Many of our native plants were limited in extent, and recovery challenged because they are slow to germinate and require cover of annual grasses to assist. We had to collect seed to help establish native seed production areas. This was a major undertaking, often best left to commercial nursuries.
Although we normally did not crop soils by cultivating, on some of our wildlife food plots or fields, we saw benefits in organic protection on areas lightly disked, or strip disked as compared to heavily disked. Perhaps similar to low till farming, leaving organic residue and some roots undesturbed. In gully recovery, we sometimes ripped deeply (18-24 inches) soils compacted through reshaping the surface or affected by pan before establishing grass cover, leading to reforestation. We had read estimates that it may take a century to reestablish an inch of topsoil, so it suggests recovery of soils is not necessarily a rapid process.
In in some of my western travels and upbringing, I learned of our vast native prairies in dry SW USA, and how they were abused and now sagebrush or other poor to sparse cover types. Whether forestry or agronomy, part of the expertise for sustainable land management includes identifying capabilities, limits and as well mitigation or opportunities on damaged lands. understanding past management history, including native grass and forest covers can be helpful in this process, as well as adjusting practices and perhaps crop type to help maintain or build organic content.
From OM review of history of some our damaged gullied lands, it is apparent that many early settlers did not recognize the damage of continued cropping, erosion, attention to control concentrated flow, and maintaining agricultural terraces in functional condition where needed. Certainly with the variety of commercial products and under intense or frequent management scenarios, one can accomplish results beyond normal land capability or aid in recovery of lands to an improved state. But to many, we have tendency to play the hand dealt, and consider investments in conservation and land recovery as undesirable to economic sustainability of ownership and management.
If the magic OM number is 1 or 2 percent, the first question is getting there, perhaps by adding OM , but then how can it be sustained? How much effort is allowed, to still consider it sustainable management? The meaning of sustainability may not be the same for everyone.
Soil Organic Carbon (SOC) is a major indicator of soil fertility in the tropics.
Generally, Walkley and Black, 1934 method is used for the estimation of SOC.
Most of the Indian Soils are deficient in Organic Carbon.
The critical range of OC for Indian Soils is 0.4 - 0.7 %
So after testing, the status of OC in the soils here is categorized as
Low < 0.4 %
Optimum 0.4 - 0.7 %
High > 0.7 %
So, soils with more that 1% OC sounds promising, but the other local edaphic features needs to be considered there.
Maybe see thse two recent papers:
Article Global meta-analysis of the relationship between soil organi...
Chapter The Role of Soil Organic Matter for Maintaining Crop Yields:...
Dear Hugo Fernandez-Mena
take into account that a classification of the soil organic matter is hard irrespective of other ancillary information. Mediterranean areas have usually low SOC (
Just a consideration to avoid a mistake with the thresholds. Please, do not mix in the discussion the concepts of soil organic carbon (SOC) with the soil organic matter (SOM) because a 1% of SOM could be approx. a 0.5% of SOC.
Dear colleagues,
Another point to the nice one highlighted by Eduardo would be soil depth. We should focus on a standard layer (30 cm depth? 40 cm depth?) given the usual stratification of soil organic carbon.
One to 2% soil organic matter is an extremely low level which would not optimize crop production. The level 2% soil organic carbon and 2.5% is approaching levels which could optimize the crop production environment. Under prevailing environment the prevalent erosion rates and cultivation practices would probably result in very low soil organic matter levels. Cultivated soils in cradle of our civilized areas can be among the most depleted based on eons of depletive practices which have resulted in erosion and loss organic matter. Uncontrolled overgrazing and lack of comprehensive soil conservation plans are among the many issues.
Thank you all for your contributions.
SOC may be a good proxy to look for information in the scientific literature. Still, farmers in our conditions tend to refer to SOM (in % of organic matter) when they work with soil analysis and agronomist extension. The translation of one indicator to other remains useful (SOM=SOC*1,72 as generic).
Concerning C storage, I'm not sure if it could be linked since I'm referring to the C in the first arable horizon, therefore not the whole soil itself. As Daniel Plaza-Bonilla mentioned, 30-40 cm.
There are still, some open questions about this topic.
Hugo Fernandez Mena is right in the confusion of having soil organic matter and soil carbon as two nearly synonymous terms which are different in quantity.
I generally use soil organic matter as it more commonly used in the United States while it appears that in scientific community and Europe soil organic carbon is more widely used.
In terms of soil organic matter we get a decreasing return on investment by raising from 0 to 5% soil organic matter and after that point results are plateaued.
If you take soil organic matter and divide by 1.72 it will convert to soil organic carbon in reverse taking soil organic carbon and multiplying by 1.72 will convert to soil organic matter.
The biggest results for soil organic matter accrual is at the lowest levels and the laws of diminishing returns are showin in soil orgranic matter plant response curves.
- Badges
- Science topic
- Similar topics
- Agricultural Science
- Soil Science
- Soil Analysis