“Healthy soils are high performing, productive soils” – USDA leaflet
This is an interesting and very relevant quote for me. The main reason being, how do you define a “healthy soil”?
“Soil health is a combination of physical, chemical, and biological properties that impact the function and productivity of the soil” – a different USDA leaflet
Is this a good definition? It is certainly a bit wooly, but then it is an extremely complex system, and one that we do not yet fully understand. I would love to be able to quantify “soil health”, in order to show whether or not it is something to be strived for on a commercially run farm.
Perhaps the best marker for soil health is organic matter levels. Indeed, SOM is linked with all three factors from the above definition,
- Physical – higher SOM levels can mitigate or eliminate the effects of compaction, as well as improving water infiltration and holding capacity.
- Chemical – SOM binds plant nutrients in the soil, stopping them from leaching away
- Biology – SOM is a food source for microbes, which in turn create plant-available nutrients
If soil health = SOM, my life is a lot easier. It is fairly easy to measure, and when you can measure something, you can manage it. But is this too simplistic? Perhaps. Any viws on this would be much appreciated.
I started the day by visiting Jason Miller, an agronomist with the USDA (that’s the US Department of Agriculture by the way). As is probably obvious, I took home a wad of printouts. Here are a few quotes from them, with my thoughts added.
- “Research at Michigan State University indicates that a 1% increase in SOM offers a 12% increase in crop production potential” This is a massive number, but does tally roughly with our farm, where adjacent fields with differences in SOM levels of ~2% can vary in yield by 20%.
- “Using 1% SOM as a baseline level, the total long term value of a 1% increase could be estimated at $24/acre for the nutrient value and available water holding capacity” This is data from South Dakota, and so not necessarily applicable to other areas. But if you consider $24 is currently around 6bu of maize, then a 3% rise in SOM levels would be worth 18bu, equivalent to raising yields by perhaps 20%. How easily can we increase SOM?
- “A typical acre of soil 6 inches in depth weighs about 1000 tons. One percent organic matter equates to 10 tons of organic material…it takes at least 10 pounds of residue to decompose to 1 pound of organic material” If we want to increase our SOM levels by 3%, then it will take 300 tons per acre of residue, or in real money, 670t/ha (Don’t forget, these are short tons they talk about, not metric tonnes). That is A LOT, and something doesn’t add up. David Brandt claims to have increased SOM levels from 0.5% to 5% on his farm. He started farming in 1971, so in 40 years he must have added 25t/ha of plant residue to the soils every year. In actual fact he says he can make that change in about 7 years, which is 140t/ha/yr. Neither of these seem plausible to me if you consider that a wheat crop may produce 14t/ha of above ground material, double that when you include roots too.
- “Mycorrhizal fungi numbers reduce under wheat, canola [oilseed rape] and lupins. A low level of mycorrhizal colonisation in plants is also associated with high available phosphorus levels in the soil” We all know the importance if these fungi, but our conventional wisdom is that wheat does associate with it. I wonder who is correct?
- “Reducing or eliminating tillage…can save fuels costs. A 50% reduction in fuel costs at $4/gallon would come to a $10,000 annual saving on an average 1,200ac farm” That is $8/acre, or 2bu of maize/ac. This is equivalent to a 1-2% yield increase. A useful amount, but it does rely on no-till providing the same or better yields than conventional tillage.
- “[Dwayne Beck’s farming system means] more profitability and stability” This appears to be categorically true in the conditions of his farm. It is unclear if the same happens in Europe, where we have much stabler results anyway, and profits are often shown to be higher in tillage systems. There are many trials in the US that have been running for decades comparing tillage and farming systems, we seem to have no comparable ones in the UK.
- “The biggest key to aeration, infiltration, drainage, and resilience is the formation of excellent soil structure and macro-pores. These are cut or destroyed by even light tillage” This is an important point, as there will be times when some form of cultivation will result in a better and more profitable crop, in the short term at least. How quickly soil can bounce back from these events is important to know when making tactical decisions.
- Cover crops create a canopy that in turn yields a microclimate ideal for microbes that break down stubble. As a result, organic matter increases, and soils are warmer and drier at the surface” This can be a catch-22. We want residues, and we want cover crops. The solution is perhaps to make sure cover crops are high in carbon, so that they do not break down too fast. We struggle in a British climate to grow lignified covers, which are the only types that will leave significant surface residue.
- “Microbes with active living roots are what builds organic matter” I think this is correct. SOM is built primarily by roots, whereas the main benefit of surface residue is to protect living plants from moisture loss and weed competition in the shorter term.
After leaving Pierre (that’s a town, not a person), and heading north, I took a little detour to see another farmer, Robert Salverson. He is another maize, soybean and wheat farmer, but outside of the day job he started up a service company for the oil fields in the north west of South Dakota; obviously an enterprising sort of guy.
It is interesting to talk to these guys about my plans for trying some grain maize in the UK. They all think it is crazy to plant winter wheat immediately afterwards, because of the fusarium risk. I am hoping that our usual, robust, fungicide program will take care of that. The alternative is to grow a second spring break crop, which could be a good option in fields where black grass is a problem.
Robert showed me a field that was until last year virgin, never farmed, prairie. It is now planted with a crop of soybeans. Since the advent of roundup ready crops, this process is much simpler than it used to be, and the transformation to clean cropping land only takes a couple of years.
I asked him whether he felt bad about destroying prairie land; “Hell no! [or words to that effect]” was the response. I can’t help but feel it is a shame to lose this type of land to monoculture cropping, if for no other reason than the steady decline in biodiversity. However, people in glass houses should not throw stones, and the only reason that we do not do the same at home is that it has already been done in generations past!
The only real option if we are to continue producing economically viable crops is to attempt to be as benign to the environment as possible within our system. A good way to do this might be through the use of no-till, cover crops, and livestock integration. Someone should do a Nuffield Scholarship on it…