In a previous post, I argued that green manures require big biomass to achieve benefits that outweigh the damage done by tilling the crop into the soil. This is the point-in-time need, but what about over time? I believe that some benefits of green manures are cumulative, they build on the changes that the previous green manure crop made. To get this cumulative effect, the green manures must be close enough in time so that the changes from one crop have not dissipated before the next green manure crop is used. So, the crucial factors may be both the biomass produced by a green manure crop AND how often you produce that biomass in a field.
Here is my hypothesis for why frequency, along with biomass, is a key factor in this system:
My example system is potatoes following mustard green manure crops, but the same idea may be effective in other systems. You can read a case study of a farmer in this system published in June 2017.
A mustard green manure, when incorporated, increases the soil’s ability to suppress a pest. Here the pest is the disease/nematode complex “potato early dying.” However, the suppression effect is not permanent; it increases to a certain level but after time decreases as the green manure effects on the soil fade.
The pest pressure in the field is also important, shown here as A or B. If the pest suppression ability, related directly to green manure biomass and perhaps incorporation methods, rises above the disease level, then we see suppression, but if not, no suppression even though we have changed the soil. So higher pest pressures may mask the changes of one green manure crop.
A and B could also represent the use of a potato cultivar that is more or less resistant to potato early dying. Line A would be a less resistant cultivar needing a higher level of suppression in the soil, while a more resistant cultivar, line B, would not need as much suppression.
In a 4-year rotation (Figure 1a), where green manures are only used before potatoes, the level of pest suppression may fade to pre-green manure levels after 2+ years without a green manures. In this case, the second green manure crop gives results similar to the first crop. There would be no cumulative effect.
Now consider what could happen if we come back with another green manure crop before the increased pest suppression level dropped back to pre-green manure levels (Figure 1b). The peak level of pest suppression would surpass that from the first green manure crop. If continued, the cumulative effect would continue to increase pest suppression levels in the soil.
A good hypothesis can explain observations. When I first had this idea about cumulative effects, I liked it because it would account for the results I have seen in the field. Those using their first green manure crop generally do not see much pest suppression, although they still improve their soils. Even those who persevered in using mustard green manures did not find the success we were measuring at the Gies farm (McGuire 2003). The difference? At the time of our first testing of the Gies fields, the soils had received the benefit of 3-4 green manure crops, each separated by just one year of potatoes. All the other growers were using at least a 3-yr rotation and sometimes longer. These counterintuitive benefits we see in a shorter crop rotation could be explained if the pest suppression ability of the soil is cumulative when green manures are grown more often.
There are some potential problems with doing this on farms. Pests that are not affected by the green manure may get worse in a short rotation; however, this was not evident on the Gies farm fields. Also, potato processors stipulate the length of the crop rotation in their contracts with potato farmers. They would have to be convinced of the benefits of shorter rotations with green manures before they will change this.
However, there are also some potential benefits. Growing potatoes more often should increase profits if yield and quality can be maintained. Farmers doing this could concentrate potato production on their best fields, leaving other less productive fields to less-valuable crops. Eventually, as pest suppression in the soil builds, fumigation could be eliminated which would also increase profits. In addition to profits, the soil under the shorter rotations should improve as we saw in the Gies fields.
So, what’s next? Testing this hypothesis through on-farm research is feasible with support from farmers and processors – the field setup and work required is not difficult. It would, however, take a long-term effort to see how many green manure crops it takes to increase pest suppression on fields with different soil types and crop history. Check back with me in 10 years.
McGuire, A.M. 2003. Mustard Green Manures Replace Fumigant and Improve Infiltration in Potato Cropping System. Crop management. doi: 10.1094/CM-2003-0822-01-RS.