Crop rotation is a time-proven practice. Compared to monocropping—the continuous production of the same crop year after year—growing two, three, or even more crops can help with pest management. Research has also found that it can increase nutrient cycling, soil organic matter levels, and yields. These benefits are thought to stem from the diversity of the crops in a rotation. However, new research suggests that there is another factor: active crop time.
ACTIVE CROP TIME INCREASES YIELDS
Active crop time is the percent of time in a rotation when crops are actively growing. It includes time in cash crops and cover crops but does not include the time when a crop is in the field but not photosynthesizing, such as when a corn crop is drying down.
Garland et al. (2021) looked at active crop time in a large data set from 155 fields (conventional farming) across a large swath of Europe. They searched for links between active crop time, crop rotation diversity, soil diversity, soil function, and crop yields. They found only weak links between crop diversity, the soil factors, and crop yields. However, soil function, bacterial diversity, and crop yields were more strongly linked to active crop time. But why?
MECHANISMS
It could be the increased carbon flow from increased photosynthesis. C inputs drive processes in the soil. Therefore, short-term benefits of active crop time are most likely related to maintaining a continual flow of carbon through the soil (Just-In-Time Soil Health, Neal et al. 2020). Active crop time has the additional benefit of incorporating root exudates into the influential factors, something which is missed by focusing on crop diversity only.
Garland et al. notes that active crop time represents “a constant source of carbon and nutrients.” The authors suggest this carbon flow through the soil may be more important than increases in soil organic matter: “… carbon accumulation in cropping systems does not necessarily have a positive impact on yields,” (Wood et al. 2016).” Another hint that this may be the case: the research found links between soil function and increased microbial biomass, a result we’d expect if active crop time increases carbon flow to the soil.
Supporting this, McDaniel et al. (2014) found that adding a cover crop to a rotation increased both soil C and N levels even more than increasing crop diversity. The interaction of crop rotation benefits from active crop time and crop diversity would also explain why studies are inconsistent in their observed rotation effects on soil properties. If crop diversity is increased while holding active crop time and carbon inputs constant, then soil improvements are less likely. And the flow of C may be just as important as the accumulation of C in soil organic matter (Janzen 2015).
CROP DIVERSITY VS ACTIVE CROP TIME
All this makes me wonder: did past studies that attributed increased yields to crop rotation diversity actually observe the effect of increased active crop time? This could occur where adding crops to a rotation also increases active crop time. For example:
- Adding winter wheat to a rotation of summer cash crops.
- Adding cover crops to any rotation compared to rotations without cover crops.
However, adding crops does not always increase active crop time. If the added crops have the same growing season length as the crops already in a rotation, the active crop time does not change. And adding crops with shorter growing season would decrease active crop time: for instance, adding dry edible beans to continuous corn. These interactions between crop rotation diversity and active crop time could explain inconsistent results in studies looking only at increased crop rotation diversity. For example, a rotation of 2-3 crops with cover crops may produce better results than a 6-crop rotation without covers (See Figure 1).
Figure 1. Active crop time across four rotations. Rotations 1 and 2 have the same active crop time but increasing crop diversity. Rotation 3 adds an overwintering crop, which increases active crop time. Rotation 4 increases active crop time even more with cover crops.
In a quick survey, I found several studies that credited increased yields to crop diversity but could also be, at least partly, because of increases in active crop time (Hunt et al. 2019; Marini et al. 2020; Davis et al. 2012; Lehman et al. 2017).
TRADEOFFS
There are always tradeoffs:
- Markets may favor rotations with few or even just one crop.
- Added crops require additional management and equipment.
- Cover crops can be difficult to insert in some rotations (Blanco-Canqui 2021).
- In drier regions, water availability may limit increased active crop time (Nielsen et al. 2016). Nor will mixtures help here (Nielsen et al. 2015).
WHAT TO DO?
Garland et al. is one study. It will take time for additional research to verify their findings, determine where similar results can be expected, and sort out the important factors involved. In the meantime, increasing active crop time can have other benefits besides increasing yields. The most basic is keeping the soil covered to minimize erosion. This is in line with the regenerative agriculture principle, “maintain living roots in soil” and with “maximize photosynthesis and minimize tillage.” Increased active crop time may also produce more stable crop yields (Sanford et al. 2021).
Increasing your cropping intensity is the key to getting any of these benefits. Ask yourself if any of these options make sense in your system:
- Cash crops that use more of the growing season, like winter wheat.
- Double-cropping: two cash crops per year.
- Relay crops.
- Cover crops: increase active crop time without having to expand markets.
- Perennial crops.
- Planting green.
- Irrigation to allow crops to be grown in dry regions or seasons.
Source : wsu.edu