As an irrigated cropping systems agronomist I work on ways to sustain agriculture. In doing this, I have come to realize that there are certain requirements that agriculture must meet to produce food and to keep producing food (yes, fiber too, and other non-food products, but mainly we are concerned with food production).
Here are my essentials for sustaining agricultural production:
- Protect the soil
- Maintain soil fertility
- Use water efficiently
- Protect the crop
I view these as a hierarchy: if #1 is not met, the lower requirements do not mean much, but once #1 has been met, we can move to #2, provided that does not threaten #1, and so on. Each component is required, but not sufficient; all of them are needed. Below, I elaborate on each requirement.
1. Protect the soil
We must minimize erosion, as that is the greatest threat to the soil. This is the top priority because erosion is not easily fixed. We minimize erosion by protecting the soil surface from wind and rain. Here are the main practices used:
- Maintain crop residues. This is the best way to keep the soil protected.
- Minimize tillage. Tillage reduces soil cover leaving it more prone to erosion.
- Avoid compaction. Fixing compaction requires deep tillage to fix, which increases the potential for erosion.
2. Maintain soil fertility
The export of food from farm fields is the overriding goal of agriculture. However, this export, especially with an increasing portion of our population concentrated in cities, is also an export of nutrients from fields. These nutrients must be replaced by an equal import of nutrients, otherwise agriculture becomes a mining operation.
The one exception to this export-import rule is nitrogen. By using legumes, we replace the exported nitrogen with nitrogen fixed from the air. All other nutrients (phosphorus, potassium, micronutrients) must be replaced through inputs. This is why low-input agriculture has never made much sense to me, for it either means low yields, or that you are mining your soil.
There are two sides to soil fertility maintenance. First, replacing the nutrients that are exported in food:
- Application of fertilizers. These can be organic, synthetic or a mix of both. Eventually, we will have to figure out how to recycle nutrients that are exporting in food to cities, especially for phosphorus, which we will not be able to mine forever as it is found in limited amounts in a few locations on earth. This means we will have to use biosolids, or figure out how to keep humanure separate from all the other drain waste.
Second, we must improve the efficiency of our nutrient use:
- Cover crops. These can scavenge nutrients and prevent them from leaching.
- Slow-release fertilizers. Nutrients become available over time reducing the risk of leaching from soils.
- Precision farming. Nutrients can be are applied where they are needed, at the time they are needed, and in the quantity needed.
- Crop rotation. Some sequences of crops are more efficient that others in nutrient use.
- Reduce erosion. Erosion is the loss of soil, but also of nutrients. Erosion can be reduced by the methods in #1 above.
3. Use water efficiently
Whether the water comes from clouds or from sprinklers or drip emitters, we must use it efficiently. Many people are stunned by the amount of water that crops use, especially when compared to use by human populations, but a large amount of this water, called transpiration, goes through plants and is directly related to production. What is not needed is evaporation, from the soil surface. Here are the practices that can be used to minimize evaporation:
- Keep the soil covered. This practice performs double duty in #1 and #3. It reduces evaporation, leaving more water in the soil for transpiration. It also improves water infiltration, reducing wasteful (and erosive) runoff.
- Minimize tillage. Each tillage pass opens up the soil to increased evaporation. No-till, strip-till and other reduced tillage practices can save a significant amount of water by both maintaining crop residues on the soil surface and avoiding evaporation due to soil disturbance.
- Improve irrigation practices. Irrigation scheduling and use of drip irrigation, where feasible, and sprinkler irrigation, can reduce water use compared to furrow irrigation systems.
4. Protect the crop
This is last, because the above essentials are required first to produce a crop worthy of protection, but once this is done, once we have a highly digestible, nutritious crop, the pests will come, either to take advantage of the fertile soil (weeds), or to feed on our crop directly (diseases, insects, and other pests).
For various reasons, I place weed control above that of diseases and insects. Here are the main practices used:
- Crop rotation. A diverse crop rotation is hard to beat for benefits to the entire system.
- Competitive crops. Anything that gets the crop growing quickly; variety choice, optimum planting dates and methods, soil fertility immediately around the seed, stale seedbed planting, etc.
- Cultivation (tillage to kill weeds). Before herbicides, this was required, and although there are no-till methods that do not use herbicides, they are difficult to manage in many situations. So, the advantages of cultivation must be weighed against the disadvantages it produces for #1 (protect the soil) and #3 (use water efficiently) above.
- Herbicides. These have great benefits, not only for controlling weeds, but for both #1 and #3 above. Again, the benefits should be weighed against possible water pollution, human health effects, and effects on non-target organisms.
The top three practices used for weeds, can also be used for managing diseases and insects, depending on the details of the specific pest. In addition, and within Integrated Pest Management, there are many practices that can be used to avoid and manage pests, including:
- Crop choice. Plant pest- and disease-resistant varieties.
- Field sanitation. This provides an example of how this hierarchy can guide decisions. Field sanitation often requires removal of crop residues, which threatens #1 above. Since this is lower in the essential hierarchy, it should generally be avoided.
- Pesticides (fungicides, soil fumigants, insecticides, etc.) I am not going to go into all the details and differences among these pesticides, only that there are large differences, especially in terms of the disadvantages of the use of these materials, such as risk of water pollution, human health risks, and non-target or off-site effects. Each situation should be evaluated according to this hierarchy, weighing the benefits and risks, and where they occur in this list of essentials. For some pesticides, the decision, either to use it or not, may be easy, but there are many that may fall in the middle. Here, economic, market, and society forces must play a part.
I think it is important to keep the ranking of these essentials in mind as we think about sustaining agriculture. It is easy to start arguing about a point in #4 in a system while deficiencies remain in #1 or #2. Remember, the higher ranked essentials must be met before we start on those lower. Another reason for the ranking is the large differences in the effects of these factors on crop yield when they are not met. For example, if the soil is eroded and you are trying to grow on subsoil, the production could easily be 1/10th of that of a non-eroded soil. Even fertilizers cannot easily boost yield on heavily eroded soils. Similarly, protecting an underfertilized crop from pests is much less effective than protecting one that has all the nutrients that it needs. The ranking also represents the temporal importance of each essential factor; soil loss has long-term effects, but mismanagement of pests affects only that year’s yield (weeds and certain soilborne pests are exceptions). Finally, it is much more difficult to reverse the order and prioritize these requirements upwards; for instance, with lots of inputs, you can maintain the fertility of an eroded soil, but the inputs are much less effective than they would be on a soil protected from erosion.
Another way to look at this is through yield gaps, the gap between current yields and potential yields. There are two strategies to pursue to close yield gaps; first by addressing yield limiting factors, and then by addressing yield reducing factors. In this hierarchy, #1-3 are taking care of yield limiting factors (by using yield-increasing practices), while #4 addresses yield reducing factors (by using yield-protecting practices).
Finally, coming from an agronomist, this applies best to annual crops, and applies only to field production, not the entire food system. That’s how I view the essentials of agriculture – how about you?