Scavenger crops have been recognized for over a century as crops that could live off of soils that had become too depleted for standard crops to thrive on. The most basic scavenger crop was hay, grasses and clover that could grow in depleted soil and be harvested to feed animals. Clover was a particularly useful scavenger crop, as it was good eating for the livestock and also fixed nitrogen in the soil. By letting a field 'lie fallow' and producing a crop of hay, the soil was able to recharge and regain its former productivity.
But the modern scavenger crop is a quite different sort of beast. While in the past scavenger crops helped the soil recharge, nowadays many farmers have nearly the opposite problem; modern farms use so much fertilizer (either nitrogen fertilizer or manure) that nitrates leach into the soil, and may pollute groundwater and field runoff. Nitrates can cause serious health problems for humans, and are toxic to fish and other aquatic creatures as well. They can also lead to eutrophication, algae blooms, and anoxia. But scavenger plants can help! They are planted after a heavily fertilized crop has been harvested, and absorb the excess nitrates.
The primary defenses against nitrate leaching are careful moderation of fertilizer usage, avoiding fertilizing fields when water runoff is high, and avoiding over-irrigating the fields. But even with these careful measures, some nitrates escape the primary crop, and a scavenger crop may be used to 'catch' them.
Barley, rye and alfalfa are common scavenger crops. They have deep roots, allowing them to catch nitrates that have seeped too far into the ground for other plants to catch. Alfalfa is so nitrate-hungry that it is used to plant fields around pig lagoons to help dispose of the runoff. Wheat can also be used as a scavenger crop; while it does not have very impressive roots, it works very well as a winter crop. Any scavenger crop than can be planted immediately after harvest has added value to a farmer, as producing two crops a year it much more profitable than simply alternating crops.
While scavenger crops are generally food crops, any sort of cover crop will reduce nitrate leaching, even a good thick cover of weeds. Legumes such as soybeans are less effective than other types of plants, because they do not need to absorb as much nitrogen from the soil. Most types of grasses are more than twice as effective in stopping nitrate leaching than are soybeans. Soybeans do keep nitrogen in their leaves and root nodules rather than leaving them to seep into the groundwater, but the nitrogen will return to the ground after the plants die1.
Scavenger crops are particularly important for corn (maize) and many vegetable farmers, as these plants have comparatively shallow roots. In order to both keep their primary crop and scavenge nitrogen, many corn farmers are experimenting with types of relay cropping, in which the scavenger crops are planted between the rows of corn before the corn is harvested.
The more effective use of nitrate fertilizers not only saves the farmer money, but is good for the environment. Nitrogen fertilizer is made using fossil fuels, which we are running low on. Farming practices that use fertilizers more efficiently are very good things. Just as importantly, fertilizer runoff has become a very big problem. The Gulf of Mexico dead zone is a prime example of what happens when thousands of farms all contribute a little fertilizer to the environment. Agricultural runoff has the potential to do great damage in any ecosystem it appears in. Farming techniques like planting scavenger crops, relay cropping, and crop rotation can help reduce our abuse of fertilizers and keep the Earth healthy.
1. Confused? Soybeans do fix nitrogen in the soil, and planting crops like clover and soybeans can replace nitrogen lost from the soil. They have diazotrophs living on their roots that turn N2 into nitrogen compounds that are useful for plants. But if there is an excess of nitrogen in the soil these plants will rein in their nitrogen fixing, and use the nitrogen that is present in the soil. Soybeans can be a useful scavenger crop because not only will they adjust their nitrogen intake as needed, but also because they will leave a lot of their nitrogen in the soil after harvest. This is great for sustainable agriculture, but can be a problem for farmers who are trying to maintain correct nitrogen levels through careful administration of fertilizers.