What Farmers Can Learn from Their Soil in the Off-Season

After a growing season, when corn, soy, broccoli, and Brussels sprouts have been harvested and sold, fields appear to lie in rest for the next year’s planting. But underneath the surface—and sometimes on top—there’s more going on in the off-season than meets the eye.

With advanced precision agricultural data, it’s possible to get a picture of the vibrant activity below the surface. Using that information, farmers can understand the current state of their soil’s health, create strategies around fertilizer application in the off-season, and consider crops that flourish even in cooler weather.

The Debate Over Off-Season Fertilizer Application

After a harvest, fields still contain the remaining nitrogen fertilizer applied before and during the main growing season. Similarly, compost added to crops will still be releasing nitrogen after the crops are gone. Up on the surface, crop residue left on the field—husks, stems, leaves—becomes a source of organic matter full of important plant nutrients, as well as a buffer against the wind and weather that could cause erosion.

Because there is some nitrogen left in the soil, and because there’s so much variety in climate and soil type, there are a range of opinions about when and what to add to fields during the off-season. Temperature and pH, in particular, will factor into how much nitrogen currently in the field will turn into usable nitrate by the next season. Combining end-of-season readings from the in-ground soil probes with their own knowledge about their soil can help farmers decide about what they might use as amendments during the off-season.

Here are some of the options:

• Monitoring existing nitrogen. Based on the end-of-season soil sensor readings, farmers might decide that the nitrogen still in the field from chemical fertilizer or compost is all that’s needed. This might depend on the expected off-season temperatures, expected rainfall, and pH. Monitoring what is happening in the off-season can show farmers how much the concentration is dropping and then how much to add at the beginning of the following season. Having a sense of how the nitrogen at the end of the season will make it through the off-season is key, particularly whether it will cycle into usable forms or run off due to high rainfall or snowmelt, or poor soil structure.
• Applying synthetic fertilizers. Urea, the most widely used solid nitrogen fertilizer worldwide, dissolves in water, so it needs irrigation or rainfall to move into the soil. In dry climates, urea remaining on the surface would be lost into the air. But at temperatures below 60°F, ammonia gas is greatly reduced. Urea can be readily nitrified—that is, converted to nitrate (NO₃)—even when applied late in the fall, and it can be quite susceptible to denitrification or leaching the following spring. Anhydrous ammonia (AA) applied in the fall does not nitrify as quickly and has been found, in some studies, to offer the soil an advantage over urea. In many cases, nitrogen applied in the fall will leach out of the soil by the time spring arrives.
• Applying compost or organic fertilizers. Adding compost supplies the soil with many new nutrients. Farmers can watch the effects of organic amendments in two ways: first, by monitoring the fields to see the levels of nutrients the compost has contributed; and second, by observing CO₂ levels, evidence that the soil’s microbes are metabolizing the addition and making nutrients available to the next season’s plants. Early CO₂ readings can give a farmer a preview of the field’s future fertility.

Off-Season Crops

More and more farmers are realizing that the end of the main growing season doesn’t have to coincide with the end of crops for the year. For many, planting cereal and grass cover crops like rye and crimson clover has become an annual ritual for protecting the soil’s surface while encouraging nitrogen fixing below.

But others are experimenting with ways off-season crops can add to the profitability of a farming business while strengthening soil health. In California, for instance, recent seasons have seen farmers planting oil seeds like canola, giving the state’s vast agricultural regions a chance to see the soil benefits of cover crops and crop rotation while bringing a usable crop to market.

Stephen Kafka, an agronomist at the University of California, Davis, worked with several California farms to plant canola and camelina, oil seeds that do well in the cooler season in areas of Australia with similar climates, according to the Soil Science Society of America (SSSA). Kafka and his researchers planted dozens of varieties of the soil seeds and then watched how they performed in the off-season, from October to June. The seeds, which are processed into edible oils, biofuels, and animal feed, have a market in the U.S.

“They tend to be high-value, but water-demanding,” Kafka told SSSA. “Cool season crops have three advantages: the cooler temperatures allow plants to grow without losing as much water through transpiration as crops that grow during hot weather. There is also less evaporation of moisture from the soil. Lastly, the cool season is when California, in particular, gets most of its rainfall—so cool season crops benefit from this direct source of water.”

Likewise, pulses like peas or lentils can fix nitrogen over the off-season, helping to reduce farmers’ need to add nitrogen at the beginning of the next growing season.

Agricultural technology that monitors fields in the off-season grants farmers accurate information about what’s going on underground—information that previously would have come from informed estimations. Whether used as a more accurate reading on end-of-season soil health, as a way to be more precise about taking care of soil between planting, or as a push to experiment in off-season planting, in-ground soil sensor probes can help farmers make quick, smart decisions about how to foster soil health even in the off-season.

For annual cropping systems, we recommend removing the Teralytic’s wireless probe from the ground before harvest. For perennial cropping systems, it’s best to remove before the average soil temperature drops below freezing. For more information on using the Teralytic’s wireless in-ground soil probe, contact us.