Feed Crops Day by Day

Sufficient nitrogen and phosphorus must always be available, even when daily uptake is low, which makes placement important. ( Lindsey Benne )

It’s possible to apply exactly the right rate of seed, fertilizer, water (if you irrigate) and pesticides, and plant the best hybrid or variety, on every soil management zone—literally every acre—you farm. “Growing a better crop boils down to doing a better job of managing the Big 3—sunlight, water and nutrients,” says Ken Ferrie, Farm Journal Field Agronomist. While varying inputs isn’t exactly easy, understanding your crops’ daily needs and these basics about nutrients and water makes it manageable. (We’ll cover sunlight in the future.) 

Time and Place Phosphorus and Nitrogen to Maximize Yield Potential

Computer models available on the internet can predict nutrient needs at various points in the growing season. Use timing and placement (two of the 4Rs—the others are the right product and the right rate) to meet those requirements.

Because phosphorus (P) is a key component in cell elongation and division, or growth, availability is crucial during plants’ vegetative growth stages. “But the availability of phosphorus early in the season is related more to soil temperature and biological conditions than to soil test values,” Ferrie says. “That’s why we get a strong response to starter fertilizer during cold springs, even in soils that test high in phosphorus.”

Very little P is available to plants until soil temperature reaches 65°F or higher and stays there. At that point microbes become active and release P from the soil. “At that stage the amount of ortho-phosphate, the form plants can take up, is directly related to soil test values,” Ferrie says.

After the soil has warmed, you can use soil test values to calculate your total application rate. “If the soil test value is high, you don’t need to broadcast any phosphorus,” Ferrie says. “But remember, plants will still need phosphorus in starter fertilizer as long as soil temperature stays below 65°F.”

Using variable-input technology (VIT) to apply the right amount of P in each management zone keeps some areas from getting too high and others from getting too low, which is good for the crop and the environment. “In zones where the soil test shows you need to apply phosphorus, count the amount applied in starter as part of the total,” Ferrie notes.

VIT can help you master one of the most challenging, and vital, aspects of crop production—nitrogen (N). “With nitrogen, you must consider immobilization, mineralization, losses and the timing of plant uptake,” Ferrie says.

“While plants need only a small amount of N early in the season, letting them run short, even for a day, might reduce yield potential,” he adds. “Use timing and placement, such as banding nitrogen near the row as you plant, to meet their needs early in the season. The rapid-growth stage requires volume and timely application. After pollination, plants need only enough to get to the finish line. Avoid overapplying because that might leave a surplus in the soil, which can be leached into water supplies.”

VIT uses design protocols to meet crop needs in every management zone. “Pull soil and tissue tests to verify your program and record your results,” Ferrie says. “Computer modeling can help, but there’s no substitute for boots on the ground.”

Base Yield Goals On Your Crop’s Water Supply

Whether dryland or irrigated, corn’s water usage is determined by the evapotranspiration rate. “You can’t pump water through a plant,” Ferrie says. “It must be pulled through by evaporation off the surface of the leaves. Plants can stand in water all day, but if there’s a low evapotranspiration rate, they won’t pull it up.”

Water makes yield. It transports nutrients to the leaves, where photosynthesis turns them into food; then the movement of water distributes that food throughout the plant.

Various computer models, available on the internet, predict the amount of water required to produce a crop. The model used by Kansas State University agronomists shows corn needs to transpire 25" of water per acre, and soybeans must transpire 24", during the growing season. “Remember,” Ferrie says, “this is not the amount of rainfall but the amount of water transpired through the plants.”

To dial in your yield goal for a management zone, let’s work through this example.

In the Midwest, the early tassel to mid-milk period usually spans from June 25 to Aug. 20. During that time, corn requires 13" or 14" of water. Look at the 30-year weather history for your area to see what you can expect for rainfall. In the Midwest, it’s usually 8" to 9".

“If all that rainfall infiltrates into the soil (and some of it probably will run off), you will be 4" to 5" short of what the crop needs,” Ferrie says. “The rest of the water must come from the soil or irrigation. Some Midwestern soils hold 2½" of useable water per foot to a 3' depth. That’s 7½" of water, so there’s no problem growing corn in that soil.”

But what if your soil holds only ½" of usable water per foot to a depth of 2', or 1" of water total? “Your corn crop will be 4" short of water during a critical time,” Ferrie says. “If you’re 4" short, and there are 27,154 gal. in every inch of water, that means you’re 108,616 gal. short. At 3,000 gal. per bushel, that’s 35 bu. worth of water.

“We can’t change the water-holding capacity of soil (at least not quickly), or the amount of rainfall, so we must reduce our yield goal in that zone by 35 bu. per acre—from 250 bu. to 215 bu. per acre, for example. That means we can lower our population by 6,000 plants per acre—for example, from 36,000 to 30,000.”

Take your own observations into account as you make adjustments from zone to zone. “Tracking yield in dry and wet years will go a long way in helping identify a soil’s water-holding capacity,” Ferrie says. “You also can compare yield and topographic maps.”

To zero in on population, plant strips of various populations and see how the crop responds. “On hilltops, decrease the population,” Ferrie suggests. “If you see double ears, increase the population. If the plants are short and seem stressed, cut back the population further.”

Water Requirements to Produce a Crop

Tools are available to vary the rate of every input for every soil type in a field. Ready yourself for variable-input technology by following along with this seven-part series at bit.ly/VIT-series