“When guys have 200-bu. corn and 70-bu. beans they’re taking about 130 lb. of phosphorus and 152 lb. of potash/potassium out of the soil [in that two year rotation],” says Bob Perry, general manager, Perry Agricultural Laboratory, a soil testing lab in Bowling Green, Mo. “A lot of folks are just putting on 100-lb./100-lb.—all of a sudden they’re behind.”
Crops need phosphorus in full form early for root development and as the plant grows to promote healthy stalks, stems and flower production. Potassium plays a vital role in plant growth as well, and deficiency can result in stunted growth, defoliation and weakened response to weather changes such as drought.
“When we have poor fertility we’re much more susceptible to any deviation in weather,” Perry says. “Going through any stress is harder when you don’t have the fertility you need.”
Perry recommends soil to have at least 50 lb. of phosphate per acre (25 ppm) and 300 lb. of potassium per acre (150 ppm) each season. “Before farmers apply nutrients each crop season, they need to take soil tests and past yields into consideration,” explains Sally Flis, director of agronomy at The Fertilizer Institute. “If they haven’t tested in a while they should—it helps determine if there are certain fields or areas of fields they need to focus on more.”
Use soil tests and tissue tests as the road maps to good fertility. In addition, keep yield goals in mind as they affect nutrient levels needed for the crop. Take a look at the chart on the right to see how much nitrogen, phosphorus and potassium it takes on average to grow 1 bu. of corn.
“With higher yields crops remove more nutrients so you’ll want to monitor fields year over year more closely,” says David Hardy, North Carolina Department of Agriculture and Consumer Service soil scientist. Pay attention to soil type and monitor its potential for leaching, he adds.
In addition to keeping soil tests up-to-date, Hardy recommends farmers check on their crops throughout the season by pulling plant tissue samples. Tissue tests give a snapshot of how a crop is using nutrients and how the fertility program is working.
“I’ve gotten where I rely a whole lot on tissue samples—it gives me an idea of what’s going on,” says North Carolina corn, soybean and cotton farmer Curtis Furr. He plants wheat immediately following corn and says it’s important to double check the crop has what it needs because the corn often pulls much of the nutrients before wheat is planted.
Pulling tissue samples is easy, Furr says. Walk the field and pull 20 samples from 20 different plants scattered throughout. Send in the samples for analysis and wait for the results. “It’s not very expensive either—a $10 sample could save you thousands.” Simply relying on what’s been done in the past might not suffice this year. Each bushel pulled out of the field means nutrients are depleted from the soil, and if nutrients are not replenished you cap the crop’s yield potential based on what genetics and management could produce.
“The maximum corn yield is over 500 bu. per acre so we have the genetics to achieve that—the rest is how we manage,” Perry says. Wallets might be tight, but you could lose more money in yield loss if you don’t cater to the crop and soil’s nutrient requirements.