Not all soils are created equal, and that simple realization will go a long way in helping you determine yield goals and fertilizer needs.

“Whatever you’re growing, you need to take into consideration what your land is capable of producing,” says Charles Mitchell, Auburn University Extension agronomist.

“In Alabama, we have a lot of varying soil types with different mineral and chemical properties and yield potentials, so we need to take into consideration all these factors.”

For example, growers need to ask themselves if their land is capable of producing 200 bushels of corn per acre. Sometimes, says Mitchell, the answer will surprise you.

“I never thought you could produce 200-bushel corn on Black Belt Prairie soils in Alabama. But the Dee family in Pickens County has proven me wrong on that, on several occasions. Annie Dee has asked me what nutrients to put out to produce 300-bushel corn, and I can’t tell her.

Some soils are well-drained and others are not, and that’s an important consideration,” he says.

In Alabama, the most limiting factor in crop production is water, according to Mitchell. “It suddenly hit me a few years ago why we are so successful in growing cotton in Alabama, but we’re a total failure at growing dryland corn, and that’s because corn is a determinate plant.

“It’s genetically set to produce a crop in a specific number of days. If anything slows down the growth — if anything affects photosynthesis during the span of 120 days or so — you’ll lose yield, and yield potential will decrease.

“Whenever you see corn wilting, that means photosynthesis has shut down. When corn is wilting it is no longer producing sugars, and it is no longer growing. So if it’s wilting today, that’s one day out of 120 or a little less than 1 percent yield loss already. If it wilts for 10 days, that’s a little less than 10-percent yield loss, assuming everything else is perfect,” he says.

Cotton, on the other hand, is an indeterminate plant, so it’ll just keep growing, he adds.

“If cotton goes through a drought, it’ll start to set bolls whenever it starts to rain. You might lose some yield, but it can compensate for some of those days of drought. Peanuts are the same way, but soybeans are less so.”

These differences have been evident, says Mitchell, on the Old Rotation, a crop fertility experiment begun on the Auburn University campus in 1896.

“Before we installed irrigation, we were probably averaging 70 to 80 bushels of corn per acre, and doing everything we knew how to do.

“Now, with irrigation, we’re pushing 200 bushels per acre each year. With soybeans, we were down to 30 to 40 bushels per acre and now we’re up to 60 to 70 bushels. The difference with cotton has not been as dramatic as with corn or soybeans.

One thing we can do to increase our yield potential is to irrigate. Eliminating water as a yield-limiting factor is a first step, but it’s not always easy.

What is Liebig's Law of the Minimum?

You just can’t escape Liebig’s Law of the Minimum in crop production, says Mitchell. This concept was originally applied to plant or crop growth when it was found that increasing the amount of plentiful nutrients did not increase plant growth. Only by increasing the amount of the limiting nutrient was the growth of a plant or crop improved.

“Whatever is the most yield-limiting factor, in whatever field you’re managing, you have to take care of that first before you can reach your yield potential,” says Mitchell.

Rarely are nutrients the most limiting factor, although they can be, he adds. “Fifty years ago, it definitely was nutrients, and that’s when soil testing began. We can throw nutrients out there, and in some cases, there’s even too much nutrient.”

Researchers, including Mitchell, conducted a cotton survey in central Alabama about 10 years ago looking at various growers’ fields.

“We took soil samples and leaf samples from about 300 fields of cotton. Rarely were nutrients not in the range that we like to see. Soils tested high in most nutrients and there was hardly ever a problem with tissue analysis. Our farmers are doing a great job soil-testing and managing nutrients.”

The research, however, did find that 67 percent or two-thirds of the fields had a hard pan or a traffic pan, says Mitchell. “Fifty-percent of the fields had soil organic matter less than one-half of 1 percent. That’s not a soil, that’s dirt. By definition, soil must contain some organic matter. And that’s why they had hardpans — they didn’t have the structure there to maintain that in-row subsoil slit whenever it was cut.”

At that time, he says, about half the growers were planting a cover crop. “But they were killing the cover before it made any substantial growth in the spring, so they weren’t putting back any organic matter.

“But at least they were planting a cover crop. If we went back today, I think it would be a different situation. We’ve learned the value of organic matter to soil quality.”

As far as nutrients, there are about 16 to 18 elements that all plants need and three of them come from air or water, says Mitchell. “Water is critical, making up 80 percent of a plant’s green weight. Without water, you can forget everything else. That’s why irrigation is a key to high yields.”

If you believe in it, global warming actually is helping in the production of carbon dioxide or C02, says Mitchell.

“We know C02 is increasing, and that can only help plants. Some of the high yields we’re seeing today could be due to some of the increased C02 in the atmosphere. We’re taking care of the carbon, hydrogen and oxygen — the rest of them come from the soil or must be added as fertilizers.

“Plants need relatively large amounts of nitrogen, phosphorus, and potassium. These nutrients are referred to as primary nutrients and are the ones most frequently supplied to plants in fertilizers.”