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Fertile soil isn’t enough; nutrients must still be available to plant

Farmers attending the Channel Seed Achievement Series workshop in Dodge City in early December got a lesson on soil fertility and productivity from Fred Vocasek, senior laboratory agronomist for Servi-Tech.

“If you are looking at your crop and it is nutrient-deficient, the answer may not be as simple as adding fertilizer,” he said. “The problem may not be the amount of nutrient in the soil, but the ability of the plant to access those nutrients.”

Understanding soil testing, he said, is both a science and an art. “The science is in the numbers,” he said. “The art is predicting how those numbers are going to translate into nutrient availability to a growing plant.”

Soil texture is important, he said, because clay soils hold more water and have more surface area on soil particles to hold nutrients.

“Remember — plants do not eat their food, they drink it,” he said. “There has to be water in the soil to dissolve the nutrients and make them available to the plant.”

Plants also uptake nutrients only through the hairs that grow at the tip of the root, he said, making the feeding zone fairly small. If you have soil conditions that restrict root growth, you have even less available space to provide the nutrients.

Key Points

• Agronomist gives lesson in soil tests and plant nutrition.

• Plants don’t eat their food, they drink it.

• Timing and conditions need to be right for the maximum benefit.

“Different nutrients have different levels of mobility,” he said. “Nitrate will move a long distance, but phosphate has to be close to the root.”

In the soil-test lab, he said, scientists attempt to emulate the feeding process for a season in a short period of time by evaluating the pH level of the soil, its texture, its nutrient content and the presence of trace minerals and micronutrients that may affect overall fertility.

He said scientists in western Kansas over the last several years have seen a gradual decline in the pH level of soils, and urged farmers to monitor closely for the possibility of needing to add lime to the soil.

“The problem with more acid soil is that aluminum becomes more active in acidic conditions, and active aluminum stops the growth of the root tip,” he said. “Even in fertile soil, the plant can’t thrive.”

Too much lime is also a problem, he said. Excess lime impairs the activity of herbicides and can also make the plant iron-deficient. Iron-deficient plants can’t form chlorophyll, he said.

Organic matter is also critical to plant nutrition, Vocasek told the group, because it supports the living soil, the microbes and earthworms.

Nitrogen fertilizer changes form very rapidly in the soil, he said, and microbes play a key role in that change. Ammonia nitrogen changes to ionized ammonium, he said. One type of bacteria then grabs a hydrogen atom to form nitrite; another grabs an oxygen atom and forms nitrate, which is the nutrient that can be absorbed by plants.

“This whole process, over time, gradually increases soil acidity,” he said. “To offset that, we recommend applying 2 to 3 pounds of lime for every pound of nitrogen fertilizer you use.”

He said timing of anhydrous application is also important. The soil must be warm — microbes are most active and most able to convert ammonia to nitrate at soil temperatures of 90 to 120 degrees F. And it needs to have moisture, but not be too wet.

“If you can ball it up in your fist and it holds together but doesn’t lose water, that’s about right,” he said. If conditions aren’t right, you will lose the ammonia before it is converted, he said.

“When you are applying, if you can smell the ammonia — you’re losing it,” he said.


NUTRITION LESSON: Farmers attending the Channel Seed Achievement Series workshop in early December gained greater understanding in how plants are nourished.

This article published in the January, 2012 edition of KANSAS FARMER.

All rights reserved. Copyright Farm Progress Cos. 2012.