Tips for Understanding Soil Fertility Recommendations
 

 
Soybean production in West Tennessee

An ongoing study at the UT AgResearch and Education Center at Milan is comparing the sufficiency/slow build soil fertility philosophy (UT recommendations) and the maintenance/building philosophy (a commercial lab’s recommendations) for soybeans (shown) and corn.  Photo courtesy UTIA.

 
Row crop and forage producers, as well as homeowners, should have their soil’s fertility tested to determine their crop or garden’s fertility needs. However, Lori Duncan, a row crop sustainability specialist with University of Tennessee Extension, explains that not all soil tests or fertilizer recommendations are the same.

“The University of Tennessee has a soil testing lab and makes fertility recommendations, but we are not the only organization in the state that does so. Other organizations do not necessarily use the same soil testing procedures or philosophies when making fertilizer recommendations, which can lead to confusion for our Tennessee producers and homeowners.” Duncan said.

The expert goes on to explain: “Fertility recommendations are based on one of three different philosophies: maintenance, building and sufficiency.” Duncan said. “Sometimes testing organizations use a combination of these philosophies to make a fertilizer recommendation.”

Under the maintenance philosophy, nutrients are applied at the rate of crop uptake regardless of soil test value. In the building approach, heavy rates of nutrients are applied with the intention to build soil test values to a high or very high soil test rating. Duncan says most commercial laboratories and agribusinesses in Tennessee use either the maintenance or building philosophies or a combination of the two.

The sufficiency concept is based upon filling the gap between the native soil supply of a nutrient and what it takes on average to maximize yield. The University of Tennessee soil fertility recommendations are based on the sufficiency concept in conjunction with a slow-building philosophy in relating soil test values to fertility recommendations.  “Research conducted by the nation’s land-grant universities has consistently shown that this is typically the most profitable approach to manage nutrient inputs in crop production systems,” said Duncan.

An ongoing study at the UT AgResearch and Education Center at Milan is comparing the sufficiency/slow build philosophy (UT recommendations) and the maintenance/building philosophy (a commercial lab’s recommendations) using variable rate applications of P2O5 and K2O and uniform nitrogen rates. In 2014, sufficiency/slow build yielded an average 204 bu/acre of corn at a fertilizer cost of $0.75/bu, while the maintenance/building philosophy yielded an average 209 bu/acre of corn at a fertilizer cost of $0.90/bu.

In 2015, the differences for soybean production were more dramatic. Sufficiency/slow build yielded an average 49 bu/acre of soybeans at a fertilizer cost of $0.51/bu, while the maintenance/building philosophy yielded 50 bu/acre soybeans at a fertilizer cost of $1.34/bu.

An ongoing study at the UT AgResearch and Education Center at Milan is
comparing 
results from different soil fertility philosophies.

Philosophy

2014 Corn

2015 Soybeans

Yield (bu/A)

Fertilizer Cost ($/A)

Fertilizer Cost ($/bu)

Yield (bu/A)

Fertilizer Cost ($/A)

Fertilizer Cost ($/bu)

Sufficiency/Slow Build

204

152.96

0.75

49

25.21

0.51

Maintenance/Build

209

187.36

0.90

50

66.91

1.34


 
“No one philosophy is right or wrong,” said Duncan. “When crop prices are high, a producer might tend to use a different philosophy than when crop prices are low and fertilizer prices are high. Whether the crop is grown on rented ground can also make a difference to a producer’s business model,” said Duncan. “Fertilizer application rates above the recommended rates have the potential to reduce profits as well as have negative impacts on the environment,” she said. “Producers need to make a decision based on what is most profitable for their operation and be aware of current environmental issues so they can adopt sustainable management practices and avoid potential regulations or legal consequences in the future.”

The UT Extension Soil, Plant and Pest Center provides basic soil analysis, which includes lime and fertilizer recommendations, for the modest fee of $7 per sample. Basic analysis results options are normally available within 1 – 2 days of sample receipt. Your county UT Extension office can provide soil boxes for shipping and can also assist with your sampling or production questions.

To learn more about collecting and submitting soil samples and the other services of the UT Soil, Plant and Pest Center, visit the center’s website ag.tennessee.edu/spp. You may also contact the center at 615-832-5850 or by email to the soilplantpestcenter@tennessee.edu

Through its mission of research, teaching and extension, the University of Tennessee Institute of Agriculture touches lives and provides Real. Life. Solutions. ag.tennessee.edu

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Contact:

Lori Duncan, UT Extension Row Crop Sustainability Specialist, 865-974-7111, laduncan@utk.edu

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