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Manage P&K in no-till cotton

WHILE PHOSPHORUS and potassium act about the same no matter how the soil is tilled or not tilled, management techniques can change in no-till cotton.

Research by University of Tennessee plant scientists Forbes Walker and Michael Essington shows that phosphorus and potassium tend to be at highest concentrations near the soil surface in no-till fields (Milan Field Day Tour Report, 2001). Over the long-term, potassium can become more concentrated in the planted row in no-till fields, rather than middles.

That means soil sampling should be done carefully to make certain it truly represents the field.

Samples should be taken at random locations, with an equal number coming from within and between the rows, Walker and Essington say. Samples should be taken to a depth of 6 inches.

Phosphorus tends to stay in place. But in sandy soils, those with well-defined macro-pores, and where phosphorus has been repeatedly added for many seasons, it can leach through the soil profile.

“The movement of P tends to be enhanced by the association of P with mobile organic materials and clay particles,” Walker and Essington say.

“In no-till systems, P tends to accumulate in the upper 1 to 2 inches of the soil profile, whereas in tillage systems it is found throughout the plow layer.”

But that does not necessarily reduce crop uptake of phosphorus in no-till. Roots can still easily get to the phosphorus, feeding the plant.

Potassium is more mobile than phosphorus. But potassium-fixation can make it unavailable to crops. Fast-fruiting cotton varieties grown on west Tennessee soils have high potassium requirements, Walker and Essington say.

That means you'll need to concentrate on making sure there's enough potassium in the soil to keep the crop at maximum production through the season, no matter what tillage system you use.

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