Preplant P and K considerations for winter wheat

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Winter wheat planting is quickly approaching and considerations for preplant P and K applications should be made according to soil-test results and the intended summer rotation. The majority of wheat in Arkansas is double-cropped with soybean and therefore guidelines have been developed that allow producers to apply all of their P and K for both wheat and soybean prior to wheat planting. Winter wheat is one of the most P responsive crops produced in Arkansas and close attention should be given to soil test P levels and fertilizer application rates. Early season P deficiency will lead to stunted plants and lack of tillering, which ultimately can have a significant impact on wheat yield. Application of P fertilizer can be made until the Feekes 5 growth stage with little reduction in yield, but preplant incorporation of P is the best management strategy. Although wheat does not respond to K fertilization to the same extent as soybean, K is still an essential nutrient for maximum wheat yield production. The following recommendations for P and K are based on rotation, either winter wheat alone or double-cropped with soybean, and soil test P and K level.

University of Arkansas Division of Agriculture guidelines for P and K fertilizer recommendations are based on the following:

1) A 4-inch deep soil sample

2) The Mehlich 3 soil test method

3) The nutrient concentrations of the extracts determined using an inductively coupled plasma atomic emission spectrophotometer.

The majority of silt loam soils in the Delta Region of Arkansas test in the “Low” soil test P category resulting in a recommended P2O5 application rate of 70 units and 90 units per acre for wheat and wheat double cropped with soybean, respectively (Table 1). Soybean is not as responsive to P fertilization as wheat and therefore only an additional 20 units of P2O5 is required to maximize yields for wheat and soybean produced in a double-crop rotation versus winter wheat alone.  Recommended K rates for wheat and wheat double cropped with soybean are also listed in Table 1. Much like soil test P the majority of the silt loam soils in the Delta Region have “Low” soil test K levels and the recommended K2O application rates are 90 and 120 units per acre for wheat and wheat double-cropped with soybean, respectively. Soybean is highly responsive to K fertilization and therefore additional K is required to maximize soybean yields in a double-cropped system versus wheat planted alone. Wheat straw/stubble will generally contain 15 lb P2O5 and 100 lb K2O per acre. Much of the K taken up by the wheat crop remains in the stubble following harvest and stubble management can influence the availability of K for the successive soybean crop. These recommendations are based on stubble management practices, such as no-till, stubble incorporation by tillage or burning of wheat residue as these practices tend to return the majority of the nutrients contained in the stubble back to the soil. Burning wheat residue is a common practice and much of the P and K in the stubble is retained in the ash and is potentially available to the soybean crop. The one consideration for burned wheat residue is whether or not the ash stays in the field, because ash that leaves the field results in a loss of nutrients. In future blogs we will discuss how to identify nutrient deficiencies in winter wheat and N management. Be sure to follow the Soil Fertility Program on Twitter @UARK_SoilTest and like us on Facebook for updates and Soil Fertility News (www.facebook.com/UARKSoilTest).

Table 1. University of Arkansas phosphorus (P) and potassium (K) fertilizer recommendations for wheat and wheat followed by double-cropped soybean based on Mehlich-3 soil test P and K (as determined by ICAP).

Nutrient Soil Test Level Soil test Concentration Winter Wheat Wheat &

 

Double-crop Soybean

 

 

ppm P

- – - – - – - – lb P2O5/acre – - – - – - – -

Phosphorus

Very Low

≤15

100

120

Low

16–25

70

90

Medium

26–35

50

50

Optimum

36–50

0

0

Above Optimum

≥51

0

0

 

 

ppm K

- – - – - – - – lb K2O/acre – - – - – - – -

Potassium

Very Low

≤60

140

180

Low

61 – 90

90

120

Medium

91 – 130

60

80

Optimum

131 – 175

0

60

Above Optimum

≥176

0

0

† Wheat followed by double-crop soybean P and K fertilizer recommendations include the fertilizer amounts for both crops.  The cumulative fertilizer rate can be applied in the fall.


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