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Nitrogen Phosphorus Sulfur Chloride pH

Phosphorus (P) Fertility Management

Wheat is very responsive to fertilizer P applications on soils that do not provide adequate amounts of this essential nutrient. Phosphorus is second only to nitrogen as the nutrient that most commonly limits wheat growth and development.

Across the Great Plains region, there are large acreages of wheat that do not receive adequate fertilizer P, and consequently, profitability is sharply reduced. For some fields, applying adequate fertilizer P is more important than fertilizer nitrogen. About 0.5 pounds of P205 is removed with each bushel of wheat.

 

P Deficiency Symptoms. In the center plot, no phosphorus was applied. Phosphorus was applied to the plots on either side. While purpling of stems and lower leaves is often cited as the most common symptom of P deficiency, stunted growth and poor tillering are the most visible early indications of P deficiency in wheat. Purpling of leaves will occur, but is not always apparent in wheat. As wheat approaches heading, thin, poorly tillered stands and delayed maturity are typical of P deficiency. Heading and maturity may be delayed several days if inadequate supplies of P were available to the plant. Early shortages of P in wheat result in substantially reduced root system development and stunted overall plant growth. Because of its importance in root growth and development, shortages of P in winter wheat often result in increased susceptibility to winter injury. Inadequate early root development due to inadequate P nutrition also increases the susceptibility to moisture stress. In addition, adequate P is needed for tillering, head formation and grain filling.

The recently revised "Soil Test Interpretations and Fertilizer Recommendations" bulletin (MF-2586) provides detailed information on P recommendations for wheat.  The accompanying figure provides an overall summary of suggested P management in Kansas. Both ‘sufficiency’ and ‘build-maintenance’ guidelines are provided to Kansas producers. An abbreviated set of P recommendations can be found below.

 

Phosphorus Application. Wheat is known to respond well to applications of phosphorus on soils testing low or very low in available phosphorus. Wheat plants do not tiller well under severe phosphorus deficiency and often are more subject to winterkill.

Phosphorus for small grains may be applied broadcast incorporated, preplant injected in concentrated bands, or banded at planting. Band applications of this plant nutrient with the seed at planting or injected preplant are generally recognized as being more efficient in the year of application than broadcast treatments, particularly when low rates are applied to soils low in extractable phosphorus. The efficiency of broadcast applications, regardless of soil pH and P soil test levels, can be improved by incorporating the phosphorus fertilizer into the soil. This allows the plant to continue to use phosphorus even when the surface is dry.

Another alternative would be to band apply both nitrogen and phosphorus fertilizer before planting. Dual application of N and P with a tillage implement has the advantages of saving time by combining fertilization with tillage and of placing the phosphorus in the root zone for good utilization. This application technique has been found to be equal in efficiency to starter applications at planting time.

Liquids and solids as well as varying chemical forms of phosphorus (e.g. ortho- and poly-phosphates) are available on the market. Research conducted by Kansas State University researchers has shown that all are agronomically equal when applied at the same phosphate rate and by the same method of application. Selection of a phosphorus source, therefore, should be made on the basis of application equipment requirements, how the application technique for a specific P fertilizer fits in to the overall management plan and product availability in the marketplace.

 


 

Phosphorus Sufficiency Recommendations
for Wheat1

Bray P-1
Yield Goal (bu/A)
Soil Test 30 40 50 60 70
ppm - - - - - - - - - - - lb P2O5/A - - - - - - - - - - -
0-5 50 55 60 60 65
5-10 35 40 40 45 45
10-15 20 25 25 25 30
15-20 15 15 15 15 15
20+ 02 02 02 02 02
Crop
Removal3
15 20 25 30 35
Wheat Sufficiency P Rec = [46+(Yield Goal x 0.42) + (Bray P x -2.3) + (Yield Goal x Bray P x -0.021)]
    If Bray P is greater than 20 ppm, then only a NP or
      NPKS starter fertilizer suggested
    If Bray P is less than 20 ppm, then the minimum P
      recommendation = 15 lb P2O5/A
 

Phosphorus Build-Maintenance Wheat Recommendations5


Bray P-1
6-Year Build Time Frame
Yield (bu/a)
Soil Test 30 50 70
ppm - - - - - - - - - - lb P2O5/A - - - - - - - - - -
0-5 68 78 88
5-10 53 63 73
10-15 38 48 58
15-20 23 33 43
20-304 15 25 35
30+ 02 02 02
Phosphorus Build-Maintenance Rec =
{(20 - Current P Soil Test) x 18} + P2O5 Removal In Crop
           Years to Build

 

 

1  Crop P & K recommendations are for the total amount of broadcast and banded nutrients to be applied.  At low to
    very low soil test levels applying at least 25 to 50% of total as a band is recommended.
2  Application of a NP, NPK or NPKS starter fertilizer may be beneficial regardless of P or K soil test level, especially
    for cold/wet soil conditions and/or high surface crop residues.  Do not exceed N + K
2O guidelines for fertilizer placed
    in direct seed contact.
3  Crop removal numbers provided for comparative purpose only -0.50 lb P
2O5 and 0.30 lb K2O per bushel of harvested
    wheat.  If crop removal exceeds nutrient applications, soil test levels are expected to decline over time.
4  Recommended amounts of P
2O5 and K2O are based on crop nutrient removal at the indicated yields (0.50 lb
    P
2O5/bu and 0.30 lb K2O/bu).
5  The six year timeframe shown is an example only.  Build programs can be over longer timeframe, however,
    build-maintenance recommendations should not be less than crop sufficiency based fertility programs.