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Preseason Phosphorus and Potassium Applications – Satisfy Nutritional Needs for High Yields
Author: Nathan Slaton, Professor & Director of Soil Testing

The nutritional needs of soybean must be satisfied to produce high soybean yields. Conventional soybean fertilization practices in Arkansas usually focus on phosphorus (P) and potassium (K) fertilization, which is the topic of this discussion arranged in a question and answer format.

How accurate are soil test results in predicting whether soybean yields will be increased by P fertilization?
Over the past eight years, we have had about 40 trials evaluating soybean response to P fertilization, most of them on loamy soils. We have found that the Mehlich 3 soil test does a good job of identifying soils that do not require P fertilization.

This simply means that we have high confidence (nearly 100%) that soybean grown on soils with ‘Optimum’ or ‘Above Optimum’ soil test P levels (less than 35 ppm or 70 lb/acre) will show no yield increase from P fertilization.

We are equally confident that soybean will seldom respond to P fertilization in what we call the ‘Medium’ soil test level (26-35 ppm or 52 to 70 lb/acre). By definition we expect soybean yields to be increased from P fertilization on soils that test below optimum in P [‘Low’ (16-25 ppm or 32 to 50 lb/acre) or ‘Very Low’ (less than 16 ppm or 32 lb/acre)]. Unfortunately, soybean does not always respond to P fertilization when soil test results are Low or Very Low. Our research shows that soybean yields are increased on about one-fourth of our soils that test Low in P with yields increased by 5 to 10%.

For soils testing Very Low in P, soybean yields are increased on about one-half of these soils with increases ranging from 10 to 25%. The likelihood of soybean yield being increased by P fertilization increases considerably as soil test P drops to less than 10 ppm (or less than 20 lb/acre).

Were phosphorus fertilizer recommendations for soybean changed for the 2011 cropping season?
Yes, as of January 1, 2011 we reduced the amount of P recommended in the Very Low, Low, and Medium categories by 20 lb P2O5/acre. Although we do not expect a significant yield increase from P fertilization on soils having a Medium soil test P level, some fertilizer is still recommended to help replace a portion of the P that will be removed by the harvested crop.

Soil Test P
Availability Index

Recommended P Fertilizer Rate Equivalent amount of


lb P2O5/acre

lb 0-46-0/acre



80 174










>50 0


Soil P was extracted with the Mehlich-3 soil test method

How accurate are soil test results in predicting whether soybean yields will be increased by K fertilization?
In contrast to soil test P, the Mehlich 3 soil test method does an excellent job of assessing soil K availability for soybean.

Research trials conducted in silt loam soils having Low (61-90 ppm or 122 to 180 lb/acre) or Very Low (less than 61 ppm or less than 122 lb/acre) soil test K levels showed soybean yields were increased at 94% of these sites with yield increases from optimal K fertilization averaging 35 to 60% above the yield of soybean receiving no K.

Soybean yields were increased at 46% of the sites having a Medium soil test K level (91-130 ppm or 182 to 260 lb/acre) with the average increase being about 10% above the yield of unfertilized soybean. For soils having an ‘Optimum’ (131 to 175 ppm or 262 to 350 lb/acre) or ‘Above Optimum’ soil test K level, we expect no significant yield increase from K fertilization. However, the University of Arkansas does recommend that 50 lb K2O/acre be applied on soils having an Optimum soil test K level.

This recommendation serves two purposes. First, for fields that are soil sampled using the field average approach (one or two composite soil samples per field) it reduces the risk of yield loss from K deficiency in field areas that may test below optimum. Secondly, this recommendation serves to replace a portion of the K that will be removed by the harvested soybean, helping to maintain the soils K fertility. Understanding how K fertilizer rates change from one level to another and properly interpreting the expectation Garden of returns from fertilization will aid in making the appropriate decision for each field.

How much P and K are removed by each bushel of harvested soybean?
The most common average values of Hello P and K removal per bushel of harvested soybean are 1.4 lb K2O and 0.8 lb P2O5. These values are equivalent to 2.3 lb 0-0-60 fertilizer and 1.7 lb 0-46-0 fertilizer that are removed for each bushel of harvested soybean. Arkansas research has shown that these average values are appropriate.

How much P and K total uptake is needed to produce each bushel of soybean?
The aboveground portion of the soybean plant must accumulate the equivalent of about 5.0 lb N, 1.0 cheap jerseys lb P2O5, and 3.8 cheap nba jerseys lb K2O to produce one bushel of soybean.

I applied my P and K fertilizer in late fall of last year. Is it still available or do I need to add more?
Over the last three years (when fall weather allowed), we have established several trials evaluating crop nutrient uptake and yield response to the time and rate of P and K application. The limited amount of data that we have shows that fertilizer application rate is Dvorac more important than the time of application.

This is consistent with what most of the published literature shows. Our primary concern with fall application is that we have observed both P and K deficiencies in fields that had fertilizer applied before planting, which suggests that some soils have the capacity to rapidly fix P or K into unavailable forms, the P and K could be lost (sandy soils), or that the applied fertilizer rate was simply insufficient.

That said, there are some good guidelines to follow in determining whether to apply P and K in the fall or spring. Factors including the soil test level, soil texture, rate of fertilizer to be applied, cheap jerseys winter field management (will the field be flooded for waterfowl habitat), how responsive the following crop is to fertilization, and whether the field has had nutrient deficiency problems in the past should be considered.

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