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Have enough P, K for your soybeans?

Most soybean farmers in the Mid-South and Southeast will remember 2003 as one of their better years. Soybean yields met or exceeded past records in many states. State average yields ranged from 33 bushes per acre in Georgia to 43 bushels per acre in Kentucky.

Since 1980 — a contrasting drought year in much of the Mid-South — state average soybean yields have been increasing. The yield increase has averaged about 0.3 bushel per acre per year in Georgia and the Carolinas to 0.5 bushel per acre per year in most Mid-South states.

A number of plant nutrition questions are being asked by soybean farmers and landowners who recognize the opportunity of good markets in 2004. This article includes many of the more popular questions, and it provides science-based answers.

Q: What soybean yields are possible in the Mid-South and Southeast United States with good management?

A: Research at a number of locations across the Mid-South and Southeast has shown that soybean yields ranging from 45 to 50 bushels per acre are common, and that yields above 55 to 60 bushels per acre are within reach for many farmers using good management practices with adequate moisture or irrigation. In fact, many researchers are consistently producing yields above 70 bushels per acre with progressive management.

Q: How much phosphorus (P) and potassium (K) is removed from fields in the harvest of soybean seed?

A: Soybeans remove about 0.8 pound of P2O5 (0.35 pound of P per acre) and 1.4 pounds of K2O (1.17 pounds of K per acre) for each harvested bushel per acre. So a 50-bushel-per-acre soybean harvest removes about 40 pounds of P2O5 per acre and 70 pounds of K2O per acre.

Q: How much P and K do soybeans need to take up from the soil and from fertilizer to achieve good yields?

A: The uptake of P2O5, K2O, magnesium (Mg), and sulfur (S) by soybeans is shown in the following table:

Soybeans take up large quantities of nutrients
N P2O5 K2O Mg S
Yield lb/A taken up
40 220 38 140 16 14
55 290 53 190 22 18
70 360 67 220 28 22
Note: P and K are expressed in oxide terms to better relate to fertilizer equivalents.

Q: What are the optimum soil test P and K levels for soybeans?

A: University researchers have correlated soil test P and K levels with soybean yields. As soil test P and K levels increase, soybean yields increase up to optimum levels where P and K are no longer yield limiting.

A review of current university soil test P and K interpretations indicates there is a great deal of similarity in interpretations of the agronomic optimum P and K levels for soybeans. Most land grant university research shows that 90 to 95 percent of the maximum soybean yield is reached when extractable soil test P levels are at or above 40 to 50 pounds per acre (20 to 25 parts per million (ppm), assuming a 6-inch soil sampling depth) and extractable soil test K levels are at or above 220 to 280 pounds per acre (110 to 140 ppm) on silt loam soils, based on most common soil test extractants employed in the region. The optimum interpretations will differ for very sandy soils and for clays (consult your laboratory for specific interpretations).

Q: Are the fertilizer recommendations developed 10 to 20 years ago appropriate for the higher soybean yield expectations and reduced tillage systems used today?

A: Soybean yields achieved today are frequently much greater than the yields achieved in past fertilization and crop nutrition research. It is safe to assume that soil P and K needs today will be at least as high as predicted by past studies. The key is to strive for high soil fertility levels to remove P and K as factors limiting soybean yields. With reduced tillage (no-till, strip till, ridge-till, stale seedbed), farmers must pay very close attention to the soil sampling depth. Be sure to use a very consistent sampling depth, and sample accurately from the surface to the depth recommended by your laboratory.

In reduced tillage systems, P and K levels are often much higher in the top 3 inches than at 3 to 6 inches deep. Sampling 3 to 4 inches deep instead of 6 inches deep can result in high soil test results and may cause farmers and crop advisers to mistakenly believe soil fertility is adequate.

Q: How much will soil test P and K change in one year after a high-yielding soybean crop (like in 2003)?

A: Soil test levels — determined by different soil extractants and methods — are just indicators of the relative fertility level, and they do not measure the exact amount of nutrients accessible to a given crop during a growing season. On silt loam soils, it generally takes 6 to 14 pounds of P2O5 per acre to raise the soil test P level by 1 pound per acre, and it takes 4 to 8 pounds of K2O per acre to raise the soil test K level by 1 pound per acre. So, it may take similar P2O5 and K2O removal rates to lower soil test P and K levels by 1 pound per acre.

A 50-bushel-per-acre yield may lower soil test P by about 4 pounds per acre and soil test K by about 12 pounds per acre. Years of harvest removal of nutrients without replacement fertilization can take a heavy toll on soil fertility and limit the yield potential.

Research by Iowa State University scientists showed that without P fertilization for 10 years, a corn-soybean rotation cut soil test P levels in half: From the initial 84 pounds per acre down to about 40 pounds per acre. Farmers who have not collected soil samples on their fields, or who have not fertilized in recent years might be surprised to learn how much they have mined soil P and K.

Q: What kind of yield response can be expected from P and K fertilization on low fertility soils?

A: Response to adequate P fertilization on low P soils can range as high as 11 to 18 bushels per acre per year. Response to K on low K soils can exceed 10 bushels per acre per year. If 60 pounds of P2O5 per acre and 90 lb of K2O per acre were applied on a low P and K soil, with soybean prices at $8 per bushel, the net return could exceed $100 per acre (assuming costs: $0.27 per pound of P2O5, $0.16 per pound of K2O, and an application cost of $4.50 per acre).

Q: Is all the fertilizer P2O5 and K2O used up the year it is applied?

A: On average, about 10 to 20 percent of the applied P is taken up by the crop the year it is applied. The remainder, because of P chemistry reactions in the soil, is stored in the soil for future crop use. Plant recovery of the applied K may range from 30 to 60 percent during the season of application. The remainder is held on soil cation exchange sites and in clay minerals for subsequent crop use.

Q: Cotton responds well to K applications, but I have heard that soybeans actually remove more at harvest than cotton. Is that true?

A: A cotton crop yielding 1,200 pounds of lint per acre takes up about 170 pounds of K2O per acre (142 pounds of K per acre), with about 50 pounds of K2O per acre (42 pounds of K per acre) actually removed from the field at harvest in the seed and lint. A 50-bushel- per-acre soybean crop also takes up about 170 pounds of K2O per acre, with 70 pounds of K2O per acre removed from the field at harvest in the seed. So, even though the K uptake by cotton and soybeans at these yield levels is the same, about 40 percent more K is removed in the harvest of soybeans, compared to cotton.

Phosphorus and potassium are essential nutrients for all crops and animals. Adequate amounts of these nutrients enable soybeans to:

  • capture sunlight and convert it to energy more efficiently,
  • increase root growth for efficient uptake of soil moisture and other nutrients,
  • increase the absorption of N2 gas from the air by symbiotic bacteria in nodules on roots, to nourish plants with nitrogen necessary for making amino acids and protein,
  • resist damage from several plant diseases, and
  • yield more, which will result in a higher profit potential.

Cliff Snyder is southeast director for the Potash & Phosphate Institute.

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