Soil Testing Episode 2 Phosphorus and Potassium

December 5, 2019

12.5.19 | Soil Testing Part 2 from Central Valley Ag on Vimeo.

Phosphorus (P):  Phosphorus is tested by different methods, and those methods make a big difference. Midwest Labs tests both P1 Bray and P2 Bray; each test gives us useful information. The P1 test is the primary number used for making a phosphorus application recommendation, while the P2 test helps us refine that recommendation. CVA likes to see P1 levels at approximately 25 ppm. Levels under 20 ppm indicate P is likely to be a limiting factor in corn, wheat, or soybean production. At P1 levels of 20-30 ppm, enough phosphorus fertilizer should be applied to at least replace the P removed by the crop. P1 levels over 30 ppm can receive less P fertilizer than the crop removes and remain productive. Soils should be retested every 2-4 years to make sure levels are not declining below established thresholds. P2 levels are best at approximately two times the P1 level. The P2 test gives an indication of reserve phosphorus and the potential of unusually high P tie-up in the soil. P1 to P2 ratios of greater than 1:4 indicate a strong likelihood of applied phosphorus being tied up in unavailable forms in the soil. In these cases, banding applied P should be considered to limit soil contact. These levels depend a little bit on each grower’s yield goals. If a grower is shooting for yields of over 250 bu/acre of corn, we will probably increase the P1 Bray target level slightly. 
When soil pH levels exceed 7.3, Midwest Laboratories will often run an Olsen Bicarbonate phosphorus test in addition to the P1 and P2 Bray tests. Carbonates and bicarbonates contained in a higher pH soil sample may neutralize the extract solution used in the Bray tests making these extractions less effective. The interpretive ranges for the Olsen Bicarbonate test are lower than the P1 Bray test. A good rule of thumb is to multiply the Olsen test result by 1.4 and use the resulting number in a P1 Bray based recommendation.

Some laboratories use the Mehlich 3 phosphorus extraction using colormetric detection. This analysis method will often have slightly higher results compared to a Midwest Lab’s Bray P1 extraction. A good conversion of M3 colormetric P to a comparable P1 Bray result would be to multiply the Mehlich 3 value by 0.75 or 0.80. If the other lab is using the Mehlich 3 extraction and P detection with an ICP instrument, expect the P value to be significantly higher. In this case, multiply the M3 ICP P value by 0.60 to get a Midwest Labs P1 Bray comparable value. If these adjustments are not made, Central Valley Ag’s recommended guidelines based on the P1 Bray method from Midwest Labs will often result in significant under fertilization.

Potassium (K):  At CVA, we like to see K levels near 200 ppm to minimize the chances that a lack of K will limit crop production. That number varies slightly with cation exchange capacity because the clay particles in a finer soil (higher CEC) can tie up K, making it unavailable for crop needs. A range of 150 ppm for coarse soils (sand) to 240 ppm for fine-textured soils (clay) should meet a crop’s needs under most conditions. Sometimes we focus more of our discussion in high yield environments or sandy soils on how many times should we apply K during the growing season. We know high yielding crops can use as much as 12 lbs of K2O per acre per day. Sometimes it makes sense to supplement a little of the soil’s base fertility with an in-season application just before the plant reaches reproductive stages when daily growth is greatest. 


Posted: 12/5/2019 5:14:10 PM by Kelli Reznicek | with 0 comments

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