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Phosphorus decisions, Is it worth cutting P?

With the current conditions and input cost many wheat producers are considering cutting back on inputs. I can’t disagree with the plan, but I would caution against what you cut. If you have read any of my past blogs, or seen me speak, you should know I’m all for cutting back on pre-plant nitrogen (N). Based on some recent trials I would not argue cutting the potassium (K) side, but phosphorus (P) that’s another story that we will walk through in this blog.

First and foremost, soil testing is the key to P management. If your soil test is below the critical threshold for the test you use, 32.5 for Mehlich 3 (M3P), then you need to add phos. We have enough work that shows current recommendations work for P in wheat. Reeds paper Evaluation of incorporated phosphorus fertilizer recommendations on no-till managed winter wheat Link to Paper goes over soil test recommendations in no-till and the recent double crop soybean project Double Crop P and K Blog highlights the importance of P fertility, regardless of yield level. Also if your soil test is below a 5.5 and you haven’t limed (Liming is the best solution, Band-aids not so cheap Blog ), then the next best option is adding additional P to alleviate the aluminum toxicity Band-aids for low pH Blog. In-short if the fields soil test P and or pH is below optimum you should not forgo P application.

But the primary reason I am writing this blog is for those looking at fields with composite soil test that is right around the critical thresholds, and they are trying to make the call on to apply P or not to apply P. Even on fields with soil test values in the good level, I am usually in favor of banding in-furrow fertilizer wheat, but not because of the same reasons I am for corn. With corn you are planting in cool soils and the availability of nutrients like P is lower in cool wet soils. For wheat cold soil isn’t the concern until we reach the end of the planting window. It will serve as a bit of a “pop-up” as the crop comes out of dormancy in the spring. I have also seen little to no value of N applied in furrow. I see same response to DAP (18-46-0), MAP (11-52-0), and TSP (0-46-0) when all applied at same rate of P. Meaning it was the P not N making the difference.

For me the reason I still recommend getting a little phosphate out even when the soil test comes back is that the great majority of fields have a large range of variability. Looking at a set of 650 grid sampled fields across Oklahoma and Kansas it showed on average soil pH 6.0 and M3P was 34 ppm. Both pH and P are at adequate/optimum levels. However, the average is usually somewhere between the low and high point and in this data set and the range of soil pH was 1.8 units and the range in M3P was 67 ppm. That meant on average of the 648 field with pH values the average difference between low pH and high pH was 1.8 units and the difference between low P and high P was 64 ppm.

Summary of grid soil sample data from fields in Oklahoma and Kansas. Data shared by participating farmers and consultants. Data presented is the number of fields in summarized for each variable, the Average value is what we expect as the average composited field value, the Range is the average difference between the min and the max of all fields.

The field below is from Kingfisher county and was sampled at a resolution of 10 acres per sample. This is a fairly course resolution for grid sampling but provides a great view of how variable our soils can be. The field average pH is 5.3, which is below optimum but our aluminum tolerant wheats would be able to handle fairly well. For the P the average is 22 ppm which needs about 18 lbs of P2O5 to max yields. If the farmer applied a flat rate of 20 lbs there would be significant forage loss on about 65% of the field, for grain only about 45% of the field due to underapplication of P. Note that low P and low pH are not correlated well, meaning the areas low in pH are not always low in P.

Example of a grid soil sampled field from west central Oklahoma. Field sampled at a 10 ac resolution. Even at such a course sampling; soil pH averaged 5.3 with range of 4.7 to 6.8, Soil test P average 20 ppm with a range of 7 to 40 ppm.

Banding P makes it more efficient because it slows the rate of tie. However, we have plenty data that says broadcast applied P is still a great option, even after planting. So what are my take homes from this blog?

First: If you are grazing wheat get down 40-50 lbs of N pre. But I have plenty of data the pre-plant N on grain only wheat is not needed. I have the same amount of data that shows the only value of in-furrow N for grain only is that it forces you to plant more seeds, because it just lowers stand.

Second: When it comes to wheat pay attention to Phosphorus and soil pH. Even our acid tolerant wheats preform better in neutral soil pHs, especially forage wise.

Third: A composite soil sample is an AVERAGE of the field. If your average is right at the ok level (pH of 5.6ish and M3P of 30 ppm), then half of your field is below optimum and will benefit from P.

Fourth: If you can band P great, but if you cant broadcast is still a viable option. Do Not Skip P when soil test says there is a need.

Questions or comments please feel free to reach out.
Brian Arnall b.arnall@okstate.edu


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