Soybean School: Think P and K for pods and yield

If you’ve been regularly scouting soybeans and noticed some potassium deficiency symptoms — yellowing on leaves — now is a good time to check the number of pods in those spots.

“What we often don’t do is compare the number of pods later on, and of course that’s what yield is made of — pod number, seed size,” says Horst Bohner, Ontario Ministry of Agriculture, Food and Rural Affairs soybean specialist in this episode of Soybean School.

In the video, Bohner shows us some fertilizer trials where pod numbers give us some proof of concept. One side of the field has 50 pounds actual of phosphorus (P) and potassium (K), and the other side of the field, no fertilizer applied. The field with fertilizer applied, plants had an average of 35 pods per plant, whereas the side with no fertilizer had 28 pods per plant.

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The number of pods is the main thing that can be manipulated through proper fertilization, which will result in higher yield. There were more pods set and less pod abortion with the fertilized soybeans.

“My theory is that one of the reasons the plant aborts so many of its flowers — up to 75 per cent in a normal year — is because it doesn’t have enough nutrients in the plant at the right time,” says Bohner.

Phosphorus and potassium provide that one-two punch for soybean nutrition. About 70 pounds of actual potassium is removed through the crop versus about 40 pounds of phosphorus (per acre). When formulating fertilizer blends, Bohner recommends a two-thirds to one-third mix of phosphorus and potassium, unless phosphorus is already high in the soil test. The two nutrients seem to work synergistically, particularly at the beginning of the crop’s growth, when root growth and nutrient uptake is happening.

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