| [Image: sweet sorghum emerging] |
| [Image: applying fertilizer] |
| [Image: sweet sorghum field] |
On Monday of this week (April 21) we noticed that many leaves of the sweet sorghum plants had a purplish tint to them (see picture 1). This purple hue did not just show up on one of the varieties in the trial but was present in all of the varieties. Therefore, we can rule out a varietal explanation of the expression of the purple color. Generally, when a purple tint like this shows up on the leaves of a sorghum plant, the first thing that comes to mind for most agronomists is that there is a potential phosphorus deficiency problem occurring. The purple color that you see expressed along the margins of the leaves and beginning to show up between the veins of the leaves is caused by the expression of the plant pigment anthocyanin. The expression of this pigment is generally associated with phosphorus (P) deficiency in many plants.
Generally, for a P deficiency, these symptoms would show up on the older leaves first while the newer leaves would not show the symptoms. The reason is that P in the plant is mobile and can be translocated from the older leaves to the younger, more important leaves of the plant. That is why many plants tend to show the symptoms on the lower leaves and not the upper, newest leaves. However, this was not the case in our plants. We saw that almost all of the leaves, new and old, showed the symptoms. One reason could be that all of the P in the plant had already been assimilated into permanent cell constituents, such as nucleic acid. This, in turn, would then make the P non-mobile. However, this is truly speculation at this point. To accurately determine the potential of a nutrient deficiency, we need to evaluate the results from a recent soil test. Unfortunately, because this test was implemented on a new field that was not intended to be used when we were conducting our fall soil samplings, we do not have these results. However, we did pull some samples at planting and we are awaiting the results now. For the most part, the soils at the Rice Station South Farm are not P deficient. So why would these symptoms show up? Well, one reason might be because of the lack of rainfall and cool temperatures that we have had since planting the test. The lack of moisture and cool temperatures can both contribute to slow uptake of P in plant roots, causing a temporary P deficiency, even if the soil has an adequate P supply. Warmer weather and some rain this weekend would most likely alleviate the problem. Nonetheless, 200 pounds per acre of 8-24-24 were surface broadcast on Monday by our research fertilizer applicator (picture 2). The fertilizer was then incorporated by cultivation (picture 3). When we get the soil test results, we will know for sure if this is the case, and I will be sure to share the results with you. If we do get that rain this weekend, the sorghum should move a lot next week. Until then…
Dustin Harrell