Thanks to growers’ collaboration and help from Superior Ag, we collected plant tissue samples from 12 watermelon fields at different crop growing stages in Southern Indiana in the 2020 season. In this article, I want to discuss two of the issues that standout from these tests.
One is the generally low or deficient Magnesium (Mg) levels across majority of the samples. In highly leached sandy soils, magnesium levels are usually low. Magnesium uptake is strongly influenced by soil pH. It is most available to plants at pH between 6.5 to 8.5 in sandy soils. With that said, if soil pH was lower than 6, it is almost certain we would see Mg deficiency in plant tissues. Magnesium is a mobile nutrient in plants, symptoms of deficiencies appear first on old leaves, as yellowing between the veins. Magnesium can be supplied through dolomitic limestone. But dependency on Mg release from dolomitic lime may not be adequate, especially if the limes were applied in spring, in which Mg availability may be limited to the second or third crop. Results from this survey illustrate the need for supplement Mg fertilizer application in watermelon production, especially when soil test indicating Mg was in the low range.
The second observation is the generally low Potassium (K) levels in plant tissue samples. Potassium is involved in maintaining plant water movement. When K is deficient, cell wall and stems are weakened; sugar and starch tend to accumulate in leaves rather than in fruit and roots. Potassium plays many important roles, deficiency of potassium directly affects yield and qualify of the crop. The adequate range of K for watermelons on the dry weight basis is 4.0-5.0% at flower stage, 2.5-3.5% at small fruit stage, and 3.5-4.5% at older fruit to harvest stage (Bryson et al., Plant Analyaiss Handbook III). The survey showed K levels in most watermelon samples were lower than the recommended ranges, this is particularly true toward the later part of the season. In healthy and recent developed leaves, nitrogen (N) to K ratio is about 1:1. However, in most of the samples, N is at sufficient or excessive levels; and the percentages of N are much higher than that of K at all cropping stages. Levels of K and N are known to be closely related. Without sufficient K, N tends to be high in plant tissues. Abundant N may increase plant sensitivity to diseases while K combats this effects; abundant N stimulates rapid and soft growth while K encourages growth of firmer tissue; negative effect of high nitrogen on fruit quality are also countered by K. Results from this survey indicate nitrogen may not be a limiting factor for watermelon crops at least for 2020 season, but K could be.