Figure 2. This cantaloupe field displayed symptoms initially thought to be disease related. Upon further investigation, however, it was determined that severe magnesium and potassium deficiencies were the problem, and resulted in significant crop loss. Photo by Josh Freeman
By Josh Freeman (University of Florida, IFAS Extension, Assistant Professor, Horticultural Science) and Mark Reiter (Associate Professor and Extension Specialist, Virginia Tech)
Many growers live with the mindset that vegetable crops need a tremendous amount of calcium to promote strong growth and prevent fruit disorders like blossom-end rot (BER). There is no doubt BER can cause serious yield loss, especially in tomatoes, and calcium deficiency is the culprit. However, annual applications of calcium may not be necessary to prevent it if ample calcium is already in the plant-soil system.
BER is a fruit disorder where cell walls break down because of a lack of calcium in the developing fruit. However, in many cases BER is not a result of a calcium deficiency in the soil. BER often develops on plants grown in soil high in calcium. In cases where calcium is not limited in the soil, calcium has been limited in the plant because of inadequate irrigation. Large quantities of calcium is moved into the plant through water via mass diffusion from the roots. If plants are drought stressed and are not taking up adequate water, then the plants are not taking up adequate amounts of calcium either.
While growers have good intentions on preventing yield zapping disorders by applying additional fertility, they can often cause other yield inhibiting issues which we have seen over the last several growing seasons. Keep in mind that too much of anything is not good for plant growth; whether it is rain, sun, heat, nitrogen, calcium, etc.
Cations (positively charged nutrients) bind to the soil based on the cation strength, with calcium cation strength equal to magnesium and stronger than potassium. Therefore, large additions of calcium will displace potassium on the soils cation exchange capacity “bank” and can be leached by excess irrigation, rainfall, etc.
Prophylactic applications of calcium applied as lime or gypsum that are not based on soil test results could create deficiencies in other positively charged nutrients (cations), primarily magnesium and potassium. These cations compete for limited binding sites (very limited in our sandy soils) on soil particles and for uptake by plants. An example below (Figure 1) demonstrates preferential uptake and availability for a paired soil and plant analysis. The soil analysis indicated high potassium and magnesium in the soil but the tissue test resulted in low potassium and magnesium in the plant. Plant analysis for calcium was sufficient. In this case the tomato plants were in good shape with no underlying conditions such as nematode infestation or soil-borne pathogens to inhibit root function. The preferential uptake is where calcium is taken up by the plant and not potassium or magnesium. Deficiencies of both potassium and magnesium can have significant consequences for plant growth and development.
Figure 1. These data demonstrate the preferential uptake and availability for a paired soil and plant analysis. Note the soil analysis indicated high potassium and magnesium in the soil but the plant tissue analysis indicated low potassium and magnesium in the plant.
Figures 3. Cantaloupes displaying symptoms of severe magnesium and potassium deficiencies. Photo by Josh Freeman
In 2014 and 2015, symptoms were observed in commercial cantaloupe fields in the panhandle that initially resembled plant disease (Figures 2, 3, and 4). Upon further investigation, no pests or pathogens were observed. After plant and soil analyses were conducted, it was determined that severe magnesium and potassium deficiencies were the problem and resulted in significant crop loss.
Figures 4. A closer view of cantaloupe leaves displaying symptoms of severe magnesium and potassium deficiencies. Photo by Josh Freeman
Plant analysis of symptomatic plants is shown in Figure 5. Notice the excessive calcium and sulfur plant tissue concentrations; both are the major elements found in gypsum (CaSO4). University of Florida nutrient management recommendations (http://edis.ifas.ufl.edu/cv101) state that calcium applications to soils testing above 300 ppm (600 lb/acre) with mehlich-1 extraction will likely not result in crop response.
Figure 5. Plant analysis of symptomatic plants. Note the excessive calcium and sulfur plant tissue concentrations.
It is not uncommon to see soil tests from commercial vegetable farms testing nearly four times what is deemed sufficient (Figure 1). When soil conditions reach this state, it may be difficult to remedy potassium or magnesium deficiencies in plants. It will take years of removal through vegetable harvest to draw soil calcium levels down. Farmers experiencing these issues should take soil tests and follow the recommendations for fertilizer applications. If calcium has displaced potassium and magnesium in the upper soil profile where most vegetable roots are found (0 – 12 inches in depth), future fertilizer additions will be necessary to replace these cations on the soil exchange complex. An accurate soil test will help you make this decision. Overall, if you have sufficient soil calcium and are experiencing BER, irrigation should be the first farming practice you investigate. UF/IFAS recommendations for irrigation management can be found in the publications listed below:
Principles and Practices of Irrigation Management for Vegetables
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