A fertilizer plan designed to reach top yields ensures adequate plant nutrition is available and often relies on ideal weather conditions.
There are times, however, when hot, drought conditions demand even more nutrients than a pre-plant soil analysis might suggest; particularly for magnesium and potassium. Crops may be even more at risk if mid-to-late-season yellowing of crop leaves has occurred in the past.
Magnesium and Potassium Functions
Sufficient levels of Mg and K are necessary for setting seed and grain and filling kernels as these activities shift the leaf’s carbohydrate-producing system into high gear, moving newly manufactured sugars from the leaf into the developing crop and roots.
Adequate K is also necessary for proper opening and closing of the leaf’s stomata, which controls the intake and flow of carbon dioxide (CO₂). K deficiencies cause stomata to close, which shuts off vital supplies of CO₂ needed for efficient photosynthesis. Also, Rubisco, the most important enzyme involved in photosynthesis, is found at low levels in Mg and K deficient plants.
Impact on Photosynthesis
Studies show that insufficient Mg and K reduces the efficiency of photosynthesis, and the plant can no longer use all of the sunlight striking its leaves for energy. The excess solar energy begin to form molecular oxygen which makes the plant more susceptible to leaf-damaging reactive oxygen species (ROS), or build-up of free radicals that can damage the leaf through photo-oxidation; this can be identified by yellowing appearance seen in some fields at the height of summer heat and dry conditions. However, photo-oxidation caused by Mg deficiency is commonly mistaken as water stress allowing it to remain an unidentified reason for reduced yield.
Mg and K are required for the translocation of carbohydrates (sugars) produced by the leaves to other parts of the plant such as grains, stems, shoots and roots. Studies show that leaves deficient in either Mg or K are likely to have massive accumulations of carbohydrates due to impaired flow of sugars through the plant’s vascular system, which can inhibit photosynthesis even further.
Implications
While stressed plants deficient in nitrogen or phosphorus do not show signs of carbohydrate loading in the leaves, crops growing in very bright sunlight for long periods may have an increasing demand for Mg and K. To support critical sugar translocation through the plant, plants may need higher amounts of Mg and K during their reproductive growth stages to maintain maximum photosynthetic activity and avoid photo-oxidation damage.