Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Growers, consultants, and soils laboratories have noted that plant tissue tests on several vegetables (such as watermelon) have been showing lower than expected levels of potassium (K) and calcium (Ca) in plant tissues this year, even though soil levels are high.
There are a number of possible causes for these lower than expected tissue test results. High rates of nitrogen applied to vegetable crops can often reduce the levels of K and Ca in plant tissue. High nitrogen promotes foliage growth and more leaf area. This can have a dilution effect on K and Ca as there is less available proportionally to supply the new leaves.
The use of fertilizers high in ammonium and/or urea (which quickly released ammonium) can cause a temporary suppression of K and Ca uptake because ammonium is a competing cation. This suppression lasts until the ammonium is converted into nitrate in the soil by nitrifying bacteria. In drip irrigated vegetables where Urea Ammonium Nitrate (UAN) solutions are used as the nitrogen source during regular fertigation, this suppression can last throughout much of the season. The use of fertilizers with calcium nitrate and potassium nitrate as the nitrogen source can eliminate this competitive effect.
Very high levels of K fertilization can also reduce Ca uptake and excess magnesium can interfere with both K and Ca uptake.
In addition to dilution effects and cation competition, use of acidifying nitrogen fertilizers such as UAN or ammonium sulfate will drop the soil pH. When soil pH drops below 5.3, root function can be negatively affected, which will further limit K and Ca uptake. This can occur if soil pH is marginal to begin the season. It is common practice to lime fields on a 3 year rotation throughout the region. In the third year before the next liming, many fields fall into this marginal pH category.
Lower than normal K and Ca in leaf tissues can also be related to high temperatures and plant stress. In periods with extreme high temperatures, plant stomates close earlier in the day, transpiration is reduced, and K and Ca uptake are reduced because less water is being taken up by the plant.
Managing plant tissue K and Ca requires balancing fertilization. Where high nitrogen rates are being used to push high production, additional K should also be added in equal or higher amounts than nitrogen (1:1 to 1:2 ratio). This is particularly true for fruiting crops such as tomatoes, peppers, watermelons, and cantaloupes. Additional fertilizer calcium will also be needed for crops susceptible to blossom end rot.