Banking Potassium: Getting A Bit Ahead On Tomato Consumption

One of the greatest challenges in growing tomatoes in an intensive production system is keeping up with the plants high consumption of potassium during fruit production.
Banking Potassium: Getting A Bit Ahead On Tomato Consumption - Articles
Banking Potassium: Getting A Bit Ahead On Tomato Consumption

Very often some of the first fruit are yellow shouldered. The heavy consumption of potassium actually starts about 2 weeks before the first flowers are visible. The concept of 'banking' potassium or applying extra a bit earlier seems to be in order to reduce packing house losses.

Before you start applying extra potassium, your irrigation / fertigation solution needs to be at the correct pH. Tomatoes and peppers extract the most nutrients at a pH of 6.2-6.5. For growers pulling irrigation water from limestone aquifers, this will probably mean the addition of acid on a constant basis to lower your water's pH. Get your water tested at a laboratory for pH and alkalinity expressed as ppm bicarbonate. Since the pH scale is logarithmic, each 1 point on the scale is a 10X difference in pH. A two point difference is 10 x 10 or 100X. This means that small variances from the ideal pH of 6.2-6.5 can result in major nutrient deficiencies. Most growers use either sulfuric acid or citric acid. Organic growers can use citric acid. Use the online alkalinity calculator to get your acid concentrations. Be sure to follow all directions on the calculator and pay careful attention to the pull down menus on the input side to get the correct recommendations.

Since we need to start the application of higher levels of potassium earlier than flowering in order to bank some and have high enough levels for the first fruit, tissue analysis should start as soon as your plants are large enough to collect full sized mature leaves from and not cripple your plants. Make the change to a higher potassium ratio fertilizer about 2 weeks prior to what you've been doing in the past and start a weekly foliar application of potassium at the same time. Be sure your fertilizer is compatible with foliar application.

Growing great tomatoes that are full flavored and have low losses in the packing house requires keeping tissue potassium levels above 3%. From before the first blossoms to the last harvest, tomato plants (and peppers as well) require huge amounts of potassium to produce the most flavor and prevent yellow shoulders / gray wall.

Although I look at every nutrient level, those most important to packout are the following:

  1. Tissue N levels should be at about 4% as fruit are developing. Higher levels can create soft fruit, more foliage, and fewer fruit.
  2. Tissue K levels need to be above 3% for tomatoes and peppers to produce the highest quality fruit and to keep creating new blossoms.
  3. Ideally Ca should be at about 3% and Mg at 0.8-1%. This prevents cracking and produces fruit that are tough enough for packing and shipping.

In retrospect, although I've always recommended bi-weekly tissue testing to growers due to the cost of the tests, I now recommend weekly sampling that starts as early as the plants can handle the cut.

If you plowed down 30-50% of your soil test recommended potassium, calcium and magnesium when getting your soil ready and work to keep these levels at their optimum values during production, you should be able to increase yields and reduce packing house losses.

Prepared by Steve Bogash, former extension educator.