Updated: May 8, 2019
Several recent soil analyses around Knoxville have shown trace amounts of Phosphorous (P) and Potassium (K). Phosphorous is critical for a variety of cell and organ functions in plants. Deficiencies in P, especially in cherry and other stone crops can cause poor fruit production and slow growth. Limited potassium (K) can cause discoloration (purpling and/or bronzing) of leaves and a 'warped' fruit. These responses to a lack of K in the soil are similar to responses to various pests. Potassium is also critical to proper functioning of a plants' immune response. Without sufficient K, plants will catch every passing 'cold'.
One way to differentiate pests (or biotic) causes from nutrient (abiotic) causes is to look at growth-rate. Growth-rate can be measured by comparing this seasons internal length with last seasons or with what you find in the same plant growing at a greenhouse. Shortened internodal length means you're looking at nutrient deficiencies. These are not easy things to identify for most people.
The soils I have been testing are on old agricultural land. That makes sense because some agricultural practices (e.g. tilling, crop type and rotation) will extract P and K fairly rapidly. So, what to do?
The solution depends on the pH of the soil and the clay content. In the soils I've been working with lately, the pH is very basic (alkaline) so additions of P and K should be rapidly adsorbed by the soils. However, plants can only access a small percentage of what is applied so be patient. Your applications of P and K should be moderate with the goal of taking up to a year to get your soils back to a balanced NPK.
Slow release fertilizers and additions like cottonseed meal and blood meal should be applied in the spring. Applying a heavy dose of any fertilizer in the fall or winter can cause the plants to grow when they should be going into or in dormancy. Don't apply during heavy rain or the rainy season as erosion or percolation through the soil (though not necessarily an issue with clay soils). You'll lose the nutrients you applied and waste money. February and April additions of NPK are the best time to apply. Retest soils in the early summer to determine the next application and the application rate.
Soil nutrients are complex; that's why I'm here and other like me. Hire someone who knows soils and plants do a nutrient analysis and come up with a plan of action that will be effective and save you money.
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