Soil Test Result Questions
Hello! My home soil test (from perennial garden) has above optimum levels of phosphorus (97 ppm) and magnesium (420 ppm). The potassium level is just below optimum (124 ppm). Calcium is high (2666 ppm) although there is only a value without an explanation. I looked up 'normal' range which I believe is approximately 700-1300 ppm. There is a recommendation for fertilizer and to add nitrogen which I will do. The pH is 7 and organic matter at 11% is adequate.
My questions: 1) what are the issues with above optimum P and Mg? What causes that? Could I and should I bring it down? If so, how? 2) Is the elevated level of Ca an issue? If so, can I do anything to fix it? What problems does it cause?
I had a lot of disease and insect problems with my perennial flowers this summer and last and I wonder if it will be improved with these recommendations. Another thing I've noticed the last few years is what looks like black mold and/or soot on my milkweed pods and other plants, on both the leaves on the plants and on the ground. I also have many very old oak trees that drop leaves which I allow to decompose. Could that be the cause? Thank you so much for your help!
Oakland County Michigan
The soils in Michigan are often naturally high in magnesium, phosphorus and calcium. This is largely due to how the soil formed from the parent materials millions of years ago.
1) what are the issues with above optimum P and Mg? What causes that? Could I and should I bring it down? If so, how?
Answer: There are no issues with above optimum levels of phosphorus and magnesium. They occur naturally in the soils here in Michigan,and there is no practical way to reduce the amounts. These plentiful nutrients will be taken up by your plants in the amounts they need, as they need them.
2) Is the elevated level of Ca an issue? If so, can I do anything to fix it? What problems does it cause?
Answer: No, high levels of calcium naturally occur in our soils, too. There is nothing to ‘fix’ here either, as the plants will take up the amount of calcium they need.
Disease and insect problems each have to be addressed for the specific condition and on the specific plant. Plants that have proper nutrition, correct amounts of sun, water, correct soil pH, and good air circulation will tend to be healthier than plants that are lacking in these. Healthy plants are more resistant to diseases and better withstand attacks by insects.
Some disease issues can be held at bay by correctly mulching around the base of plants so as to reduce soil (which contains fungi) splashing up onto leaves during rain or watering. Watering so as to keep leaves dry is also a good practice. Leaves that stand wet for long periods of time, especially overnight, tend to have more fungal issues. Most plants want the first 1-2 inches of soil to be dry or nearly so before watering them again.
The blackened areas on milkweed and other plants sounds like a condition called sooty mold. When aphids feed on plants in high populations they exude a sticky substance called honeydew. This sticks to plant parts, and anything else around the plant, and a black fungus grows on it. (See links below)
Oak leaves are good mulch and a good source of organic matter. However, no mulch should be deeper than 2-4 inches or air and water may be restricted from reaching the root zone. Oak leaves shredded by mowing over them make an even better mulch, than leaving them whole, because they will release their nutrients faster, and will not form dense mats that can prevent water filtering through.
Next season as you monitor your plants and begin to see symptoms of insects or disease, please take pictures and submit them as questions to us. It is better to take action as soon as you see damage start. Of course, not all insects are bad, some are predator insects that feed on the bad ones.
I am including links that discuss these topics in more detail.
If you are interested in the science of why soils have certain nutrients, here are some articles:
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