One of my three sweet cherries in the food forest has for the last two years lost its leaves a little earlier than the other trees. A slight difference, I dismissed it as stress from converting its nitrogen source, as it came in soil filled with synthetic N from a nursery unlike my other trees that were bare root.
This year, it happened far earlier and another, the healthiest of the three that was planted bare root, began the same yellowing, browning, curling, and dropping of leaves from oldest to just sprouting.
Potassium deficiency seems to be the culprit, as the leaf loss began mid season, about July for the one, and August for the other, accompanied by lack of rain.
It’s not so much deficiency so much as unavailability. You see, potassium is naturally in the soil –in most cases a lot of it, but it’s chemically bound as rock to other elements. Rain and to a small extent plant roots use acids that slowly free up the potassium, making it available for use. When rain stops falling, the supply of acids goes down, without the acids there’s no release of potassium.
We have been without rain for several weeks now. And before this lack of rain we had too much –daily rain for several weeks. This actually makes the problem worse because sodium becomes more active when there’s too much potassium, and just like drinking sea water causes dehydration so active sodium makes water unavailable to plants. So no wonder I’m seeing a deficiency.
Other plants need high amounts of potassium for good health such as clover, which also browned and dried up in that part of the food forest, whereas ragweed (Ambrosia) is just getting really happy, as it likes a low potassium environment.
Of course, several chemicals may be at work showing the same symptoms, but they do fit the bill for potassium. Whatever the culprit, the remedy, as far as I am concerned, is about the same.
The weathering of rock releases plenty of nutrients including potassium. Once released, organic matter and humus grab hold of these released nutrients and hold them with a weak bond plants can access. When organic matter is lacking, these nutrients leach, or get bound up as this quote from a study in upper New York points out:
Only a trace of acid-soluble phosphorus occurs in the plow layer and upper B horizons. At lower depths, the phosphorus content is as much as 12 ppm. The pH increases with depth from 5.3 in the surface soil to 5.7 in the B a. Nitrogen averages 0.13 percent in plow zones, while organic matter averages 4 percent. As expected, organic matter is closely related to nitrogen in these soils. It is also related to exchangeable potassium: the amount of mineral colloidal matter in coarse sands is too meager to have much effect on the supply of potash [the usable form of potassium] in ionic [usable] form. Hence, most of the available element is held on organic colloids.
Organic colloids is organic matter and humus, aka decomposed mulch or compost. You get a higher yield of humus from woody material containing lignin than green lush material, hence my interest in woody N fixers.
Thankfully, my oldest Amorpha is right by the healthier cherry tree, so help is on the way.
Adding a few more mulch makers around the tree would in the long run benefit this supply of organic matter. Comfrey would be especially good for this as its roots go deep to where potassium and phosphorus leaches, as the quote infers. Comfrey is also said to accumulate more of these elements than most plants, which when cut would decompose along with organic matter.
See this study on the effectiveness of buckwheat for scavenging and making phosphorus more available. Some study organizations such as this one suggest it affects more than phosphorus. Rye is mentioned here for nitrogen and potassium scavenging.
This spring I added more swales in the food forest –just small ones that I dug with a shovel. These allow more rainwater to soak in, releasing more nutrients for a longer period of time. I’ve noticed this is only slightly longer though, if wilting is any indicator. The greater benefit plants get is more water when it’s around, which does boost growth noticeably. More growth makes more mulch, which makes more humus, hence water and nutrient retention is improved by swales in a direct as well as roundabout way.
So this is the plan I have for now. I’ll keep you posted on its effect.