At the end of the 2025 season, soil from four beds was sampled and sent to the UMass extension for analysis. (Thanks, Mike!) These are the reports:
- Bed A (peas, pole beans) S251117-108
- Beds B-C (eggplants, fava beans, cucumbers, squashes, watermelons) S251117-109
- Bed D (bush beans, soybeans) S251117-110
- Bed 1 (tomatoes) S251117-112
A guide on how to interpret the results is here. There are other references at the end of the reports.
Soil organic matter ranged from 4.8% to 5.4%, which is fine, even in the “new” perimeter beds. Salt and lead content were all low. We did not order the extra test for nitrogen, but we know we always need to add nitrogen, mainly fertilizer and manure.
pH in all the beds was 6.1-6.5, which is a little on the low side for crops that like sweeter soil, like the brassicas. Reference here. Potatoes however prefer a lower pH, ideally 5-5.5, so we should never lime that bed. (High pH promotes scab.) It would be good to have a reliable pH tester at the garden so we could tweak the pH for each specific crop.
The magnesium level was high in all the beds except A. Both dolomitic limestone and wood ash add magnesium to the soil, so we should not use dolomite limestone and be careful using wood ash. High magnesium can disrupt the absorption of other nutrients.
In all the beds except B-C, the potassium level was low or very low, so it’s likely the garden is generally a bit deficient, and might explain the poor performance of the tomatoes and beans. A good organic source for potassium is wood ash, but it must be used with caution because it can raise pH rapidly and can be caustic. We should never add wood ash where potatoes are to be grown. This article recommends using 1/4 to 1/2 cup of wood ash per tomato plant, but more research should be done.
Each report includes a recommendation for adding lime (to raise the pH a little), nitrogen, phosphorus and potassium. They are all close enough that we can probably apply them across all the beds. A general application of wood ash would raise the pH and potassium, so should be good for all the beds except where the potatoes are going. If we are concerned about the high magnesium, we could apply only non-dolomitic limestone and elemental potassium (potash) instead of wood ash. Without testing every bed and knowing the exact preference of each crop planted in it each year, it’s impossible to fine tune the soil improvement.
