A great deal of time and money is spent on soil nutrient testing in gardens. Soil nutrients needed for optimal health vary from plant to plant. Further, its not just N, P, K, but 40+ other nutrients and micro-nutrients which provide for optimal growth and health. This begs the question, “What happens to plant health when these nutrients are not present or abundant enough?” Turns out the plants themselves display nutrient deficiencies in some key ways, and with a bit of research, one can skip the soil testing and go right to the plants themselves for an analysis.
The first consideration one may make regards the plants that willingly establish themselves in the area being surveyed. By willing establish I mean volunteer- not planted by us, humans. When plant volunteers in a place, its telling us it is either more able to cope with the soil, and so out-competes other plants. For instance, where white clover appears, soil is typically low in nitrogen; if plantain appears the soil is often cultivated, wet and clayey, so slightly acidic. Other examples include buttercup (clayey, wet soil), stinging nettle (abundant moisture, high nutrients typical of alluvial soils) and dock (wet, acidic, clayey and often magnesium depleted). A less than ideal garden soil may still bear good crops if one knows what to plant; for instance, beans, beets, carrots, parsnips, peas, radishes, sage, and thyme all will tolerate and even perform well in poor soil conditions.
Following this notion, we chose two varieties of radish to do some observation based soil analysis in the Learning Garden: Red Cylindra and Pink Globe Radishes. This wasn't a scientific survey, but a naturalist one; science could easily be employed to produce a knowledge of the soils nutrients by comparing observational data of soil growth with plant material testing, soil testing and so on. However, a quick observational survey suffices to tell us quite a good bit about our soil in the Learning Garden.
As a bit of background, the soil in the Learning Garden is the garden mix sold by Great Western Supply. The soils they produce have been in beds at the garden for 1-3 years. The Garden Mix is a combination of 1/3 screened sandy loam, 2/3 mushroom compost (cow manure). Worm tea, a liquid produced from worm castings, was applied at seeding, upon germination and then bi-weekly. This ruins any 'control' we might have used in a formal scientific study aside, but still gives us some insights.
Radishes above below are from three amended beds. The top row are Red Cylindra radishes planted in the NE bed which has the shallowest soil (4"), least light and most temperature variance; the Red Cylindra and Pink Globes in the bottom row are from the two beds farthest west, with deeper soil (4-6", the most light and least temperature variance.
Radishes on the top row are pithy, with air pockets and fibrous roots, and which had 'bolted', aka, put on flowers before producing edible vegetation. Pith and bolting can be a sign of several factors; shallow soil, late harvest, spotty watering, nitrogen over-fertilization and too much heat. Early flowering is typically stress related, meaning a lack of nitrogen, soil depth, or spotty watering. We can eliminate late harvest and too much heat, and its not likely the soil is overly fertile, but the bed these were harvested from is only a few inches deep, preventing the tap from developing properly and causing the plant to ACT like it is poorly watered. So these radishes are telling us “MORE SOIL!”, and with that bed being just 3-4 inches in depth, I tend to agree.
The bottom row has some nice specimens which are crisp, juicy, almost an inch in diameter. Among them are roots that went woody before developing, barely a pencil width in diameter. This can be a sign of overcrowding, or nitrogen and phosphorus over fertilizing. However, we saw no leaf burn that is typical of nitrogen over-application. Judging by the quality of the roots that did form, and the broadcast method of planting the seed, its a fair wager that the soil is good shape but the plantings were a bit too dense in spots- meaning the gardener needs to thin some crop!
Overall the soil at the Learning Garden appears to be in good shape. by continuing to mulch, feeding worm tea and applying manure from Brother Edmund's chickens on occasion- and adding depth to some of the beds!- the soil here will be good for years to come. For more info on radishes and other plants as soil nutrition indicators, see:
COMING SOON: Companion planting with Corn, Beans and Squash in a Composting Raised Bed