Breakfast, Lunch and Dinner - Soil fertility by the seasons
Growing and observing plants and trees over a period of many years has enabled me to see different natural cycles in action. While the change of seasons is the most obvious and familiar of natural cycles, my scientific curiosity had me wondering about soil fertility. Surely, little plantlets, like the tiny Philodendron seedlings in the picture at right, needed a different mix of nutrients than mature plants, blooming plants, and seeding plants did (see pictures below). I mused about the changes in soil nutrient levels that must be going on in forest, field and stream on a seasonal basis.
In part, what started me in this exploration was a little experiment I conducted while working at a large foliage nursery.. While there, I had the opportunity to run a study in which I was using Dracaena massangeana cuttings and three different fertilizers, 14-14-14, 20-6-12, and 14-26-6. The plants in this little test have a variegation that consists of a broad lighter green stripe down the middle of each leaf. At the time, i was curious as to what effect, if any; the differing element levels would have on the variegation pattern.
While I knew what three fertilizer blends were included in the study, I did not know, initially, which blend was applied to which test group. All the blends looked essentially the same, but were numbered so as to distinguish them.
The color of P
After a month, I could see an obvious difference in one of the groups. The color banding was much more distinct, and the green areas of the leaf were a deeper, richer green than those plants in the other test groups. At the end of the study, I found that the greatest color contrast was in the group receiving the 14-26-6, or the highest phosphorus. The next best color was in the 14-14-14, and the palest or least distinctive coloration was in the 20-6-12 group, or the highest nitrogen. This got me to thinking that, perhaps, changes in soil nutrient levels coincided with differing growth stages in certain plants. So I ran a few informal tests of my own, and discovered that giving a variegated ginger a high nitrogen fertilizer yielded a pale green leaf, while giving the same type of plant a high phosphorus mix yielded bold white borders on deep green leaves. I knew right then that I was onto something.
As a plant physiologist, I was aware of the role that phosphorus played in plant metabolism, so I reasoned that the high phosphorus blend would benefit tuber production in plants that developed such storage organs. I chose Amorphophallus as my test subject, and found out that when you run a test like this, you need to be prepared to sacrifice a few pots. The tubers grew so robustly that, at season's end, the pots, which were plastic, were either severely distorted or broken open altogether by the unbridled tuber growth! Additionally, the following Spring, these precocious large tubers were more apt to bloom than those not receiving the extra phosphorus.
Timing is Everything
Seeing these results led me to ponder about how a plant like Amorphophallus would grow under natural forest conditions. In the active growing season, nitrogen levels would be highest, promoting foliage production. As the season wore on, nitrogen would be depleted gradually and come more into equal balance with phosphorus. Late in the season, nitrogen would be at its lowest level, while phosphorus would be highest. I proceeded to test this out in the pot, giving high nitrogen fertilizer in the Spring, a 14-14-14 mix in summer, and then switching to a 14-26-6 blend in the Fall, when the tubers were most actively growing. I was astonished at the difference in growth I observed when compared to a plant receiving one fertilizer analysis year round. This was no competition; the seasonal cycling of nutrients produced the largest, most robust plant and tuber, hands down.
Cycles within cycles
Of course, the soil composition is not uniform throughout, and the presence of small plants, shrubs, trees, and grasses will have a significant effect on nutrient levels in any given plot of land. Trees, with more widespreading root systems, will have access to higher nitrogen levels for a longer period of time than small herbaceous plants, whose entire life cycle from seed to seed set occurs over just a few months. Localized root zone fertility levels will change rapidly while wide-area levels average out to a slower change over time. This is reflected in the growing periods of different types of plants. Trees may bank phosphorus in the Fall to use for blooming in the Spring, right before the foliage growth period. This would be similar to the pattern for tuberous plants, whose stored food enables immediate blooming upon emergence, with foliage growth and gradual restocking of food stores occurring through Summer and Fall.
Keeping these observations in mind can help us to maximize the growth of plants in our gardens and farms. By learning from nature and applying that knowledge, we can grow our crops in a manner that more closely approximates the age-old natural cycles of sprouting, growth, maturation, and dormancy. After all, don't we have one food for breakfast, in the morning, another for lunch at noon, and yet another for dinner, when we are preparing for a night's rest?
Photo credit: LariAnn Garner, Aroidia Research
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