GROWTH PATTERN OF WOODY PLANTS
Trees and shrubs do not seem to benefit from spring applications of phosphorus (and other nutrients) to the same extent as other types of plants. This is no doubt due to their different growth pattern. That tremendous canopy of leaves produced so quickly in spring by a mature elm could not possibly be manufactured from foods absorbed from cold, wet spring soil, in which most nutrients would be locked up and unavailable. Instead, this growth comes from food stored in tissues a year before. Maple sap is a case in point. Its sweetness as it is tapped for maple syrup in late winter comes from natural sugars stored the previous summer.
Growth of trees begins with elongation of terminal buds and leafing out of dormant foliage buds long before bacteria can begin their work in the cold soil around the tree roots. It is not uncommon for maples, for example, to finish their flowering and produce their first leaves before the last traces of snow have disappeared. Twig elongation and production of new leaves continue without interruption until about August first north of the Ohio River, and for about two weeks longer south of that line.
QUICKLY AVAILABLE IN SOLUTION
About August first, most trees will shed a few leaves as though anticipating autumn. Most of the foliage, however, continues to function in food manufacture, but twigs stop growing in length. Instead, they begin to swell in girth. This indicates a storage of starches and sugars in the wood, a process which continues until frost kills all foliage. Almost the last act of the growing season is a withdrawal of all food from the leaves and a halt to chlorophyll formation, thus bringing on the pageant of fall foliage color.
If soluble phosphorus is applied in summer, just before twigs start to increase in diameter, it will be stored along with elaborated starches and sugars, ready for next spring's burst of growth. If, however, it is applied in spring, it will not affect growth a great deal and, by August, it will be locked up and of little use to the tree.
POTASH: THE THIRD "ESSENTIAL"
Potash is classed with nitrogen and phosphorus as one of the three essential (major) fertilizer elements. This seems surprising in light of the relatively small amounts of potash removed from the soil by some crops. A 25-bushel per acre oat crop, for example, removes only 5 pounds of potash. Yet this element is highly important to several basic functions in plants.
It helps check the tendency of nitrogen to produce soft, rapid growth. It is essential to formation of starch and sugar and to transport of these materials inside plants