Stewart:
Fortunately, plants are pretty good at sorting out their nutrient supply
needs, so long as we supply adequate amounts. The ratio of needs does chnage
during the growing season, but unless you are in a very sandy soil, we can
usually provide most (or all) of the annual needs as a soil application and let
the plant take what it needs when it needs it.
Foliar application may be helpful as a supplement, but I have never been very excited about trying to feed all of the nutrients that way. Leaves are designed for CO2 and H20 exchange; roots are better equipped for meeting mineral nutrient requirements. I have been surprised at how effective the pumpkin roots are at taking up water and nutrients for a fairly large plant. Research has shown pumpkins need relatively high amounts of K and N. As you note, P requirement is higher around pollination time. This is similar in most plants---when they shift to the reproductive stage, there is a high P requirement. P is required as the backbone of the genetic code (DNA), and all energy conversion processes in the plant, so any time there is a lot of celll growth, there is a high P requirement. K is essential for all cell membrane transfers---such as water and nutrient uptake, opening and closing of stomates, etc. When K is deficient, the stomates become less effective in controlling H2O loss from the leaves, and CO2 assimilation. The roots become less able to take up water and nutriuents. K is also critical in maintaining diesease resistance. N is a major component of genetic material (DNA) and proteins and amino acids.....so again, whenever there is rapid growth or reproduction (seed and fruit development), there is a high N requirement.
The "salt" effect is a chemical reaction of certain materials. When an acid and a base react, a salt is formed. If that concentration is too high, it can burn the leaf tissue. High salt content can also burn roots, but if the nutrients are applied to the soil, the salt splits apart into component ions and is buffered by the soil, setting up an equilibrium situation so that nutrients are slowly released as the plant takes them out of soil solution.
Bottom line.....I would follow a good soil application program for fertilizer, supplementing with foliar during critical times if you like, but don' t depend on leaf absorption to meet the plant's needs. The products like Miracle Grow have low-salt forms of nutrients, and you will usually pay more per pound of actual nutrients, but you also get some safety factors that you may be willing to pay extra for. It is a good high quality nutrient source and the added cost will be negligible unless you get into large acreages. The convenience and local access may well be worth the added cost.
I am probably rambling on more than you want to know on this. Not knowing your biology and chemistry background, I don't know how much to go into this. You don't have to know all of the details to grow a good crop, just be sure there is an adequate supply of nutrients and let the plant sort out its needs. Then the limiting factor is likely to be water-----which is probably easier for most of us to supply and monitor.
Good luck for a BIG ONE!!!
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