David M. Kopec, Extension Turfgrass Specialist
Foliar feeding, or the application of fertilizer minerals by spraying turf with a fertilizer solution, is a much talked about practice. Although superintendents have been practicing foliar feeding for years, there has been a frenzy about converting fertilizing greens solely with foliar fertilization, at the replacement of standard granular applications.
Whats all the hub-ub, Bub?
The foliar craze has come about, mostly due to use of the new super bentgrass and super (ultra-dwarf) bermudagrasses for greens. It's for sure that the high shoot densities of these grasses require more surface cultivation to control thatch build-up, and smoothness. However, there is a tendency to say that these higher density greens grasses (both bentgrass and bermudagrass) need more fertilizer. Whether this is true or not, remains to be proven. Regardless, as part of an management scheme, it is desirable to avoid spikes of growth by applying small amounts of fertilizer on a frequent basis (spoon feeding) to grasses that grow aggressively and have high shoot densities.
It is often times easier to apply small amounts of fertilizer as a spray, rather than granular fertilizers (even greens grade materials). Also, there is no fertilizer pellets removed by mowing events. Note that you can still loose fertilizer from foliar feedings, if you don't water in before mowing.
How does foliar feeding work for a plant?
The fertilizer elements applied to turfgrass leaves are absorbed through tiny cracks or pores in the surface of the leaf surface in the wax layer. These pores are very, very small tubes, and are lined with water. They are called transcuticular pores.
These pores are less than one nanometer in opening size (one billionth of a meter long). Because they are small, only small mineral particles can be taken in. Thus, small particles like urea nitrogen can enter, but not large particles like iron cheleates! These pore channels are also lined with negative channels, so they are attracted to positively charged fertilizer particles such as ammonium, potassium, magnesium and sodium, and tend to repel anions, like nitrate, phosphate, and sulphates. Likewise, nitrogen based on urea or ammonium can be transported through these leaf pores. These are the general rules.
If the pores get flooded with a large concentration of anions inside the tubes, then they can absorb some nitrate and phosphates. This is limited, however.
Foliar fertilizer does not penetrate the stomates of leaves. The inner walls of the stomates (water control valves for leaf cooling) are covered with globs of wax, to repel outside water from entering the stomates, themselves. Also, foliar absorption is actually greatest at night (when stomates are closed). This shows that stomates play no role in foliar feeding.
The concentration of stomates on turf leaves does indirectly affect foliar-feed uptake. As the number of stomates increases, there are more micro pores appearing on the leaf between stomate cells. Under these conditions, the pores appear to be more permeable and numerous than other micro pore (cracks) on the leaf surface. These cracks (between the stomates) are larger in size and can allow passage of large size metal cheleates (like cheleated iron) and pesticides.
A curious observation is that foliar feeding always seemed to work better (for me) on cool season grasses, than bermuda! Well, cool season grasses have higher concentrations of stomates (to keep the leaves cool!). This makes sense and now I know why!
After the long journey and struggle to enter the leaf, nutrient uptake into cells is much like uptake within plant cell roots. Only now there is no restriction based on root condition. Therefore, foliar-feeding offers some advantage when root activity is limited for any reason.
There is some evidence that a certain percentage of foliar uptake of fertilizer requires plant energy. This occurs only when the fertilizer element is inside the plant, and moves from cell to cell.
As a process, foliar fertilization is not a plant efficient process! Leaf uptake of nutrients is only a tiny fraction of what is taken up in the soil solution by roots. This is why you still need to water in foliar-feed sprays, so they reach the roots. This is why spoon feeding requires lots of closely spaced foliar applications.
Likewise foliar applications of iron cheleates makes sense since there is essentially no time lost due to soil water uptake via the root system. Ferrous sulphate applied as a foliar feed works well also, because the leaf uptake portion is quick. However, once washed in a large part of the iron from ferrous sulphate will be tied up in the soil because of our high soil pH conditions.
Likewise, urea applied as a foliar-feed will green up turf faster than when applied as a granular (or spray when watered in immediately).
Lastly, for the reasons explained above, periodic foliar-feeding has a great burn potential to the foliage. If you wanted to apply monthly fertilizer requirements in two spray applications, you would almost certainly burn the turf at some point. Fertigation is applying 30 days of fertilizer in 15-30 water applications! That's the difference between foliar feeding and fertigation.
1.Foliar-feeding works at the plant level by the entrance of preferred mineral elements through special pore or cracks in the waxy leaf surface layer.
2.The preferred elements for foliar-feeding include iron (and iron cheleates), urea nitrogen, ammonium nitrogen, potassium and magnesium.
3.Cheleate applications of iron (or other cheleate mixes) work better on cool season grasses than bermuda, but responses occurr in both cases. (my own opinion and observations).
4.Foliar applications of fertilizers need to be irrigated in before mowing, for best results.
5.Since leaf penetration takes place better at night, apply a foliar feed spray during the day, let it sit, irrigate late in the A.M. morning hours, shortly before the morning mowing, when possible.
6.Credit when credit is due. Information from Dr. Richard Hull, Turfgrass Physiologist, University of Rhode Island.
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