Some products advise spraying them on the leaves of your trees – a process known as foliar feeding. At first glance this makes no sense, as plants synthesise everything they need from nutrients obtained from the soil and air and these nutrients come up with water through the roots and xylem. And leaves haven’t evolved for nutrient uptake, they have evolved for photosynthesis.
But could this actually work? Well, in order for the nutrients in foliar feed to be useful to plant cells, they would need to both penetrate the leaf and enter the cell.
Can substances on a leaf surface enter the leaf itself? For the most part they can’t as leaves are covered by a protective waxy layer known as the cuticle (described in the post Leaf Structure). One of the main roles of the cuticle is to stop pathogens and other environmental stressors entering the leaf.
But as so often happens with systems of mind-bending complexity like plants – it’s not that simple. For one thing, we know leaves have stomata which allow gas to enter the leaf. But it also turns out that the rest of the cuticle isn’t completely impregnable. The cuticle has tiny pores in it at the base of trichomes (hairy projections) and glands – these range from 0.45 to 1.18nm in diameterref. One study did indeed find that dissolved nutrients can enter the cell through these pores in the cuticle: “penetration of ionic compounds can be fairly rapid, and ions with molecular weights of up to 800 g mol(-1) can penetrate cuticles that possess aqueous pores.” The key term here is ‘aqueous’ – the pores need to be wet in order for nutrients to enter through them. For example carnivorous plants use this process to bring nutrients in from their traps via the pores in glandsref.
A great article summarising the physics of nutrients entering a leaf is here – they conclude that it’s easier for positively charged ions (calcium, magnesium, potassium, ammonium-form nitrogen) to enter via the cuticle pores whilst it’s not as easy for negatively charged ions (phosphorous, sulfur, nitrate-form nitrogen). Similarly smaller molecules or those with a smaller positive charge are easier to translocate around the plant – including ammonium, potassium, and urea. Larger molecules will stay close to their point of entry, including calcium, iron, manganese , zinc and copper. Another study states that younger leaves are less able to transport nutrients out and so applying foliar feed to developing leaves may result in the nutrients staying within the leaf (which perhaps is an effect one might want to achieve?)ref
So it seems that some amount of foliar feed may be able to enter via the cuticle’s aqueous pores, and a subset of this may be able to move around the plant.
But what about the stomata? Previous studies have said that “the combination of cuticular hydrophobicity, water surface tension and stomatal geometry should prevent water droplets from infiltrating the stomata”.ref (ie. water can’t get through stomata) but apparently dissolved ions can in some circumstances, because the ions change the surface tension properties of the liquid. This study ‘confirmed the stomatal uptake of aqueous solutions’ref; but also said this depended on whether the aqueous solution was chaotropic (reducing water tension) or kosmotropic (increasing water tension). So it’s easier for the ions on the left to enter via the stomata, and harder for those on the right.
But once in the leaf, can nutrients be used by plant cells? It seems so, in some cases, but the evidence is extremely varied and there are many different variables to untangle.
A research study was conducted by ‘Christmas Tree Specialist’ Chad Landgren for the Oregon Department of Agriculture in 2009 comparing foliar feeding to other forms of nutrient applicationref. They tested a range of approaches on blue spruce, Atlas cedar and four varieties of fir (abies), in pots and in the ground, using application methods including “helicopters, mist blowers and various backpack sprayers”. Their conclusions were: “Each conifer species and site are potentially different with regard to nutrient needs and response. Blue spruce appears rather “immune” to foliar application… Nordmann fir appeared to pick-up some of the foliar fertilizer… on other sites, no treatment (soil or foliar) appeared to move the foliar nutrient content levels.”
In another paperref the author concludes that “foliar application of particular nutrients can be useful in crop production situations where soil conditions limit nutrient availability.” and that fruit can benefit from direct sprays, but also that “applying fertilizers to leaves (or the soil) without regard to actual mineral needs wastes time and money, can injure plant roots and soil organisms, and contributes to the increasing problem of environmental pollution.”
And then of course it’s not just the leaves themselves. We now know that there is a phyllosphere – a symbiotic community of microbes in and on the leaves which perform a whole range of functions for their hosts, one of which includes producing cytokinins
that can be bioactive within the plant. If foliar feeding increases these bacteria, there may be effects throughout the plant not just on the leaf.ref
The message from all of these seems to be that foliar feeding may work for leaves or fruit with specific mineral deficiencies which need to be corrected in-situ, if the nutrient in question can get through the cuticle or stomata. Or for plants which have environmental reasons for not being able to access nutrients through their roots (like pH?). But there needs to be a specific requirement in a specific location on the tree for it to make a difference – and it will be dependent on the species, environment, nutrient etc. In most cases I would say it would be better to provide the roots with the requisite nutrients instead.