How are nutrients taken up by the crop?

Two primary points of entry through the cuticle:

Stomata - Openings in the cuticle that open and close to control the exchange of gases and transpiration (water loss). Foliar-applied nutrients can pass through stomata and thus through the cuticle. However, this process is inefficient as stomata are primarily located on the underside of leaves. Also, stomata have the ability to open and close throughout the day and are frequently closed during optimal foliar spray times.

Polar Pores - Areas where water absorbing regions in the cuticle form hydrated pathways that allow diffusion of polar compounds, like certain nutrient sprays, through the cuticle and into the leaf. 

Many foliar fertilizers on the market are inefficient at being taken up by the leaves for a number of reasons. Foliar fertilizers must be soluble in order to readily move through the cuticle. Additionally, many fertilizers on the market (such as humates and lignosulfonates) are restricted from uptake via the polar pores due to their relatively large molecule size. Charged compounds such as salts (nitrates, chlorides, etc.) will interact with the inherently negative charge of the leaf surface, interfering with foliar uptake. Chelation of metals with carbohydrate-based PAC neutralizes the charge of positively charged ions preventing tie-up on the leaf surface.

Polar pores and the associated aqueous pathways through the cuticle have diameters that range from 0.5-5 nm. This means that solutes such as urea (0.88 nm) and PAC chelates (0.77 nm) are readily able to pass through polar pores, whereas larger molecules such as lignosulfonates or suspended oxides and carbonates are not. The efficient use of polar pores requires fully dissolved true solutions of small, uncharged chelates and complexes; all conditions met by PAC Technology.

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Photo Courtesy of NutriAg

Source: NutriAg