Field trial samples (Mg-Sol, Mg-Sol_Cu, NAC-ZnO, NAC-ZnO_Cu, NAC-ZnS, NAC-ZnS_Cu) and commercial controls (Zinc nitrate, MagnaBond and Kocide) were tested for rainfastness following a published protocol [1] with minor modifications.
Briefly, citrus leaves were collected from citrus seedlings, washed with DI water and air dried. Then, leaf surface area was measured using the LeafByte App. Leaves were individually sprayed with treatment at the same rate they are applied in the field. Same rate was used for Zn and Cu controls. After air drying, leaves were individually dipped in a conical tube containing 30 ml of DI water for 30 seconds under agitation in order to simulate rain (Wash 1 – W1). Leaves were dried and the washing procedure was repeated twice in order to have W2 and W3. A fourth wash (W4) was done using 30ml of ethanol-acid solution for 60 seconds under agitation aiming to remove all the remaining metal on the surface of the leaves. Then, the metal concentration in each wash was measured by Atomic Absorption Spectroscopy. The total metal content per leaf was calculated by summing W1, W2, W3 and W4 content and dividing by the leaf surface area. The percentage of metal released in each wash was calculated by dividing the wash metal content by the total metal content. Triplicates were done and the average was used to report the results on Table 1.
In terms of Mg release, around 60% of the total magnesium content was released in the first wash (W1) for Mg-Sol and Mg-Sol Cu. The presence of Cu in Mg-Sol_Cu seems to have increased the amount of Mg release in W2 and W3 in comparison to Mg-Sol.
In terms of Cu release, Mg-Sol_Cu also released around 60% of the Cu in the first wash (W1) while NAC-ZnS_Cu released more than 80% of the Cu in W1. NAC-ZnO_Cu showed similar behavior of Kocide, releasing only 12% of the Cu in the first wash. The remaining Cu found in W4 for both Kocide and NAC-ZnO_Cu was around 87%, indicating Cu can be sustained released from those samples.
In terms of Zn release, NAC-ZnS and NAC-ZnS_Cu released 90% of the Zn in the first wash, while NAC-ZnO, NAC-ZnO_Cu released around 70% of the Zn, similarly to the control Zn nitrate.
[1] Impact of (nano) formulations on the distribution and wash-off of copper pesticides and fertilisers applied on citrus leaves. M Kah, D Navarro, RS Kookana, JK Kirby, S Santra, A Ozcan, S Kabiri. Environmental Chemistry 16 (6), 401-410 (2019).