How much kaolin is too much?

Red and white kaolin on the class panes we use to measure transmittance.

One important question that growers have been asking is how much kaolin can be on a leaf before having negative consequences from too much shading. If you are considering applying another layer, and the original layer hasn’t been washed off, it is important to know how much kaolin can be on a leaf before it begins to have a negative effect. Because we have some idea of how individual leaves respond to different light levels.

To answer this question we measured transmittance with the equivalent of different rates of kaolin. You can see the response plot below. One important feature of the response is that increasing rates of kaolin beyond 50 lbs/acre doesn’t increase the degree of shade to the same extent that rate increases below that level do.

So how much shade is beneficial for citrus plants? We can start by looking at the the maximum amount of light that these rates allow to reach the leaf.

Citrus leaves saturate at approximately 700 umols/m2 s, which means that additional light does not increase photosynthesis. With red kaolin, at the highest expected sunlight intensity (about 2200 umols), 25 lbs/acre reaches the level of shading reduces the light intensity of the exterior leaves to the saturation level, but the white kaolin doesn’t reach the level of 25 lbs/acre of red until 100 lbs/acre of white, though 50 lbs per acre reaches close to saturation.

Given these differences in intensity, we still don’t know why plants covered with the red seems to grow slightly more than those treated with the white. However, levels of red greater than 25-30 lbs/acre risk excessive diminishing light levels to exterior leaves, which means that they may not be able to maximize photosynthesis.

There can be a lot of complexity in terms of how much light reaches further into the canopy, which is part of how kaolin increases whole plant photosynthesis. However, it would seem that the risk of overapplicaton of white kaolin is low. You should be careful, though with repeated applications of red kaolin, unless previous applications have been mostly washed off.

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