Does Green Sand Inhibit Root Growth?????
First off, wow! Quite the response from the Twitterverse on this subject. I was not expecting such an engaged conversation to commence over the blog, but the constructive dialog is certainly welcome, and after all, is why a lot of us use the outlet.
I'll be as transparent as possible when presenting our situation to the audience, and try to cover all questions that came up.
A) The layer that was created and has caused the root inhibition at a depth of 1.75" in our profile was a 65 sieved sand. The sand was painted green and then kiln dried.
We absolutely believe that the physical properties that correlated with the sand could, and probably are, contributing to the root growth inhibition. Another observation of this sand, but only when painted green, was when it was wetted it became an almost paste-like substance. Without the paint, the sand did not do this. This "pastiness" was part of the reason we moved away from the sand which was supported by university researchers. We're not big fans of trying to grow roots in paste.
Again in regards to the particle size alone, we do know of other situations where that same material (sometimes green, sometimes white) was used for routine topdressing from grow in, and from those individuals standpoints, was a contributor in creating a poorer quality plant.
B) We fully support the comment made that there could be many different factors leading to the root inhibition. Topdressing rate, paint/pigment, particle analysis, kiln dried, all could be playing factors.
We understand that paints and pigments may act differently based on numerous things. Does a pigment coat, and bind to the sand particle in the same manner as paint? I have purchased green sand that was dyed with a paint, but not kiln dried. After a few irrigation cycles and a good rainfall, all of the green had been washed off the sand. It's my understanding the kiln drying of the material helps to solidify binding of the paint to the sand particle. Again, it makes sense that those paint molecules are blocking CEC exchange sites, but so far there is no research we know of to support that. If you choose for this to be your thought process, regardless of particle size, there would be some negative effects from the paint.
If you sprayed pigments over sand, would the pigments wash away after rainfall or irrigation? Again, I am not sure, I have not tested this. And if a paint or pigment is washing off, would your plant potentially be taking it up in the rhizosphere? And would that “washing” alleviate any potential CEC deficiencies on the sand particle? Again, I do not know, but it is definitely something to take into consideration.
And yes, "a visible layer does not necessarily mean a physical layer". Sure, if the green sand was a 45 sieved sand it would obtain the same physical properties of the white 45 sieved sand we use now and potentially may have not caused any issues. So yes, there is a chance that the same situation could have arisen if white 65 sieved sand was used in the same quantity over that same period of time. What we do know is that our ISTRIC tests show that the green 65 layer has not effectively inhibited our infiltration rates in that 2" lift of profile. So if water is infiltrating the layer adequately, what is causing the roots to stop growth at this interface?
There is not much research I know of to support the negative effects of kiln dried sand, but the principals behind it may be contributing negatively. We all know glass is made from sand. I have heard wetting agent product developers say that kiln drying sand is essentially an initial step in creating glass. Basically, heating the sand to those temperatures causes that sand to become more of a glass like substance (however small in quantity), and develop more hydrophobic qualities. Obviously this could be an issue with root growth and penetration.
We could create an entirely new blog and conversation on the negative health effects of kiln dried sand or the positive topdressing qualities it presents, but for now we won’t go there. I digress.
So, is it the paint or the physical properties? In this situation, we cannot positively say. And we are not blaming one or the other. We simply know what the physical characteristics of the sand were, that it was painted, kiln dried, and that it created a negative layer in our profile that inhibited root growth. To alleviate any doubt we went away from both of the characteristics to improve the plant health and have seen a much more positive response in the plant. Also, going back to the original post, when the layer was removed, roots thrived. Using dyed green 45 sieved sand may not cause any issues, but we would rather not chance it.
While we are still awaiting more research to come about dyed sands, there is some limited research on the effects of directly applying paints and pigments to greens. GCSAA published this research 15 months ago. Hopefully this may help you formulate some more of your own ideas and opinions on whether or not dyed green sand is a suitable choice as your topdressing material.
Assistant Golf Course Superintendent