- Author: Brad Hanson

This week I received an email request for information on the soil concentration of herbicide necessary to have a phytotoxic effect. This is a simple-sounding question and I was able to confidently say “Well, that depends….”.

Of course this isn’t a simple question at all because of the great differences among herbicides, soil types, environments, and in plant susceptibility not to mention complicated interactions among the factors. As an example, in the herbicide treatment table IPM guidelines for Pear (http://www.ipm.ucdavis.edu/PMG/r603700311.html) it is obvious that herbicides can be active at quite different concentrations. In this table, there is up to a 200-fold difference in application rates between rimsulfuron (0.03 lb ai/A) and diclobenil or oryzalin (4-6 lb ai/A) which can give between 4 and 10 months of residual control of various weeds.

When thinking about herbicide concentrations in soil or when interpreting lab results it is useful to be able to estimate the likely starting concentration using a couple “rules of thumb”.

- One of these is the estimated weight of 2 million lbs for a 6-inch acre furrow slice (a furrow slice is the depth of tillage for a moldboard plow that “slices” and inverts a strip of soil with each pass. One acre furrow slice is 43,560 ft2 by 6 inches deep).
- Given this estimate, if we applied 4 lb active ingredient per acre and then incorporated it 3 inches deep (1 million lbs), we would have started with approximately 4 parts per million (4 lb ai in 1,000,000 lb soil).
- We can get a little closer to a given situation if we know the actual measured bulk density of soil in the field. Most agricultural soils have a bulk density of 1.2 to about 1.7 g/cm3 (75 to 106 lb/ft3). Soil surveys or tools like this one: http://www.bio-formllc.com/obt-soilbulkdensities.html also can be used to get a closer estimate.
- If we have a sandy loam soil with a bulk density of 100 lb/ft3 (1600 kg/m3 which is the same as 1.6 g/cm3) and apply the same 4 lb ai/A herbicide and incorporate to 3 inches (0.25 ft) we could estimate an initial herbicide concentration of 3.67 ppm.

A couple of important reminders about estimating herbicide concentrations this way:

This is an ESTIMATE of INITIAL concentration and really is a “ballpark” figure. Actual concentrations will be affected by rates of application, incorporation (dilution) depth, and time since the herbicide was applied.

- The effect of application rate should be obvious – if all other factors are the same, twice as much herbicide will result in twice the initial concentration.
- If the herbicide is incorporated more deeply due to tillage or movement with water the concentration will be reduced or diluted. This can contribute to failures of some herbicides. Similarly, if the herbicide is incorporated more shallowly or not at all, the concentration will be relatively higher (less dilution).
- Finally, the time since application will affect the concentration of herbicides in soil because of degradation and other dissipation processes.

With soil residual herbicides, the initial concentration is usually considerably higher than the amount necessary for herbicidal activity. This is the reason for the residual activity – the concentration remains above the threshold level for a period of time even as degradation processes begin (see the example figure). I put this figure together for discussing problems with too much persistence or too little persistence resulting in poor weed control performance or rotational crop injury. I'm glad to say that herbicides

*usually*work the way they are supposed to!

These estimates can be a useful starting point when thinking about herbicide concentrations in soil or when interpreting analytical results. However, the actual concentrations necessary to cause phytotoxic activity can vary dramatically among plant species and herbicides. That’s a discussion for another day.

Lets' take your example of a 1 sq ft by 4 inch (1/3 ft) garden bed and combine it with the soil bulk density of 100 lb/ cubic foot in the 4th bullet in the post above. Based on that, your (very small) bed will have about 33 lbs of soil (1 sq ft x 0.33 ft deep x 100 lbs).

However, to estimate the initial herbicide concentration in that amount of soil, we'd need one more piece of information which is the herbicide application rate. With this information, usually expressed on a per acre basis for large scale applications or on a per 1000 sq ft scale for smaller applications, it is relatively simple to calculate how much formulated product was applied per square foot. From there, you can use the percentage active ingredient in the formulated product to figure out how many lbs of herbicide active ingredient could be distributed in the 33 lbs of soil in your example.

So, let's go back to my example above. Assuming you applied a herbicide at a 4 lb ai/A rate - 4 lb/A x A/43560 sq ft x 33 lb (1sq ft x 0.33 ft x 100lb) = approximately 0.00000278 lb active ingredient per lb soil or 2.78 ppm.

Of course if you applied a lower use rate product, say 1 lb ai/A, your starting concentration would be 1/4 of that or 0.69 ppm

Hope that helps you with your estimation.

Brad

How can I determine the herbicide concentration in the soil based on the following information :

- Active ingredient = 50% SC per L

- Application rate = 4L/ha

- Area of micrcosm applied = 0.25m square

- Depth of soil examined = 10cm

- Spray volume = 1L

- Volume of herbicide diluted = 10ml

- Soil type = Sand(46.8%), Clay(27.2%), Silt(26.0%)

That being said, here's the mathematical steps I'd take:

1 1 L product has ____ g of AI (you're missing this information)

- 4 L product per hectare

- 1 hectare = 10,000 sq meters

- 0.25 sq meters per microcosm

- one microcosm has 25,000 cubic centimeters (25 x 25 x 10 deep)

- 1 cubic centimeter weighs about 1.5 g (from the range in the post above, you can correct if you more about your specific soil)

If you can figure out how many grams of AI in your herbicide product, you can multiply through all these conversions and end up a reasonable estimate of grams of AI per gram of soil.

Good luck.

Brad

Actually the herbicide is atrazine (Atraforce) sold as a liquid formulation in 1L container. On the container it's written " Active ingredient = Atrazine 50% SC. " 10ml of the herbicide was diluted in 1L clean water and was sprayed on soil dimension of 0.5m x 0.5m (0.25m square). Using the initial information provided, how can I determine the herbicide concentration in the soil at 10cm depth.

1. You have 500 g of atrazine per 1000mL of Atraforce (500SC)

2. You applied 10 mL of Atrafoce to your plot (the spray dilution doesn't matter in this case).

3. Mathematically, you applied 5 grams of atrazine to the area.

4. We calculated earlier that you probably have just under 10,000 grams of soil in your plot (25cm x 25 cm x 10 cm deep x 1.5 g per cm3 = 9375 g). Let's use 10,000 g for easy calculation

5. So, you would have started with about 5 g atrazine in 10,000 g of soil.

6. Scale this up to 5 divided by 10,000 is 0.00005 g atrazine per gram soil.

7. More conventionally, you'd scale this to parts per million which gives you an estimated starting herbicide concentration of roughly 500 parts per million (ppm).

(0.2 ml Atraforce / 1 L water) x (500 g atrazine ai/ 1 L Atraforce) x (1000 mg ai atrazine / 1 g atrazine ) = 100 mg atrazine ai per 1 L water.