THE USE OF HERBICIDES ON PEAT SOILS
- Peat soils are an ideal medium for weed growth due to moisture holding capacity, physical condition and aeration.
- It is difficult to kill weeds by cultivation because of the moist soil conditions which encourage rerooting.
- Many soil-applied herbicides are reduced in activity due to strong adsorption on organic colloids.
The relationship of soil organic matter content to herbicide dose required for equal reduction in growth of a test plant is shown in table 1. The herbicides shown represent a weakly adsorbed herbicide (TCA), a moderately adsorbed one (2, 4-D) and a strongly adsorbed one (chlorpropham). The tremendous increase in dose required for chlorpropham as one goes from a substrate with no organic matter (silica sand) to one with 72 per cent (peat soil) is typical of strongly adsorbed herbicides. Comparing the mineral soil with 3.5 per cent organic matter and the peat soil it can be seen that the dose must be increased over five times to obtain equal activity on peat.
Several commonly used herbicides that are known to be strongly adsorbed by organic matter are listed in table 2. To use such herbicides on peat soils it is necessary to apply high doses and expect residual control only of very sensitive weeds. For example, chlorpropham is commonly applied in the U. S. A. at six pounds active ingredient per acre for weed control in onions grown on peat soils. Even at this high dose, weed control is limited to very sensitive species such as Polygonum spp., Stellaria media and Portulaca oleracea.
Another approach is to use these herbicides post-emergence to weeds wherever they have post-emergence activity. Such activity often can be increased by various spray adjuvants or oil carriers.
A few herbicides are not strongly adsorbed by organic matter and thus are of great value on peat soils. Some of these are listed in table 3. Propachlor is widely used in Europe for weed control in onions and some other crops on peat soils and a close relative (allidochlor) is the most extensively used herbicide in the U. S. A. for onions grown on these soils. Because of the value of weakly adsorbed herbicides on peat, more information should be obtained on other herbicides that may belong in this group.
In view of the problems already discussed it is clear that seldom will one herbicide selectively kill all weeds present in a particular field. Therefore, combinations of herbicides have become even more important on peat than on other soils. Table 4 shows the value of a combination