Weed Management in Horticultural Crops

HERBICIDE PERSISTENCE IN SOILS

Herbicides applied to the soil are directly affected by the soil characteristics. For effective action, the chemical must remain active near the root zone to kill both germinating seeds and deep rooted plants.

Factors affecting herbicide persistence in the soil:

1. Leaching:

The extent of leaching is determined mainly by solubility of the herbicide in water, the amount of water passing downward and adsorption relationship between the herbicide in the soil.

2. Volatility:

The ester forms of 2, 4-D is volatile and the vapours or fumes frequently cause injury to the neighboring susceptible crops.

3. Micro-organism decomposition:

Organic herbicides are decomposed by soil microorganism. A warm moist, well aerated and fertile soil is most favourable to micro-organisms. Soil pH also influences microorganisms. Medium to high pH accelerates the action of bacteria. Soil rich in organic matter and clay content hold the herbicide for a longer time than the sand or sandy soils.

4. Adsorption on the soil colloids:

Different soil types have different adsorptive capacities. The rate of herbicide application is correlated with adsorptive capacity of soil. For examples:

a. Soils high in organic matter require large amounts of pre-emergence and soil sterilants herbicides for weed control.
b. Soils high in clay contents require more herbicides quantity than sandy soils for pre-emergence or soil sterilant weed control.
c. Soils high in organic matter or clay contents tend to hold the herbicide for longer time than sands. The absorbed herbicide may be released so slowly that the chemical is not effective as herbicide.