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Water, soil and soil-applied weed control

Application/Equipment, Formulation/Technology, Pre-emergents 101

Know your soil type Soil texture is defined as the percentage of sand, silt, or clay particles. Understanding the differences between these three overarching aggregates and how water behaves with these particles is the first step to understanding how soil-applied herbicides work. Among other applications, knowledge of soil texture can be important when diagnosing in-field issues such as drought stress, compaction and crop diseases. Soil texture can be determined in several ways, from manually performing an analysis (e.g. using the ‘soil ribbon’ method), referring to Canada Land Inventory maps or taking soil cores for particle size analysis by a reputable soil lab. How water moves and interacts with different soil textures Three different forces dictate how water moves through porous materials like soils:
  • Gravity
  • Capillary action (movement between pores)
  • Soil solution (also called osmotic force, or movement across membranes).
Water can move by gravity alone (saturated flow), by a combination of gravity and capillary action (unsaturated flow), or as vapour in dry soils. Water itself has two forces at play. Adhesion, which is the attraction between water molecules and solids, and cohesion, the attraction between water molecules themselves. Both forces are why water droplets visually appear to cling together and onto the surfaces of solid objects. Since the amount of surface area varies between soil types, the behaviour of water also varies. Making soil-applied herbicides work Product rate, soil pH, organic matter levels, as well as soil moisture and texture all influence how well herbicides work in soil. As rain washes soil-applied herbicides (mainly focusing on Groups 13, 14, 15 characteristics) from crop stubble into the soil, an active layer forms in the upper soil profile. This herbicide solution binds to soil and organic matter particles. That means soils with higher clay or organic matter content have greater binding potential compared to sand soils, thus herbicide is more readily available. What happens in sand? The larger particles characteristic of sand soils means there is less surface area, or fewer binding sites, which means the herbicide’s active ingredient is more readily available for plant uptake. A lower rate can be used as a result. If the soils dry completely, however, that will not be unavailable (adsorbed to soil) for plant uptake until rain puts it back into solution – assuming the active has not fully degraded. In extremely wet conditions, the active is readily available and not binding to the coarse soil particles. This gives it greater mobility to move further in the soil profile.  If it moves into the root zone of germinating crop seedlings, temporary crop sensitivity can result. What happens in clay? Smaller clay particles result in more adsorption or product tie up. With more of the active ingredient being tied-up, higher rates need to be used. Since clays also retain more water, however, it is possible for active ingredients to stay within the soil water solution for longer dry periods. There is also less downward movement in overly saturated conditions.

Considering a pre-seed herbicide application? Ask yourself these four questions first:

  1. What is the soil texture of the field you are working in?
  2. Are you using the appropriate rate for the soil type?
  3. When assessing herbicide performance, it is helpful to know how much and how frequently has the field received moisture?
What affects the availability of the chemistry you are using? Soil-applied herbicides can vary in how soil parameters may affect them. The difficulty of in-crop weed control, ongoing resistance issues, and a variety of other factors make soil-applied pre-seed herbicides a practical and powerful choice for western growers. Understanding how these products, soil texture, and water interact with each other helps ensure the right rate is achieves maximum effect. Want to know hear more from the experts? Watch Rachel Evans & Nolan Kowalchuk, technical sales managers with FMC, highlight the ins-and-outs of pre-seed herbicide management.  
Weed Control