<unk> s are classified/grouped in various ways; for example, according to the activity, timing of application, method of application, mechanism of action, chemical family. This gives rise to a considerable level of terminology related to<unk> s and their use. Intended outcome * Control is the destruction of unwanted weeds, or the damage of them to the point where they are no longer competitive with the crop. * Suppression is incomplete control still providing some economic benefit, such as reduced competition with the crop. * Crop safety, for selective<unk> s, is the relative absence of damage or stress to the crop. Most selective<unk> s cause some visible stress to crop plants. * Defoliant, similar to<unk> s, but designed to remove foliage (leaves) rather than kill the plant. Selectivity (all plants or specific plants) * The basis of selectivity is based on physical or biological factors. Some biological factors include morphology, physiology, metabolism, or biochemical factors. *There are some climatic factors affecting absorption including humidity, light, precipitation, and temperature. Foliar applied<unk> s will enter the leaf more readily at high humidity by lengthening the drying time of the spray droplet and increasing cuticle hydration. Light of a high intensity may break down some<unk> s and cause the leaf cuticle to thicken, which reduces absorption. Precipitation may wash away or remove some foliar applied<unk> s but it will increase root absorption of soil applied<unk> s. Drought stressed plants are less likely to translocate<unk> s. As temperature increases,<unk> s' performance may decrease. Absoption and translocation may be reduced in very cold weather. *Selective<unk> s control or suppress certain plants without affecting the growth of other plants species. Selectivity may be due to translocation, differential absorption, or physical (morphological) or physiological differences between plant species. Surfactants alter the physical properties of the spray solution and the overall phytotoxicity of the<unk> , increasing translocation. 2,4-D, mecoprop, and dicamba control many broadleaf weeds but remain ineffective against turfgrasses. * Non-selective<unk> s are not specific in acting against certain plant species and control all plant material with which they come into contact. They are used to clear industrial sites, waste ground, railways, and railway embankments. Paraquat, glufosinate, and glyphosate are non-selective<unk> s. Timing of application * Preplant: Preplant<unk> s are nonselective<unk> s applied to soil before planting. Some preplant<unk> s may be mechanically incorporated into the soil. The objective for incorporation is to prevent dissipation through photodecomposition and/or volatility. The<unk> s kill weeds as they grow through the<unk>  treated zone. Volatile<unk> s have to be incorporated into the soil before planting the pasture. Agricultural crops grown in soil treated with a preplant<unk>  include tomatoes, corn, soybeans, and strawberries. Soil fumigants like metam-sodium and dazomet are in use as preplant<unk> s. * Preemergence: Preemergence<unk> s are applied before the weed seedlings emerge through the soil surface.<unk> s do not prevent weeds from germinating but they kill weeds as they grow through the<unk>  treated zone by affecting the cell division in the emerging seedling. Dithiopyr and pendimethalin are preemergence<unk> s. Weeds that have already emerged before application or activation are not affected by pre-herbicides as their primary growing point escapes the treatment. * Postemergence: These<unk> s are applied after weed seedlings have emerged through the soil surface. They can be foliar or root absorbed, selective or nonselective, and contact or systemic. Application of these<unk> s is avoided during rain since being washed off the soil makes it ineffective. 2,4-D is a selective, systemic, foliar absorbed postemergence<unk> . Method of application * Soil applied:<unk> s applied to the soil are usually taken up by the root or shoot of the emerging seedlings and are used as preplant or preemergence treatment. Several factors influence the effectiveness of soil-applied<unk> s. Weeds absorb<unk> s by both passive and active mechanisms.<unk>  adsorption to soil colloids or organic matter often reduces its amount available for weed absorption. Positioning of the<unk>  in the correct layer of soil is very important, which can be achieved mechanically and by rainfall.<unk> s on the soil surface are subjected to several processes that reduce their availability. Volatility and photolysis are two common processes that reduce the availability of<unk> s. Many soil applied<unk> s are absorbed through plant shoots while they are still underground leading to their death or injury. EPTC and trifluralin are soil applied<unk> s. * Foliar applied: These are applied to portion of the plant above the ground and are absorbed by exposed tissues. These are generally postemergence<unk> s and can either be translocated (systemic) throughout the plant or remain at specific site (contact). External barriers of plants like cuticle, waxes, cell wall etc. affect<unk>  absorption and action. Glyphosate, 2,4-D, and dicamba are foliar applied<unk> . Persistence * Residual activity: An<unk>  is described as having low residual activity if it is neutralized within a short time of application (within a few weeks or months) – typically this is due to rainfall, or by reactions in the soil. An<unk>  described as having high residual activity will remain potent for a long term in the soil. For some compounds, the residual activity can leave the ground almost permanently barren.
Herbicide
Amebicide
Fungicide
Prebiotic 
Deodorant
Herbal medicine
Insecticide
Pesticide