Weeds often reduce crop production as they compete with the crop for nutrients, water, light, gases and space.There are numerous losses due to weeds and they are varied in nature. Losses due weeds and the cost of their control constitute one of the largest expenditure in production of crops and vegetables. These losses are almost equally divided between reductions in crop yields and costs of controlling weeds.About one-half of all tillage done on farms is for control of weeds. Weeds reduce land value, lower crop quality and animal products; result in spread of disease causing animal deaths and offer habitat to insects and opportunity to multiply.Weeds cause problems in water management by clogging ponds, recreational lakes, irrigation and drainage ditches and interfering with navigation. Some weeds may impair the health of human beings on contact or by allergenic reactions from pollen.
Weeds also cause expenses in non-agricultural areas such as roads, railroads, utilities, industrial sites, parks, lawns and forests. One of the important developments in fertilisers in recent years is their use as pesticide carriers (herbicides, insecticides, fungicides). While fertilisers and pesticides are distinct entities and usually are treated as such, pesticides can no longer be excluded from any broad discussion of fertiliser technology and usage. Insecticide and herbicide additives to fertilisers play an important role in the technical and economic position of the industry.For efficient use of nutrients, crops require excellent weed and insect control. Without proper control of weeds and insects investment of growers in fertilisers will be wasted. An actively growing weed is more easily killed with herbicides than one stunted from a nutrient deficiency or for any other reason. This is because a herbicide generally moves into a plant through the vascular tissue. The more rapidly accumulates at the site of action the less likely it is to be detoxified before destroying the plant.Many weeds compete for fertiliser nutrients better than the crop species. Therefore, starter fertiliser is generally placed in furrows adjacent and slightly beneath the planted seed to give the crop maximum opportunity to assimilate it. Later application of fertiliser may be injected into the soil when the crop root system and the growth rate have developed sufficiently. Broadcast application fertilisers on to the soil surface have often benefit weeds more than crop.
Weeds are also influenced by soil pH, so changes in soil reaction can be made to inhibit some troublesome weed species such as Dandelions. Other weeds as red sorrel appear to tolerate acid soil quite well.Proper use of nitrogenous fertiliser has been helpful in weed control. Nitrogen encourages more rapid early vegetative growth and increased crop competitiveness and shading to associate weeds in some instances.Soil fertility does affect insect populations. Over 35 years ago, it was found that grasshopper survival and reproduction were lower when grasshoppers were fed on corn with high nitrogen content. There will be fewer green bugs per gram of foliage and less plant damage where nitrogen is included in the nutrient combinations, with the exception of nitrogen and calcium combination.