Fertilizer time-controlled release

In contrast, changes in the solvent evaporation rate may also contribute to product improvement in paints. Small quantities of properly selected solvents can improve physical properties and the appearance of paints. Other applications of residual solvents include time-controlled release of fertilizers and production of materials with controlled morphology ... 

Polyesters are also used in controlled-release forms of drugs and agricultural products such as fertilizers and herbicides. By coating the active material with a polyester selected so as to degrade over time, the material is released gradually rather than all at once. 

The effect on the enviromnent deriving from the use of fertilizers and pesticides is an issue of global concern. Controlled release (CR) is a method by which biologically active chemicals are made available to a target species at a specified rate and for a predetermined time. The polymer serves primarily to control the rate of delivery, mobility, and period of effectiveness of the chemical component. 

The periodic administration of chemicals such as drugs, pesticides, fertilizers and herbicides results in a cyclic variation of their concentration with time. Consequently, the concentration levels may vary from dangerously high to ineffectively low levels. Much research carried out over the last 15 years has been aimed at developing techniques for controllably releasing these chemicals. 

Weeds compete with crop plants for valuable nutrients. The traditional method for controlling weeds is to plow them under the soil, where in decomposing they release the nutrients they absorbed while they were alive. Plowing also aerates the soil, but it is either labor-intensive or energy-intensive and can lead to topsoil erosion. In the early 1900s, farmers noted that certain fertilizers, such as calcium cyanamide, CaNCN, selectively kill weeds while causing little harm to crops. This prompted a broad search for chemicals that act as herbicides. Today, a farmer can choose from hundreds of herbicides, many tailored for a specific weed. Farmers in the United States apply almost 600 million pounds of herbicides annually, which is about three times more than the amount of insecticides they apply. 

Some forms of pest resistance involve. hanging the pest rather than the host plant. The best example of autocidal resistance is the sterile male technique used to control. nsects. Scientists rear thousands of males of a particular insect species under controlled conditions, then sterilize them with x-rays. These sterilized males are released in the wild to mate with females of the same species. Since the males are sterile, mated females do not produce eggs, causing populations to drop drastically over time. (The technique is restricted to insect species with females that only mate once.) This method is most successful when the sterile males compete aggressively with the natural population of fertile males. A working example of autocidal resistance occurs at the border of Mexico and California, where sterile males of the Mexican fruit fly are released to help control populations and to prevent the pests from entering California. 

Urea is used as a feed supplement for cattle, up to 8 lb/yr per head of cattle. In liquid fertilizers, urea raises the upper limit for stable nitrogen solutions. In solids it prevents caking due to the formation of ammonium chloride in mixtures of ammonium and potassium fertilizers. Ureaforms which are condensates of urea and formaldehyde, can be used as time-release fertilizers when it is desired to release fertilizing urea at a controlled rate, such as on a golf course [8]. The main use of solid, prilled urea is as side-dressing. 

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