Effects of different extract conditions on water dissolution rate of coated controlled/slowed-release fertilizer

Using water to extract coated controlled /slowed-release fertilizers(CRFs/SRFs) and calculate the amount of released nutrient in stated time was an popular method for evaluating nutrient release characteristic of CRFs/SRFs. For different researchers and manufacturers, however, the extract modes and conditions were much different. So in this paper the effects of different fertilizer ( nutrient) -water ratio 1 : 100 , 1 : 20, 1 : 5 ( Fert. -water ratio ) , 1 : 100 , 1 : 50 ( N-water ratio) , extraction modes (continuous and renewing eluant) , temperature (25℃,35℃,60℃) , and eluant pH(using pH 5.0 and pH 7.0 phosphate buffer)on water dissolution rate of coated CRFs/SRFs with different materials were studied. All the fertilizer were extracted for 7 days and their dissolution rate were measured every day. The results showed that the fertilizer concentration in the solution continuously increased under continuous extraction. The fertilizer concentration of the eluant was an important factor affecting on nutrient release of CRFs/SRFs. So enlarging fertilizer-water ratio, renewing eluant could decrease this effect. Temperature had a very important effect on nutrient release of CRFs/SRFs. For both organic and inorganic material-coated fertilizers, increasing temperature could significantly speed up water dissolution rate. But too low or high extraction temperature was not good because too low temperature would spend more time for the CRFs/SRFs to release their nutrients and too high temperature would conceal or reduce the differences of both rate and model of nutrient release among the CRFs/SRFs because the nutrient of the fertilizer released very fast in 1 to 7 day. The fertilizer type and fertilizer-water ratio had significant effects on pH values of the eluants. For the 5 tested fertilizer types,the trends of pH changing were different under continuous extracting and renewing extraction mode in seven days. Under continuous extraction, the pH value increased continuously as time prolonging, the highest pH was about 2.81 unit higher than the lowest one from 1 to 7 day. But under renewing extraction, the pH value decreased a little as time prolonging. The release rates of CRFs/SRFs in pH5 phosphate buffer were much higher than that in pH7 phosphate buffer. Therefore, for CRFs/SRFs, using a certain acid-ascent pH buffer, periodic renewing the eluant or under dynamic flow and same N (P, K)-water ratio at about 30℃ was more reasonable.