A Rapid Technique for Prediction of Nutrient Release from Polymer Coated Controlled Release Fertilizers41
release expected for most CRF, i.e., 3 to 6 months. Me-
dina et al. [17] used a laboratory procedure to predict N
release rate of several slow release fertilizers. The
method include extractions in 0.2% citric acid solution at
4 temperatures; i.e. 2 h at 25˚C, 2 h at 40˚C, and 20 h at
50˚C, and 50 h at 60˚C. They reported that the above
accelerated laboratory extraction procedure was success-
ful in predicting N release rate of some slow release fer-
tilizers. However, this method was not satisfactory for
some other type of slow release fertilizers. Hence the
need for a rapid nutrient release evaluation for verifica-
tion of the product label nutrient release duration. An
alternate approach is desirable to establish correlation
between the nutrient release at high temperature (in a few
d) vs. that at prevailing soil temperature during the grow-
ing season (several d to months). This correlation can be
used to predict the rate and duration of nutrient release at
ambient soil temperature by using the nutrient release
measurement over a short duration at high temperature.
Dai and Fan [18] and Dai et al., [19,20], evaluated the
nutrient release from two resin-coated N, P, K fertilizers
(Trincote 1 and 2) at 25˚C, 50˚C, 60˚C, 70˚C, 80˚C and
90˚C. They used the calibration of nutrient release at
80˚C and 25˚C as a model to predict the nutrient release
rate at 25˚C (in d) using the release results at 80˚C (in h).
The objective of this study was to develop and validate a
rapid test for prediction of nutrient release at 25˚C from
polymer coated CRF products by using the measured
nutrient release rate at 100˚C temperature.
2. Materials and Methods
Two CRF products used in this study included: polymer-
coated urea (PCU); 43% N and polymer-coated N, P, K
(14-14-14) (PC_NPK) fertilizers. Nitrogen release from
these products was determined in water over 220 d at
25˚C, and 220 h at 100˚C.
2.1. Nutrient Release at 100˚C
A constant temperature extractor (Model HKQT, assem-
bled at Shandong Agricultural University, Taian, China)
was used to determine the nutrient release characteristics
from the above two CRF products in deionized water at
100˚C. The extractor consisted of air-tight incubation
chambers (500 ml), stainless steelwire mesh containers,
water bath, and temperature and pressure regulators. Ten
grams of each CRF was placed in one of six wire mesh
containers which submerged into 250 ml deionized water
in incubation chamber in three replications. The incuba-
tion chamber was preheated to 100˚C at 100 kPa. The
extractions (50 ml) were collected at various time inter-
vals from 1 to 220 h following incubation for analysis of
total N in the extract using a total N analyzer (Liqui-
TOCII, Elementar Americas, Inc., Mt. Laurel, NJ). At
each sampling, the remaining extract was depleted and
another 250 ml deionized water was added for the sub-
sequent extraction.
2.2. Incubation at 25˚C
Nutrient release characteristics from CRF products in
free water at 25˚C was evaluated by following the pro-
cedure described by Dai et al. (2006). Ten grams of CRF
product was weighed, sealed in nylon mesh bags, placed
into plastic bottles containing 250 ml deionized water,
and bottles were incubated at 25˚C. Each treatment was
replicated three times. Nutrient release at various sam-
pling time, over 220 d, was measured by sacrificing three
bottles per treatment at each sampling time. Total N in an
aliquot of the total extract was measured, as described
above, and total N released from ten grams of the product
at each sampling time was calculated.
2.3. Model Development
Total duration for over 90% release of nutrients at 100˚C
is in the range of several h or few d as compared to sev-
eral d or months for similar magnitude of nutrient release
at 25˚C. Therefore, a calibration between the nutrient
release rates at 100˚C and 25˚C can be used to predict the
nutrient release rate at 25˚C by measuring the release rate
at 100˚C. This can be accomplished by the following
steps:
Determine nutrient release rates in water for a given
CRF product at 100˚C and 25˚C, until at least 80% to
90% of total nutrients are released at the respective tem-
peratures.
Develop relationship between the cumulative nutrient
release as percent of total nutrients in the product (Y) and
time (X) at each temperature:
2
111111
Y=A +BX +CX [1]
where Y1 = cumulative release at 100˚C; X1 = release
time (in h); A1, B1, and C1 are constants
2
22 22 22
Y=A +BX +CX [2]
where Y2 = cumulative release at 25˚C; X2 = release time
(in d); A2, B2, and C2 are constants.
From the above equations, we can calculate the time
required for release of different percentages (P1, P2, P3,
P4 and P5 etc.) of total nutrients as Z21, Z22, Z23, Z24 and
Z25 etc. (in d) at 25˚C; and Z11, Z12, Z13, Z14, and Z15 etc.
(in h) at 100˚C. Notice that the percent of total nutrient
released is similar for a given pair of release times at two
different temperatures, i.e., Z21 and Z11, Z22 and Z12 and
so on.
Using the above paired values, we can then establish
relationship between the nutrient release time at 25˚C as
a function of that at 100˚C as follows:
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