A Rapid Technique for Prediction of Nutrient Release from Polymer Coated Controlled Release Fertilizers

doi:10.4236/ojss.2011.12005

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Open Journal of Soil Science, 2011, 1, 40-44

doi:10.4236/ojss.2011.12005 Published Online September 2011 (http://www.SciRP.org/journal/ojss)

A Rapid Technique for Prediction of Nutrient

Release from Polymer Coated Controlled Release

Fertilizers

Shengsen Wang1, Ashok K. Alva1*, Yuncong Li2, M in Zhang3

1USDA-ARS, 24106 N. Bunn Road, Prosser, WA99350; 2University of Florida, Tropical Research and Education Center, Homestead,

USA; 3Shandong Agricultural University, College of Resources and Environment, Taian, China.

Email: *[email protected]

Received July 28th, 2011; revised August 15th, 2011; accepted August 21st, 2011

ABSTRACT

Controlled release fertilizers (CRF) are produced with different rates and durations of nutrient release to cater to dif-

ferent crops with wide ranges of nutrien t requirements. A rapid technique is needed to verify the lab el specifications of

nutrient release rate and duration. Polymer-coated urea (PCU) (43% nitrogen [N]) an d polymer-coated N, phospho rus

(P), potassium (K) (PC_NPK; 14-14-14) fertilizer products were used in this study. Soil incubation of the above CRF

products at 25˚C showed that 63.6% to 70.8% of tota l N was released over 220 da ys (d). At 100˚C in water 100% of N

release occurred in about 168 to 216 hours (h). Regression equations were developed for cumulative nutrient release as

a function of release time separately at 25˚C and 100˚C. Using the above regressions, the release duration for a given

percent nutrient release at each temperature was calculated. These values were then used to establish a relationship

between the release duration at 25˚C as a function of that at 100˚C. This relationship is useful to predict the release

duration at 25˚C of an unknown CRF product by conducting a rapid release test in water at 100˚C. This study demon-

strated that a rapid nutrient release test at 100˚C successfully predicted nutrient relea se rate and duration at 25˚C, for

polymer coated fertilizers. Therefore, this rapid test can be used to verify the label release rate and duration of most

CRF products.

Keywords: Nutrient Requirement, Nutrient Management, polymer Coated Fertilizer, Slow Release Fertilizer

1. Introduction

Potential benefit of controlled release fertilizers (CRF) is

the ability to manipulate the rate and duration of nutrient

release, so that the product can be applied once a year to

supply the nutrient requirement over the entire annual

growing period [1,2]. The duration of nutrient release can

vary for several months depending on the coating speci-

fication and duration of crop growth. Although coating

can be applied on any nutrient granules, much of the in-

terest and justification for coating is on nitrogen (N)

source. This is due to the complexity of N transforma-

tions in the soil that ultimately produces nitrate (NO3-N)

form, which is highly vulnerable to leaching down the

soil profile if not taken up by the plant roots. Under con-

ditions that favour downward transport of water below

the root zone, the NO3-N can be carried by leaching wa-

ter deeper into the vadose zone and can contaminate

surficial aquifer. Hence, there is a need to develop tech-

niques to mitigate NO3-N leaching below the rootzone.

Most of the studies on nutrient release by the CRF prod-

ucts are based on the N release [3,4]. In a coated N,

phosphorus (P), potassium (K) fertilizer the release of N

was faster than that of P and K [4].

Verification of nutrient release pattern of CRF is criti-

cal for evaluation of effectiveness of these fertilizers for

supplying plant nutrients according to the crop needs and

the duration of crop growth. Despite a variety of predic-

tion models and methods to evaluate the nutrient release

[5-10] being developed in the past, there is no consistent

and standardized method being recognized to date [10-

12]. However, these predictions were relying heavily on

the characteristics of the coating materials. Nutrient re-

lease of polymer-coated CRF is primarily temperature

dependent [4,13-16]. However, verification of nutrient

release pattern and total duration at the ambient soil tem-

perature is not feasible due to the prolonged duration of

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:

111111

Y=A +BX +CX [1]

where Y1 = cumulative release at 100˚C; X1 = release

time (in h); A1, B1, and C1 are constants

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:

A Rapid Technique for Prediction of Nutrient Release from Polymer Coated Controlled Release Fertilizers

13.2. Predicting Total N release at 25˚C by Using

Measured Release at 100˚C

Z=P + MZ + NZ [3]

where Z2 = release time (in d) at 25˚C; Z1 = release time

(in h) at 100˚C; The relationship between cumulative percent release of N

and release time is described by quadratic or cubic func-

tions both at 25˚C and 100˚C for both products (Table 1).

The regression equations are highly significant with R2 >

0.98. Using these regressions, the times required for a

different percent release of total N were calculated at

both 25˚C and 100˚C (Table 2).The Z1 and Z2 values for

each CRF product were then used to establish regressions

between release time at 25˚C as a function of that at

100˚C for each product (Table 1). These regressions

were also highly significant with R2 > 0.998.

P, M, and N are constants.

Therefore, once we know the release time at 100˚C for

a given percent release of nutrients from an unknown

CRF product, we can use equation #3 to calculate the

release time at 25˚C for the same percent release.

3. Results and Discussion

3.1. Release Characteristics of Two CRF

Products at 25˚C and 100˚C

At 100˚C (Figure 1), 100% of total N in PCU and PC_

NPK was released in about 220 hours. In contrast, at

25˚C only 63.6% to 70.8% of total N from the above two

products were released over a period of 220 d. At 100˚C,

percent N release from PCU was generally greater than

that from PC_NPK at any given time throughout the in-

cubation, except the initial 7 h. At 25˚C, percent N re-

lease was greater from PCU than that from PC_NPK

during first 100 d. Subsequently, however, the trend was

reversed until the end of 216 d incubation period. The

percent release of N increased at a rapid rate during the

first 56 d at 25˚C, followed by a slow rate during the rest

of incubation.

3.3. Application of the above Model to Predict

Total N Release at 25˚C

If we have an unknown coated product of somewhat

similar coating characteristics, a fast release test can be

conducted at 100˚C. The release time for a given percent

of total N at 100˚C can then be used in either equation in

Table 1 (depending on PCU or PC_NPK) to predict the

time required to release the similar percent of total N in

soil at 25˚C. Thus, the nutrient release pattern at 25˚C

can be predicted by using a fast nutrient release test at

100˚C within h or a few d.

Figure 1. Cumulative release of nitrogen (N) as percent of total N from polymer-coated urea (PCU) and Polymer-coated N,

phosphorus (P), potassium (K) product (PC_NPK) in wate r at 25˚C and 100˚C.

A Rapid Technique for Prediction of Nutrient Release from Polymer Coated Controlled Release Fertilizers43

Table 1. Relationship between cumulative percent release of total nitrogen (Y1 and Y2) and time (in days (X2) for 25˚C and in

hours (X1) for 100˚C), and equation for calculation of nutrient release at 25 ˚C using the release time at 100˚C for any given

percent release.

Fertilizer Temp. Cumulative percent release of total N vs. timeR2 Release time at 25˚C as a function of that at 100˚CR

25˚C 23

222

Y6.381 0.874X0.006X0.00002X 21

0.988 2

Z12.897 3.431Z0.198Z  0.998

PCU

100˚C 2

111

Y3.446 2.618X0.025X0.00007X 3

0.984

25˚C 2

Y4.408 0.492X0.001X 0.990 2

211

Z45.908 14.424Z0.510Z0.007Z  0.998

PC_NPK

100˚C 2

111

Y5.471 1.920X0.014X0.00003X 0.989

PCU = Polymer-Coated Urea; PC_NPK = Polymer Coated N, P, K fertilizer; All regressions are significant at P ≤ 0.001.

Table 2. Calculated time (from Equations 1 and 2) for dif-

ferent cumulative release of N (as percent of total N) for two

controlled release fertilizers.

Time required for respective release

PCU PC_NPK

Cumulative re-

lease of N (per-

cent of total N) 100˚C 25˚C 100˚C 25˚C

h d h d

10 3.4 4.5 4.4 9.6

20 6.7 14.5 6.9 27.6

30 9.0 31.2 11.1 64.9

40 13.4 73.6 19.3 92.1

50 20.1 136.1 27.7 116.9

60 25.2 198.5 37.7 160.0

1 Z2 Z

1 Z

PCU = Polymer coated area; PC_NPK = Polymer coated N, P and K.

Unlike the past methods of prediction of nutrient re-

lease from CRF products, the method described in this

study is rapid, reproducible, and requires no chemicals

for extractions. This method also integrates the properties

of coating material in determining the nutrient release at

ambient temperature in the soil. No tedious extraction

and analytical technique are required, except analysis of

total N in the water. Therefore, the proposed method can

be readily adapted by the fertilizer manufacturer or dis-

tributors for accurate labelling of the CRF release rate

and duration.

4. Conclusions

This study demonstrated that 100˚C nutrient release test

in water was useful for prediction of nutrient release rate

and duration at 25˚C. Therefore, a quick test done at 7 to

10 d is useful to predict the CRF release characteristics at

25˚C.

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