Controlled release fertilizers (CRFs) insure controlled release of nutrients due to their coating. The aim of this study was to investigate the effect of nutrient solution parameters as well as time on the evolution of NO3, NH4, PO4 and K concentrations. For this reason, 0.5 g of Multicote fertilizers 14-14-14 and 15-7-15 dissolved in 100 mL and 300 mL of deionized water respectively. The solutions remained at 24oC and their pH was adjusted once during their preparation at 5.5, 6.0 and 6.5 or adjusted every three days for a 24 days period. A linear empirical mathematical model was developed for the prediction of the above mentioned nutrients concentrations (CX) in relation to the remaining (Vs) and the removed (dVs) volume of the nutrient solution, its pH and time (t) from its preparation. The model output compares favorably with data for the prediction of the concentration of these nutrients.
Controlled release fertilizers (CRF) were introduced in the global market in recent
years, in an attempt on behalf of the fertilizer industries to find ways to cover
different needs either of crops, or of the producers themselves
In experiments concerning the use of CRFs in free water solutions and water
saturated substrates, it is referred that the type of medium affect the nutrients release
rate
This paper examined the effect of some major characteristics of the solution such as pH, volume and temperature, the type of CRFs and the time from solution preparation on the alterations of nutrients concentrations. The results were used to develop an empirical mathematical model for the prediction of the evaluation of NO3, NH4, PO4 and K concentration in aqueous solutions. 2
For the purposes of the experiment, two types of Multicote fertilizers 14-14-14 and 15-7-15 (100% coated) were used for the preparation of two groups of aqueous solutions (six solutions in each group). Each solution in the first group had 100 ml volume and was prepared with the use of 0.5g of 14-14-14 fertilizer, while in the second group each solution had 300 ml volume and was prepared with the use of the same weight of 15-7-15 fertilizer. In two of the solutions from each group, the pH was initially (t0) adjusted to 5.5, 6.0 and 6.5. In half of the above mentioned solutions the pH was adjusted once during their preparation (no buffered solutions), while in the other half solutions the pH was adjusted every three days (buffered solutions), in the initial value (namely 5.5, 6.0 and 6.5), with the addition of NaOH or HCl. During the experiment period the solutions were kept at a fixed temperature (24°C). The concentrations of NO3, NH4, PO4 and K, the volume of the solutions remained after sampling and the pH were measured every three days for a 24 days period. The above measurements performed using LAMOTTE Smart 2 colorimeter, volumetric cylinder and HI991300 Portable pH/EC/TDS meter.
Data were statistically analyzed by analysis of variance (ANOVA) using Statgraphics Centurion XVI. Duncan’s multiple range test was used at a significance level of 0.05.
3.1
The pH evolution in no buffered solutions, where the pH was adjusted once at t0 at 5.5, 6.0 and 6.5, is shown in Figure 1. However in these solutions the initial pH value and the type of CRF used for the preparation of the solution seems to affect significantly the evolution of pH during the experiment period.
In the above mentioned solutions with 300 ml volume prepared with the use of 157-15 fertilizer, pH varied slightly from the initial pH. As shown in Figure 1, when pH was initially adjusted to 5.5 or 6.0 it stabilized after fifteen days from the solution preparation close to 5.7, while in solution where the pH was adjusted initially to 6.5, it remained almost stable to the initial pH. In contrast, the pH in solutions with 100 ml volume prepared with the use of 14-14-14 fertilizer, increased significantly during a period of three days after its preparation. After this period the pH in all solutions stabilized at a value which was also relative to the initial pH of solutions. In solutions where pH was initially adjusted to 5.5 it stabilized at 6.0, while in solution where the pH was adjusted initially to 6.0 or 6.5 it stabilized at 6.5. The highest pH stability over most of the experiment period of solutions prepared with the use of 15-7-15 fertilizer compared to those prepared with the use of 14-14-14 fertilizer, may be due both to the type of the fertilizer and the volume of the solution.
In consequence pH adjustment was more efficient in solutions with volume 300 ml prepared with the use of 15-7-15 fertilizers than in those with volume 100 ml prepared with the use of 14-14-14 fertilizers, when it was attempted every three days, as presented in Table 1.
Fig 1. Evolution of pH in no buffering solutions prepared with 14-14-14 (▬) and 15-7-15 (- -) fertilizer, where the initial pH was adjusted to 5.5 (■), 6.0 (●) and 6.5 (▲).
In specific, both pH stability and the efficient pH adjustment observed in solutions prepared with the use of 15-7-15 is due probably to the lower content in P2O5 this type of fertilizer has, compared to 14-14-14 fertilizer. It is a notable property of phosphate fertilizers to develop an alkaline pH in aqueous solutions when they are in presence. In addition the ability to predict the pH evolution of aqueous solution prepared with the use of CRFs, is of particular importance when it concerns nutrient solutions for soilless cultures.
The results of the measurements concerning the alteration of NO3, NH4, PO4 and K concentration in solutions where the pH was adjusted every three days to the initial value, reviled a similar evolution of the concentration of these elements, regardless to the type of CRF used for solutions preparation. However, the concentrations that reached each one of the nutrients were significantly affected by the pH of the solution and probably by their volume (Figure 2). As seen in the above mentioned figure, both concentrations of NO3 and K increased significantly during a period of nine days from t0 where 14-14-14 fertilizer was used for nutrient solution preparation (Figure 2 A1 and B1) and three days from t0 where 15-7-15 fertilizer was used for the same purpose (Figure 2 A2 and B2). The concentration of the above mentioned nutrients decreased during the next six days and it reached the minimum value twelve to fifteen days from t0, where 14-14-14 fertilizer was used for nutrient solution preparation (Figure 2 A1 and B1), and six to nine days after t0, where 15-7-15 fertilizer was used for the same purpose (Figure 2 A2 and B2). A similar alteration pattern was observed for both nutrient elements during the next period until the end of the experiment.
The concentration of NH4 in solutions prepared with different CRF type, altered following almost the same pattern (Figure 2 C1 and C2). The pH and probably the volume of the solution seems that also in this case affected the rate and the concentration increment as well as the higher concentration level that NH4 reached in these solutions.
However similar higher concentration levels of NO3, NH4 and K were observed in the solutions prepared with the different CRFs, probably because of their similar composition concerning N and K. The decrease of NO3, NH4 and K concentration shown in Figure 2 may be attributed to the KNO3 and NH4NO3 complexes, formatted in the solution, when the concentration of NO3, NH4 and K was increased. In that solutions precipitations were visible.
In contrast PO4 concentration was continuously increased after solutions preparation except from the solution with volume 100ml prepared with the use 1414-14 having pH 5.5 where a decrease of PO4 was observed fifteen days after t0 (Figure 2 D1 and D2). This may be occurred to KH2PO4 and NH4H2PO4 complexes formed in the solution due to the high concentration of K and NH4. In that solutions precipitations were visible. In addition the above complexes formation may be favored because of the higher content in PO4 that 14-14-14 fertilizer has.
According to the above, since pH affect significantly the release, and therefore the concentration of nutrients in the solutions, may become an alternative to the temperature in order to control efficiently the nutrients elements concentration in nutrient solutions used in soilless culture. 3.3
The above mentioned results reveal that the pH, the volume of the solution, the time
from their preparation and the type of the CRF fertilizer play a significant role on the
evolution of NO3, NH4, PO4 and K concentration. Specifically the pH of the aqueous
solution and undoubtedly their volume seems to affect both the nutrients release rate
and the level of their concentration, regardless to CRF type used for their preparation.
The time is another parameter that also undoubtedly affect the release rate of the
nutrients since in CRFs fertilizers coating membrane plays exactly this role, to
control the nutrients release in a course of time
Taking into account all the above mentioned parameters, an empirical mathematical equation to estimate the NO3, NH4, PO4 and K concentration in aqueous solutions prepared with 14-14-14 and 15-7-15 CRF fertilizers was developed. The form of the equation if the following:
CX = a + b * Vs + c * dVs + d * pH + e * t (1) Where: CX = the concentration of NO3, NH4, PO4, K at time t
Vs = the sum of the volume removed by sampling dVs = the volume of the solution removed in each sampling pH = the pH at time t t = the time (in days) from the preparation of the solution
To calibrate the model described with equation (1), measurements performed in 100 and 300ml solutions prepared with the use of 14-14-14 and 15-7-15 CRF, were used. Statgraphics Centurion XVI software was used in order to estimate the a, b, c, d and e parameters used in equation (1) and presented in Table 2 and 3.
Figure 3 shows the 1:1 linear correlation between measured and estimated from the equation (1) values of the concentration of NO3, NH4, PO4 and K in aqueous solutions prepared with the use of 14-14-14 and 15-7-15 CRF. The correlation of the following values is linear, since the equation describing the relation between the measured and calculated values has the form of y = a * x + b, in which a and b do not differ statistically from the values 1 and 0 respectively (Gauch et al, 2003). 4
In this work similarities concerning the evolution of NO3, NH4, PO4 and K concentration were reviled among aqueous solutions with different volumes, which were prepared by using CRFs with different composition. In all these solutions parameters such as pH, the volume of the solution, the type of CRF fertilizer used for their preparation, as well as the time from their preparation affected significantly the rate of nutrients release and the evolution of nutrients concentration.
Based on the above mentioned parameters an empirical mathematical model was developed and calibrated in order to predict the concentration of NO3, NH4, PO4 and K in aqueous solutions prepared with the use of CRFs. The model output compares favorably with data for the prediction of the concentration of these nutrients.
Although most of the researchers refer that temperature is the most important factor that influence the diffusion and therefore CRFs nutrients release, the above measurements revealed that pH might affect significantly the release, precipitation and the final concentration of nutrients in aqueous solutions.
However additional research is necessary to identify other parameters that may affect the release of nutrients from the CRFs in aqueous solutions. This information could be used to improve the mathematical model efficiency. With the perspective of using CRFs to prepare nutrient solutions, the above mentioned model can be used to determine the optimal solution management. Fig 2. Alteration of NO3, NH4, PO4 and K concentration, in solutions prepared with the use of 14-14-14 (▬) and 15-7-15 (- -) fertilizer, where the pH was adjusted in the initial value 5.5 (■), 6.0 (●) and 6.5 (▲). Acknowledgments. We are thankful το Haifa South East Europe Ltd company for granting the necessary quantities of Multicote fertilizers to carry out the experiment.