Nutrient Cycling @ 1997 Kluwer
179
in Agroecosystems 46: 179-187, 1997. Academic Publishers. Printed in the Netherlands.
Effects of temperature on nutrient releasefrom slow-releasefertilizers I. Commercial and experimental products
M.E. Engelsjord’, 0. Fostad’ & B.R. Singh2 ‘Department of Horticulture and Crop Sciences, Agricultural University of Norway, PO.Box 5022,1432 is, Norway 2 Department of Soil and Water Sciences, Agricultural University of Norway, l? O.Box 5028, 1432 is, Norway (*Corresponding author) Received
27 December
1995; accepted
in revised
form 30 July 1996
Key words: electrical conductivity, leaching, nitrogen, pH, phosphorus, potassium, release pattern, slow-release fertilizers, temperature
Abstract We studied the effect of temperature on the release of N, P, and K from slow-release fertilizers (SF@). The study was conducted in micro-lysimeters filled with moist peat medium. Increasing the temperature from 4 to 12 “C slightly increased N release from three different slow-release N (SRN) carriers with different particle sizes and coating thicknesses. At 21 ‘C! the rate of release was significantly different than the other two temperatures. Urea formaldehyde (UF), sulphur coated urea (SCU) and coated calcium nitrate (CCN), incubated in sphagnum moss peat, released between 3 and 20% of the applied N in six weeks. For eight synthetic and organic NPK carriers, the release pattern was similar to UF and SCU. However, the leaching losses of N from the NPK fertilizers were up to twenty times more than for the SRN products. Except for Osmocote @ and Duna, which released 30-40% of the applied N as mineral-N within six weeks, all other slow-release and slowly mineralized NPK carriers acted like readily water-soluble compound NPK. Temperature did not affect the nutrient release from NPK fertilizers. Introduction Fertilizer use efficiency can be increased by minimizing losses due to leaching, surface-runoff, ammonia volatilization and denitrification. Under ideal soil fertility management, nutrients are supplied through fertilizers at times and in amounts to meet the need of the plants above that available from the soil. To achieve this objective, there have been efforts in the last two decades to develop fertilizer products which release nutrients at rates and concentrations to allow plants to maintain maximum growth but minimize loss (Goertz, 1991; Hauck, 1985). Slow-release fertilizers (SF@) with nutrient release patterns which are slowed by solubility, mineralization rate or coatings, are primarily used on ornamental plants, high value vegetable and fruit crops and turfgrass.
Compared with conventional soluble fertilizers and fertilization techniques which result in a fast green-up and a growth flush, SRF have a longer duration of nutrient release, and thus provide the possibility of better utilization of applied nutrients. Lower risk of N loss by leaching and runoff from soil, reduction in chemical and biological immobilization reactions that decrease the supply of plant available N, and reductions in both quantity of fertilizer used and in frequency of applications, are among the advantages of SRF reported in the literature (Hauck, 1985; Waddington, 1990). Most SRF are N products or coated urea. Joshy (1979) listed high N content, high reaction rate and lower cost per unit N, as the most important factors in choosing modified ureas. Nitrogen is released from sulphur coated urea by degradation of the wax/oil in flaws of coating and diffusion of dissolved urea through flaws in the coating (Waddington, 1982). In contrast
180 to sulphur coated water soluble urea, the condensation products of urea and formaldehyde need microorganisms for mineralization. Interest has also developed to coat soluble fertilizers which contain P and K in addition to N (Hauck & Koshino, 1971). The nutrient release mechanism of products like Osmocote ® are somewhat different from products coated with S. Many natural products (manure, compost, blood meal etc.), which are slowly mineralized, have also been added to soils to supply nutrients. Nutrient release from fertilizers can be influenced by soil pH, soil temperature, microbial activity, moisture content and granular size (Allen, 1984; Jarrell & Boersma, 1979; Oertli, 1973; Prasad et al., 1971; Turner & Hummel, 1992; Wilkinson, 1977). Among these factors, temperature is a particularly important factor under Norwegian conditions. Diffusion through partially permeable barriers and break-down of pollymeric or condensation products by microorganisms are strongly temperature dependent. As a result, little N release has been measured from SRF under cool soil conditions, producing slow and unacceptable turf growth (Waddington, 1990). Several methods have been used to predict the rate of nutrient release from slow-release materials. Incubation of fertilizers with soils under both laboratory and field conditions, and release patterns of SRF in free water have been employed. Predicting nutrient release under field conditions based on free water leach tests, is reported to be difficult (Shaviv & Mikkelsen, 1993) and the tests have also been thought by some to be inadequate predictors of agronomic response (Goertz, 1991). Lunt and Oertli (1962) reported that the mineral-N release during incubation in soil usually correlated well with crop response data. Although the N release from SCU has been reported to be similar in water and in moist soil, differences in N release due to soil moisture levels have been observed (Dawson and Akratanakul, 1973; Giordano & Mortvedt, 1970; Hauck & Koshino, 1971; Waddington, 1982). Dawson & Akratanakul (1973) reported that the N release from SCU decreased with increasing soil water content. Giordano & Mortvedt (1970) and Hauck & Koshino (1971) related the decrease in N release with increasing water content to greater stability of the sulphur coating under saturated than under unsaturated soil conditions. Formation of ferrous sulfide under saturated conditions may seal the particle surface and slow urea release two to four weeks after incubation (Hauck & Koshino, 1971)
A wide variation of temperature in different parts of Norway during the growing season, make it important for studies on the effect of temperature on nutrient release from slow-release and slowly mineralized fertilizers to be conducted. This study measured the release of both mineral-N, P, and K from commercial and experimental slow-release and slowly mineralized fertilizers at different temperatures. Experimental products prepared and developed by Norsk Hydro a.s. and their co-operators, were compared with slowrelease N fertilizer products available on the market, in an effort to find a slow-release NPK product with desirable properties and composition. The fertilizers tested were intended to be used for nursery plants and for maintenance of green areas (lawns and sports turf). Since peat is a common growth medium in nurseries, the release of nutrients from slow-release and slowly mineralized carriers were measured in this medium under greenhouse conditions.
Materials and methods
One experiment to determine N, R and K release from different slow-release and slowly mineralized carriers, was conducted in a growth chamber. The three slowrelease N products examined were; Lesco ® sulphur coated urea (SCU), and HydroformTM urea formaldehyde (UF) and coated calcium nitrate (CCN) manufactured by Norsk Hydro a.s. Osmocote ® manufactured by Sierra Chemical Company, Carbols from SvusGlass Institute, and processed chicken (Duna) and pig (Promest neu) manures, as well as sulphur coated NPK (SC-NPK) and compound NPK from Norsk Hydro a.s., were the NPK fertilizers. Polypropylene vials (volume of 300 cm3), placed up-side-down on specially constructed stands, were used as micro-lysimeters. From the outlet opening there was a tube connected to a collector can. The lysimeters were filled with fertilizers and moist sphagnum peat media (Huminal ® ) through a 3.14 cm 2 inlet opening. The peat medium was little to medium decomposed (H2 to H4) according to von Post's scale (Sveistrup, 1984) with a bulk density of 0.07 kg dm -3, nutrient deficient, and had a pH value of 4.0. Prior to fertilizer incubation, the peat was limed with 2 g dolomitic limestone per liter, similar to the practice followed in nurseries. The initial and final pHs in the peat were not measured. The micro-lysimeters were placed in dark growth chambers at 4, 12 or 21 °C. Both inlet and outlet openings were covered to minimize the N loss by ammonia volatilization. Table 1
181 shows the different fertilizers tested and their rates of application. The fertilizer treatments were replicated twice. The lysimeters were leached once a week for six weeks, starting one week after fertilizer application, by gently adding 150 ml of tap water to each microlysimeter. Subsequent to a complete drainage, the volumes of leachates were measured and samples were collected for analysis. The NHJ- and NOs-N concentrations were measured using Flow Injection Analysis (Tecator Application Note, ASN 15 l-02/1992 and ASN 62-02/1983). Both total P and K were determined photometrically by the standard Norwegian methods (1984,1993). The percentage of added nutrients released were calculated from the leachate volumes and the nutrient concentrations, minus the corresponding amounts for a control lysimeter with no fertilizers. Measurements of electrical conductivity of leachates (ECw) were done with a Philips PW 9529 Conductivity Meter. The data on nutrient release, pH and ECw from the fertilizer treatments, were statistically evaluated using an analysis of variance procedure. A Ryan-Einot-Gabriel-Welsch Multiple Range test (REGWQ) was used with a significance level of p=O.O5 using the GLM procedure of SAS (SAS Institute Inc., 1987).
5
.-.a-.-.-.
0 0
1
2
3
Incubatmn
4
pemd
5
6
(weeks)
Figure la. Effect of temperature on cumulative mineral-N release (a, b) and electrical conductivity of leachate (c,d) from slow-release N fertilizers at 4 ‘C (a,c,) and 21 ‘C (b,d). Critical range value for REGWQaos at each date is calculated for the amounts released and not for the cumulative values. - 0 - UF, HD-10+25; -----*UF, HD-8+16; - -X - UF, -10+25; - - + - - UF,-6+14; P SCU-16; + SCU-12; - U- - CCN-T140; + CCNT70
Results and discussion N release from Nfertilizers 0
The N release rates were not significantly different at 4 and 12 “C for any of the SRN fertilizers, but they were significantly higher at 21 “C than the two lower temperatures. On average, less than 10% of the N applied was released as ammonium and nitrate within six weeks from UF, SCU, and CCN fertilizers at 4 and 12 “C (Figure 1). The release increased to 15-20% at 21 “C for UF and SCU, and to 50% for CCN (T70). The four UF products were slowly mineralized, and a slight temperature dependent release of N was observed. On average, between 7 and 15% of the applied N was leached by six weeks at 4 and 12 “C but the amount almost doubled at 21 “C. The highest release rate was observed two weeks after incubation. After three weeks, the mineralization of N from UF granules dropped to < 1% at 4 “C, and < 4% at 21 “C. Only a small increase in N release from SCUs was observed between 4 and 12 “C (data not shown). After six weeks a cumulative N recovery in the leachate
1
2
3
lncubatmn
Figure
4
period
lb.
5
6
(weeks)
Cont.
C I
Figure
lc.
Cont.
182 Tuble I. Types off fertilizers,
nutrient
Fertilizer
composition N
and the rate applied P
K
Rate
-----w-
(g L-l)
Urea formaldehyde,HD-lo+25 Urea formaldehyde,HD-8+16 Urea formaldehyde, -lo+25
39 39 38
3.8 3.8 3.9
Urea formaldehyde, -6+14 Sulphur coated urea, 16% S Sulphur coated urea, 12% S Coated calcium nitrate, T140
38 37 31 12
3.9 4.1 4.1 12.5
Coated
12
12.5
N,fertilizer
calcium
NPKfertilizer Duna (poultry Promest neu Osmocote@ Carbols-WO Carbols-W2 Carbols-W5
nitrate,
manure) (pig manure) Plus (34 months) (without waterglass) (with 2% waterglass) (with 5% waterglass)
Compound NPK Sulphur coated NPK,
0
1
2
3
4
T70
5
5.1% S
6
Incubation period (weeks)
Figure
Id.
Cont.
was 18% at 21 “C, which is twice the amount found at 4 and 12 “C. Similar to UF, the highest rate of N release from the SCUs was observed after two weeks of incubation at 4 and 12 “C. At 21 “C, the N released from SCUs was more constant during the six weeks period. The release pattern for SCUs used in this study is similar to that reported by others (Allen et al., 1971; Hashimoto & Mullins, 1979), although the magnitude and temperature effect differed. The differences in the effect of temperature in N release between this study and those of others are due to different temperatures
5
1.6
2.2
6.7
5 15 18 12 11
1.7 5 4 7 5
4.8 11 8 10 10
6.7 10.0 8.3 12.5 13.6
21.6 15
2.2 6.5
11.6 12.5
6.8 11.0
and materials used in the studies. Relatively higher moisture contents in the peat in this study may partly explain the low rate of N release from SCUs. Under these circumstances there are no organisms to remove the sulphur coating. Similar to UF and SCU, N release from CCN was slightly different at 4 and 12 “C (data not shown). At 12 “C, significantly higher amounts of the applied N were released (Figure 1). As expected, the release of N was higher for T70 (48%) than for T140 (12%). Overall, the total mineral-N recovery in the leachate of SRN was low. Some NH4-N may convert to gaseous ammonia and be lost (Singh & Nye, 1986), or may be adsorbed to humus colloids and thus resulting in less leachable N in the percolating water. Electrical conductivity and pH of leachates Low release of N from SRN fertilizers at 4 and 12 ‘C was also reflected in low ECw values (< 0.5 mS), while the ECw ranged from 0.2 to 2.5 mS at 21 ‘C (Figure 1). The CCN (T70) product produced much higher ECw of the leachate than from CCN (T140) and urea-based SRN. Higher concentrations of salts in the leachate also resulted in lower pH values (Table 2). There was a significant drop in the pH from one week to two weeks
183 after incubation for all the SBN carriers. For SCU this fall in pH can partly be explained by a release of S from the coating. The effect of temperature on leachate pH was only minor. Mineralization of N from organic N and inorganic compounds is known to have an acidifying effect (Mengel & Kirkby, 1987). Microbid decomposition of soil organic matter (peat medium) results in H+ production, and also NH3 and H2S. The latter compounds can be oxidized in the soil to strong inorganic acids, resulting in soil pH drop. The leachate pH in the control treatment, however, did not show any particularly decreasing or increasing tendency during the first two to three weeks. This is an indication of very low microbial activity in the moist peat medium. The drop in leachate pH for the fertilizer treatments suggests that a nitrification of ammoniacal-N released from the fertilizers occurred. Even though nitrification takes place slowly in very acid soils (Harris, 1988; Mengel & Kirkby, 1987), nitrifiers in soils of low pH may be adapted to these conditions (Harris, 1988). The change in leachate pH after three weeks was only small, indicating that the peat system stabilized after a very short time.
a
Figure 2~. Effect of temperature on cumulative mineral-N release (a,b) and electrical conductivity of leachate (c,d) from slow-release and slowly mineralized NPK fertihzers at at 4 ‘C (a,c) and 21 “C (b,d). Critical range value for REGWQ(),tts at each date is calculated for the amounts released and not for the cumulative values. - * - Duna: * Promest neu; d Osmocote Plus; - H - Carbols-WO; JtCarbols-W2; d Carbolsw5; - 4 - Compound NPK; - 0 - SC-NPK.
b
N release from NPKfertilizers
The water soluble compound NPK fertilizer released approximately 95% of the applied N in three weeks (Figure 2) and the rate was not affected by temperature. The release pattern of N from SC-NPK, containing a 5.1% sulphur coating, was generally similar to that of the compound NPK. In the SC-NPK about 60% of the applied N was recovered in the leachate after one week, showing that the sulphur coating had no slow release effect for this fertilizer. The N release rate through the coating of Osmocote@ was slow but increased linearly with time, with 3% released after one week to about 40% after six weeks at 21 “C (Figure 2). Although the release of N from coated fertilizers is effected by soil temperature which affects diffusion (Boon, 1981; Oertli, 1980; Wilkinson, 1977), a temperature effect was not observed in this study for most of the products. Because of nutrient mobility in wet peat medium, diffusion did not control N release. A slight effect of temperature was, however, seen for Osmocote@ . Two different water glass products manufactured by Svus-Glass Institute (former Czechoslovakia) with different granule sizes and high levels of P and K were tested but the data is not presented. The initial
0
1
2
3
4
5
6
lncubatmn period (weeks)
Figure
2b. Cont.
‘“T
C
0
1
2
3
4
Incubatmn period (weeks)
Figure
2~. Cont
5
6
184 Table 2. Leachate
pH from slow-release
N fertilizers
(average
2
Incubation 3
Fertilizer 1
for three temperatures, period
4, 12 and 21 “C)
(weeks) 4
5
6
UF,HD-lo+25 UF,HD-8+16
8.55a rt 0.27’ 8.35a f 0.25
7.50a f 0.24 7.50a f 0.20
7.20a f 0.19 6.90a f 0.49
7.43a f 0.46 7.13a f 0.69
7.15a f 0.40 7.07a f 0.57
6.95a f 6.60a f
UF, -lo+25 UF, -6+14 SCU-16 scu-12
7.87ab 7.9Oab 7.4Obc 7.97ab
7.45a 7.35a 7.23a 7.32a
6.97a * 0.23 6.73a f 0.38
7.08a f 0.32 6.90a f 0.41
6.72a f 0.95 6.65a f 0.97
7.08a f 0.27 7 23a f 0 34
CCN-T140 CCN-T70
6.67~ f 0.58 5.87d f 1.19
7 10a 7.05a 4.92b 4.6Ob
7.55a 7.27a 5.OOb 4.65b
7.20a 7.25a 5.23b 4.6Ob
7.42a 7 20a 5.35b 4.67b
+ f f f
ZMean f standard deviation, po.05. Mean leachate pH of control:
0.20 0.20 0.36 0.32
* & & &
0.29 0.36 0.54 0.43
4.85b EIZ0.20 4.47b + 0.36
Mean values
followed
*
2 3 4 Incubatum period (weeks)
Ftgure
f f k A
0.67 0.44 0 14 0.42
a coloumn
ZIZ 0.38 & 0.33 f 0.39 f 0.45
are not significantly
f f f f
0.51 0 57 0.36 0.55
different
at
4.78 f 0.46.
I
1
0.23 0.24 0.16 0.38
by the same letter within
d
0
f + + +
1.13 1.25
= -
5
6
2d. Cont.
raw materials of the melted fritted glass were Kolaapatite, potash and grinded sand. Because the fertilizers contained only P and K, N (as pulverized ammonium nitrate) was incorporated additionally to the fritted glass products by the manufacturer. The products were prepared with 0, 2, and 5% water glass, respectively. The temperature dependent nutrient release, which was observed for some of the PK-types, were not found for the NPK-types. Water glass coating did not have any effect on N release in peat. Between 70 and 100% of the applied N was collected in the leachate after six weeks (Figure 2). The highest N release was observed for the Carbols 12-7-10 product with 2% water glass, while the “untreated’ Carbols 18-4-8 released the lowest N amount. These results seem to suggest that the water glass treatment may have changed the characteristics of the fertilizer granules, giving higher release rate than the untreated granules. However, the thickness of the water glass coating affected the N release slightly
over the whole incubation period. Similar to the compound NPK fertilizer, 40-50% of the N applied was released between one and two weeks after incubation. Only small amounts of the mineral-N were recovered in the last three weeks of the study. In this study, two manures containing about 5% N were evaluated. The N release from the poultry manure (Duna) was slower than the release from the pig manure (Promest neu). The N release from these products was 30% and 70% after six weeks of incubation. Although the temperature did not effect release from the Promest neu, a slight increase in N release from Duna was observed at 21 “C versus 4 “C. Duna showed a slow and steady mineralization pattern similar to Osmocote@ , but the Promest neu mineralized Xl-60% of the applied N within two weeks. Hibjdrg (1989) found a similar N release pattern for a dry poultry manure product. The relatively large concentrations of mineral-N found in the leachate from all treatments in the beginning of the experiment, may not be entirely related to N release from the fertilizers applied. The weakness of using peat moss as an incubation medium is that its mineralization process may be affected by N application. Added N may cause a more rapid degradation of the moss than without, causing more N to be mineralized from the moss (priming effect). P and K release from NPK fertilizers
The release rates of P and K were different for the eight NPK products, with Osmocctes@ and Carbols products giving significantly lower values (Tables 4 and 5). Less than 20% of the applied P was collected in the leachate after six weeks. The corresponding values for K var-
185 Tub/e 3. Leachate pH from slow-release and slowly mineralized NPK fertilizers (average for three temperatures, 4, 12 and 21 “C) Ferttlizer Duna Promest neu Osmocote@ Plus Carbols-WO CarbolsW2 Carbols-W5 Compound NPK SC-NPK
Incubation period (weeks) 3 4
1
2
6.27a i 0.7tz 5.18ab f 0.93 5.2Oab f 0.91 5 67ab f 0.79 5 58ab f 0.39 5 58ab f 0.85 4.75bc rt 0.65 3.92~ IIT0.15
5.35a ZIZ0.46 4.80ab f 0.62 4.40b rfr 0.24 4.77ab IIZ0.18 4.63b f 0.10 4.78ab * 0.22 4 22b & 0.53 4.55b & 0.24
6.35a f 0 51 5 37bc ?c 0.70 4.57d f 0.24 5.45bc f 0.40 5.35bc f 0.16 5.55b jl0 31 4.78cd IIZ0.46 5.42bc h 0.27
6.70a f 0.32 5 72bc f 0.58 4.68d f 0.23 6.15ab f 0.23 5.98bc & 0.15 5.97bc * 0.55 5.35c f 0.48 5.70bc f 0.17
5 6.65a * 0.42 6.OOabf 0.60 4.90~ f 0.26 6.43ab & 0.48 6.27ab * 0 19 6.67a f 0.29 5 73b + 0.63 5.85b f 0.14
6 6.90a f 5 93ab f 4 88b * 6.37a * 6.48a * 6.17a * 5.95ab * 5.85ab f
0.36 0.77 0.46 0.76 0.22 1 23 0.51 0.23
z Mean f standard deviation. Mean values followed by the same letter within a coloumn are not stgmficantly different at p=O.O5. Mean leachate pH of control: 4.78 + 0.46. Tuble 4 Cumulative amounts of phosphorus in leachate in % of the P applied (average for three temperatures, 4, 12 and 2 I ’ C)
Fertdizer Duna Promest neu Osmocote@ Plus Carbols-WO Carbols-W2 Carbols-W5 Compound NPK SC-NPK
Incubation period (weeks) 3 4 5
1
2
1.9 0.8 0.6 1.2 1.6 1.0 17.4 57.6
16.3 14.8 3.0 4.5 64 2.8 63 2 79.4
30.2 31 0 64 8.2 10.6 5.0 84.9 85.3
39.4 43.9 9.3 11.6 14 3 6.7 95 5 88. I
45.2 53 0 120 14.3 16.9 8.0 101.0 90.2
6’ 49.7bY 59.6b 14.8~ 16.4~ 19.2~ 9.lc 104.5a 91.8a
‘End of experiment. YValues followed by the same letter are not significantly different at p=O.O5.
ied between 19 and 60%. A relatively high P release from the readily soluble NPK compound suggest that P adsorption in the peat medium was very low and does not account for the low recovery. Low P adsorption by organic matter rich soils relative to mineral soils is reported in the literature (Mengel & Kirkby, 1987). Pig manure released significantly more P and K than the poultry manure. Temperature did not have a significant effect on P and K release from these products. Electrical conductivity and pH of leachates The ECw decreased when the mineral-N release decreased to near 0, and thus the mineral-N release was related to ECw. Compared to SRN fertilizers, relatively higher rates of N release were observed from the NPK fertilizers. In addition to P and K release (Tables 4 and 5), N forms were responsible for higher ECw values in the leachate from NPK fertilizers. While the ECw values were relatively constant for the
SRN fertilizers, a rapid decrease in ECw occurred at two weeks with the NPK’s. This corresponded to the release patterns (Figure 2 and Tables 4 and 5), in which high amounts of N, P and K were found in the leachate after two weeks of incubation. No significant effect of temperature on ECw or pH of the percolate was observed for either of the NPK fertilizers. There were large variations in pH of the leachates from different products (Table 3) with Duna having the highest pH values (> 6.0) and Osmocote@ the lowest values (< 5.0). There was an increase in leachate pH with incubation time for NPK products, similar to that for the CCN products. After a significant drop two weeks after incubation, the pH of the leachate water increased during the incubation period (Table 3).
186 Tuble 5. Cumulative amounts (average for three temperatures
of potassium in leachate 4, 12 and 21’C)
Fertilizer
Duna Promest
Incubation
neu
Osmocote@ Carbols-WO Carbols-W2 Carbols-W5 Compound SC-NPK
Plus
NPK
2
3
4
5
8.6 20.1
30.2 60.9
44.0 76.2
53.0 83.7
60.1 89.3
66.8~~ 93.3a
1.5 14.0
5.9 43.2
11.0 54.4
15.3 58.1
19.1 60.5
22.7f 62.8~
11.0 7.2 21.6 52.6
31.1 22.1 61.7 73.0
37.8 27.4 71.8 77.0
40.8 29.3 74.8 78.9
42.9 30.8 76.2 80.4
44.9d 32.4e 77.3b 81.8b
Conclusions
are not significantly
Allen
There was consistently higher N release at 21 “C than at 4 and 12 “C for all products. Organic fertilizers, as well as compound NPK fertilizers coated with S or water glass suspensions, gave significantly higher N release than products with only N, and thus the latter had more slow-release characteristics. The release of P and K from the NPK fertilizers was not affected by temperature. Both Carbols and Osmocote@ released P and K more slowly than the other NPK fertilizers. Only the Osmocote@ product released N, P or K in a controlled manner. Because the other experimental NPK products released N too rapidly as compared to the SRNs and Osmocote@ , further improvements need to make them as slow-release products for nursery plants and green areas.
Acknowledgements The authors wish to thank the Research Council of Norway and Norsk Hydro for awarding a fellowship to the senior author and for financial support of this research project. The technical assistance of KarlMartin Kalfjos (deceased), Ann-Helen Kalfjos, Ellen Zakariassen and Sissel Alvestad is gratefully acknowledged.
References SE (1984) Slow-release nitrogen fertilizers. (ed) Nitrogen m Crop Production pp 195-206. SSSA, Madison, WI
period (weeks)
1
ZEnd of experiment. YValues followed by the same letter p=o.o5.
Allen
in % of K applied
In: Hauck RD ASA, CSSA and
6=
different
at
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