Problems in the Use of Nitrogen Fertilizer for Upland Crops in Multiple Cropping Systems
Consumption of Fertilizer in Taiwan
Fertilizer consumption in Taiwan has been increasing steadily over the past four decades. Current estimates indicate that Taiwan's farmers use approximately 1.3 million mt of fertilizer each year, equivalent to 0.4 million mt of nutrients. Before 1970, almost 80% of the total fertilizer distributed was used for rice production. With a rice surplus and the adoption of a crop diversification program, the fertilizer used for rice has declined to less than 40%, while 60% is now applied to upland crops (Ku 1987) ( Fig. 1). The average rate of consumption of N-P 2O 5-K 2O in Taiwan, at 269-78-112 kg/per hectare of arable land, or 188-55-79 kg/ha per crop, is one of the highest in the world.
Upland Crops and Excessive N Applications
The response of upland crops to fertilizer N is generally higher than that of rice, as shown by the much lower yield indices of the -N plots with upland crops compared to those with rice ( Table 1).
The productive efficiency of fertilizer N in fertilizer experiments is thus generally higher for upland crops than for rice ( Table 2).
The higher productive efficiency when fertilizer N is applied to upland crops may give the impression that problems related to nitrogen use are less important for upland crops than for rice. However, the situation is in fact the reverse. Fertilizer applications for upland crops are much higher than the recommended rates, and are often in excess of the ability of the crop to absorb them. Since the response of crops to nitrogen is generally high, farmers are inclined to use higher rates of nitrogen than are required.
As seen in Table 3, the yields of upland grain crops in the farmer's plots ( = check) are lower than those in the demonstration plots, despite the higher rates of nitrogen applied. The higher efficiency of fertilizer-N in the demonstration plots was generally because of better application techniques and related cultural practices.
The rate of N applied to vegetables, in particular, is far higher (often 3-5 times) than that absorbed by the crops. ( Fig. 2). Vegetable crops receive such heavy applications of fertilizer partly because heavy applications of nitrogen fertilizer give rapid growth of leafy vegetables, with consequent good quality and high prices in the market. Furthermore, the majority of vegetables are harvested during the vegetative growth period, during which a higher concentration of N is needed up to the time of harvest.
Fruit trees in Taiwan also generally receive high rates of N. Examples of the rates of N applied in citrus orchards are shown in Table 4. N is applied in the form of both chemical fertilizer and organic manure. Large quantities of various kinds of beancake, meal and livestock manure are applied as organic fertilizer, with a fairly rapid rate of decomposition in the soil. Consequently, the amount of the nitrogen applied is estimated to be six to twelve times higher than that removed by the harvested fruit.
High Residual N in Soil after Upland Cropping
Amount and form of residual N
Fertilizer N added in excess of crop requirements accumulates in the soil, predominantly in inorganic forms. The content remaining in the soil after upland crops have been grown is usually high, because of excessive application of N fertilizer. The residual inorganic nitrogen in the top (0-30 cm) layer of soil when corn was harvested from a plot to which 180 kg/ha of N had been applied was 257 kg/ha ( Table 5). It is not clear why the difference in the amount of residual inorganic N in the plot to which 60 kg/ha of N had been applied and that receiving 180 kg/ha exceeded the difference in the amount of applied N given to these plots. However, the fact of the high residual N left in the soil must be recognized.
The amount of residual inorganic nitrogen in strawberry fields was also great, i.e. 309 kg/ha, as shown in Table 6. In all cases, nitrate was the major constitutent of the inorganic nitrogen. The level of residual inorganic nitrogen in fields producing vegetable crops was also high. One example, a field of Chinese leeks after one year of cultivation and given different levels of N, is shown in Table 7.
Some of the nitrogen fertilizer applied, in addition to that remaining as inorganic nitrogen, must have been immobilized in the form of organic N. The percentage of this fraction in the residue is lower than the fraction of inorganic under excess chemical N applications. The level of organic nitrogen remaining from organic fertilizer applications, on the other hand, can be high when there is a high rate of application.
Amount of Residual N Related to the Rate of N increase in application rate. However, according to Broadbent and Carlton (1978), this generalization is not valid with regard to optimum nitrogen fertilization for field grain crops. Much evidence indicates that the efficiency of fertilizer-N absorption remains relatively constant with increasing rates of nitrogen fertilizer, at least until the level at which the maximum yield is obtained (Fried 1978). If the levels of applied N then continue to increase, the uptake rate tends to fall rapidly, while the yield does not increase further but in fact decreases. Thus, maximization of yield is not contradictory to the maximization of nitrogen uptake efficiency and the conservation of energy. Crop yields are maximized at N application rates with maximum uptake efficiency and at these input rates, little fertilizer N is retained in the soil. Residual inorganic N in the soil increases at application levels above that value ( Fig. 3).
If fertilizer applications were always at the level to give an efficient response, as shown by Broadbent and Carlton, losses of N from N-fertilized cropland would be minimal. However, maximum economic efficiency of production usually requires above-optimum inputs of N fertilizer (Keeny 1982), particularly with regard to vegetable crops. As a result, N recovery by agronomic crops is seldom more than 70%, and the average value is probably nearer to 50% (Olsen and Kurtz 1982). In addition, the optimum rate of fertilizer application is influenced by a multitude of site-specific conditions and by the weather (Keeney 1982). Our experimental results also indicated that residual inorganic N in the soil increased with the rate of N applied, in spite of the fact that the yield was still increasing ( Fig. 4). Residual N in Soil and Its Impact on the Succeeding Crop and Environment
A long-term fertilizer trial conducted over 48 years has indicated that the loss of nitrogen is marked under the subtropical and tropical conditions of Taiwan.
The data in Table 8 indicates that of the 8075 kg of N/ha added over 48 years to the plot treated with NPK fertilizer (total of 85 crops, each with 95 kg of N/ha), only 25% was absorbed by the rice plants. On average, 4.5% was left in the soil, and about 70% was lost from the soil. The same tendency, although to a lesser extent, was observed in the plot treated with farm-yard manure. Similar experiments conducted in temperate countries usually show that a significantly higher percentage of the nitrogen added accumulates in the soil. The turnover of organic carbon in soil is apparently more rapid in tropical and subtropical countries than in temperate regions.
Residual N in soil which is present in the form of nitrate is commonly lost through leaching, or denitrification, or a combination of these.
The loss of N from leaching in wet rice cultivation is usually small, but losses from soil producing upland crops can be quite high. Leaching losses range from 13 to 102%, and generally averaged 25 to 50% of the N applied in most cropping situations in southern California under irrigation (Legg and Meisinger 1982).
The loss of the residual N in the form of nitrate through denitrification is significant when the soil after upland cropping is flooded and prepared for rice cultivation. As shown in the previous table ( Table 5), the content of nitrate N was high in the plot with a high N level compared to that with a low N level at the harvest of corn. However, the yields of paddy rice in these plots, to which no fresh fertilizer N application was made, are much the same, showing no residual N effect from the previous crop ( Table 9).
A tracer study in the same field also indicated that the uptake by the rice crop following corn of residual fertilizer N from the soil is small, only 1.4-1.9% of that added to the previous crop, while the amount remaining in the soil was also small (about 14%) (Lian 1990b).
The nitrate nitrogen leached from the soil may pollute groundwater, while that which runs off may pollute surface water. It has been reported that in Techi Reservoir in Central Taiwan, eutrophication exacerbated by fertilizer run-off from surrounding orchards and vegetable gardens is a serious problem (Hsieh et al. 1989). An increase of the nitrate content in the groundwater of certain areas under intensive cropping has also been reported (Chi 1975).
Flooding followed by plowing, levelling and draining is considered to be an effective way of avoiding salt accumulation on the soil surface after continuous cropping of vegetables with high fertilizer inputs. However, it has been pointed out that the N 2O emitted during the denitrification process may contribute to the depletion of the ozone layer in the stratosphere (Keeney 1982).
On the other hand, the residual N in the form of organic N or the crop residue itself is mineralized, released and utilized during the growth of the succeeding crop, particularly at the later stages of growth. The residual nitrate nitrogen is also utilized by the succeeding crop when there is continuous upland cultivation. The residual effects of heavy nitrogen applications to vegetables grown before rice can be carried over to the subsequent rice crop from flower initiation to the heading stage (Hayami 1975). If the supply of nitrogen is too great, ripening of the rice is retarded and there may be lodging.
Furthermore, the acidity induced by the fertilizer N must be neutralized sooner or later, if good soil condition and productivity are to be maintained. It is generally accepted that 1.8 kg of effective lime is required to neutralize each kg of NH 4-N applied in fertilizer (Olson and Kurtz 1982). The induced acidity is greater with larger excess applications of N which result in a higher level of residual N. Efforts to Minimize and Utilize Residual N
Since the amount of residual N generally increases with higher rates of N application, many scientists have suggested that N fertilizer applications be restricted to the rates that are significantly below the economic optimum in order to minimize pollution from fertilization (see e.g. Keeny 1982). Such a proposal, however, may not be acceptable to farmers, who may lose the opportunity for higher yields and profits.
The adoption of an appropriate cropping system, combined with soil testing which measures the residual N after upland cropping, and adjusting the application of fertilizer N to meet the needs of the subsequent crop, may be able to both reduce and utilize the residual N.
Although studies on soil testing for paddy rice have been carried out for some time, practical application is limited, since fertilization techniques are quite well established among farmers in Taiwan and fertilizer is readily available. However, the amount of residual N after upland cropping is quite variable and may have an important impact on the succeeding crop and the environment, so that the application of soil testing to upland cropping, particularly under a multiple cropping system, merits attention.
A suitable cropping system may be enough to utilize residual N and increase N use efficiency. For example, the cultivation of sweet corn after strawberry is considered effective in utilizing the high residual N after the strawberry harvest. In plastic greenhouses, the cropping of amaranth over the summer and the rotation of other types of vegetables during the rest of the year may reduce problems caused by the application of too much fertilizer to vegetables (Chang and Liao 1989). More studies of the N balance in various cropping systems are needed.
Factors Affecting Efficiency of Applied N and Yield in Crop Production
Fertilizer nitrogen applied to soil is absorbed and used for the production of yield (i.e. the harvest organ). Efficiency of fertilizer N can be formulated as follows (Yoshida 1981) (see Formula 1).
Efficiency of Fertilizer N Absorption in Relation to Yield
In most cases, yield is proportional to the N in the plant, which implies that nitrogen uptake is often insufficient for a crop to attain its potential yield in a given environment ( Fig. 5 Fig. 6 Fig. 7). This insufficiency is usually due to either the limited amount of applied nitrogen, or to low efficiency in absorption. The relationship between crop yield and the efficiency of fertilizer N absorption is as follows:
- With limited applications of nitrogen, yield is directly related to the rate and the efficiency of absorption. Variation in the efficiency of absorption between different crops is great (10-70%), depending on soil conditions and the method and timing of application. Application which minimizes the loss of nitrogen in soil increases crop absorption and yield.
- In many cases, ample amounts of fertilizer N, which are more than the crop requires, are applied in the field. Even so, the amount of nitrogen absorbed by the plant may still be insufficient for the crop to attain its yield potential, because of low efficiency of absorption. Low efficiency in such cases is caused by the unfavorable condition of the plants or the soil, rather than by the nitrogen supply.
As well as those examples mentioned in the previous section, some of the ways in which N uptake efficiency in corn production can be improved are as follows: Spacing of Plants
A significant positive interaction between the rate of nitrogen application and density of planting was found. In the fall corn crop, a maximum yield of 8.9 mt/ha was obtained with dense plant spacing (70 x 20 cm, i.e., 71,400 plants/ha) and heavy rates of N (150 - 200 kg/ha) ( Fig. 5). There was a good correlation between yield and the amount of N absorbed. N absorption was higher with denser planting and heavier N applications. In-row Subsoiling and Deep Placement of Fertilizers
In-row subsoiling with deep banding of fertilizers was found effective in drained paddies converted to corn production. The compactness of the subsoil, which tends to restrict root development, was improved by this practice, so that the efficiency of fertilizer-N absorption was improved and the yield increased (see Fig. 6). Timing of Application
The timing of N applications is an important factor affecting the efficiency of nitrogen absorption, because the time interval between application and crop uptake determines the length of exposure of the fertilizer nitrogen to loss processes such as volatilization, denitrification and leaching. Efficiency is also affected by the timing of uptake itself, as the behavior relating to grain production differs according to the growth stage at which the nitrogen is absorbed by the plant.
The uptake was low when N was banded basally at sowing (28%). It was highest (58%) when N was top-dressed (banded) and covered by up-hilled soil when corn was at the knee-high stage ( Table 11). Although uptake was also low for N which was top-dressed at tasseling (also banded, but without covering soil), the percentage distribution of absorbed N to grain was higher when N was applied at this stage than when it was applied basally at sowing, or at the knee-high stage. This may be related to a higher grain production efficiency when nitrogen is absorbed during this stage (see below).
Top-dressing of nitrogen when rice is at the panicle initiation stage is a common practice (Su 1975). It is also effective for corn, as was shown in a field experiment in which corn was treated with differently timed applications. N topdressing at tasseling had a favorable effect. Both the efficiency of fertilizer-N uptake, and the ability of the N absorbed to produce yield (NE), were promoted by the top-dressing of N at tasseling, as shown in Fig. 7 (compare left and right) and Fig. 8.
Efficiency of Absorbed N in Producing Yield
There is considerable variation in the efficiency of absorbed N in producing yield (i.e., NE), although this is not as marked as the variation in efficiency of absorption. There is variation between different cultivars of the same species, and even more, between crops of different species.
Efficiency generally declines with increased fertilizer N applications. It is this type of response which determines the productivity of crops ( Fig. 9). Varietal differences in nitrogen response and NE between traditional rice varieties and improved ones are mainly due to the differences in their nitrogen uptake and leaf morphology (Taraka et al. 1964). The higher NE of potato and sugar beet compared to other crops, on the other hand, is a result of their longer period of sink activity (Tanaka et al. 1984). Thus, the factors affecting NE are mostly genetic, although environment, as well as the interaction between the genetic character of the variety and the environment, are also important. The growth of the crop is closely affected by these factors, resulting in different patterns of growth. As the pattern of nitrogen uptake during growth is the main factor which can be manipulated to affect the growth pattern, the timing of nitrogen applications not only improves the absorption efficiency but also the NE.
The ability of absorbed nitrogen to produce grain or straw varies according to the growth stage at which the nitrogen is absorbed (Ishizuka 1980). The nitrogen absorbed at different stage of growth affects the harvest index of nitrogen, HI (N), and the nitrogen concentration of the harvest organ, N% (h.o.), as indicated by the following formula (Tanaka et al. 1984).
NE = HI(N) / N% (h.o.)
As mentioned above ( Table 11), the percentage distribution of absorbed N to grain is higher when N is applied at tasseling rather than basally at sowing or at the knee-high stage. Results are even clearer when maize was grown by water culture and treated with 15N tagged fertilizer at various growth stages (Lian and Chang 1991). The greater distribution to grain of the N absorbed at tasseling causes a higher HI (N), which in turn may be related to a higher NE.
Top-dressing with nitrogen at the tasseling stage (corn) or the panicle initiation stage (rice) usually promotes a higher NE, particularly when application rates are low.
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Source: the same experiment as in Fig. 7
Q. ( Y.D. Park)
The average rate of nitrogen fertilizer applied in Taiwan is one of the highest in the world. Is there any program to reduce this? What are your recommended application rates based on?
A. No harmful effects have been observed when fertilizer is applied at the recommended rates. The higher rates used by farmers give lower yields than we get from our demonstration plots. To some extent, the additional fertilizer does not have a favorable effect on yield, since the excess is not used.
Recommendations are usually based on many experiments carried out on a range of different soils. For example, there has been intensive research on rice production, and the rates of application of N, P and K are based on these experiments correlated with soil testing analysis. A similar system operates for upland crops.
Q. Why is that with higher rates of N application, the production efficiency goes down?"
A. It is a generally believed that both absorption (uptake) efficiency and NE (the efficiency of absorbed N to produce yield) generally fall with higher rates of N application. However the details are not so simple.
According to Broadbent, as cited in this paper, the uptake efficiency does not begin to fall until N applications reach the optimum rate to give maximum yield, though an example which may contradict this was given in the paper.
NE also generally decreases with an increase in N uptake, as a consequence of higher rates of N application. This decrease, however, is smaller for improved varieties than for traditional ones, as far as rice crops are concerned. Varietal differences are mainly due to the difference in nitrogen uptake and leaf morphology. The timing of nitrogen applications, which regulates N uptake during the growth process, and particularly the improvement of varietal characters on nitrogen uptake and leaf morphology through plant breeding, has had tremendous success in minimizing the tendency of NE to decrease with higher rates of N application in recent years.
Index of Images
Figure 1 Fertilizer Consumption for Main Crops in Taiwan
Figure 2 The Relation between the Amount of Nitrogen Fertilizer Applied to Vegetables and Other Crops, and the Amount of Nitrogen Absorbed by the Crop
Figure 3 Yield, Crop Uptake of N, and Residual N in Soil after 1975 Crop Harvest, California, (60 CM Irrigation)
Figure 4 Corn Yield, Crop Uptake of N, and Residual N in Soil after 1989 Crop Harvest
Figure 5 Nitrogen Response of Corn at Two Plant Spacings in Different Locations, and Relationship between Nitrogen Absorbed and Yield. (1983 Fall Crop)
Figure 6 Nitrogen Response of Corn at Two Different Depths of Fertilizer Placement, and Relationship between the Nitrogen Absorbed and Yield (1983 Fall Crop)
Figure 7 Nitrogen Response of Corn under N Applications, and Relationship between N Absorbed and Yield (1985 Fall Crop)
Figure 8 Relationship between Nitrogen Absorbed under Different N Applications, and Efficiency of Absorbed Nitrogen to Produce Yield (Ne)
Figure 9 Relationship between Nitrogen Accumulated in Plant (N (Plant)) and Efficiency of Nitrogen (Ne) in Various Crops at Graded Fertilizer-N Levels.
Table 1 Effect of Fertilizer Applications on Yield of Major Crops
Table 2 Recommended Application Rates of Fertilizer Elements for Major Field Crops in Taiwan and Their Productive Efficiency (Average Yield Increment from Each Unit of Application)
Table 3 Nutrient Rates and Yields of Various Crops Obtained in Large-Scale Fertilizer Field Demonstrations in Taiwan
Table 4 Comparison of the Amount of Fertilizer N Conventionally Applied to Citrus, and That Removed by the Crop and the Fruit Harvested
Table 5 Residual Inorganic Nitrogen in the Soil at Harvest of Corn to Which Had Been Applied Different Rates of Nitrogen, in a Rice-Corn Cropping System. (January 1990)
Table 6 Residual Inorganic Nitrogen in the Soil at the Harvest of Strawberry in a Strawberry-Corn Cropping System (April 1991)
Table 7 Content of Inorganic Nitrogen in the Soil at Various Depths after One Year of Cultivation of Chinese Leek (February 1974)
Table 8 Estimated Loss of Added N after a 48-Year Long-Term Field Trial
Table 9 Yields of Paddy Rice after Corn in a Rice-Corn Cropping System (1ST Crop 1990)
Table 10 Percent Uptake by Rice Crop Following Corn of the Residual Tagged Fertilizer Nitrogen (1ST Crop 1987)
Table 11 Effect of Time of Application on Percent Uptake and Distribution in Grain and Stover of Tagged Nitrogen at Harvest of Corn (1986 Fall Crop)