Causes and consequences of overfertilization in orchards.
Weinbaum, S.A., R. S. Johnson, and TM. DeJong

J. Horticultural Technology 1(2): in press. 1992

Reviewer's note: Nitrogen is the element most often applied in orchards to enhance tree growth and production. For most fruit crops, increasing N rates above an optimal level often results in increased yields, but also deterioration of fruit quality. Conversely, yields in other tree crops (especially nut crops) may increase with little or no reduction in quality. There is a point of diminishing returns, however, beyond which the income received for the product does not cover the cost of the extra fertilizer. More importantly, the extra N applied before this point is reached can represent an environmental cost in terms of nitrate contamination of groundwater. The effect on groundwater is the primary focus in this extensive review paper (115 references).

Overfertilization with N is defined as "the application of N in excess of the tree/vine capacity to utilize it for optimum productivity." This practice can lead to a buildup of N in the soil, where it becomes vulnerable to loss by denitrification (which produces volatile nitrogen gases) and leaching. Tree and vine crops may be more susceptible to such losses than other crops. In one San Joaquin Valley study, the proportion of applied N that was removed in harvested crops was found to be substantially lower for tree crops than for other crops (Table 1). A study in Fresno County showed that more Soil nitrate accumulated below the root zones of orchard crops than at similar depths in other crop classes.

Fruit Tree Response to Overfertilization

Overfertilization is associated with a number of adverse consequences in orchards in addition to its negative impacts on the environment. First, excess N can result in increased vegetative growth which "accentuates shading within the tree and negatively affects flower bud development, fruit set, fruit quality, and shoot survival." Second, overfertilization in stone fruits delays fruit maturity and can lead to uneven ripening of fruit on a tree and on individual fruits. Third, high N inhibits proper fruit coloration in many species. Finally, several physiological disorders may be accentuated by overfertilization; susceptibility to disease and insect pests may be influenced as well.

Factors Contributing to Overfertilization

The authors describe three factors which must be more fully understood in order to prevent overfertilization: 1) the efficiency of fertilizer N use, 2) other sources of plant- available N, and 3) the tree requirements for N (demand).

Fertilizer N Use Efficiency (NUE) is the proportion of fertilizer N recovered by the plant relative to the total amount of fertilizer N applied. The NUE for most California fruit growers is currently less than 25 percent, i.e. they apply more than four times the amount of N recovered from the land in the plant. Such a low NUE results in increased losses, including leaching to groundwater. Overwatering increases nitrate leaching (especially in coarse-textured soils), which encourages the grower to apply high rates of fertilizer N. Thus, prevention of nitrate pollution requires not only reducing N rates, but also minimizing deep percolation. To reduce deep percolation, careful monitoring of soil moisture and knowledge of evapotranspiration rates are essential. Reducing fertilization rates increases the percentage recovery of N by trees and reduces the potential for leaching of residual fertilizer N. Other practices that can increase NUE and reduce nitrate leaching include split fertilizer applications, low-volume irrigation and fertigation.

An important factor affecting NUE is the timing of N fertilization. Winter applications were used in the days of dryland farming to carry N into the root zone. Fertilization took place in the winter because of the increased availability of labor and because it was believed that this N would be readily available for uptake by trees in early spring. However, extensive research has shown that N used in the early spring growth of trees comes from uptake from the soil prior to leaf fall. This N is stored in perennial tissues over the winter and is translocated to developing blossoms during the early stages of growth resumption in spring. Because little N is taken up from the soil until the period of rapid shoot growth in the spring, winter-applied N is vulnerable to leaching.

NUE may also be affected by alternate bearing (alternating heavy and light crops each year). During a heavy production year, root growth is limited; consequently, the capacity for N uptake can be reduced. (Reviewer's note: More N is removed in the crop in a heavy crop season than in a light production year. However, the evidence indicates that the ability of the tree to take up N from the soil is reduced when a heavy crop is present. Thus, high N rates applied during a heavy crop year, in anticipation of excess N removal, may be wasteful and polluting. Implications for N management strategies in light of these phenomena are currently under investigation.)

Plant-Available N is the chemical form of soil N (mostly nitrate) that can be absorbed readily by plant roots. Plant- available N makes up less than 3 percent of the total soil N; the rest is immobilized in the soil organic matter and is available only as organic matter de-composes. In some cases, there may be sufficient plant-available N from non-fertilizer sources to maintain tree productivity for extended periods (i.e., 4-6 years). For example, N in organic matter (some of which may be from previous fertilizer applications) can mineralize and become available.

Irrigation water is another source of nitrate-N. Nitrate in well water has increased in many parts of California. In the San Joaquin Valley, such irrigation water commonly supplies 70-100 kg N per hectare (approx. 60-90 lbs. per acre) each year to tree crops. Thus, in order to reduce groundwater contamination, it is recommended to consider irrigation water as a N source and to adjust fertilizer rates accordingly. In some cases, sufficient N is present in the irrigation water to satisfy the N requirements of the crop without further N supplementation. [See Hirschfelt, Components 1(3), 1990.]

Tree N Utilization (Demand). Proper N fertilization requires that you know the amount of N removed by a crop in a season. As shown in table 2, this figure can differ by as much as 1000 percent between species. The N contained in other plant parts, such as leaves and prunings, is insignificant compared to that removed in the crop. With almonds and walnuts, for example, only 4-8 lbs. N per acre are removed in prunings. A program of N fertilization based solely on crop removal, however, may not lead to optimum production since other environmental and management factors influence NUE.

N uptake and tree response can decrease progressively with incremental increases in N application rates. This decrease may indicate luxury consumption of N; more often, however, the capacity of plants to take up N decreases as maximum productivity and vegetative growth occur. The decline in N uptake leads to excess accumulation of nitrate in the soil which can move with percolating water to groundwater.

Diagnosis as a Guide to Fertilization

This section begins with an analysis of grower motivations behind the selection of fertilization practices. It is not possible to accurately predict N requirements for optimum productivity over a diversity of sites. Nonetheless, diagnostic possibilities and relevant information exist to keep N losses to a minimum; yet most growers are inclined to ignore these resources. Management practices are often based on tradition (e.g., winter fertilization), testimonials (e.g., perceived cause and effect relationships of neighboring farmers), and convenience (e.g., uniform fertilization of a field with two soil types). A sound approach to N fertilization involves the diagnosis of tree N status and soil N availability, knowledge of crop N demand, and consideration of other site-specific variables.

Diagnostic methods for determining N requirements include tissue testing and soil testing. Soil tests can reveal the need for less N fertilizer if high levels of residual nitrate are detected. In general, however, soil tests do not account for the spatial variability in soil nitrate concentrations, root distribution, and soil N mineralization potential. Also, even if soil N levels are low, tree N reserves may be used to produce an adequate crop in a given year.

Tissue tests are more indicative of N availability than soil tests. In orchards, the total N concentration (percent dry weight) is determined from leaves sampled in mid-summer. Leaf analysis in citrus is largely responsible for a 50 percent reduction in N fertilization rates in orange groves in some areas of California. However, an informal survey indicated that less than 20 percent of tree fruit growers statewide perform leaf analysis annually. Furthermore, variability exists among laboratories and advisers in the accuracy of results, the basis of interpretation, and the ensuing recommendations. It is also noted that commercial crop advisers often have a vested interest in selling fertilizers.

If tree N status is initially low, leaf N concentrations increase significantly with the application of fertilizer N. However, leaf analysis results are relatively insensitive to excessive N fertilization rates. In one study, doubling the N application rate to apples failed to increase leaf N significantly. High leaf N levels (i.e., above the sufficiency threshold) usually indicate overfertilization and thus an increased potential for leaching and volatilization of N. Yet little research has been carried out to address the upper limit of the N sufficiency range.

According to the authors, the goal of maximizing economic profitability has led to environmental degradation. The high ratio of fruit value to fertilizer cost has encouraged overfertilization. They predict that the price of fertilizer would have to increase by more than 200 percent before growers will use fertilizers judiciously. While many practices have been recommended to minimize N losses, it will take legislative action to produce the desired results unless all participants take a more proactive approach.

Reviewer Comments

From a political perspective, regulations restricting N fertilizer usage will almost certainly be enacted if nitrate levels in groundwater continue to rise. Will growers wait until they are forced to change? The answer is likely to be yes, particularly if fertilizers remain relatively cheap. Many growers are concerned about groundwater contamination. Yet it is difficult to risk reductions in yield, especially when the difference between profit and loss may be just an extra shot of fertilizer away. The question of responsibility further complicates the problem. There are many sources of nitrate contamination and each grower's contribution is usually small. For these reasons, it seems likely that growers will need strong economic incentives or regulations to persuade them to maximize the efficiency of N fertilizer use.

One practice not mentioned in the article that has potential for reducing nitrate pollution is the use of cover crops, including resident vegetation. Nitrate leaching potential is usually greatest during the winter, when N uptake by tree roots is low and rainfall is highest. Winter cover crops (especially nonlegumes) can capture much of the nitrate that would otherwise leach. Since the amount of soil residual nitrate captured by a cover crop is proportional to the plant biomass, the cover must have significant biomass by early winter in order to be effective. Cultivating or mowing the cover would be appropriate in early spring, particularly during drought periods.

References

Birdsall, J.J., P.H. Derse, and L.J. Teply. 1961. Nutrients in California lemons and oranges. J. Amer. Dietetic Assn. 38:555-559.

Miller, R.J. and R.B. Smith. 1976. Nitrogen balance in the southern San Joaquin Valley. J. Environ. Qual. 5:274-278.

Mullins, M.G., A. Bouquet and L.E. Williams. 1992. Biology of the Grapevine (in press).

For more information write to: (Weinbaum) Department of Pomology, University of California, Davis, CA 95616.

(CI-SWN.069) Contributed by Chuck Ingels