Comparative Study of Established Organic and Conventional Tomato Production Systems in California
This four-year study compared 20 commercial farms representing a broad range of production practices in terms of conventional fertilizer and pesticide use, reliance on organic soil amendments and biological pest control. The farms were compared in terms of physical, chemical, and biological soil characteristics; nitrogen inputs, water use; productivity; insect pests; disease severity; and costs of production.
The project represents a unique synthesis of multiple site comparisons and measurements of a range of biological and environmental parameters that can serve as indicators of underlying ecosystem processes. By using multivariate statistical approaches, we were able to separate management-induced effects on ecological characteristics from confounding variation due to other factors such as soil type, temporal effects, and surrounding vegetation. The farms included in this study were selected to maximize similarity with respect to soil texture, climate, surrounding vegetation, farm scale, crops produced and marketing strategies.
A. Quantification of soil, crops and economic parameters in different tomato production systems:
1) To document and compare general soil characteristics, crop development, and disease severity in farming systems ranging from organic (ORG) to conventional (CNV) in the Central Valley of California.
2) To determine whether organic practices improve soil structure or moisture retention - characteristics leading to improved water infiltration, root development and water extraction by the tomato crop.
3) To compare insect damage, levels of biological control by natural enemies, and examine relationships between vegetable crops and pest populations in organic and conventional systems.
4) To conduct a financial analysis of the different tomato production systems.
B. Multivariate analysis of soil and crop parameters in the different production systems:
1) To determine the principle components from ten soil and plant parameters that will explain most of the variability among sampling sites in organic and conventional systems.
2) To determine whether plant growth (shoot dry weight and yield), and root rot severity are affected by soil organic matter, N mineralization rate, microbial activity, pH, available soil nitrogen, water-stable aggregation, and soil electrical conductivity.
3) To determine whether organic soil management provides adequate nitrogen for optimal plant growth as indicated by tissue and petiole NO3, analysis, by increasing soil N-mineralization potential.
4) To determine whether arthropod damage is related to plant tissue nitrogen content.
Annual Progress Report for 1992 (and Final Report Conclusions)
Applying multivariate techniques to evaluate groups of soil characteristics showed that management effects were more important in distinguishing between Org and CNV sites than differences related to parent soil type.
These management-induced differences in the soil environment were quite robust, especially considering the range of management practices and soils included in the study. The differences also reflect the fundamental nature of the differing fertilization strategies. In organically managed systems decomposers are needed to process the organic residues and make nutrients available to the crop, while under CNV management, nutrients are applied directly in mineral form. Other increased frequency of bare fallows in CNV fields, could also affect microbial activity and N turnover. Despite wide variation in amounts and types of amendments added, ORG soils were clearly characterized by greater microbial activity and potentially mineralizable N, in conjunction with small instantaneous inorganic N pools.
Organically managed soils for tomato production are characterized by high microbial activity and potentially mineralizable N, small instantaneous inorganic nitrogen and suppression of the corky root diseases of tomatoes.
The most important differences in soil conditions observed in this study -- namely in NO3- pools, N mineralization potential, and microbial activity -- are known to affect a variety of plant diseases, particularly root diseases. High N concentrations may increase or decrease plant disease severity depending on the pathogen, the form of N applied, and its effects on the rhizosphere pH. Although it has been known for many years that organic amendments such as composts and green manures often reduce plant disease, this reduction has only recently been associated with an increase in microbial activity.
In our study, two soilborne tomato diseases were most prevalent: corky root and Phytophthora root rot, caused by Phrenochaeta lycopersici and Phyotophthora parasitica, respectively. Incidence and severity of both diseases were less in ORG than in CNV fields. Although disease severities were generally too low to affect tomato yield, there was sufficient disease to show interesting relationships with various soil and plant characteristics.
Our results suggest that organically managed soils may by suppressive to P.lycopersicim, which is knows to cause corky root disease. In addition to biological disease suppression on ORG farms, lower N concentrations in tomato plants from these farms may have rendered the plants more resistant to corky root. Increased susceptibility to root rots at high fertilization rates has been reported previously. Preliminary data from greenhouse studies suggest that both mechanisms, increase biological control and reduced susceptibility due to lower N concentrations in tomato tissue, are involved in corky root suppression in organically managed soil.
Plant Growth and Fruit Yield
Tomato production, expressed as fruit dry weight, was not significantly different in ORG and CNV fields (nested ANOVA, p>.2, n=17). Yields varied about 4-fold within each group, indicating that variation in location and specific practices within groups were more important determinants of yield than ORG and CNV management per se. Previous on-farm comparisons of other crops have shown comparable yields from ORG and CNV farms, yet comparisons on field stations report more instances of lower yields in ORG treatments. This discrepancy between on-farm and experiment station comparisons may be due to the inclusion of data from fields that were only recently converted to ORG management, since significantly smaller yields occur during the first few years of transition to ORG production for a variety of reasons.
Pest management on tomato foliage and fruit in ORG and CNV fields did not differ significantly. Foliar damage by each of the major pest groups (flea beetles, caterpillars, and leafminers) varied between 20 and 95% but was distributed evenly among both types of farms.
In contrast to the comparable pest damage patterns on CNV and ORG farms, timing of the crop had a significant impact on the damage sustained by the tomoto crop (nested ANOVA, p<0.05 for caterpillars and stinkbugs on fruit, and thrips, flea beetles and caterpillars on leaves). Damage by thrips and stinkbugs tened to be greatest on the early crop (April transplants), but damage by caterpillars was significantly lower on tomatoes transplanted in April compared to those transplanted in May or June. Thus, damage patterns to tomatoes changed over the season due to the phenology of major pests, but did not differ with respect to management category (ORG vs CNV).
One possible mechanism for the maintenance of relatively low insect damage levels on ORG farms in compensatory biological control by the natural enemies of tomato pests. In contrast to the homogeneous damage profiles among the farms, the natural enemy communities (predators such as lady beetles and spiders and parasitoids such as ichneumon wasps) associated with the tomato crop on ORG and CNV farms were distinct. Species richness of predators and parasitoids was 46% greater, on average, and natural enemies were 43% more abundant on ORG farms than on CNV farms. However, herbivore densities were only 5% greater, on average, on ORG farms and represented 27% more species.
The absence of synthetic agrichemicals in organically managed tomatoes did not result in a significant change in yield. Other studies comparing ORG and CNV agriculture also show that productivity on established organic farms is not generally reduced. The comparable yields obtained by organic growers are all the more significant, considering that little formal research has been conducted to optimize cultural practices or select for suitable crop genetic characteristics appropriate for organically-managed cropping systems.
Furthermore, this study underscores the potential for enhanced ecosystem processes to compensate for chemical inputs in some agricultural systems. Indeed, agricultural practices which control or minimize biological diversity and circumvent ecosystem processes may actually create a need for many inputs currently used in CNV systems.