# Predicting Net Returns of Organic and Conventional Strawberry Following Soil Disinfestation with Steam or Steam Plus Additives

^{1}

^{2}

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## Abstract

**:**

## 1. Introduction

#### Previous Literature

## 2. Materials and Methods

#### 2.1. Field Trials

#### 2.1.1. Production System and Timing

#### 2.1.2. Treatments

^{−1}(2288 m

^{3}·ha

^{−1}soil treated). This is 6.42 L·m

^{−3}or 1.55/105 BTU/m

^{3}. Baker (1957) listed a figure equivalent to 1.48/105 BTU/m3 to raise soil temperatures from 15.6 to 71.1 °C with steam, so our results are comparable. Machine, fuel, and labor costs were estimated at U.S. $13,521 per hectare, based on the single-bed prototype. Steam was injected and mixed into the soil through a bed shaper equipped with two rototillers, each with 24 steam injection tines delivering steam through injection nozzles in the tines, which were distributed at 90_ spacing about the tiller circumference and _10-cm spacing along the tiller shaft. Steam was also introduced into the bed shaper from the sides and top. The bed shaper was adjustable in pitch and height, resulting in steam being delivered at _25- and 35-cm depths, and also from 18 cm above the surface. The cross-sectional area of the formed bed treated was 36 by·91 cm (81-cm top width, 102-cm bottom width, 36-cm height) or 0.33-m

^{2}cross-section. The volume treated was 2460 m

^{3}·ha

^{−1}. Water was supplied to the steam generator through a 400-m-long hose reel, and was softened using commercial ion exchange canisters for boiler longevity (Culligan Water Conditioning, Salinas, CA, USA). Insulation was used to maintain heat in the bed for a few minutes. This was accomplished by towing an insulating foam blanket (Rubberite Cypress Sponge, Santa Ana, CA, USA) behind the steam applicator.

^{−1}pelletized MSM (Farm Fuels, Inc., Watsonville, CA, USA) at TCR 2012/13, SJR 2012/13, McFadden 2013/14, and with 2245 kg ha

^{−1}at Spence 2014/15, Fuji 2014/15, and TCR 2014/15. This timing exposed the MSM to the heat of the steam treatment.

^{−1}.

#### 2.2. Economic Data and Methods

^{−1}and averaging the annual means of daily low and high prices.

^{−1}. The resulting cultural costs were 37,472 USD ha

^{−1}for conventional production, excluding the cost of pre-plant soil disinfestation. For all but one organic trial, the cultural costs were 36,862 USD ha

^{−1}. One organic trial (TCR) had a lower planting rate of 39,865 plants ha

^{−1}, reducing its cultural costs to 38,236 USD ha

^{−1}. Harvest costs were 23.97 USD per 8-container 3.6 kg tray for organic production and 21.36 USD for conventional production. (The difference per tray is because the lower organic yield increases the time for a picker to fill a tray.) The cost of steam is from [14] and includes operational costs and depreciation for the steam machine. Unlike broccoli residue or other byproducts used for anaerobic soil disinfestation, MSM is a purchased input.

^{−1}) were calculated using the above information as follows:

_{i,t}= p

_{i}∗ y

_{t}− c

_{t}− w

_{i}

_{i}corresponds to the price for the type of strawberries produced in trial t (organic or conventional) in the season it was conducted. The cultural cost (USD ha

^{−1}) for trial t (organic or conventional) is denoted c

_{t}, and the treatment cost (USD ha

^{−1}) is denoted w

_{i}. The calculation of net returns (USD ha

^{−1}) and all subsequent statistical analyses were conducted using Python 3.8.3, with all prediction models being estimated using the scikit learn library, version 0.23.1.

#### 2.3. ANOVA and Regression Analysis

^{−1}in structural models, estimated separately for the organic and conventional datasets (Equation (1)). Net returns (NetReturns) were regressed on 0–1 dummy variables for each treatment (Steam and Steam + MSM) and T − 1 of the T trials, indexed by t (D

_{t}). The coefficient on each treatment dummy variable (β

_{1}and β

_{2}) measures the difference in net returns ha

^{−1}between that treatment and the untreated control. The coefficient on the dummy variable for trial t (γ

_{t}) measures the difference in net returns between that trial and the omitted trial T, which serves as the base. These coefficients identify any impact of a treatment regardless of the heat outcomes achieved in the treatment.

#### 2.4. Predictive Analysis

## 3. Results

#### 3.1. Descriptive Statistics

#### 3.1.1. Heat Variables: Heat Duration and Maximum Temperature

#### 3.1.2. Yield

#### 3.1.3. Costs

^{−1}into cultural, harvest, and treatment costs. Because costs are averaged across all replicates in all trials for each production system and treatment, and trials differ in their treatments and number of replicates, the cultural cost is not constant across treatments in the organic trials. Cultural cost is a larger share of total costs for the control than for either steam or steam + MSM because there is no treatment cost for the control. Harvest cost increases with yield, so a higher yield reduces the share of treatment cost in total cost. In the conventional trials, the harvest cost accounts for more than half of the total cost for the two treatments: 52% (steam) and 57% (steam + MSM). In the organic trials, harvest cost is around 70% of total costs.

#### 3.1.4. Net Returns

^{−1}) by trial and treatment chronologically. Trials are labeled by type of production system (O for organic or C for conventional), trial name, and season. Each treatment is plotted in the same color across all trials.

#### 3.2. ANOVA

#### 3.3. Linear Regression

#### 3.4. Predicting Net Returns as a Function of Maximum Temperature and Heat Duration

#### 3.4.1. Single Variable Analysis

#### 3.4.2. Joint Analysis

^{−1}to 182,584 USD ha

^{−1}. The replicate with 73,921 USD ha

^{−1}in net returns achieves a maximum temperature of 55 °C, so its value on the x-axis in the figure is difficult to compare to the iso-net return lines. The adjusted R

^{2}is 0.34.

^{−1}and the maximum net returns are 22,232 USD ha

^{−1}. The iso-net returns line labeled 0 is the set of combinations for which net returns are 0 USD ha

^{−1}for that pair. As one moves left and up from that iso-net return line, net returns increase. The adjusted R

^{2}is 0.74.

#### 3.4.3. Predicting Net Returns by Treatment

^{2}= 0.62). Figure 7 plots iso-net returns lines and the observed maximum temperature–heat duration pairs. These lines now represent the increment in net returns over the untreated control with the labeled value. Observed pairs are represented by blue dots. Examining those lines, incremental net returns increase with the maximum temperature throughout the observed range; the value of the iso-net returns curves increase as one moves to the right. Heat duration has very little impact, as shown by the nearly vertical iso-net returns curves.

^{2}of 0.41. Examining the observed maximum temperature–heat duration pairs, the increments in net returns are roughly the same for pairs that are low in both dimensions as ones that are high in both dimensions. Net returns are positive in the observed range, although the iso-net returns curves on either side of the observed range are negative (Figure 8).

^{2}= 0.733), although a two-degree polynomial is a very close second (R

^{2}= 0.729) (the two have the same value if rounded to two digits after the decimal point.) Again, maximum temperature is the primary determinant of the increment in net returns ha

^{−1}. In contrast to steam treatments in the organic production system, however, higher maximum temperatures are associated with lower increments in net returns in the conventional system (Figure 9).

^{2}is −1.09. The negative R

^{2}means that the model does not predict each observation as well as the sample average of net returns does.

#### 3.4.4. Net Returns at Mean Maximum Temperature and Heat Duration

#### 3.4.5. Net Return-Maximizing Maximum Temperature and Heat Duration

^{−1}. For organic steam + MSM treatments, the corresponding maximum temperature is 62 °C and the corresponding heat duration is 44 min, leading to an increment of 88,396 USD ha

^{−1}. These heat durations are noticeably shorter than those conducted in the field trials (Table 2), indicating that the value of increasing heat duration was negative.

^{2}of the quadratic model is 0.70, compared to the 0.71 R

^{2}of the linear model. For the conventional steam only observations, the R

^{2}of the quadratic model is 0.729, compared to the 0.734 R

^{2}of the linear model. The two round to the same value when considering only two digits to the right of the decimal point. Given the closeness in fit, we also compute the optimal maximum temperature and heat duration for these models. For all conventional observations, the optimal maximum temperature is 69 °C with an optimal heat duration of 84 min, resulting in a net return increment of 20,916 USD ha

^{−1}over the net returns to the control. For conventional steam only observations, the optimal maximum temperature is substantially higher at 76 °C, and the optimal maximum heat duration is much shorter at only 23 min. The difference in net returns was 17,337 USD ha

^{−1}more than the control.

## 4. Discussion

## 5. Conclusions

## Supplementary Materials

^{−1}): Organic, Steam; Table S2: Mean Squared Error (MSE) for Fitting of Increment in Net Returns (USD ha

^{−1}): Organic, Steam + MSM; Table S3: Mean Squared Error (MSE) for Fitting of Increment in Net Returns (USD ha

^{−1}): Conventional, Steam; Table S4: Mean Squared Error (MSE) for Fitting of Increment in Net Returns (USD ha

^{−1}): Conventional, Steam + MSM; Table S5: Mean Squared Error (MSE) for Fitting of Absolute Net Returns (USD ha

^{−1}): Organic, Steam; Figure S1: Iso-net return curves for absolute net returns (USD ha

^{−1}): organic, steam; Table S6: Mean Squared Error (MSE) for Fitting of Absolute Net Returns (USD ha

^{−1}): Organic, Steam + MSM; Figure S2: Iso-net return curves for absolute net returns (USD ha

^{−1}): organic, steam + MSM; Table S7: Mean Squared Error (MSE) for Fitting of Absolute Net Returns (USD ha

^{−1}): Conventional, Steam; Figure S3: Iso-net return curves for absolute net returns (USD ha

^{−1}): conventional, steam; Table S8: Mean Squared Error (MSE) for Fitting of Absolute Net Returns (USD ha

^{−1}): Conventional, Steam + MSM; Figure S4. Iso-net return curves for absolute net returns (USD ha

^{−1}): conventional, steam + MSM.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 1.**Maximum temperature (°C) and heat duration (minutes) pairs: all steam and steam + MSM treatments. Each dot represents the maximum temperature and heat duration for a single trial replicate.

**Figure 2.**Maximum temperature (°C) and heat duration (minutes) pairs by production system. Each dot represents the maximum temperature and heat duration for a single trial replicate. Replicates in each trial are a different color. The size of each dot corresponds to the yields achieved by that replicate.

**Figure 3.**Net returns (USD ha

^{−1}) by treatment and trial, ordered chronologically. O = organic, C = conventional. Each treatment is assigned a specific color across all trials: the control is blue, steam is magenta, and steam + MSM is green. Not all trials included all treatments.

**Figure 4.**Polynomials by degree for organic system: individual heat variables. (

**A**) shows the optimal fitted model for predicting net returns (USD ha

^{−1}) as a function of heat duration (minutes) (adjusted R

^{2}= 0.54). (

**B**) shows the optimal fitted model for predicting net returns as a function of maximum temperature (°C) (adjusted R

^{2}= 0.67). In each panel the blue dots indicate observations, and the line plots the fitted model. All steam and steam + MSM replicates included.

**Figure 5.**Polynomials by degree for conventional system: individual heat variables. Panel (

**A**) shows the optimal fitted model for predicting net returns (USD ha

^{−1}) as a function of heat duration (minutes) (adjusted R

^{2}= 0.61). (

**B**) shows the optimal fitted model for predicting net returns as a function of maximum temperature (°C) (adjusted R

^{2}= 0.57). In each panel the blue dots indicate observations, and the line plots the fitted model. All steam and steam + MSM replicates included.

**Figure 6.**Iso-net returns curves for organic (

**A**) and conventional (

**B**) strawberries. Each line is the set of points that results in net returns (USD ha

^{−1}) with the labeled value. Blue dots plot observed maximum temperature (°C)–heat duration (minutes) pairs. All steam and steam + MSM replicates included. R

^{2}= 0.34 (organic) and R

^{2}= 0.74 (conventional).

**Figure 7.**Iso-net returns curves for the increment in net returns (USD ha

^{−1}) relative to the untreated control: organic, steam. Each line is the set of points that results in an increment in net returns with the labeled value. Blue dots plot observed maximum temperature (°C)–heat duration (minutes) pairs. The equation above the figure reports the estimated coefficients. R

^{2}= 0.62.

**Figure 8.**Iso-net returns curves for the increment in net returns (USD ha

^{−1}) relative to the untreated control: organic, steam + MSM. Each line is the set of points that results in an increment in net returns with the labeled value. Blue dots plot observed maximum temperature (°C)–heat duration (minutes) pairs. The equation above the figure reports the estimated coefficients. R

^{2}= 0.41.

**Figure 9.**Iso-net returns curves for the increment in net returns (USD ha

^{−1}) relative to the untreated control: conventional, steam only. Each line is the set of points that results in an increment in net returns with the labeled value. Blue dots plot observed maximum temperature (°C)–heat duration (minutes) pairs. The equation above the figure reports the estimated coefficients. R

^{2}= 0.73.

**Figure 10.**Iso-net returns curves for the increment in net returns (USD ha

^{−1}) relative to the untreated control: conventional, steam only. Each line is the set of points that results in an increment in net returns with the labeled value. Blue dots plot observed maximum temperature (°C)–heat duration (minutes) pairs. The equation above the figure reports the estimated coefficients. R

^{2}= −1.09.

**Table 1.**Summary of trials. Includes season, production system, which treatments were included, and the rate of the mustard seed meal (MSM) amendment.

Trial (Season) | Production System | Steam Included | Steam + MSM Included (MSM Rate) |
---|---|---|---|

MBA (2011/12) | Conventional | Yes | No |

Spence (2011/12) | Conventional | Yes | No |

SJR (2012/13) | Conventional | Yes | Yes (3368 kg ha^{−1} pelletized MSM) |

TCR (2012/13) | Organic | Yes | Yes (3368 kg ha^{−1} pelletized MSM) |

MacFadden (2013/14) | Conventional | Yes | Yes (3368 kg ha^{−1} pelletized MSM) |

Fuji (2014/15) | Organic | No | Yes (2245 kg ha^{−1} pelletized MSM) |

Spence (2014/15) | Organic | Yes | Yes (2245 kg ha^{−1} pelletized MSM) |

TCR (2014/15) | Organic | No | Yes (2245 kg ha^{−1} pelletized MSM) |

**Table 2.**Maximum temperature (°C) and heat duration (minutes) by treatment and production system. Includes mean and standard deviation and the total number of replicates across all trials used to calculate the descriptive statistics.

Treatment | Replicates | Maximum Temperature (°C) | Heat Duration (Minutes) | ||
---|---|---|---|---|---|

Mean | Standard Deviation | Mean | Standard Deviation | ||

Conventional | |||||

Steam | 16 | 81.5 | 9.7 | 84.8 | 47.9 |

Steam + MSM | 8 | 71.9 | 12.6 | 49.1 | 43.4 |

Organic | |||||

Steam | 9 | 67.0 | 13.5 | 98.1 | 107.2 |

Steam + MSM | 16 | 66.0 | 11.7 | 91.6 | 95.4 |

**Table 3.**Yield (t ha

^{−1}) by treatment and production system. Includes number of trials, total number of replicates from those trials, mean, standard deviation, and coefficient of variation.

Treatment | Trials | Replicates | Mean | Standard Deviation | Coefficient of Variation |
---|---|---|---|---|---|

Conventional | |||||

Control | 4 | 24 | 27.0 | 11.0 | 0.4 |

Steam | 4 | 16 | 33.1 | 16.0 | 0.47 |

Steam + MSM | 2 | 8 | 44.9 | 6.1 | 0.13 |

Organic | |||||

Control | 4 | 25 | 44.6 | 14.8 | 0.32 |

Steam | 2 | 9 | 70.4 | 14.4 | 0.19 |

Steam + MSM | 4 | 16 | 68.8 | 13.0 | 0.18 |

**Table 4.**Disaggregated cost (USD ha

^{−1}) by production system and treatment. The share of total cost is included in parentheses.

Production System | Conventional | Organic | ||||
---|---|---|---|---|---|---|

Cultural Cost | Harvest Cost | Treatment Cost | Cultural Cost | Harvest Cost | Treatment Cost | |

Treatment | ||||||

Control | 37,472 (0.38) | 60,069 (0.62) | 0 (0.00) | 38,468 (0.23) | 126,191 (0.77) | 0 (0.00) |

Steam | 37,472 (0.36) | 53,708 (0.52) | 12,355 (0.12) | 38,484 (0.22) | 127,990 (0.72) | 12,355 (0.07) |

Steam + MSM | 37,472 (0.29) | 72,792 (0.57) | 17,139 (0.13) | 38,459 (0.21) | 125,178 (0.69) | 17,139 (0.09) |

**Table 5.**Net returns (USD ha

^{−1}) by treatment and trial. Net returns calculated as gross revenues minus treatment costs and cultivation costs.

Control | Steam | Steam + MSM | |
---|---|---|---|

Conventional | |||

MacFadden 2013/14 | −3591 (A) | 13,630 (B) | 16,037 (B) |

MBA Pic-Clor 2011/12 | −20,570 (A) | −20,179 (B) | |

SJR 2012/13 | −39,107 (A) | −11,571 (B) | −16,303 (B) |

Spence 2011/12 | −36,827 (A) | −44,433 (A) | |

Organic | |||

Fuji 2014/15 | 11,174 (A) | 95,803 (B) | |

Spence 2014/15 | 80,554 (A) | 148,256 (B) | 142,806 (B) |

TCR 2012/13 | −30,667 (A) | 88,909 (B) | 87,714 (B) |

TCR 2014/15 | 37,804 (A) | 130,872 (B) |

**Table 6.**Split-plot ANOVA: strawberry net returns (USD ha

^{−1}) by production system. The trial is the primary factor and the treatment is the secondary factor.

Treatment | Net Returns | Groups |
---|---|---|

Organic | ||

Steam | 121,880 | A |

Steam + MSM | 114,299 | A |

Control | 27,023 | B |

Conventional | ||

Steam + MSM | −133 | A |

Steam | −16,638 | B |

Control | −23,799 | C |

Coefficient | Standard Error | t Stat | p-Value | |
---|---|---|---|---|

Intercept | 39,719 | 9102 | 4.36 | 7.62 × 10^{−5} |

Steam | 91,369 | 9414 | 9.71 | 1.67 × 10^{−12} |

Steam + MSM | 89,239 | 7612 | 11.72 | 3.98 × 10^{−15} |

Fuji 2014/15 | −30,849 | 11,693 | −2.64 | 0.01 |

Spence 2014/15 | 28,264 | 10,353 | 2.73 | 9.08 × 10^{−3} |

TCR 2012/13 | −56,048 | 10,459 | −5.36 | 2.92 × 10^{−6} |

Adj. ${R}^{2}$ | 0.86 | |||

DW | 1.98 | |||

JB | 0.405 | |||

Cond. No. | 6.51 |

Coefficient | Standard Error | t Stat | p-Value | |
---|---|---|---|---|

Intercept | −46,251 | 3548 | −13.03 | 2.36 × 10^{−16} |

Steam | 11,242 | 2981 | 3.77 | 5.03 × 10^{−4} |

Steam + MSM | 17,503 | 3849 | 4.55 | 4.55 × 10^{−5} |

MacFadden 2013/14 | 44,686 | 4082 | 10.94 | 7.04 × 10^{−15} |

MBA Pic-Clor 2011/12 | 20,255 | 4554 | 4.45 | 6.25 × 10^{−5} |

SJR 2012/13 | 12,543 | 4082 | 3.07 | 3.72 × 10^{−3} |

Adj. ${R}^{2}$ | 0.81 | |||

DW | 1.02 | |||

JB | 0.90 | |||

Cond. No. | 6.37 |

**Table 9.**Polynomials by degree: predicting net returns (USD ha

^{−1}) as a function of heat duration (minutes) and maximum temperature (°C) by production system.

Degree of Polynomial | Average Mean Square Error | |
---|---|---|

Organic | Conventional | |

1 | 5.52 × 10^{9} | 4.22 × 10^{9} |

2 | 2.53 × 10^{9} | 3.08 × 10^{9} |

3 | 1.02 × 10^{10} | 3.42 × 10^{9} |

4 | 2.82 × 10^{11} | 4.76 × 10^{9} |

5 | 2.69 × 10^{16} | 8.47 × 10^{11} |

6 | 3.72 × 10^{16} | 2.20 × 10^{15} |

**Table 10.**Increment in net returns (USD ha

^{−1}) by treatment at mean maximum temperature (°C) and mean heat duration (minutes).

Treatment | Difference in Net Returns ha^{−1} * | Net Returns ha^{−1} ** |
---|---|---|

All organic | 77,393 | 104,404 |

Organic, steam only | 90,642 | 117,654 |

Organic, steam + MSM only | 88,120 | 115,132 |

All conventional | 13,494 | −10,295 |

Conventional, steam only | 9013 | −14,776 |

Conventional, steam + MSM | 21,212 | −2579 |

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**MDPI and ACS Style**

Michuda, A.; Goodhue, R.E.; Hoffmann, M.; Fennimore, S.A.
Predicting Net Returns of Organic and Conventional Strawberry Following Soil Disinfestation with Steam or Steam Plus Additives. *Agronomy* **2021**, *11*, 149.
https://doi.org/10.3390/agronomy11010149

**AMA Style**

Michuda A, Goodhue RE, Hoffmann M, Fennimore SA.
Predicting Net Returns of Organic and Conventional Strawberry Following Soil Disinfestation with Steam or Steam Plus Additives. *Agronomy*. 2021; 11(1):149.
https://doi.org/10.3390/agronomy11010149

**Chicago/Turabian Style**

Michuda, Aleksandr, Rachael E. Goodhue, Mark Hoffmann, and Steven A. Fennimore.
2021. "Predicting Net Returns of Organic and Conventional Strawberry Following Soil Disinfestation with Steam or Steam Plus Additives" *Agronomy* 11, no. 1: 149.
https://doi.org/10.3390/agronomy11010149