Modeling the Interaction of Plastic Film Mulch and Potato Canopy Growth with Soil Heat Transport in a Semiarid Area

: Plastic film mulch is an important agricultural technology to reduce water evaporation and modify the soil thermal conditions for crop production. The optical properties of plastic film mulch and the crop canopy growth are both key factors impacting soil heat transport in the soil-film-canopy-atmosphere ecosystem. In this study, a process-oriented model was developed to better understand the interaction among the plastic film mulch, potato ( Solanum tuberosum L.) canopy growth, and soil thermal conditions. Canopy growth, photosynthetically active radiation transmittance, net radiation, soil heat flux, and temperature were monitored in a two-year plastic mulch field experiment in Wuwei (Gansu Province, China). Results showed that the simulation of daily soil surface temperature had a good performance with 2.8 and 1.5 °C of root mean square error (RMSE) for the transparent film mulch (TM) and black film mulch (BM), respectively. Moreover, the simulation of the daily net radiation and soil heat flux model indicated reasonable fluctuations with potato phenological development with the daily R 2 ranging from 0.89 to 0.98 in 2014 and 2015 for the TM and BM treatments. It was shown that the canopy temperature under BM was greater than that in TM treatment, and the maximum value difference could be up to 7 °C during the early potato growing period, which implied that the BM may perform better in modifying the canopy thermal condition. The model could provide heat distribution information for plastic film choosing in potato field to avoid heat stress.

The two widely used plastic films are black plastic film and transparent plastic film. Although both black plastic film (with high shortwave absorptance) and transparent plastic film (with high shortwave transmittance) can raise soil temperature [22], the effects of these two plastic films on plant growth can be contrary. For example, some researchers report that the use of black plastic film mulch leads to higher crop yield than transparent plastic film mulch [12,23,24]. Others report that transparent plastic film mulch results in higher crop yield than black plastic film mulch [25,26]. Such contradictory results are possible due to the complexity of heat transport in the soil-film-canopy-atmosphere system. Heat transport can be affected by the extent of contact between mulch and soil, the optical properties of the plastic film, and the geometry of the mulched surface [27].
Models have been developed to study heat transport considering different influencing factors since field experiments are time consuming and laborious. For example, some models have been developed to describe heat transport between soil, transparent plastic film mulch, and the atmosphere with energy balance equations considering the impacts of soil characteristics and the optical properties of mulch [28][29][30]. One validated heat transport model considers the effects of the optical properties of different plastic film mulches and the air gap between plastic film mulch and soil surface [6]. Since these models have limitations in simulating heat transport in field crops with plastic film mulch, other models have been developed. For example, Wu et al. [31] proposed a model to simulate soil temperature and water content in winter wheat (Triticum aestivum L.) with transparent plastic film mulch. This model mainly focuses on the distribution of soil temperature and water content. Yang et al. [32] added a plastic film mulch layer model into the SHAW model, constructed by Flerchinger et al. [33] and Flerchinger and Pierson [34], to consider the effects of plastic film mulch on heat transport in maize (Zea mays L.).
Although these models have been developed to study the effects of plastic film mulch on heat transport in fields with or without crops, some problems still need to be solved with new models. Heat stress, affecting the above-ground and below-ground parts growth of potato [35,36], can cause the low potato yield [37]. It is necessary to know the heat distribution for plastic film mulch choosing to provide optimal heat conditions for potato growth. One important problem is how to simulate the heat transport as the potato plant canopy changes. Zhang et al. [11] observed that the effects of transparent and black plastic film mulch on soil temperature in potato fields can be different in two growing seasons because of the different canopy growth. How can the interaction of canopy growth and the optical properties of plastic film mulch on heat transport be considered in the model?
The objective of this study was to develop a model to simulate the radiation, soil heat flux, and mulch surface and soil surface temperature at different potato growth stages considering the interaction of canopy growth and the optical properties of plastic film mulch. Leaf area index, canopy coverage, and photosynthetically active radiation transmittance were introduced in the model to account for the effects of plant canopy growth on heat transport. The model can be used to predict radiation, temperature, and soil heat flux in potato field and explain the effects of different plastic film mulches on potato growth during different growth stages.

Study Area and Experimental Design
The study area was located at the Shiyanghe Experimental Station (N 37°52′, E 102°50′, altitude 1581 m), China Agricultural University, Wuwei, Gansu Province, on the border of the Tenger Desert with a typical continental temperate climate. The area has sufficient heat resources and limited water resources with mean annual temperature 8 °C, annual accumulated temperature (>0 °C) 3550 °C, annual sunshine duration 3000 h, annual precipitation 164 mm, pan evaporation 2000 mm, and groundwater table 5-30 m below ground surface [38]. The soil texture is sandy loam with mean soil bulk density 1.53 g/cm 3 at 0-1.0 m depth [11].
To calibrate and verify the model, experiments were conducted in potato fields from April to August in 2014 and 2015. Two soil surface treatments were compared: transparent plastic film mulch (TM) and black plastic film mulch (BM), each with a thickness of 0.008 mm. The soil surface of each treatment was fully covered with polyethylene (PE) plastic film. Each treatment was replicated three times in a randomized complete block design.

Agronomic and Irrigation Practices
As the agronomic and irrigation practices have been described in detail by Zhang et al. [11], only a general introduction is presented here to avoid repetition. In the first year the potatoes were planted on 22 April and harvested on 21 August 2014. Since the weather was warm earlier in 2015, the potatoes were planted on 15 April and harvested on 20 August. The plot size was 6 m long and 5.6 m wide with 7 beds (0.8 m wide and 0.2 m high). The potato crop was irrigated using a drip irrigation system with drip tape placed under the plastic mulch on the center of the beds. Irrigation was initiated when the soil matric potentials measured with tensiometers reached −25 kPa. The specific irrigation information has been shown by Zhang et al. [11].

Measurements
Ten plants in each plot were labeled for height measurement every 7 days (fewer than 7 days during vigorous plant growth). The leaves of five plants in two plots were collected to measure leaf area index in 2014. Total leaf weight and the weight of 30 typical leaves were measured. To reduce labor intensity only the area of the 30 typical leaves was measured with the AM300 (ADC BioScientific Ltd., Hertfordshire, UK) a portable leaf area meter. The total leaf area per plant was calculated using the ratio between the weight of the 30 leaves and the total leaf weight.
The fractional canopy coverage, proportion of the ground covered by potato canopy (Pcc), was determined with digital images which were taken at a height of 2 m above ground using a Canon IXUS 132 digital camera (Canon Inc., Tokyo, Japan) [39,40]. The images were taken at 12:00-13:00 to minimize shadows. A typical region (1.6 m × 0.9 m) containing two rows (row spacing 0.8 m) with 6 plants (plant spacing 0.3 m) was selected in each plot to take photographs. The plants were captured in the images using quick selection tool, magic wand tool, and lasso tool of Adobe Photoshop CS5 (Adobe Systems Inc., San Jose, CA, USA). Then, the images were processed into black and white binary image (black pixels denote plants) with the Adobe Photoshop CS5. The proportion of black pixels in the image, read from the Adobe Photoshop CS5 histogram represented the fractional canopy coverage. The example of images before and after processing is shown in Figure 1. The photosynthetically active radiation transmittance of the potato canopy (Tpar) was measured with SunScan Canopy Analysis System (Delta-T Devices Ltd., Cambridge, UK) at 12:00-13:00 on a sunny day. Given the inhomogeneity of the potato canopy, the SunScan probe was placed on the bed (perpendicular to the row direction) at 7 random positions for each plot. The average of the transmittance was recorded.
Temperatures on the plastic film mulch surface, on the soil surface, and at 0.05 and 0.1 m soil depths were measured with sensors in the middle of the beds in one replication of each treatment. The sensors on the plastic film mulch surface, on the soil surface, and at 0.05 m soil depth were 0.25-mm copper-constantan thermocouples (ST10, Beijing Unism Technologies, Inc., Beijing, China). The sensors at 0.1 m soil depth were soil temperature/water sensors (FDS120, Beijing Unism Technologies, Inc.).
Weather variables were measured with a standard automatic weather station (HOBO H21-001, Onset Computer Corp., Cape Cod, MA, USA), 2 m aboveground. The measurable weather variables consisted of global radiation, wind speed, relative humidity, air temperature, atmospheric pressure, and precipitation. Global radiation and air temperature has been shown in previous report of Zhang et al. [11]. Net radiation above the canopy was measured with net radiometers (NR Lite2, Kipp&Zonen, Delft, Netherlands), installed in the middle of one replication of each treatment.
Soil heat fluxes at 0.05 m soil depth in the middle of the bed were monitored in one replication of each treatment using soil heat flux plates (HFP01, Hukseflux, Delft, The Netherlands). According to Giambelluca et al. [41] the soil heat flux on the soil surface can be estimated as follows: where G is the soil heat flux on the soil surface (W/m 2 ) (W is the amount of energy received per second), The temperature sensors were connected to a data logger (SMC6108, Beijing Unism Technologies, Inc.). The net radiometers and soil heat flux plates were connected to another data logger (CR1000, Campbell Scientific Inc., Logan, UT, USA). The measurements were sampled at 10 s intervals and the averages were stored every 10 min.

Energy Balance Equations
The energy transport in potato fields with plastic film mulch is illustrated in Figure 2. According to Ham and Kluitenberg [6], these assumptions are made for the model: (1) Soil evaporation is neglected because of the full plastic film mulch. Although soil evaporation still occurs at the hole of plant emergence, the evaporation is quite small compared with plant transpiration; (2) There is no heat stored in the plastic film mulch and plant; and (3) Heat transfer in the process of evaporation/condensation between plastic film mulch and soil is neglected. Thus, the energy balance of the plant canopy can be expressed as follows: where Rnc is the net radiation absorbed by the plant canopy (W/m 2 ), Hca is the sensible heat flux from the plant canopy to the atmosphere (W/m 2 ), λT is the latent heat flux of the plant canopy (W/m 2 ), and Hmc is the sensible heat flux from the plastic film mulch to the plant canopy (W/m 2 ). According to Ham and Kluitenberg [6], the energy balance of the plastic film mulch can be defined as follows: where Rnm is the net radiation absorbed by the plastic film mulch (W/m 2 ), and Csm is the conduction from the soil surface to the plastic film mulch (W/m 2 ). The energy balance of the soil surface can be defined as follows: where Rns is the net radiation absorbed by the soil surface (W/m 2 ), and G is the soil heat flux at the soil surface (W/m 2 ).

Governing Equations for Net Radiation of Each Layer
The radiation transfer is more complicated in the soil-film-canopy-atmosphere system than the soil-film-atmosphere system. The physical representation of the shortwave radiation transfer can be described in Figure 3.
The shortwave radiation Rsc, Rsm, and Rss absorbed by the plant canopy, the plastic film mulch, and the soil surface can be described as follows: where Rs is the global radiation (W/m 2 ); αsc and αsm are the shortwave absorptances of the plant canopy and plastic film mulch, respectively; ρsm and ρss are the shortwave reflectances of the plastic film mulch and soil, respectively; and τsc and τsm are the shortwave transmittances of the plant canopy and plastic film mulch, respectively.

As shown in Figures 4 (a)-(d)
, the longwave radiation Rlc, Rlm, and Rls absorbed by the plant canopy, the plastic film mulch, and the soil surface can be described as follows: where σ is the Stefan-Boltzmann constant [W/(m 2 °C 4 )]; Tsky (set equal to the air temperature) [6], Tc, Tm, and Ts are the temperatures of the sky, plant canopy, plastic film mulch, and soil surface (°C), respectively; εsky, εc, εm, and εs are the emissivities (or infrared absorptances) of the sky, plant canopy, plastic film mulch, and soil surface, respectively; ρlm and ρlc are the longwave reflectances of the plastic film mulch and plant canopy, respectively; and τlc and τlm are the longwave transmittances of the plant canopy and plastic film mulch, respectively.  Then, the net radiation Rnc, Rnm, and Rns absorbed by the plant canopy, the plastic film mulch, and the soil surface can be expressed as: ( where ρa is the density of air (kg/m 3 where rST is the mean stomatal resistance (s/m), rb is the mean boundary layer resistance (s/m), and LAI is the leaf area index.

Governing Equations for Sensible Heat Flux
The sensible heat flux can be defined as follows: where Tm is the temperature of the plastic film mulch (°C), r a a is the aerodynamic resistance between the plant canopy and the reference height (s/m), and r m a is the aerodynamic resistance between the plastic film mulch and the plant canopy (s/m).
The aerodynamic resistances r a a and r m a are given by Shuttleworth and Wallace [43] as follows: where hr is the reference height (m), u is the wind speed at the reference height (m/s), n is the eddy diffusivity decay constant when the ground is fully covered by crop, k is von Kármán's constant, hp is the plant height (m), d is the zero plane displacement when the ground is covered by crop fully (m), and z0 is the roughness height when the ground is covered fully by a crop canopy (m). The d and z0 can be given as follows: The r a a (0) and r m a (0) can be expressed as follows: where z0′ is the roughness height when there is no crop (m).

Governing Equations for Soil Heat Flux
The soil heat flux G suggested by Horton and Chung [44] is as follows: where Ts is the temperature of the soil surface (°C), Tl is the temperature of the first vertical node below the soil surface (°C), Δz is the distance between the soil surface and the first vertical node (m), Δt is the time step increment (s), n is the time where θn is the solid phase content, θo is the organic matter content, and θ is the volumetric soil water content (m 3 /m 3 ). According to Chung and Horton [46] the soil thermal conductivity λ can be estimated as follows: where b1, b2, and b3 are empirical parameters [W/(m °C)].

Governing Equations for Conduction From the Soil Surface to the Plastic Film Mulch
The heat conduction from the soil surface to the plastic film mulch Csm is given by Ham Kluitenberg [6] as follows: where rc is the thermal contact resistance between the plastic mulch and soil (m 2 °C/W). According to Ham Kluitenberg [6] the conduction is dominated by heat transfer when the distance between the plastic film mulch and soil is less than 0.01 m. Then the rc can be defined as follows: where zg is the distance between the plastic film mulch and the soil (m), ka is the air thermal conductivity [W/(m °C)], and Nud is the Nusselt number reflecting the regime of conduction. In summary, the energy balance equations (3)-(5) can be expressed as follows:  (1 )

Parameterization for the Energy Balance Equations
As many parameters were involved in the energy balance equations, the parameterization for the net radiation, latent heat flux, and heat conduction were introduced separately.

Net Radiation of the Plant Canopy
The global radiation (Rs) was measured with the weather station. The shortwave transmittance of the potato canopy (τsc) is supposed to be equal to the photosynthetically active radiation transmittance (Tpar). According to Haverkort et al. [47] the shortwave reflectance of potato canopy can be approximated as follows: where ρsc is the shortwave reflectance of the potato canopy and Pcc is the canopy coverage. Then the shortwave absorptance of the potato canopy (αsc) is: The emissivity (or infrared absorptance) of the potato canopy (εc) is 0.97 when the ground is covered fully by the canopy [27]. The relationship between εc and Pcc can be assumed as follows: The longwave reflectance of the potato canopy (ρlc) is about 0.01 [27]. According to Ham Kluitenberg [6] the longwave transmittance of potato canopy (τlc) is approximated as follows:

Net Radiation of the Plastic Film Mulch
The shortwave absorptance (αsm) and the shortwave transmittance (τsm) of the plastic film mulch were determined through parameter calibration with the field experiment data. The shortwave reflectance of the plastic film mulch (ρsm) can be obtained by the equation: The emissivity (or infrared absorptance) (εm) and the longwave transmittance (τlm) of the plastic film mulch were determined by parameter calibration. The longwave reflectance (ρlm) of the black film mulch and transparent film mulch are about 0.01 and 0.13, respectively [27].

Net Radiation of the Soil Surface
The shortwave reflectance (ρss) and emissivity (or infrared absorptance) (εs) of soil were determined by parameter calibration. According to Campbell and Norman (page 164) [48] the emissivity of sky (εsky) is defined as follows:

Parameterization for Latent Heat Flux
The air temperature (Ta), actual atmospheric pressure (P), and relative humidity (RH) were monitored with weather station. According to Maiti et al. [49] the density of air (ρa) can be calculated as follows: where ρ0 (the air density at 0 °C and 0.1013 Mpa) is about 1.29 kg/m 3 . According to Allen et al. [50], the psychrometric constant (γ), the slope of the vapour pressure curve (Δ), the saturation vapour pressure (es), and the actual vapour pressure (ea) can be expressed as follows: where Tmin and Tmax are the minimum and maximum air temperature at a certain period of time (°C), respectively; es(Tmin) and es(Tmax) are the saturation vapour pressure at Tmin and Tmax (kPa), respectively; RHmin and RHmax are the minimum and maximum relative humidity (%), respectively. The mean stomatal resistance (rST) and the mean boundary layer resistance (rb) of the potato canopy were determined through parameter calibration with field experiment data.
As the direct measurement of leaf area index (LAI) is destructive and time consuming, the daily LAI was determined by the fraction of intercepted photosynthetically active radiation (Ipar) indirectly. According to Haverkort et al. [47] and Boyd et al. [51] the relationship between Ipar and LAI can be expressed as follows: where κ is the extinction coefficient. The κ can be determined through the measured Tpar and LAI. In this experiment κ is 0.92, close to the result of Spitters [52]. The relationship between the photosynthetically active radiation transmittance (Tpar) and Ipar is:

Parameterization for Sensible Heat Flux
The wind speed at the reference height (u) was monitored with the weather station. The plant height (hp) was determined using interpolation method with plant height measured in the field.

Parameterization for Soil Heat Flux
The temperature (Tl) of the first vertical node above the soil surface was monitored with sensors. The distance (Δz) between the soil surface and the first vertical node was assumed to be 0.1 m and the time step increment (Δt) was 3600 s.

Parameterization for Heat Conduction
The air thermal conductivity (ka) is about 0.025 W/(m °C) ( [48]; page 118, Table 8.2). The Nusselt number (Nud) is 1 [6]. The distance (zg) between the plastic film mulch and soil was determined through parameter calibration or measurement with field experiment data.

Numerical Solution of the Energy Balance Equations
After the parameterization, the energy balance equations were functions of Tc, Tm, and Ts. Equations (31)-(33) can be expressed as a nonlinear equation group F(T) as follows: The equation group (47) can be solving by Newton-Raphson iteration technique [6]. The partial derivatives of the iteration technique can be described as a Jacobian matrix F'(T) as follows [6]:

Statistical Analysis
To evaluate the model performance three statistical parameters, including root mean square errors (RMSE), coefficient of determination (R 2 ), and mean errors (ME) were applied during model calibration and validation. These statistical parameters were calculated from simulated and measured temperatures, soil heat fluxes, and net radiations using the equations as follows: where Oi and Pi are the observed and simulated values, respectively; O and P are the averages of the observed and simulated values, respectively; and N is the number of observations.

Results and Discussion
The Pcc (fractional canopy coverage) and Tpar (photosynthetically active radiation transmittance of the potato canopy) were measured to reflect the potato canopy growth ( Figure 5; Figure 6). The black plastic film mulch had greater plant canopy in the BM treatment than in the TM treatment in 2014 and 2015. It might be caused by the higher air temperature near the black mulch surface [11].

Model Calibration and Validation
Model parameters were calibrated with the temperature of the plant canopy (Tc), the temperature of the plastic film mulch (Tm), the temperature of the soil surface (Ts), the net radiation (Rn), and the soil heat flux (G) which were measured in the potato field in 2014. The calibrated parameters for the transparent and black plastic film mulch treatments are shown in Table 3. The calibrated model was validated with Tc, Tm, Ts, Rn, and G measured in the potato field in 2015. Table 3. The model parameters (shortwave transmittance τsm, shortwave absorptance αsm, emissivity εm, and longwave transmittance τlm of the plastic film mulch; emissivity εs and shortwave reflectance ρss of soil; mean stomatal resistance rST, mean boundary layer resistance rb; and distance between the plastic film mulch and soil zg) calibrated with hourly temperature, net radiation, and soil heat flux measured for the transparent plastic film mulch (TM) and black plastic film mulch (BM) in the potato field at Wuwei, China in 2014.

Model Calibration and Validation for Temperature
Generally, the model can reflect the daily fluctuations of mulch surface temperature and soil surface temperature for the TM and BM treatments by comparing the simulated and observed temperatures (Figures 7 and 8; Table 4; Table 5). In 2015, the daily RMSE of soil surface temperature for the TM and BM treatments, which were smaller than the hourly RMSE, were 2.8 and 1.5 °C, respectively. This result was similar with Ham and Kluitenberg [6] who reported that the RMSE of soil temperature was 2.4°C for the TM treatment and 1.8 °C for the BM treatment. In 2014 and 2015, the hourly and daily R 2 of temperatures ranged from 0.65 to 0.84 and 0.76 to 0.91, respectively.    (Figures 7 and 8). The larger simulation deviation might be caused by the neglect of soil evaporation from the hole (8 cm in diameter) of the plant emergence. Moreover, the temperature sensors on the mulch surface were affected by solar radiation during the early plant growth stage, which might also cause the deviation. As the plant canopy got larger, the simulation got better during the later plant growth stages. The variations of mulch surface temperature and soil surface temperature were smaller during later potato growth because of the larger area shaded by the plant canopy. Generally, the model could simulate the daily temperature variations at different potato growth stages for different plastic film mulch treatments.

Model Calibration and Validation for Net Radiation and Soil Heat Flux
The hourly simulated and observed net radiation for the TM and BM treatments were compared and the statistical hourly and daily RMSE, R 2 , and ME were calculated ( Figure 9 and Table  6; Table 7). The daily simulation of net radiation was reasonably good with daily RMSE for the TM and BM treatments 20.30 and 32.79 W/m 2 during validation, respectively. The ME of daily net radiation for the two treatments ranged from 12.96 to 36.14 W/m 2 in 2014 and 2015. The R 2 ranged from 0.89 to 0.98, which was greater than 0.8, a typical value for model performance according to Baldocchi and Wilson [53]. An under-estimation of net radiation for the TM treatment and over-estimation for the BM treatment were observed at noon in 2015 (Figure 9). Despite the simulation deviations, the model reflected the daily fluctuations of net radiation during different growth stages of potato growth.  Generally, the model reflected the daily variation of soil heat flux with daily RMSE of the TM and BM treatments ranging from 17.03 to 25.27 W/m 2 and ME ranging from −11.30 to 23.35 W/m 2 during the calibration and validation presses ( Figure 10 and Table 6; Table 7). However, the hourly and daily R 2 of soil heat flux were rather small. This might be caused by some abnormal values during soil heat flux measurement.

Canopy Temperature Analysis With the Model
The canopy temperature can affect the aboveground plant growth, especially during the early plant growth stage [11,54]. The canopy temperature was estimated with the calibrated model for the TM and BM treatments in 2015 ( Figure 11). The canopy temperature of the BM treatment was greater than for the TM treatment. The maximum temperature difference between the TM and BM treatments could be as much as 7 °C during early potato growth. The higher canopy temperature in the BM treatment could be the reason why the aboveground growth in the BM treatment was greater than in the TM treatment [5,11]. The model could be a useful tool to study the spatiotemporal distribution of temperature, net radiation, and soil heat flux in potato field with different plastic film mulches and their effects on potato growth during different growth stages.

Summary and Conclusions
A numerical model was developed to simulate the heat transport in potato field with full plastic film mulch in an arid area. This model predicted the interaction of potato plant growth and the optical properties of plastic film mulch on heat transport by utilizing the leaf area index, canopy coverage, photosynthetically active radiation transmittance, and considering the radiation transfer in atmosphere-canopy-mulch-soil system. The heat transfer parameters (net radiation, mulch surface temperature, soil surface temperature, and soil heat flux) were measured in the field experiments in 2014 and 2015 growing seasons in potato field with two conventional plastic film mulch treatments (TM and BM treatments) to calibrate (2014) and validate (2015) the model. Generally, the temperature simulation was reasonably good with daily RMSE of soil surface temperature 2.8 and 1.5 °C for the TM and BM treatments, respectively. The model reflected the daily temperature variations during different potato growth stages except during early plant growth. Moreover, the model simulated the daily fluctuations of net radiation with daily RMSE for the TM and BM treatments 20.30 and 32.79 W/m 2 during validation, respectively. The daily R 2 of net radiation ranged from 0.89 to 0.98, while the daily R 2 of soil heat flux were small because of some abnormal values during soil heat flux measurement. The simulation showed that the BM treatment had higher canopy temperature than the TM treatment. The maximum canopy temperature difference between TM and BM treatments could be as high as 7 °C during early potato growth.
In conclusion, the model could give an explanation for the interaction of plastic film mulch and potato canopy growth on heat transport. The model can be used to simulate heat transport in potato fields with different plastic film mulches in semiarid areas. The heat conditions simulation can provide guidance in plastic film choosing for potato cultivation to avoid heat stress. One of the limitations to study in the future is the cause of the simulation deviations, especially during early crop growth.