Effects of soil moisture and length of irrigation on soil wetting to deliver fumigants through microirrigation lines in sandy spodosols

doi:10.4236/as.2011.24069

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Vol.2, No.4, 533-535 (2011) Agricultural Sciences

doi:10.4236/as.2011.24069

Effects of soil moisture and length of irrigation on soil

wetting to deliver fumigants through microirrigation

lines in sandy spodosols

Bielinski M. Santos*, James P. Gilreath

Gulf Coast Research and Education Center, IFAS, University of Florida, Wimauma, USA;

*Corresponding Author: [email protected]

Received 24 August 2011; revised 22 September 2011; accepted 13 October 2011.

ABSTRACT

Soil fumigant delivery through microirrigation

(drip) lines has the potential to replace direct

soil injection into planting beds. However, wet-

ting coverage in these Spodosols must be im-

proved to increase soilborne pest and weed

control. Field trials were carried out to deter-

mine the imp a ct of soil m oisture on the exte nt of

wetting cross-sectional areas through varying

irrigation times. Soil moisture contents w ere: 1)

7% moisture (field capacity), and 2) 20% (satu-

ration), along with 2, 4, 6, 8, and 10 h of irrigati on .

Pressed beds had 70 cm tops. Drip lines had

emitters spaced 30 cm apart delivering 0.056

L·min–1 per m of row at 55 kPa, and two drip

lines were buried at 2.5 cm below the surface

and 30 cm apart from each other. Water was

mixed with a blue marking dye to analyze the

water distribution patterns. Beds were opened

at the emitters and high-resolution digital pic-

tures were taken for each treatment. Resulting

images were adjusted using photographic soft-

ware and covered areas across the beds were

determined. Regression analysis showed sig-

nificant quadratic equations for both soil mois-

ture situations, with saturated soils obtaining

the highest cross section coverage (90 and 94%

after 8 and 10 h). In field capacity beds, the ma-

ximum cross section coverage obtained was

82%. Within each soil moisture situation, there

were no differences between 8 and 10 h of irri-

gation.

Keywords: Methyl Bromide; Drip Irrigation;

Soilborne Pests; Water Management

1. INTRODUCTION

The loss of methyl bromide (MBr) as soil fumigant

has prompted a great deal of research to identify an al-

ternative to use in horticultural crops. Thus far, combi-

nations of broad-spectrum fungicides and nematicides

followed by herbicides have been shown to be suitable

options to reduce the impact of soilborne diseases,

nematodes and weeds in vegetable crops. The conven-

tional procedure to apply chemicals such as 1,3-di-

chloropropene + chloropicrin (1,3-D + Pic) and metam

sodium (MNa) has been either in-bed injections with

chisels or pretransplant broadcast applications with spe-

cialized equipment [1]. However, the cost of the equip-

ment and the risk of personnel poisoning are some of the

concerns about these application techniques.

Application of any MBr alternative requires the

maximum possible coverage of soil volume in order to

contact the target pest in sufficient quantity and time

duration to achieve acceptable control. Unfortunately,

this is not always possible, thus pest control gaps are

frequently observed. Therefore, more efficient delivery

systems need to be developed. Some recent reports have

indicated that strawberry yields obtained with MBr were

comparable with those of Telone C-35 (1,3-D + Pic

65:35; Dow AgroSciences LLC, Indianapolis, Indiana,

USA), Inline (1,3-D + Pic 61:33; Dow AgroSciences

LLC, Indianapolis, Indiana, USA), liquid and granulated

MNa, Pic, methyl iodide, and methyl iodide + Pic

(60:40). Other trials indicated that Inline® + MNa, Te-

lone-C35® + MNa, Telone II® (98% 1,3-D; Dow Agro-

Sciences) + MNa, and Telone-C35® + MNa + Bacilus-

subtilis were equivalent to MBr [2]. However, no much

research has been conducted under Florida conditions on

this subject and it needs to be further expanded.

The benefits of developing a soil fumigation system

for light sandy soils, such as are present in Florida, could

transcend current boundaries, allowing scientists to gain

a better understanding of how fumigants move in the soil

and their relationship with crop yields. Likewise, it

would impact on the feasibility of vegetable crop set-

B. M. Santos et al. / Agricultural Sciences 2 (2011) 533-535

534

tings, since costly injection machinery would not be

needed for soil fumigation, because the same irrigation

system used for delivery of water and fertilizer would be

utilized. One of the biggest obstacles to many alterna-

tives has been the onerous requirement for personal pro-

tective equipment (PPE). This PPE makes application

even more difficult in subtropical climates because of

the impact on workers. The possibilities of reducing soil

and water pollution and personnel poisoning/exposure

could be additional benefits derived from alternative

delivery procedures.

Soil texture has a close relationship with waterfront

movement in the soil profile. Heavy to loamy soils usu-

ally have slow vertical infiltration, providing greater

opportunities for liquid formulations to move laterally,

thus increasing coverage. However, in sandy soils, lat-

eral movement of water is restricted by the large amount

of macropores [3]. This has been the case for wetting

pattern trials in Georgia, where distances of 45.7 cm

between drip emitters provided the least bed width cov-

erage, with the most occurring with 30 cm between

emitters. The lowest uniformity of wetting took place

with 40.6 and 45.7 cm between emitters, and a maxi-

mum wetting of 60% of the bed volume was obtained [4].

In Florida, irrigation duration trials resulted in 45% of

the soil volume in the bed wetted after 12 h with a single

drip tube, and 85% coverage after 10 h with two lines [5].

Likewise, irrigation volumes were tested and indicated

that the use of two drip tubes only reduced the time of

flooding, even though two drip tubes increased spatial

distribution. The most bed wetting achieved with a sin-

gle tube was 50% to 60%, using 1135 L per 30 m of row

[6,7]. Huckaba et al. [8] found that bed wetting with a

single drip tape resulted in 50% coverage after 8 h, while

use of two tapes yielded 90% coverage.

To avoid erratic soilborne disease control with mi-

croirrigation, water application has to be studied consid-

ering the internal water movement in the soil profile.

Research needs to be conducted to determine the opti-

mum parameters for fumigant application through mi-

croirrigation systems in sandy soils. Distribution of irri-

gation water containing soil-active pesticides throughout

the raised soil bed may be necessary to obtain adequate

control of soilborne pests with microirrigation-applied

chemicals. The objective of this research was to deter-

mine the impact of soil moisture on the extent of water

coverage obtained through varying irrigation times.

2. MATERIALS AND METHODS

Two field trials were conducted during January and

March 2003 at the Gulf Coast Research and Education

Center (GCREC) of the University of Florida. The soils

were classified as EauGallie fine sand (AlficHaplaquods,

sandy, siliceous, hyperthermic) with 1.0% organic matter

and pH 7.3. Raised beds were pressed with a bedder and

covered with low-density plastic mulch. Bed dimensions

were 0.71 m wide on top by 0.81 cm wide on the bottom

by 0.20 m high, giving an approximate cross sectional

area of 0.15 m2. Two drip irrigation lines (T-Tape Sys-

tems, San Diego, California, USA) will emitters spaced

30 cm apart were placed on bed tops before plastic

mulch placement. Each emitter delivered 0.056 L·m−1

per minute at 0.55 bars. Drip lines were buried 2.5 cm

below the surface and 30 cm apart.

Treatments were arranged in a split-plot design with

three and six replications for January and March trials,

respectively. Soil water contents were the main plots,

while lengths of irrigation were the subplots. Water con-

tents were 7% (field capacity) and 20% (saturation).

Lengths of irrigation were 2, 4, 6, 8, and 10 h. Water

table was maintained 0.45 m below bed surface by con-

tinuously providing seepage irrigation. Each experimen-

tal unit was 6 m long. Soil moisture levels were meas-

ured by weight at the beginning of each trial. Soil sam-

ples were collected at random from three places within

each block and dried for 48 h at 120˚C.

Distribution of microirrigation water was evaluated by

using a water-soluble blue marking dye. The 0.95 L of

dye was mixed in a 208-L tank and was introduced into

every tankful of water during the various irrigation cy-

cles. This methodology has previously being tested and

provides a quick, inexpensive, and readily visible me-

thod for evaluating patterns of microirrigation water dis-

tribution [4,5]. One week after dye application, plastic

mulch was removed in two places within each experi-

mental unit and cross sections were exposed at the emit-

ters. In order to obtain darker cross sections, each cut

was allowed to dry for 2 h. Afterwards, high resolution

digital pictures were taken of each section. Resulting

images were transferred to photographic software and

covered areas were analyzed and calculated. During pre-

liminary trials, this digital methodology was adjusted to

reduce image distortion. Also, this method was validated

against a manual technique used previously, in which a

Plexiglas grid is placed against the cross sections and the

dye-marked contour is drawn [5]. The digital methodol-

ogy proved to be more accurate and less time-consuming.

The relationship between irrigation time and covered

area was examined with regression analysis. Standard

errors were used to measure differences among treatment

means.

3. RESULTS AND DIS CUSSION

There was no significant interaction between treat-

ments and trial (P > 0.05). Therefore, data from the two

trials were combined. Irrigation times significantly (P <

0.05) affected cross-sectional covered areas (Figure 1).

B. M. Santos et al. / Agricultural Sciences 2 (2011) 533-535

535535

Figure 1. Effects of irrigation time on cross-sectional covered

area of beds under two soil moisture contents. Regression

equations are y = 39.6 + 34.6x – 1.6x2, r2 = 0.91 for 20%

moisture; and y = 55.0 + 27.6x–1.3x2, r2 = 0.94 for 7% mois-

ture.

Quadratic regression equations characterized the re-

sponses of each soil moisture status (y = 39.6 + 34.6x –

1.6x2, r2 = 0.91 for 20% moisture; and y = 55.0 + 27.6x –

1.3x2, r2 = 0.94 for 7% moisture).

When irrigation was applied for 2 h, no differences

were observed in the wetted areas for either soil mois-

ture, with less than 45% of total area wetting. However,

significant changes were observed beginning with 4 h of

irrigation. In those cases, beds with 20% moisture

reached larger wetted areas at the emitters than those

with 7% moisture. When 20% moisture was present,

there were increments of 7%, 6%, 9%, and 8% at 4, 6, 8,

and 10 h of irrigation, respectively. With the soil at field

capacity and 10 h of irrigation, 82% wetting was obtained,

whereas a maximum coverage of 94% was obtained in the

saturated soil during the same period (Figure 1).

Previous reports on Florida Spodosols have indicated

that the largest covered area obtained with two irrigation

lines was 85% [5]. That finding was confirmed by these

trials, where 82% wetting area was obtained under simi-

lar conditions. However, coverage was further improved

by raising soil moisture prior to the irrigation. Therefore,

soil water status has an influence on water distribution

patterns throughout planting beds, increasing bed wet-

ting as soil moisture increases.

This finding might allow scientists to redesign strate-

gies for drip-applied fumigants in Florida Spodosols.

Further studies should be conducted to determine if dif-

ferences in wetting areas reflect positively on yields or

soilborne pest and weed control. Also, the research

should be conducted to assess the effect of increased

wetting on fumigant diffusion beyond waterfronts.

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