Assessing Wheat Crop Water Use

Transcription

1 Assessing Wheat Crop Water Use Using soil water monitoring devices in dryland agriculture Trial Results 2010 Michael Zerk Irrigation Consultant

2 Prepared by: Michael Zerk Irrigation Consultant, Rural Solutions SA Office: (08) Fax No: (08) Rural Solutions SA This work is copyright. Unless permitted under the Copyright Act 1968 (Cwlth), no part may be reproduced by any process without prior written permission from Rural Solutions SA. Requests and inquiries concerning reproduction and rights should be addressed to insert name, Business Manager, Rural Solutions SA, GPO Box 1671, Adelaide SA Disclaimer Rural Solutions SA and its employees do not warrant or make any representation regarding the use, or results of the use, of the information contained herein as regards to its correctness, accuracy, reliability, currency or otherwise. Rural Solutions SA and its employees expressly disclaim all liability or responsibility to any person using the information or advice.

3 3 TABLE OF CONTENTS 1 INTRODUCTION 4 2 EQUIPMENT AND INSTALLATION 5 Equipment 5 Installation 6 Sensor Retrieval 8 3 RESULTS 9 Crop Development 9 Crop Nutrition 10 Soil Water Monitoring Data 10 Application of Soil Water Monitoring Technology 19

4 4 1 INTRODUCTION In broadacre cropping situations, moisture availability is often the main factor in determining whether a crop will achieve full yield potential. Several models, such as the French and Schultz Potential Yield Model and APSIM Crop Simulation Model (and associated commercial interfaces such as Yield Prophet ), are available to assist farmers determine target yield. In order to achieve optimum results from such models some knowledge of soil water holding capacity and soil water at sowing is required. Soil water monitoring devices (SWMDs) are widely used as tools to assist in irrigation management decisions, but have rarely been applied in broadacre cropping situations. SWMDs are divided into two broad categories, using either tension or volumetric water content methods, and a wide range of portable and permanent devices are available in each category. Briefly, soil water tension devices indicate the effort required to extract water from soil, whereas volumetric water content devices indicate the volume of water present in the soil. This trial was proposed to see whether SWMD data could be applied to assist decision making and yield forecasting in wheat production systems. It was decided that both methods should be employed, as this allowed comparison of one sensor type to another and provided an opportunity to assess the suitability of different installation methods to soil conditions found in dryland farming situations. Two trial sites were established in paddocks near Bordertown and Wolseley in the Upper South East of South Australia (see Figure 1). One monitoring point was installed at each site in a location that represented typical field conditions according to visual survey. Bordertown Site Wolseley Site Figure 1 Trial site locations indicated by white arrows. The townships of Bordertown and Wolseley are to the left and bottom right of the image respectively.

5 5 Figure 2 Bordertown (left) and Wolseley (right) trial sites directly after sowing in May, EQUIPMENT AND INSTALLATION The SWMDs, and associated installation equipment, used for the trial were sourced from Measurement Engineering Australia (MEA), Magill, South Australia. Two types of sensor were installed to record different attributes of soil water. EQUIPMENT Soil Water Tension Soil water tension refers to the force with which water is held by soil and is expressed by the term, -kpa. When a plant removes water from the soil, it will first remove water that is held at low tension. When this easily removed water is depleted the plant must remove water held at greater tension. As water is held with greater tension, the numbers become more negative and the plant must work harder to extract water. Watermark or GB Lite sensors read the level of resistance to a small electric current passed through electrodes embedded in a granular matrix material contained in a plastic cylinder - more water in the soil and block yields lower resistance to a current. Holes in the cylinder allow for movement of water between the soil and the matrix. These sensors measure soil water tension in a range from -10 kpa (Field Capacity) to -200 kpa regardless of soil type Soil Water Capacity Soil water content refers to the amount or volume of water held in the soil and is either expressed as the millimetres depth of water held per meter depth of soil (mm/m) or a percentage of the measured volume (%). As plant roots remove water, the depth or percentage water content held by the soil reduces. SM200 sensors belong to a family of sensors that use the dielectric constant of the soil, water and air mixture to determine soil water content; as small changes in the quantity of soil water have large effects on the electromagnetic properties of the soil water media. These capacitance sensors calculate soil moisture from a relatively small volume of soil,

6 6 measuring a sphere out to about 10cm from the sensor, but provide accurate results across the complete range of soil moisture. Both of the sensor types connect to specifically designed data logging systems; GBug for GB Lite sensors and TBug for SM200 sensors. Each logger connects to four sensors and provides comprehensive data by measuring soil water at regular intervals; 15-minutes for the TBug and 2-hours for the GBug. Data is downloaded from loggers and transferred to computer with an MEA Retriever, for viewing with MEA Bug Software. INSTALLATION The same installation pattern was followed at both sites (Figure 3), with the aim to capture variation in soil moisture through the most active part of the root zone. Equipment was installed directly after sowing, May 26 (Bordertown) and 27 (Wolseley), on both sides of a randomly selected crop row at 10, 20, 30 and 40 cm below the local soil surface. Installation was completed with minimal site disturbance, as illustrated by even crop establishment at both sites (Figure 5), and during installation soil at both sites appeared moist from 10 cm below the surface to the bottom of the measured profile. This facilitated establishment with all sensors providing satisfactory readings following a short period of settling. LOGGER SENSOR Figure 3 Installation plan for soil moisture sensors. The diagram on left is an overhead view with sensors positioned along the crop row. The diagram to the right is a cross-section showing sensors staged through the profile.

7 7 Figure 4 Completed installation at the Wolseley (Left) and Bordertown (Right) sites. Figure 5 Even seedling emergence following installation at Wolseley (Left) and Bordertown (Right) sites. Sowing rows under which equipment was installed (circled) have established as well as those adjacent.

8 8 SENSOR RETRIEVAL There is always a concern that when attempting to measure something, the nature of that thing being measured is changed. For this reason sensors were removed after harvest and observed for errors in installation or function that may have created flaws in the data. A summary of these findings is presented below Soil Contact and Interaction The GB Lite sensors were of particular concern as they must be soaked in water before being installed into an auger hole with a slurry mixture. If soil is too dry following installation the sensors may never achieve good soil contact. Figure 15 demonstrates an ideal scenario, with a pair of GB Lite sensors still covered with soil and fine roots after removal. SM200 sensors are easier to install as the waveguides are inserted into undisturbed soil. Sixteen sensors in total were installed and all appeared well integrated with the soil. Figure 6 Clockwise from top left: SM200 sensor with the body wrapped in roots (circled) and worm castings attached to the waveguide (arrow); inside surface of SM200 installation auger hole with sensor markings (arrow) and roots (circled); GB Lite sensor showing signs of good soil and root interaction (fine roots in soil not visible in photo) Root Channels and Preferential Flow The auger holes required to install both types of sensor may create non-representative root cannels and facilitate preferential water flow. This is particularly of concern in moist clay soils as movement of the auger in and out of the hole tends to seal the sides. Likewise, when re-packing the hole it can be difficult to achieve a similar soil density to the surrounding soil. To minimise these risks auger holes were drilled off the vertical, soil was passed through a sieve before re-packing and holes were plugged with a mix of sand and bentonite. A satisfactory result is demonstrated in Figure 15; whereby the sensor body has embedded into the bottom of the auger hole and roots visible at the face of the hole do not appear excessively dense. Preferential flow is difficult to distinguish at

9 9 sensor retrieval; however sensor response to rainfall during the growing season (Figure 6 and Figure 11) is deemed appropriate and plants under which sensors had been installed maintain the same appearance as adjacent crop rows (Figure 9). 3 RESULTS CROP DEVELOPMENT Achieving high yield and quality targets requires establishing a crop that is responsive to sunlight, water and fertiliser inputs. Literature suggests that, given that all other conditions are favourable, the availability of water between stem elongation and flowering (anthesis) determines target yield in wheat. In this (approximately) 30-day period, tillers become spike-bearing shoots and the eventual size of these spikes is determined by the number and potential size of kernels i. It is important to note that extended periods of soil saturation may induce comparable plant stress and yield reduction to periods of moisture deficit. Varietal and growth stage details for both sites are listed in Table 1. Dates for all growth stages are based on assessment of plants located at and adjacent to the monitored site during regular visits to download soil moisture data. Additional information relating to timing of operations has been supplied by the farmer 1. Table 1 Crop Details and Growth Stages, Wolseley Bordertown Variety Gladius Derrimut Sowing Date May 25 May 22 Stem Elongation (Z30/31) August 5 August 5 Urea Application Not Recorded August 9 Third Node (Z33) August 26 August 26 Ear Emergence (Z51) September 23 September 20 Flowering (Anthesis) (Z61) October 10 October 6 Early Dough (Z83) November 4 November 4 Maturity (Z93) Early December December 5 Harvest January 3 December 29 Yield (t/ha) 4.5 Not Calculated 1 Yield records are a paddock average as supplied by the farmer. Results are provisional at the time of writing due to the staggered nature of harvest and number of grain quality segregations required.

10 10 CROP NUTRITION Tissue samples were collected at Early Booting (Z43) from around the monitored site; excluding plants under which equipment was installed. The results of analysis are presented in Tables 2 and 3 below and show crop nutrition at both sites to be adequate for normal development. Table 2 Plant nutrient analysis results for the Wolseley site, sampled September 13. Table 3 Plant nutrient analysis results for the Bordertown site, sampled September 13. Guided by these results and in the absence of plant disease symptoms, crop yield potential shall be considered a response to plant available water recorded by soil water monitoring equipment. SOIL WATER MONITORING DATA The following graphs show soil water data for the period between Stem Elongation and Flowering (Z30/31 to Z69). Figure 6 and Figure 7 show water content and tension at Wolseley, with water content and tension at Bordertown presented in Figure 8 and Figure 9. Each graph displays four traces, each representing a sensor position, that rise when soil water increases and fall when it is depleted. The nature of variation allows us to distinguish between events such as rainfall, capillary rise, crop water use and drainage.

11 11 Start of Stem Elongation Ear Emergence Start of Flowering Figure 7 Changes in soil water content (%) at the Wolseley site for the period critical to establishing yield potential in wheat (Z30/31 to Z61).

12 12 Start of Stem Elongation Ear Emergence Start of Flowering Data missing due to error in logger function Figure 8 Changes in soil water tension (-kpa) at the Wolseley site for the period critical to establishing yield potential in wheat (Z30/31 to Z61).

13 Wolseley Soil at Wolseley is near to above Field Capacity at the start of Stem Elongation, as indicated by tension readings Figure 7, with saturated conditions likely at 30 and 40 cm. Water content remains relatively constant throughout Stem Elongation (Figure 6) before starting to decline in September in response to increased crop water demand and reduced rainfall. It is likely that high yield potential has been established given nonlimiting water conditions at this time. A marked decline in soil water content is apparent between Ear Emergence and the Start of Flowering. At Ear Emergence, the slope (rate of change) of the 10 cm (red) trace in Figure 6 becomes flatter, indicating that Readily Available Water (RAW) has been exhausted. This interpretation is supported by soil water tension recorded in Figure 7 (summarised in Table 4), where values at 10 cm exceed -100 kpa. Subsequent crop water use at 10 cm is minimal; however two periods of rapid decline are observed in Figure 6 prior to flowering. The first of these coincides with the onset of soil cracking, which effectively increases the surface area of soil exposed to evaporation and decreases root-zone volume. The second event is likely to result from further cracking and air exposure (Figure 8). Similar trends to that described above are observed at all depths prior to flowering; with crop water use occurring progressively through the profile as the most readily available soil water is extracted. Soon after the Start of Flowering the rate of change in soil water capacity slows (traces are almost flat, Figure 6) and soil water tension increases to, and then exceeds, the upper range of the sensors (Figure 7 and Table 4). Table 4 Summary of the change in soil water content (%) and tension (-kpa) between Ear Emergence and Flowering for all sensor positions - Wolseley site. Figure 9 Soil cracks at Wolseley between plants (at left, indicated by arrow) tear fine roots and effectively reduce root-zone volume. 19 Oct 2010

14 Soil Water Tension (-kpa) 14 Although soil water data indicates that RAW has been depleted and surface cracks have appeared by the Start of Flowering, plants exhibit little sign of moisture stress. Literature suggests that in very dry conditions, stem extension is restricted and anthesis may occur while ears are emerging or still within the boot. Figure 9 shows that neither of these symptoms has occurred. This implies that the crop has established roots beyond the measured zone and is able to extract sufficient water to maintain development - the extent of root activity at 40 cm described previously in Figure 6 supports this assumption. As water use for the whole of the root zone cannot be measured, calculation of total crop water use through the growing season cannot be made with confidence. Figure 10 Wolseley trial site in flower 19 Oct The period between Ear Emergence and Flowering provided an opportunity to test the relationship between soil water tension readings and soil water content. Figure 10 shows -120 Determining the Relationship Between Soil Water Tension (-kpa) and Soil Water Content (%) Readings on a Drying Soil R 2 = Soil Water Content (%) Figure 11 A strong relationship was found between soil water tension (-kpa) and soil water content for readings taken at 20 cm at the Wolseley site. a near linear increase in water tension as water content declines. This comparison is only valid up to -100 kpa at this depth as soil cracks that appear beyond this point create unreliable tension values. But as the range of values is likely to extend beyond the threshold for RAW, the graph provides a viable reference for future moisture samples taken from near the monitored site.

15 15 Ear Emergence Start of Stem Elongation Start of Flowering Figure 12 Changes in soil water content (%) at the Bordertown site for the period critical to establishing yield potential in wheat (Z30/31 to Z61).

16 16 Start of Stem Elongation Ear Emergence Start of Flowering Figure 13 Changes in soil water tension (-kpa) at the Bordertown site for the period critical to establishing yield potential in wheat (Z30/31 to Z61).

17 Bordertown Soil at Bordertown is at or above Field Capacity at the start of Stem Elongation, as indicated by tension readings Figure 12, with saturated conditions likely at all measured depths. Water content remains relatively constant throughout Stem Elongation (Figure 11) before starting to decline in September in response to increased crop water demand and reduced rainfall. High yield potential is likely to have been established, with an application of urea coinciding with the start of Stem Elongation. The decline in soil water content between Ear Emergence and the Start of Flowering is not as marked as at Wolseley; for example the slope (rate of change) of the 10 cm (red) trace in Figure 11 does not flatten until (approximately) 7-days after Ear Emergence and Figure 12 shows that RAW at 10 cm is not exhausted until just before flowering. Table 5 provides a summary of soil water capacity and tension through this period and it is clear that, in comparison to the Wolseley site, the crop at Bordertown has greater access to water through Flowering. Movement in the 10 cm trace becomes erratic after the Start of Flowering (Figure 11), suggesting poor contact between the sensor and soil. An increase in soil water capacity is observed at 20 cm (blue trace) between September 29 and October 3 (Figure 11); at the same time, the rate of increase in soil water tension declines (Figure 12). This may be explained by capillary rise or lateral water movement, but there is little to no comparable change in the rest of the profile. Root activity is not as apparent between 30 and 40 cm as described for the Wolseley site and crop water use does not appear to occur progressively through the profile as expected. Instead plants access water at 40 cm more readily than at 30 cm (Figure 11) following Ear Emergence, whilst Figure 12 shows that at the end of Flowering soil water tension at 40 cm is substantially higher than at 30 cm. This may be indicative soil constraints limiting access to water at 30 cm or, as postulated for the observation at 20 cm, may indicate capillary rise or lateral movement. Table 5 Summary of the change in soil water content (%) and tension (-kpa) between Ear Emergence and Flowering for all sensor positions - Bordertown site.

18 Soil Water Tension (-kpa) 18 Figure 14 Crop appearance 19 Oct 2010 (left) and 3 Nov 2010 (right) at the Bordertown site. Crop canopy has thinned as older leaves yellow and senesce. Soil water monitoring data at the Bordertown site appears to represent crop water use more accurately than for that at Wolseley. However, as considered at Wolseley, the crop has established roots beyond the measured zone and is able to extract sufficient water to maintain development. Again, calculation of total crop water use cannot be made with confidence. A test of the relationship between soil water tension readings and soil water content is presented in Figure 14, showing a near linear increase in water tension as water content declines. The graph provides a viable reference for future moisture samples taken from near the monitored site Determining the Relationship Between Soil Water Tension (-kpa) and Soil Water Content (%) Readings on a Drying Soil Soil Water Content (%) R 2 = Figure 15 A strong relationship was found between soil water tension (-kpa) and soil water content (%) for readings taken at 20 cm at the Bordertown site.

19 19 APPLICATION OF SOIL WATER MONITORING TECHNOLOGY Results from this trial are not conclusive, but this is not to say that soil water monitoring equipment cannot provide useful information in the management of dryland farming systems. Important lessons learnt during this trial include: Sensor Selection and Placement Whatever equipment or sampling method is used to assess soil water content, results or data need to be representative of the entire root-zone. If it is possible, when installing SWMDs a sensor should be placed at depth beyond the (likely) root-zone to monitor drainage. This is important information as the calculations used to assess crop water use efficiency assume that all growing season rainfall contributes to yield. A soil survey is recommended prior to installation so that sensors can be installed at the most appropriate locations Assisting Management Decisions SWMDs may provide useful data to assist management decisions, even for decisions made outside the period of data capture. In this trial, the decision to apply urea at stem elongation did not require consultation of soil water data as moisture levels were clearly high. Data is likely to be of greater use in more marginal season conditions when split applications of fertiliser require greater consideration Soil Water Content at Sowing SWMDs are sensitive to disturbance and easily damaged by tillage equipment. Although some devices can be installed below the soil surface, permanent installations are more likely to be found in perennial farming systems. In annual cropping applications, devices may need to be installed directly after sowing (as performed in this trial) and should, if correctly installed, provide accurate results for initial water content. Samples for calculating gravimetric water content may be taken at this time to calibrate the soil water monitoring equipment. Potential benefits of using SWMDs identified during this trial include: Assess the Effect of Soil and Crop Management Practices on Rainfall Capture, Infiltration and Drainage SWMDs may prove to be useful tools for monitoring the benefits gained through adoption of different farming practices. Given accurate rainfall data (recorded at the monitored location), calibrated SWMDs can indicate soil water increase and depletion as: rainfall capture and storage, crop water use, deep drainage and potential runoff. Soil water data collected over time can therefore be used to assess changes in water supply as a function of infiltration and distribution of rainfall and amelioration of limiting soil conditions Crop Forecasting and Modelling Yield forecasting and crop modelling are not possible using SWMDs alone; however the data recorded may help to overcome problems with assumptions of the relationship

20 20 between growing season rainfall and crop water use as contained in the French & Schultz equation. Conditions that may not be properly incorporated include runoff and deep drainage (with a resultant net loss in available water that tends to increase as rainfall exceeds and optimum level) and waterlogging (which reduces the efficient conversion of water to dry matter and yield). Where crop water use is calculated by applying crop coefficients (K c ) to measurements of evapotranspiration (ET o ) SWMDs can provide a valuable cross-reference. If the rate of moisture depletion (and replenishment) between the two methods is the same, crop water use can be concluded with greater confidence. i Stapper, M (2007) High-Yielding Irrigated Wheat Crop Management, Conclusions of the GRDC Irrigated Wheat Evaluation Project, Irrigated Cropping Forum 2007.