What s The Difference Between Soil Moisture and Plant-Available Water?

Transcription

1 What s The Difference Between Soil Moisture and Plant-Available Water? John D. Lea-Cox and Bruk E. Belayneh Department of Plant Science and Landscape Architecture University of Maryland, College Park, MD USA JLC@umd.edu

2 Measuring water in Soils and Substrates What are the major differences between tensiometers (or matric potential sensors) and capacitance sensors? How do substrates affect the type of sensor you would use? Is one sensor better (more precise) than the other? What s the difference between them in cost and maintenance? Is the information from one more valuable, compared to the others?

3 Water Content in Soils Zotarelli et al, 2015

4 Empirical Determination of Field Capacity Zotarelli et al, 2015

5 But how does soil moisture (VWC) relate to plant water stress? Or more simply, When should I irrigate?

6 Determining Threshold Irrigation VWC

7 Plant Available Water Soilless Substrates RAW PAW (%) EAW (%) WBC (%) UW (%) Author Substrate kpa 0 5 kpa 5 10 kpa >100 kpa Bunt, % soil: 40% peat De Boodt and Verdonck, 1972 Fonteno, 1981 Milks et al, % perlite % pine bark: 20% Sphagnum peat: 20% concrete sand % peat: 50% vermiculite

8 Stomatal Conductance and Matric Potential Author deboodt and Verdonck, 1972; Raviv et al., 2004 Proposed Matric Potential for Optimal Growth Ψ m (kpa) 10 Kiehl et al., Milks et al, Effect of Soil Matric Potential on Photosynthesis of Tomato Caron et al., (2007)

9 Stomatal Conductance and Matric Potential

10 Water Content (VWC) and Plant-Available Water (PAW) -1,500 kpa Modified from Zotarelli et al, 2015

11 Determining Threshold Irrigation VWC -10 kpa -40 kpa

12 Strawberry Yields Wye REC (2015) Treatment Fruit Yield (g/plant) 1 Number of Fruits per Plant 1 Average Fruit Weight (g) 1 Irrigation Application (L/Plant) 1* IWP (g/l) 1* 0-10 g -30 kpa. 601 ± 13 a 56.2 ± 1.5 a 10.7 ± ± 0.4 a 31.2 ± 1.1 # of Fruits/Plant 40 > 20 g g -40 kpa 537 ± 17 ab 49.2 ± 1.5 ab 11.0 ± ± 0.2 b 31.8 ± kpa 464 ± 16 bc 45.0 ± 1.6 bc 10.9 ± ± 0.3 b 28.9 ± kpa 410 ± 18 c 34.8 ± 1.1 c ± ± 0.1 c 29.3 ± Values given are mean ± standard error of the mean (SEM). a, b, c = Means are significantly different (P<0.05). -30 kpa (Cont.) -40 kpa (DI1) -50 kpa (DI2) -60 kpa (DI3) Treatments Bruk Belayneh et al, Acta Hort, In review)

13 Sensor-controlled Irrigation Studies 13 Chrysanthemum Trial (2015 and 2016)

14 Sensor-controlled Irrigation Studies Irrigation averages for sensor-controlled and grower-controlled blocks (n=8) 13 Chrysanthemum Trial Control Set-point = 40% VWC

15 Irrigation applications to sensor-controlled and grower-controlled blocks Month Sensor-controlled Irrigation (Gallons) Grower-controlled Water Savings (%) June ,984 2, July ,742 33, August ,905 46, September ,892 22, Grand Total 93, , Growth Indices of 13" Chrysanthemum Sensor- vs. Grower-controlled Irrigation (n=5) Tissue Dry Mass of 13" Chrysanthemum Sensor- vs. Grower-controlled Irrigation (n=5) cm Grams Height Width Depth 0 Shoot Dry Mass Root Dry Mass Total Dry Mass Controlled Timed Controlled Timed

16 Sensor-controlled Irrigation Studies - Poinsettia

17 Sensor-controlled Irrigation Studies - Poinsettia Control Set-point = 45% VWC Irrigation application for sensor-controlled and grower-controlled blocks Week Date Range Sensor-controlled Irrigation (Gallons) Grower-controlled Water Savings (%) 40 9/25/16-10/1/ /2/16-10/8/ /15/ /16/16-10/22/ /23/16-10/29/ /30/16-11/5/ /6/16-11/12/ /13/16-11/15/ Grand Total

18 How meaningful is VWC, as a measurement of plant water stress? or, alternatively How low can we go, and still be certain?

19 Commercial Peat : Perlite Substrate Water Release Curve 40% VWC = -12 kpa 30% VWC = -25 kpa 20% VWC = -56 kpa 10% VWC = -180 kpa

20 Soil Water Potential Decreasing plant Available Water Increasing Plant Stress -10kPa to -100 kpa Range

21 T8 Tensiometer (2016) UMS (Germany) Soil matric potential (0 to -100 kpa) External filling Compatible with Em50R data logger (real-time data) external-refilling-field-tensiometer/

22 Soil Matric Potential T8 tensiometers (2 per treatment) Con DI1 DI2 DI3 Matric Potential (kpa) Apr Apr May May May Jun April 1 to June 10, 2016

23 Precipitation Daily rainfall recorded at the study site Con DI1 DI2 DI3 DR Matric Potential (kpa) Daily Rainfall (In) Apr Apr May May May Jun April 1 to June 10,

24 MPS-6 Matric Potential Sensor Decagon (USA) Soil matric potential range (-9 kpa to -100,000 kpa) Accuracy (±10% of the reading) from 6-point factory calibration No Maintenance or Recalibration silica based ceramic material that doesn't degrade Insensitivity to Salts sensor is accurate in salty environments, a variety of soils, Compatible with Em50R dataloggers (real-time data) Affordable 25-30% cost of real-time tensiometers mps-6-calibrated-water-potential-sensor/

25 T8 vs MPS-6 Sensor Comparisons Matric Potential (kpa) comparison between T8 Tensiometer and MPS-6 Sensor T8 MPS-6 0 Matric Potential (kpa) /28/16 11/2/16 11/7/16 11/12/16 11/17/16 11/22/16 11/27/16 12/2/16 12/7/16 12/12/16 Date

26 It s a work in progress.. Apparent that we need to use RAW, rather than PAW as a measure of plant-available water in soils and substrates We need to measure matric potential, to be more confident in irrigation set-points that keep plants out of stress We can still use cheaper, more accurate capacitance sensors, but we just need to understand what that VWC reading actually means

27 An Empirical Approach for Calculating RAW