- Capacitance probes, e.g. Enviroscans and Diviners
- Neutron probes
- Converting between soil water potential and soil water content
Influence of availability of soil moisture
- Availability of water can influence the number of tubers produced.
- A 1.89 % loss in number of tubers has been shown for each day that plots were drought stressed after emergence (drought stress = soil moisture potential drier than -25 kPa).
- Early availability of water may also influence the rate of tuber bulking.
- One of the most critical times for water management is the early part of the season.
- This requires an early start to measurement of soil moisture and an ability to make uniform application of low amounts of irrigation.
- Dry soil before emergence may restrict root growth and uptake of nutrients and stolon formation (which can start before emergence of some crops). Water management should therefore be considered from the time of planting: it cannot be assumed that soil will be a field capacity after winter.
- The period from planting to the onset of bulking is the time when crop potential may be influenced. Water stress should be avoided during this period (i.e. soil drier than –25 kPa or wetter than field capacity – waterlogged). To reduce the risk of common scab (in dry soil) or powdery scab (in waterlogged soil), water stress should continue to be avoided until the majority of growing tubers are larger than 20 mm (when it can be expected that lenticels will have become suberised).
- If insufficient water is available to irrigate for the entire season, the early part of the season is the most important time to use the water.
- Drought at any time in the season may be detrimental to tuber yield.
- Mid season drought may result in the additional problem of growth cracking.
Irrigation management at the end of the season has implications for:
- crop maturity
- tuber dry matter concentration (low to reduce bruising risk or high for processing)
- ease of harvesting
- risk of disease
Water content at field capacity of different soils
To calculate soil moisture deficit from soil moisture content
If field capacity = 15 % then:
- 15 % soil water content = 150 mm water / m depth soil
- 10 % soil water content = 100 mm water / m depth soil = 50 mm deficit
Determination of soil moisture
Whichever equipment is used to measure soil moisture content, it is important that the “full point” is determined accurately. This should be the moisture content at field capacity. If this is underestimated insufficient water may be applied during irrigation.
Capacitance probes, e.g. Enviroscans and Diviners
- These provide continuous measurement of volumetric soil moisture content, using the capacitance (electromagnetic) technique which provides an accurate measurement of soil moisture content.
- Sensors are installed at specified depths in access tubes and connected by cables to a data logger.
- Each set of sensors in an access tube comprises a “probe”.
- Correct installation of access tubes is essential.
- Sensors from several access tubes may be connected to the same data logger.
- Results provide a graphical representation of changes with time in soil moisture content.
- Output is provided as results from individual sensors at different depths and as a summation for the whole soil profile.
- This is an operator provided service and 1 m aluminium access tubes are installed in the fields monitored.
- A “full point” is estimated for each soil depth based upon “experience, examination of a great deal of data and the field”.
- The neutron probe operator may dig a pit in the area of the probe, to check soil moisture profile.
- Readings are taken at 10 or 20 cm depth increments each week.
- Results may indicate:
- soil moisture deficit at 0 – 20 cm (during “scab control” period)
- soil moisture deficit at 0 – 40 cm (during the main period of growth)
- soil moisture deficit at 40 – 80 cm (after “scab control” period)
- the amount of water used by the crop each day
- expected irrigation requirements
- projected water use
N.B. Neutron probe operators may not be agronomists, in which case advice should be limited to water use.
- Tensiometers are the only pieces of equipment which measure soil moisture potential.
- Soil moisture potential provides the best indication of how plants “sense” availability of water.
- Different types of tensiometers may be used for “spot” measurements or installed at specific depths and left in situ for the season, connected to data loggers.
- However, tensiometers do not provide a direct measurement of soil water content and are less practical for irrigation management than are capacitance or neutron probes.
Converting between soil water potential and soil water content
- Analysis of soil samples can enable conversion curves to be produced for each soil type.
- This requires an undisturbed soil sample from each soil horizon and appropriate laboratory analysis.
- Alternatively, tensiometers can be used to estimate at what soil moisture content trigger soil moisture tensions are achieved.
- Rain guns provide a relatively simple system but application of water can be uneven in windy conditions.
- Boom or centre pivot systems can provide more uniform application but it is important to ensure that the rate of water application is not greater than the rate of absorption. This is particularly important with large centre pivots.
- Drip irrigation can result in significant savings in amounts of water applied. Careful set up is required but less attention may be required during the season than for rain guns or boom irrigators.
Alternative water sources
- “Grey” water is waste water which does not contain sewage.
- Some industrial processes produce grey water which may be available for irrigation.
- Some sources might be suitable for irrigation but it is important to obtain a full analysis of the water to ensure that there are no risks of:
- disease transmission (e.g. avoid washing water form potato or vegetable production plants)
- heavy metal deposition (heavy metals accumulate in soils and this is often a limitation to use of grey water)
- Potatoes are moderately sensitive to salinity and yield declines as salinity increases (often measured as electrical conductivity, EC).
- Sodium in saline water can reduce flocculation of clay minerals and result in impaired penetration of water (the sodium adsorption ratio, SAR, provides a measurement of this).
- Build-up of undesirable ions in soil can occur if insufficient water is applied.
- If saline water is the only source of irrigation water available, it may be worth considering if:
- salinity is low
- SAR value is low
- lack of water restricts crop growth
- potatoes will be the only crop in the rotation to be irrigated from this source
N.B. It will be important to monitor nutrient levels in soil to ensure that unwanted ions are not accumulating.
Soil moisture notes
Soil conditioners 2016 (The potential for commercial soil conditioners to improve soil structure or availability of moisture)
Reducing run-off 2016 (Cultivation techniques for reducing run-off in tramlines)
Water management 2016 (Effects of “scab irrigation” regimes on crop performance)
Soil conditioners 2015 (Introduction to commercial soil conditioners for potential influence on soil moisture and structure)
Common scab and irrigation 2013 (Influence of soil moisture on Streptomyces spp, antagonists and other micro-organisms on the tuber surface)
Water use 2012 (Potential problems from restricted application of water)
Soil surfactants 2012 (Potential for soil surfactants to influence availability of water and nutrients)xcds
Irrigation practice and water quality 2011 (Results from some irrigation trials; Environment Agency guidelines for using water in a dry season; using “grey” water or saline water)
Irrigation scheduling 2010 (Use of “Enviroscan” electromagnetic probes)
Drip irrigation experiment 2010 (Further photographs of the equipment used in the drip irrigation trial begun in 2009)Introduction to
Irrigation 2009 (Drip vs boom irrigation trial)
Water and Soils Directives 2008 (Potential impact on potatoes of the legislation)
Water management 2008 (Making best use of limited water)
Drip irrigation 2003 (Assessment of drip and solid set sprinkler irrigation techniques)
Measurement of soil moisture 2003 (Evaluation of neutron probe use)
Control of soil moisture 2001 (Effects of water availability at different times in the season)
Early irrigation 2001 (Early availability of water is important for crop growth as well as suppression of common scab)
Control of soil moisture 1998 (Influence of moisture availability on crop performance: time and amount of drought stress; measuring soil moisture; relationship between soil moisture deficit and soil moisture potential)
Soil moisture measurement 1996 (Principles of measuring soil moisture with nuclear [neutron probe], electromagnetic [e.g. “Enviroscan”] and tensiometric [tensiometer] techniques)
Water management 1994 (Effects of water availability and cultivation on physiology, root growth and yield)
Seminar 2 Notes 1992 (Chapter 8: Weather forecasting includes importance of soil moisture for potatoes)
Notes from an external source