MICRO-IRRIGATION SYSTEMS FOR POT-IN-POT ORNAMENTAL NURSERY PRODUCTION - Management, Performance, and Applications of Micro Irrigation Systems - page 303

etc. SMS-based irrigation works for any type of plant, as long as the set-points for the
irrigation controller are correctly chosen and there is no variation in factors that affect
drying, such as uneven substrate compaction at potting, uneven shading, etc. Types of
sensors that have been employed to automatically control irrigation events based on
preset substrate matric potential limits include tensiometers, which measure substrate
suction directly without calibration for substrate type, salinity or temperature but have
contact issues with coarse and dry substrates [43] and dielectric sensors [28, 30]. Use
of SMS was reported to be very promising in the cultivation of plants under PIP. SMS
significantly reduced water use and improve plant growth of many crops under PIP
[8].
21.4.3 ENVIRONMENTAL-BASEDSCHEDULING
Most commonly used environmentally based irrigation scheduling methods are based
on real time or historic weather and consider various weather parameters (light radia-
tion, temperature, wind speed, relative humidity, rainfall, etc.) to estimate the evapo-
transpiration (ET) as a function of weather conditions and plant type. ET is defined
as the quantity of moisture that is both transpired by the plant and evaporated from
the substrate surface. Since the two processes occur simultaneously and are very dif-
ficult to separate, they are combined into one process [2]. In this method rainfall and
irrigation are balanced with withdrawals such as ET and runoff. The water balance
technique involves determining the change in soil moisture over time based on refer-
ence evapotranspiration (ET o ) adjusted with a crop coefficient (Kc) specific to growth
and/or developmental stage, canopy coverage, plant population and exposure [34].
Irrigation based on crop ET values requires periodic modifi cations by the grower
as the method is specifi c to plant type, growth stage, microclimate, and is subject to
variations in local conditions such as plant spacing, canopy coverage, cultivar, root
depth, and production practices [2]. Additionally, time-costly manual fi eld observa-
tions are required to develop accurate crop coeffi cients. Lack of suffi ciently local
weather data and the programs necessary to use it to schedule irrigation lead to errors
in calculating ET o and with a resulting misapplication of water [1, 2].
Scheduling irrigation based on crop ET has limited utility under nursery produc-
tion because of the diversity of crop species being grown, the lack of crop coeffi cient
information for woody shade trees [36], and the open canopy.
21.4.4 PLANT-BASEDSCHEDULING
Innovative methods based on the direct monitoring of plant water relations have been
considered to be highly relevant and allow for environmental influence. Irrigation con-
trol devices have been designed to exploit micro measurements of stem sap flow and
stem diameter [44], leaf temperature [35], plant water potential [17], and modeling
based on empirically derived plant characteristics [29] are plant-based techniques that
have been used to gauge water loss in horticultural crops. But the plant-based irriga-
tion systems are still scarcely employed in commercial operations as low plant water
status induced by high vapor pressure deficit conditions could trigger irrigation when
the substrate moisture is not limiting [13], they lack the ability to assess the volume of
water required for irrigation and are difficult to automate [21].