Agriculture Reference
In-Depth Information
Two common types of blocks are the granular
matrix sensors and gypsum blocks. The blocks
are buried and absorb water from the soil. One
common issue is the lack of good soil-to-block
contact; air pockets can provide erroneous
measurements. The blocks are connected to wires,
which plug into a portable data logger or meter
(Morris, 2006).
Gravimetric sampling is one of the more ac-
curate soil water measuring techniques and in-
volves soil sampling, weighing wet and dried
soil, and calculations involving soil bulk density.
Due to the labor and time involved, this sampling
is usually not used on a large scale.
The balance sheet method relies on known
or predicted daily evapotranspiration (ET) meas-
urements and real-time and forecasted weather
data. Irrigation is scheduled to replace all or
some of the soil water used. Computer models
are available on the Internet, or through com-
panies that have created values for specific var-
ieties grown in specific regions. This method of
irrigation scheduling can be rather accurate,
provided correct model input values are used
during calculations.
The most traditional and perhaps most reli-
able method is digging, feeling, and viewing soil
from the rooting zone of the potato. Soil is often
collected using soil probes pushed to the desired
testing depth. Although this method relies
heavily on experience, Ley
et al
. (1994) have
produced a guide that matches soil feeling and
appearance to percent ASW. Drawbacks include
reliance on judgment, which could lead to differ-
ences among multiple testers, and inability to
probe at the depth desired due to rocks and com-
paction.
Modern soil water measurement and moni-
toring methods incorporate cellular and wireless
data transfer directly from the instrument or
field to provide real-time information to growers,
enabling them to adjust irrigation systems
quickly from their computers or phones. Remote
sensing systems use buried sensors that detect
soil moisture changes, which are fed into a data
logger (Morris, 2006). The data logger sends
wireless or cellular signals to the user's phone or
computer, allowing for real-time monitoring of
multiple fields. The disadvantages of remote
sensing are the relatively high cost for each unit
(>US$1000) and the lack of field-monitoring
visitations. As with most technology, these units
should become more affordable over time;
however, it is difficult or impossible to replace
actual field visitations by an agronomist.
Although many of the soil water measuring
instruments provide good information, growers
should incorporate a mixture of instrument use
and hands-on measuring to ensure successful
production.
Plant nutrition and fertilizer
Mineral nutrients are essential for potato plant
growth and development (Kolbe and Stephan-
Beckmann, 1997a,b). Mineral nutrients found in
the soil are typically classified either as a macronu-
trient or a micronutrient. As their name indicates,
plants require and use more macronutrients than
micronutrients, but nutrients from both categories
are important to plant health. Macronutrients im-
portant for potato production include nitrogen (N),
phosphorus (P), potassium (K), calcium (Ca), mag-
nesium (Mg), and sulfur (S); however, potato plants
and tubers extract far more N, P, and K from the soil
than other nutrients. Micronutrients essential to
potato growth include boron (B), copper (Cu), iron
(Fe), chloride (Cl), manganese (Mn), and zinc (Zn).
In most situations, potatoes grown for profit
require fertilizer applications. Unless growers
are extremely familiar with their fields and rota-
tions, a pre-plant soil analysis should be used to
determine nutrient levels, organic matter, pH,
cation-exchange capacity (CEC), and other soil
characteristics. Whether manure or synthesized
fertilizer is used, crop nutrient needs are deter-
mined largely by the region, season length and
anticipated harvest date, and market choice. Re-
commended application timing and amounts
are usually based on research (Lang
et al
., 1999;
Stark and Westermann, 2003; Dampney
et al
.,
2013) or local experience.
Fertilizer recommendations typically exceed
the actual quantity needed by the plants due to tie-up
in the soil by microorganisms (immobilization)
and chemical reactions, soil leaching and vola-
tilization, excessively large gaps between plants/
poor stands, lack of in-season monitoring and
experience, and a favorable benefit-cost ratio
(excessive fertilizer applications may be less expensive
than yield losses caused by nutrient deficiencies).
Excessive fertilizer applications can damage the
environment and lead to human health issues.
