Measuring

When applying the basics of collecting Agricultural Data, you will almost constantly be required to measure. But measurement may mean different things to different people.

The International System of Units (SI):

All systems of weights and measures, metric and non-metric are linked through a network of international agreements supporting the International System of Units. The International System is called the SI, using the first two initials of its French name Système International d'Unités.

There are seven SI base units:

• The meter for distance
• The kilogram for mass
• The second for time
• The ampere for electric current
• The mole for the amount of substance and
• The candela for the intensity of light.

There are also other units of measure derived from SI – some of these that you might encounter include:

• The newton for force and the Pascal for pressure
• The joule for energy and the watt for power
• The degree Celsius for everyday measurement of temperature
• The traditional mathematical units for measuring angles (degree)
• The traditional units of civil time (minute, hour, day, and year)
• Two metric units are commonly used in ordinary life: the litre for volume and the ton (metric ton) for large masses
• Knot, units traditionally used in meteorology
• The hectare
• The bar, a pressure unit

Irrigation Aids

Irrigation aids are necessary to make the correct decisions. Water and energy are expensive and unnecessary irrigation cannot be justified. Instruments and appliances available measure one or more of the following soil water conditions.

Measuring soil moisture with gravimetric sensors: By using a soil auger soil samples can be taken at increments of 20cm to a depth of 90 to 120 cm. Samples can be taken anywhere in the same irrigation section or lateral. Soil samples from the same depth are combined and the mass is determined before and after drying. Soil samples must be dried for at least 24 to 30 hours. The difference in mass before and after drying indicates the amount of water available in the soil. The difference between field capacity and the measured water content of the soil indicates the water deficit that must be replenished by irrigation. A good mass meter is the most expensive item required.

Measuring soil moisture with a neutron moisture probe: This is a sophisticated method that requires an expensive instrument. The installation of access tubes in the soil requires the same time and effort as the collecting of one set of gravimetric samples. Producers can save by sharing the costs of this instrument. In some parts of the country, private enterprises deliver a hygrometric service and make irrigation recommendations on a contractual basis. A correctly calibrated instrument can directly indicate the soil water content in mm per 20cm increments. When the instrument is connected to a computer with the necessary software, irrigation requirements can be obtained immediately after the measurements are completed.

Measure soil moisture with electronic sensors: Like tensiometers, the sensors must be buried at various depths. The soil above the sensors must be filled up as normally as possible and the sensors must be allowed to reach equilibrium with the surrounding soil moisture before measurements are made. The change in electrical conductivity when the soil dries out is related to the water content of the soil. The sensors must be calibrated for every soil type.