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THE AIM
Most growers know their acreage on the surface, but the sub-surface is often a mystery to them. Yet this area of a few feet of soil, where the rootzone exists, is the 'factory' for crop production. The more one can learn about conditions in this soil zone, the more one can improve the whole 'crop production' operation.
This fact has long been recognised, and there are various devices ranging from the shovel, to tensiometers, gypsum blocks and the neutron probe, that can be used to measure subsurface moisture. The neutron probe is widely recognised as being the most accurate and reliable source of gathering soil moisture data today.
HOW DOES A NEUTRON PROBE WORK
The soil moisture is measured by lowering a source of fast neutrons and a detector down a hole in the soil. Holes are drilled at selected locations in the crop (usually three per monitored area), and cased with aluminium tubing to prevent 'cave-in'. Measurements are taken through the soil profile usually down to a depth of 100-120cm. Fast neutrons are emitted from the source and fly through the aluminium tubing (just like light passing through a glass window), in a football shape pattern. The neutrons collide with objects in the soil, and bounce back to a detector. The neutrons are only slowed down by hydrogen atoms as they are of similar size and mass. The detector only detects slow returning, low energy neutrons. The count on the probe screen is therefore a reflection of the number of hydrogen atoms that have been recorded at a particular depth.
The only varying source of hydrogen in a soil is the hydrogen in water. Each neutron probe has a calibration to calculate the 'volumetric water' percentage at varying depths, and once that is known a calculation can turn water percentage into mm of water at chosen depths. The transfer of hydrogen counts into water percentage and mm of water are calculated in minutes using a software package designed for use with the probe. By taking six to seven measurements through the soil profile and plotting them graphically, one is able to get an understanding of how the soil moisture is changing.
BENEFITS OF IRRIGATION SCHEDULING WITH A NEUTRON PROBE.
Better use of natural rainfall and a possible increase in area irrigated.
Reduced water logging, soil compaction and plant disease.
Better knowledge of when to commence irrigation after rain, and the ranking of fields from driest to wettest.
Reduced cost of irrigation
5. Development of a data bank for future management decisions
Irrigation Scheduling
Field Capacity Definition
Finding Field Capacity
Refill point - Definition
Finding the Refill Point
Varying Upper & Lower Limits
Determining Irrigation Efficiency
Crop Management
Soil Management
Using The Weather Station, ET Figures & The "Periscope" to schedule
The Tools for Scheduling
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