Guidelines for the correct placement of soil moisture probes

TABLE OF CONTENTS

CHAPTER                                                                                                

1. Introduction                                                                                     
2. Soil uniformity                                                                                  
3. Distance from the crop                                                                     
4. Type of irrigation system                                                                 

4.1 Overhead irrigation – Sprinklers                                               

4.2 Overhead irrigation – Centre pivot                                                  

4.3 Micro-irrigation                                                                        

4.4 Drip irrigation                                                                            

5. Miscellaneous                                                                                 

1.Introduction

Soil moisture probes are widely used in the irrigation industry to schedule irrigation accurately. When deciding on the placement of the probe, it is important to remember that the moisture probe provides information for one fixed point and that this information is extrapolated to a much larger area. For this reason, the placement of the probe is critical and, thus, must be in the most representative spot in the field. The most important factors to consider are discussed below.

2.Soil uniformity

Soil uniformity (texture, depth and elevation) is one of the first factors to consider when deciding where to place the probe within a specific irrigation block. Where the soil is uniform across the entire irrigation block, one measurement point will be sufficient to manage the irrigation of the whole block (Figure 1). The best placement position is towards the central part of the field. Avoid low-lying areas and sections of the field where the soil is shallower, as these spots could be wetter than the rest. Where soils are uniform, the area represented by one probe can be as large as 20 ha.

Often the irrigation block layouts are done without consulting soil survey maps, which leads to blocks containing two or more different soil types. In such cases, the soil moisture probe is normally installed in the most representative area of the field i.e. the soil type covering the largest area of the specific field (Figure 2). Due to the non-uniformity of soil, there will inevitably be smaller sections of the field that are either over- or under-irrigated.

Figure 1. Uniform soils – One soil moisture probe installed in a representative spot is sufficient to manage the irrigation of the whole block.

Figure 2. Non-uniform soils – Install the soil moisture probe in the most representative area A – For example with a dominant Clay soil, the smaller section of the field with sandier soil will likely be under-irrigated. B – For example with a dominant Sandy soil, the smaller section of the field with heavier clay soil will likely be over-irrigated.

3.Distance from the crop (Row crops vs Tree crops)

The next step is to place the probe relative to the position of the crop. To measure crop water use, the probe must be located within the root zone of the crop.

For row crops such as maize or wheat, the probe can be installed within the crop row (Figure 3A). Avoid installing the probe on the side of the row (towards the inter-row), as this can lead to damage due to in-field mechanical operations. Where dual rows are planted, the best position is between the two narrow rows (Figure 3B). Ideally the probe should be located between 10 to 15 cm from a plant.

Figure 3. Recommended probe placement in single (A) and dual crop row (B) configurations. The probe should be placed where the crop roots are.

For tree crops, the ideal placement is under the leaf canopy and in the area of the tree's stem. In younger trees where the root system is not yet well developed, the probe should be placed 15 cm to 20 cm from the stem and in line with the row direction (Figure 4A). As trees grow older and the stem diameter increases, the probe can be moved further away to be between 30 to 40 cm from the stem (Figure 4B).

Figure 4 A and B. In younger tree crops (A), the probe can be placed closer to the stem of the tree, while in older trees (B), which already have an extensive and well-developed root system, placement can be further away.

4.Type of irrigation system

4.1 Overhead irrigation (Sprinkler)

In fields irrigated with overhead sprinkler systems, the best placement is a quarter of the distance between the sprinkler risers and a third of the distance between the irrigation lateral lines (Figure 5). Find the closest crop row to this position and place the probe here.

Figure 5. Probe placement is recommended in sprinkler irrigation a quarter of the distance between the sprinkler risers and a third of the distance between the irrigation lateral lines. Avoid the area close to the sprinkler riser and the area in the middle where all sprinklers overlap.

4.2 Overhead irrigation (Centre pivot)

In centre pivots, the probe can be placed halfway to three-quarters of the distance from the centre of the pivot (Figure 6). Avoid the area closer to the outside edge of the pivot as large volumes of water are usually applied here, which can lead to runoff. The probe should also be placed relatively close to the pivot starting point, as moisture conditions here will indicate when to start up again (Figure 6). In cases where different sections of the pivot are planted to different crops or have the same crop but at different ages, more than one probe can be installed. In this way, each section can be managed separately (Figure 6).

Figure 6. Probe placement is recommended for centre pivot irrigation halfway to three-quarters of the distance from the centre of the pivot. Avoid the centre as well as the outer part of the pivot.

4.3 Micro-irrigation

In fields irrigated with micro-irrigation systems, wetting is usually done in a broad band along the row direction and close to the crop. The probe must be placed within this wetting band and close enough to a micro sprinkler or mister that it falls within its wetting radius. The ideal placement is halfway to three-quarters of the distance from the centre of the micro-sprinkler wetting circle (Figure 7). Where the micro-irrigation system is slightly off-centred to one side of the crop row, the probe must also be located on the same side of the crop row to prevent the crop from physically blocking the irrigation. It is important to regularly check if risers/ stakes are still perfectly upright, especially after in-field operations.

Figure 7. Probe placement is recommended for micro–irrigation systems halfway to three-quarters of the distance from the centre of the micro-sprinkler wetting circle. The probe should be placed within the wetting radius and on the same side of the tree row as the micro-sprinkler.

4.4 Drip irrigation

Where crops are irrigated with drip irrigation, the probe should be placed a quarter of the distance away from the drip emitter (Figures 8 and 9). This will ensure that the probe is located within the wetting bulb/ area and reacts relatively quickly after irrigation has commenced. For single and dual rows, the probe should also be placed in such a manner that the crop row is not between the drip line and the probe position.

Figure 8. Recommended probe placement for drip irrigation with single (A) or dual crop row (B) configuration.

Similar guidelines can be followed for tree crops irrigated with a single or double drip line (Figure 9), such as placing the probe a quarter of the distance away from a drip emitter (Figure 9).

Figure 9. Recommended probe placement for single (A) and double drip line (B) irrigation of tree crops.

5.Miscellaneous

Representative position – For row or broad-seeded crops, the area around the probe must contain healthy and actively growing plants that have not been trampled during installation. Similarly, for tree crops, the site selected must have a healthy growing tree of similar size to those in the rest of the irrigation block.

Blockages - Regular in-field maintenance checks are required to ensure that the dripper, micro-sprinkler or impact sprinkler located at the probe position is not blocked and provides irrigation according to the design specifications.

Obstructions – Avoid having excessive weeds, tall grass and hanging branches between the sprinkler/ micro irrigation and the probe position, as this will impact the response to irrigation. Stakes for misters and spinners should also always be standing 100% upright.

Infiltration – Be aware that infiltration rates can decrease over time in some soils due to soil surface crusting. Poor water quality (high Sodium levels) can also lead to a decrease in infiltration rate.

Ridges – To prevent runoff of irrigation water away from the probe position, ridges need to be quite wide, and the top of the ridge also has to be relatively flat.