Soil water sensors that display colours as thresholds for action

Figures & data

Figure 1. The relationship between detail and utility for the irrigator.

A diagram showing the colour that relates to each range of water, nitrate and salt measurements. A graphical representation of how the benefit to the farmer increases as we drop unnecessary detail.

Figure 2. The VIA colour thresholds for water (Chameleon sensor), nitrate (nitrate test strip) and salt (VIA EC meter).

Table 1. Approximate relationship between nitrate concentration and amount of nitrate-nitrogen in the top 0.5 m.

Category	Nitrate concentration (mg/L)	Nitrogen (kg/ha) over top 0.5 m
Low	0–25	0–12
Medium	50–100	24–47
High	250–500	119–237

Table 2. The Chameleon EC colours, range and meaning from soil water samples.

Category	Colour	Range (dS/m)
Very Low	Green	<1.0
Low	Yellow	1.2–2.0
Medium	Orange	2.4–3.6
High	Red	>4.0

Table 3. A summary of a simplistic and requisite simplicity for each measurement device.

Tool	Simplistic interpretation	Requisite simplicity
Chameleon sensor	Irrigated plants need to be kept wet. Keep the Chameleon in the blue zone. Red is too dry.	The colour patterns reveal the dynamics of each unique combination of soil, crop and irrigation method. Farmer response to the pattern depends on their developing experiential knowledge and the constraints under which they must operate.
WFD	The WFD indicator shows the depth of infiltration. The captured water sample represents the solute concentration at that depth.	The concentration of solute measured from the WFD is a measure of the nitrate and/or salt that is leaving the soil layer above and moving into the soil layer below during significant leaching events.
Nitrate strip	Plant growth responds positively to nitrogen fertilizer, so a high level of nitrate-nitrogen in the soil is always good.	Nitrate levels rise due to mineralization and application of fertilizers and fall due to plant uptake and leaching. When nitrate levels are high, irrigation should be reduced to reduce the chance of leaching.
Electrical conductivity	Salt is damaging to plants and should be kept low (green).	When irrigation is efficient, salt will accumulate in the rootzone and needs to be leached before reaching the salinity threshold for the crop. The ratio of salt in the irrigation water and salt in the rootzone gives an indication of the leaching fraction.

Figure 3. Common elements that make up a three-sensor array water pattern. The timing of irrigation events is shown by the arrow.

A diagram using colour patterns to show how water content changes with time in different scenarios.

Figure 4. Common elements making up nitrate patterns and the accompanying water pattern. The timing of irrigation events is shown by the arrow.

A diagram using colour patterns to show how nitrate content changes with time in different scenarios.

Figure 5. Common elements making up salt patterns and the accompanying water pattern. The timing of irrigation events is shown by the arrow.

A diagram using colour patterns to show how salt content changes with time in different scenarios.

Figure 6. A combined water, nitrate and salt pattern.

A diagram using colour patterns to show how water, nitrate and salt content changes with time in different scenarios.

Table 4. The changes in yield from survey data covering 15 irrigation schemes in Tanzania and Malawi.

Country	Irrigation Schemes	Number of Farmers	Crops monitored	Crops surveyed	Change in yield
					Same or Lower	Increase 0–50%	Increase >50%
Tanzania	8	183	323	271	25	49	26
Malawi	7	175	413	182	35	40	26

Table 5. The number of Chameleon sensors and WFD readings taken and reductions in water use, fertilizer use and conflict.

Country	Chameleon readings	WFD Readings	Reduced water use (%)	Reduced fertilizer use (%)	Reduced conflict (%)
Tanzania	15,556	901	90	86	63
Malawi	9460	118	71	5	48

Note: Malawi farmers did not reduce fertilizer use, but fewer farmers were equipped with WFDs, so they had less information. Crop choice was also a factor in Malawi. The majority of crops monitored were beans, that received little or no nitrogen fertilizer, or maize, which has a high demand for N. This contrasts with Tanzania, where more shallow rooted vegetable crops predominated. Yet in both countries, an increase in yield was associated with a decrease in water use, strong evidence that nitrate leaching was a major contributor to low yields.