Smart farming
March 26, 2020

Why you need more sensors

We at Farm21 preach that you should measure with multiple sensors (a sensor network) across your farm. In this blog we would like to explain the reasons why we strongly believe in a sensor network.

Often we get the question of how accurate our sensor is. The direct answer is that with our sensor soil moisture measurements with 3 p.p. (percent point) accuracy can be achieved when placed correctly and with the right soil type selected. This is well up to industry standards.

Now consider this. What if you accidentally stuck a sensor just near a rock, what if the sensor went right through a lump of roots or what if the bulk density of the soil is slightly different at the sensor location than anywhere else? These different conditions and variances in the soil affect the measurements*, resulting in other values than expected. It is not that the accuracy of the sensor is bad; it still measures the amount of water in a certain volume around the sensor. It is just that there is, due to the rocks, roots or anything else, less or more water present.

This is the case with ALL soil moisture sensors available on the market. No matter the price, brand or specs; all sensors are affected by the variances in soil conditions. We've put this to the test. With two different, high quality, lab research standard, soil sensors and one of our sensors we went to several fields, where we took multiple measurements on the three depths with all three sensors. Conclusion: all sensors vary. Even at the same depth, with the same sensor and just 5 cm apart, moisture readings would differ over 8 p.p.

Knowing all of this, the question that is asked next is usually; “Can you rely on a soil moisture sensor?”. The basic answer is no, you can't. A single sensor might tell you fairly accurate what the soil moisture is on a single spot, but as long as you don't have some kind of X-ray vision, to check the soil conditions, you can only be sure that the sensor tells the truth (with 3 p.p. accuracy) about that particular spot. As a grower you cannot rely on this. You want to know what the conditions are in the entire field/area you can irrigate, to make the right decisions.

According to us, there is only one simple solution to this problem. Measure with more sensors simultaneously. Two sensors tell you more than one sensor. If one sensor tells you 35% soil moisture and another tells you 30%, then the average might lie in between. Adding a third that says 31% soil moisture, brings you another step closer to a usable field average.

So far, we only focussed on soil moisture, but you can imagine that similar effects are influencing air temperature and air humidity as well. With every sensor you add you increase the certainty of the measurements. A higher certainty, gives you a better indicator to base your decisions on.**

What does this mean for the number of sensors you need? Is it needed to place a sensor every square meter? In an ideal world Yes, but in reality No. The amount of needed sensors depends on crop type, farming equipment to act on the measured insights and of course soil conditions. There is a balance between what you can optimize and the sensor density you need for that. In other words; there should be a reasonable Return Of Investment.

In short, why you need a network of sensors: more accurate data, better insights, better decisions, more optimization, more profit.

* We partly mitigated this problem by measuring soil moisture in zones, rather than in a single spot, to reduce the negative effect of rocks and other debris.

** Sensors are still function as indicators and in no way the hard truth! Yet a good indication is needed for good decision-making.

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Sander Dekker
Crop consultant precision farming