ON THE Dancing Crow farm in Washington, sunflowers and squashes soak up the rich autumn sunshine beside a row of solar panels. This bucolic smallholding provides organic vegetables to the farmers' markets of Seattle. But it is also home to an experiment by Microsoft, a big computing firm, that it hopes will transform agriculture further afield. For the past year, the firm's engineers have been developing a suite of technologies there to slash the cost of "precision agriculture", which aims to use sensors and clever algorithms to deliver water, fertilisers and pesticides only to crops that actually need them.

Precision agriculture is one of the technologies that could help to feed a world whose population is forecast to hit almost 10 billion by 2050. If farmers can irrigate only when necessary, and avoid excessive pesticide use, they should be able to save money and boost their output.

But existing systems work out at $1,000 a sensor. That is too pricey for most rich-world farmers, let alone those in poor countries where productivity gains are most needed. The sensors themselves, which probe things like moisture, temperature and acidity in the soil, and which are scattered all over the farm, are fairly cheap, and can be powered with inexpensive solar panels. The cost comes in getting data from sensor to farmer. Few rural farms enjoy perfect mobile-phone coverage, and Wi-Fi networks do not have the range to cover entire fields. So most precision-agriculture systems rely on sensors that connect to custom cellular base stations, which can cost tens of thousands of dollars, or to satellites, which require pricey antennas and data plans.

In contrast, the sensors at Dancing Crow employ unoccupied slices of the UHF and VHF radio frequencies used for TV broadcasts, slotting data between channels. Many countries are experimenting with this so-called "white space"; to unlock extra bandwidth for mobile phones. In cities, tiny slices of the white-space spectrum sell for millions of dollars. But in the sparsely populated countryside, says Ranveer Chandra, a Microsoft researcher, there is unlicensed space galore.

The farmer's house is connected to the internet in the usual way. A special white-space base station relays that signal to a shed elsewhere on the farm that sports an ordinary TV aerial. Individual sensors talk to the shed using TV transceivers with a range of more than 8km—enough for all but the biggest farms. And those transceivers are cheap: "We've already built sensors for less than $100," says Mr Chandra. "Our aim is to get them to under $15."

Microsoft is not the only organisation hoping to make agricultural sensors practical. Researchers at the University of Applied Sciences in Mannheim, for instance, have developed a sensor network that relies on a technology called software-defined radio, which uses computers to simulate an ultra-flexible, very sensitive radio receiver. And scientists at the University of Nebraska-Lincoln are working on sensors that communicate with radio waves that propagate through the soil rather than the air, and which draw their power from the vibrations generated by farm vehicles moving about on the surface.

But although such sensor data are useful, but they cannot tell you everything. To fill in the gaps, Dancing Crow uses a drone. These are getting cheaper (a basic model costs $1,000) but they require some skill to fly, and their small batteries mean limited flight times. So Microsoft's team wrote an autopilot that lets a farmer outline a plot to survey, works out the most efficient route and sends the drone on its way, reducing the time taken to cover a farm by over 25%.

The resulting imagery contains useful information on growing conditions, crop health and insect pests, but interpreting it properly is beyond most farmers. So Microsoft also developed software that runs on an ordinary laptop, and can stitch together individual pictures into a single panoramic view of the entire farm. Sensor data can be laid atop this view, and the computer can then extrapolate a handful of sensor readings into predicted values for moisture, acidity and so on at any given point.

When the nearby Snoqualmie River rises up to flood Dancing Crow farm in a couple of months, as it does most winters, Mr Chandra plans to take his technologies to India. For the very poorest farmers, even a cheap drone will be beyond their budget. He wants to see if a lower-tech solution will work just as well—simply attaching a smartphone to a $5 helium balloon and walking it through the fields.

Correction: An early version of this article misspelled Mr Chandra's name. We apologise for the error.