Background to the Invention Control of irrigation of crops is important in ensuring that yields are maximised and that quality of the crops meets the desired standards. Over-watering can give rise to bacterial and fungal infections, as well as increasing costs unnecessarily, while inade- quate watering can result in lack of growth at critical stages in the crop's development, with adverse effects on quality and yield. In addition, the availability of irrigation water is becoming increasingly restricted, and high costs are involved in its supply, necessitat- ing careful management of the use of water.

Controlling irrigation is difficult because conditions can vary not only from field to field, but also from one part of a field to another. In addition, if it is carried out on the basis of a predetermined schedule, the schedule has to be determined on the basis of expected average weather conditions, and if such conditions are not experienced in- correct watering can result. It is important therefore to control watering with a knowl- edge of the actual moisture content of the soil. It is possible to obtain measurements using a portable moisture meter, but this procedure is very time-consuming, especially if the variation of moisture content with depth in the soil is to be taken into account.

Summary of the Invention According to the invention, there is provided a method of monitoring the mois- ture content of soil, comprising measuring a first property of soil which varies with moisture content, measuring a second property of soil which varies with moisture con- tent, deriving from a respective predetermined relationship between the first property and moisture content for each of a plurality of different soil types a first moisture con- tent value for each of said soil types, deriving from a respective predetermined relation- ship between the second property and moisture content for each of said plurality of dif- ferent soil types a second moisture content value for each of said soil types, comparing

the first and second values for each soil type and selecting the pair of values for which the difference is smallest, and obtaining from the selected pair of first and second mois- ture content values a value for the moisture content of the soil.

The invention also provides apparatus for monitoring the moisture content of soil, comprising first means for measuring a property of soil which varies with moisture content, second means for measuring a different property of soil which varies with moisture content, processing means for calculating from a respective first equation for each of a predetermined number of soil types a moisture content value corresponding to the property measured by the first means and for calculating from a respective sec- ond equation for each of said soil types a moisture content value corresponding to the property measured by the second means, means for comparing the first and second values for each soil type and for selecting the pair of values for which the difference is smallest, and means for obtaining from the selected pair of first and second moisture content values a value for the moisture content of the soil.

Another aspect of the invention provides a monitoring device for use in moni- toring soil moisture content, comprising a control means and a plurality of moisture sensing elements insertable into the soil and connected to the control means, each sensing element being arranged to sense moisture content at a different depth in the soil from the other elements, the control means co-operating with the sensing elements to provide data representing soil moisture content at a range of depths, the control means being connected to a radio transmitter for transmitting the data in conjunction with an identifying code for the device.

The invention further provides a system for monitoring soil moisture content, comprising an array of monitoring devices according to the invention located across an area of land to be monitored, and a monitoring station comprising a radio receiver for receiving signals from each of the monitoring devices in the array, signal processing means for extracting from each received signal the respective identifying code and mois- ture measurement data, and data processing means for receiving the identifying code and associated data and for storing the data for each sensor in conjunction with time and date information.

Preferably, the data processing means is programmed to compare moisture content measurements with stored reference values and to control the operation of ir- rigation means in accordance with the results of the comparison.

Each sensing element is suitably provided with a pair of electrodes on the sur- face of the device, and the control means comprises an electronic circuit for testing conductivity and capacitance between the electrodes. The control means may comprise a microprocessor and may be arranged to excite and sample the sensing elements and then convert the results to digital values, encrypting the data and sending it to the transmitter.

Soil conductivity may be measured by means of an AC potential divider circuit with AC coupling to prevent any DC charge being present at the electrodes, thereby preventing electrolytic corrosion. Soil capacitance may be measured by means of a simple relaxation oscillator, with the soil forming part of the capacitor, and a reference capacitor in the circuit produces a changed oscillation whose frequency is measured as a reference. The reference capacitor is then switched into the circuit and the new fre- quency is measured.

In the system of the invention, it is not necessary for any processing of the measured data to be carried out in the monitoring devices; the data processing means at the monitoring station can be used to derive the moisture content from the two measurements.

The use of both conductivity and capacitance measurements is preferred, be- cause conductivity measurements alone, while capable of giving a good measure of moisture content, are affected by changes in salinity, while the results of capacitance measurement depend on soil structure and content. By using the two measurements, it is possible to compensate for the effects of different soils.

Additional monitoring devices may detect and signal to the monitoring station local climate factors, such as temperature, wind speed and direction, rainfall, daylight, barometric pressure and splash (caused by rainfall bouncing back on to the plants after impact with the ground).

In one embodiment of the invention, the monitoring device comprises at least one sensing element adapted to sense the concentration of selected chemicals in the

soil, for example nitrogen or phosphate concentrations, thereby allowing control of the addition of fertiliser as well as the amount of water to be added.

The monitoring devices are suitably powered by batteries contained therein, and may comprise power management/timing means to switch the device to a state of low power consumption when measurements are not being made, and then switching the device on again at predetermined intervals to sample and transmit the measurements, thereby extending the battery life and avoiding the need to change the batteries during the growing season.

The position of each monitoring device in the array across the crop may be de- termined by conventional surveying techniques, or by using of a global positioning sys- tem receiver used at the time of installation of the devices within the crop. It will be ap- preciated that it may be important to remove the devices before harvesting, in order to avoid damage to the devices. Recovery of the devices will also enable them to be pre- pared for the next growing season, for example by cleaning and replacement of the bat- teries.

The values received by the monitoring station can be processed to permit pre- diction of the moisture hoiding capacity of the soil by comparison of the two separate measurements, which have different relative characteristics for a particular soil struc- ture. The soil moisture content measured can then be used to reference a particular soil type with its associated total moisture avaiiability, which enables calculation of soil moisture depletion, the unit employed by growers to manage crop watering.

During different stages of the growing cycle, it is necessary to target the differ- ent depths according to the location of the root system. By monitoring corresponding changes in moisture content at adjacent depths, following application of water, it is pos- sible to predict depth of the roots and thus change the application schedule accordingly.

At the monitoring station, an antenna receives the transmitted radio signals from the monitoring devices, either directly from the individual devices direct or via a data concentrator and booster station. The receiver is preferably adapted to sample the signals to reject noise and to recover the data, and then to pass it to the data proc- essing means for processing. The data processing means, for example a personal com- puter (PC), is programmed to load the raw data into a database file for the crop. The

program may convert the data into measurements which are more understandable by the operator, and may permit the manual entry of additional information such as man- ual measurements, photographs, and notes. By the entry of information on the cost of inputs and the sales value of the crop, the gross margin for each area of the field may be calculated automatically. Record data may be output in the form of printed reports and graphs.

An additional control module may be added to monitor the soil moisture, rain- fall and crop growth to determine whether or not to turn the irrigation systems on.

The module may simply advise on the need for irrigation or it may be linked directly to the irrigation equipment to achieve automatic control of the closed loop type, targeting irrigation to areas of the field where more or less water is needed. The control system may thus compensate for the effects of wind drift on spray gun type irrigators.

The use of the control system of the invention may give benefits such as im- proved crop yield and quality, reduced water costs and energy costs for pumping, re- duced labour costs, the maintenance of soil fertility, thereby reducing the use of fertil- iser, the prevention of soil erosion through over-watering, and a reduction in the leach- ing of agrichemicals into water courses.

Brief Description of the Drawings In the drawings, which illustrate exemplary embodiments of the invention: Figure 1 is a perspective view of a monitoring device, in position in the soil ; Figure 2 is a diagrammatic representation, not to scale, of a monitoring and irri- gation system; Figure 3 is a diagrammatic view of an alternative embodiment of the monitoring device; Figure 4 is a block circuit diagram of the electronic components of the conduc- tivity measuring part of the device; Figure 5 is a block circuit diagram of the electronic components of the capaci- tance measuring part of the device ; and Figure 6 is a block circuit diagram of the monitoring device.

Detailed Description of the Illustrated Embodiments The monitoring device 1 has an elongate body with two portions, the first por- tion 2 being inserted into the soil generally vertically and containing the electronic com- ponents with sensing electrodes on the surface of the body, while the second portion 3 comprises a radio transmitting antenna. The surface of the first portion 2 is divided into a plurality of separate segments 4, each having a separate set of sensing electrodes, thereby permitting measurements to be obtained at different depths in the soil. This enables the penetration of water into the soil to be measured and controlled according, for example to the type of crop being grown; different crops may require different depths of moist soil in which to grow. For example, shallow-rooting crops will not re- quire irrigation to the same depth in the soil as deep-rooting crops.

Each sensing element on the surface of the body is connected to a microproces- sor within the first portion 2 which controls the activation of the electrodes in turn to obtain a sequence of measurements for the different soil depths and creates a string of data for transmission, identifying the device, the sensing element on the device, and the measurement. The string is encrypted and passed to a transmitter connected to the antenna to transmit the data to the monitoring station, hereinafter described with ref- erence to Figure 2. The microprocessor is also provided with timing means for control- ling the cycle of measurement and transmission, measuring and sending only at prede- termined intervals, typically about four hours, and then switching the microprocessor and other components within the device off or to low-power mode as appropriate so as to conserve battery power.

Figure 2 illustrates the use of the monitoring devices in agriculture. A field 20 is planted with crops which will need watering. A number of the monitoring devices 1 are distributed across the area to be irrigated to form an array of devices. It will be appre- ciated that it is not necessary for the array to be geometrically arranged; the devices 1 will be located according to the expected variation in conditions across the field. For example, where uniform conditions can be expected, fewer devices will be required, while in those areas where large variations in soil type or drainage or exposure to weather conditions can be expected, the devices will need to be more closely spaced.

The monitoring station 21 comprises a radio receiver and decoder 22 linked to a personal computer 23, which is in turn linked to an irrigation controller 24 controlling one or more supply valves 25 in the irrigation system supplying water to the field.

The alternative embodiment of monitoring device shown in Figure 3 comprises a stake 30 connected to an elongate mast 31 through a spring 32, which allows for some movement due to wind impinging on, or agricultural machinery striking, the mast, with- out the risk of the stake being worked loose and the mast then falling over. The mast 31 carries an electronics housing 33 from which flexible connecting leads 34 extend to sensor probes 35 inserted into the ground around the mast 31. Each sensor probe 35 has an electrically insulating surface layer interrupted to expose two spaced electrodes 36 and 37 thereon. The electrodes of each probe 35 are electrically connected to con- trol electronics within the housing 33 via the respective leads 34. The mast carries an antenna 38 at its upper end, connected to a radio transmitter within the electronics housing 33 to transmit radio signals back to a monitoring station. The antenna will typi- cally need to be located at a height above the ground sufficient to ensure that it remains above the crop at its maximum growth, in order to ensure that the low-power signal transmitted by it will not be attenuated or screened by the crop.

Figure 4 shows the conductivity measuring circuit in the monitoring device. A square wave alternating voltage VIN S applied to the electrodes 36 and 37 of a selected one of the probes, the use of alternating current serving to avoid any electrolytic dete- rioration of the electrodes. The output voltage V. is measured at a point between a ref- erence resistor 40 and the probe, the relationship between the input and output voltage enabling the resistance/conductivity between the electrodes of the probe to be deter- mined. The calculation is carried out by the base station computer; the monitoring de- vice merely transmits the two voltage measurements.

The capacitance measuring circuit shown in Figure 5 uses the capacitance of the soil between the electrodes 36 and 37 on the probe as part of an oscillator whose fre- quency output is measured by counting oscillations with reference to a standard 2MHz clock in a predetermined time. A reference electrolytic capacitor 50 is switched in par- allel with the electrodes 36 and 37 to provide a second frequency measurement count.

Again, the necessary calcuiations are carried out in the base station computer; the moni- toring device transmits the pair of frequency counts.

In order to minimise power consumption, and thereby extend battery life, it is necessary to limit the amount of data transmitted back to the base station by the moni- toring device. The count measurements are obtained in the device as 16-bit values, but transmission of only 8-bit values is desirable. The monitoring device's processor is therefore arranged to compare the two 16-bit counts and to determine, working from the most significant bit downwards, where the first 1-bit appears in either value. This bit and the seven following bits are then selected for this value, together with the corre- sponding eight bits of the other value. It has been found that this method maintains a good degree of accuracy in the relationship between the two truncated values.

Figure 6 is an overall schematic circuit diagram of the monitoring device. A mi- croprocessor 60 is supplie with power from a battery 61 and controls the operation of the resistance/conductivity measuring circuit 62, as described with reference to Fig- ure 4, and of the capacitance measuring circuit 63, as described with reference to Fig- ure 5. The circuits 62 and 63 are shown connected to a single pair of electrodes 36 and 37, but in practice three or four pairs of electrodes, set at different depths in the soil, will be controlled by the same circuits. A wake-up timer circuit 64 sends a wake-up signal to the microprocessor 60 at predetermined intervals, for example four-hourly, the microprocessor itself being programmed to shut itself down when the cycle of measurement taking and transmission has been completed. A radio transmitter 65 re- ceives the data from the microprocessor 60 and transmits a signal to the base station via the antenna 66.