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t" <::> 0, t.;, \1 , \ , ~'&:. ... I ~ .:; TilE .\"Ef.JTRO.\' PROllE measures the moisture content 01 the sod In rho'! plant's root .;one. .. ,..A .....r. /-r TRACTORS REL}' on Information from tJ central proces.wr u'hen Iht'.\" faSt'" ft'ct'1 (l fit'ld triets has paid for itself lime and again. When To In;gate? The neutron prohe measures the moisture content olthe soil in the pl.ll1t'S root lone. The probe';; lIst'r;; interpret re.1dings to predict (up to a \\"cek in ad,'ance) .....hen the <'fOp should be irrigated and how much wale'f the farmer should apply itl the next irrigation. The detector tube is usually lowered by one- fool inter.al:; through an access tube placed into the soil .....ccess tubes may extend as far as six fed into the soil-the maximum depth of the Mraollones- of most irrigated crops...\s the tube is lo"'Ned, a radioactiye dement 10- ca.ted inside the tube shoots out a Ja.st series of neutrons. somewhat like a series 01 sil\'er , ! r;; ~~) · t.. "\~.~ , . . ..,....:t. :J .\'" .:JJ ~ I,\"FRARE/J THER.1fD.1fETERS measure the temperature of a field's leaf canopy to help fanners decide u'hen to Irrigate. balls shot inside a pinball machine. Uke pinballs, the neutrons speed through the soil, bouncing off the hydrogen atoms in water molecules and losing momentum. They even- tually pass back through the probe's detector tube at .1 much s]o",'er speed: much like the pinballs that eventually slip slowly between the paddles. Since the neutrons are travel- ing so s]owly, the detector tube can count them. And since the hydrogen ill the soil will most likely belong to .1 water molecule. a high count of slow neutrons indicates a high mois- ture rate, while a 10'''' count in- dicates a dry root zone. Before the moisture can he measured. ,1 site must be selected lor positioning of the access tube. The access tube is sunk in an area of the Held that best represents the soils in the field. 1f.1 measure were t,1ken for a particular kind of soil that only occurs in one..eishth oj the field. and if water were scheduled tor that field \,.ithout the res I of the fjeld taken into account, irrigation wouldn't occur as efficiently as it could Remote Sensing .....nother by-product of the recent high-tech explosion is the infrared themlOmeter, which uses remote sensing techniques to t.1ke the tempera- tures of a crop canopy-the top layer 01 plant ]ea\'es. A plant's temperature is direct- ly related to its transpiration rate (the rale at which a plant gives off water vapor), so a healthy. well-watered plant Is cooler than a stressed plant be- cause more evaporative cool- ing is occurring on the surface of the healthy plant than on the surface of the stressed plant. The infrared thermometer uses this temperature to indicate the degree of .1 plant's stress. The plant temperature. the temper.1ture of the air, and the wet and dry bulb temperature (used to measure evaporative demand of the atmosphere) are all used together to determine when a plant is stressed. The technician stands about 50 teet inside the field where the temperature won't be affected by the field's edge. which is drier, thus warmer. Then the technician takes the canopy temperature, the air tempera- ture, and then .....ct and dry bulb temperature. ~leasuring is best done between noon and 2 p.m.. when the sun is higher over head. shadows are at a mini- mum, and transpiration is at its peak. Leveling a Field With Light ..... third tool of irrigation efficien- cy, also a child of the space age, is las("r le,'('lin~. Convention.llly furro",'ed fields aren't levelled, just precision graded, which means Ihat while water can run along the furrows without puddling, some areas of the field are nevertheless going to be either wetter or drier than other areas. Thus some of the crop is going to be over watered, some of it under watered. Both over and under watering reduce crop yield.