f d the row unit weight also produces a down force shown by the arrow f g acting through the center of gravity of the row unit the force f g varies over time as the level of product in the seed hopper 28 and chemical hopper 30 changes during operation of the machine 10 these two downward acting forces f d and f g are counter acted by upward forces acting on the row unit the opener penetrates the soil and has a force f o acting upward on the opener when the opener 20 is fully penetrating the gauge wheels 24 will be in contact with the soil and a soil reaction force f r acts upward on the gauge wheels an additional upward force on the row unit includes the force f c acting on the closing wheels 38 a minimum soil reaction force f r is desired to ensure that the opener is fully penetrating the soil to the desired depth if the opener is not fully penetrating the gauge wheels will not touch the soil and f r will be zero thus some level of force f r greater than zero is desired to ensure there is full penetration the magnitude of the force f r can be measured by a sensor or load cell in a variety of locations on the row unit one example is a load sensing pin 46 in the gauge wheel depth adjustment link 48 adjustment link 48 bears against and resists upward movement of the pivot arm 50 supporting the gauge wheels 24 a suitable load sensing pin is shown.