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Don’t wait until the ground is dry to water your yard, garden

There are just a couple of weeks left before we’re beset with the worst weather of the year. That’s my opinion of course, but the heat and dryness of July and August do get to me.

Some folks will take to watering their lawns and gardens before long. I water my ever-shrinking vegetable garden some. The roses are well-mulched to make it without irrigation. Centipede grass is deemed drought-tolerant: mine sometimes gets crispy but perks up with the next shower.

It was while working with a soybean management program years back that I came to two conclusions about irrigation. First, it is expensive. Secondly, we should never wait for the soil to get dry before turning on the water. In farming, the reason for investing in irrigation wells, pumps and delivery systems is to ensure plants never are in soil too dry. The key is to timely irrigate to prevent dry soil.

The first year we worked the soybean program here, the ag engineer and his computer at Starkville had us irrigate 11 times during the summer. That sort of blew my mind and the cooperating farmer was thinking the same might happen to his crop budget. By the late September harvest, though, those 11 waterings paid off.

Soil, be it in a soybean field or flowerbed, loses water in three ways. Gravity pulls it downward. Evaporation pulls it upward and out the ground. And the water used by plants for growth eventually evaporates out of the leaves, i.e. transpiration. That sounds simple enough, but there are quite a few things that cause water to move around underground other than those three.

Temperature, soil texture, organic matter and even soil color affect the movement of water in the soil. And water doesn’t just move up and down. It moves horizontally in the soil also. There is a bunch of science jargon that lays out how pressure in soil changes and moves water around.

For us garden-minded, let’s just go with the fact water moves from the dampest to the driest spots down there. Yeah, soil pressure causes that, but wet-to-dry is what matters. 

Going back to that soybean irrigation project, there were two methods we used to determine when the soil at the roots was going too dry before it did. One was with the use of soil tensiometers that measured pressure changes by way of vacuum tubes. The other, which constituted my bi-weekly chore, was by way of buried gypsum blocks embedded with electrodes and wires sticking out of the ground. I hooked up a small meter to the wires and recorded the readout. We recorded and transmitted the measurements in “bars” of pressure.

Technically, pressure was measured in kilopascals, but the layman “bars” was more user friendly.

I would email in the readings and get back a message to either turn on the water or hold off for now and check again in a few days. I always phoned the farmer with either the costly instructions to begin watering or his preferred “wait until we read the blocks again.”


Terry Rector writes for the Warren County Soil and Water Conservation District.