Should we be applying water now? June 1
As I was checking on some possible probe placement locations in the country I was amazed at the number of pivot systems applying water. Since many growers use pivot irrigation now to apply fertility I considered that might be why these systems were running. And in a few cases I believe that was true. But most of them were moving far too slowly for normal fertigation passes and there didn’t appear to be any fertilizer tanks near the pump. Applying N didn’t appear to be the main reason for systems early start up. So, I stopped and asked one of the growers why he was running the pivot now if he wasn’t fertigating. He said, “it looked like it was as getting dry so I thought I would give the field a shot of water”. This field is mostly a clay loam soil texture and had over 5 inches of rain not more than 10 days ago. The corn was V-3 to V-4 so it would only be using .03 to .05” per day. With all that in mind I didn’t think it could be that dry. I hand probed the field and found that although the top 4” was not wet but still moist. everything below that was very wet.
Since we already had several probes installed close to this field I checked the moisture status on those units and found that we were about 30% depleted in the top 4” and were at or above Field Capacity in all the zones below that through the 3-ft. profile; understandable considering the recent abundant rainfall and minimal crop water use. Plus, I could easily see that water was migrating from the lower, wetter part of the profile upwards towards the drier zone on top.
I contacted the grower I had talked to and shared what I found with the probes around him concerning the abundant moisture below the surface and said, “Thanks, but it can’t really hurt to give it a drink now so I’m going to go ahead with my 1” application I started.”
As I was driving away I thought about what he said and realized that, based on all the units I could see running, many others probably have that same opinion. So, let’s look at what the ramifications are from watering early when your crop doesn’t need it.
First is the cost of the application. Recent studies have shown that, depending on energy and pumping costs, the additional costs of a 1” application on a pivot could around $12/acre inch or near $1600 for each application. And, these numbers don’t add in the additional costs that result from labor and added wear and tear on the pivot.
Second, soil cannot long term hold more water than field capacity will allow. What that means is any water we add to this field, after it fills up the top 4” to FC, will simply drain through the profile. And, since N is a mobile nutrient that moves with water, the drainage will take the N with it. Each additional inch of water that is applied above the needs of the crop will leach about 8 pounds of N from the profile. This can mean another additional loss of between $2.50-$5.00/ acre depending on the costs of N. And some of the N will eventually end up in our ground water. Certainly, not what anyone wants to see.
Thirdly, plant root systems need oxygen to remain healthy and function the way they are supposed to. When water enters the soil, it does so by pushing air out. So, soils above FC will have roots growing in an anaerobic environment. This causes the metabolism of the entire plant will slow down, and water and nutrient uptake will decrease, until optimal (FC) conditions return. Lower plant metabolism means less sugar production and eventually less yield. In addition, keeping the root zone wet increases microbial growth and causes the formation of compounds that are toxic to plant growth
These are great reasons not to over water, but the most important is to make sure that a healthy, deep root system develops early in the year. Most crops are very resilient to early water stress and have very little if any detrimental yield effects if water is a little short early in the growing season. By making sure we only apply water based on the needs of the crop and the water storage capabilities of the soil we encourage those roots to grow deep. Then when the critical crop growth stages are reached (reproduction for most crops) we have a deep, healthy root system that can tap into the water stored in the lower part of our profile. Without access to that reserve, or help from Mother Nature with rain, most irrigation systems will not be able to keep up with the crops water needs.
So, the answer to the question concerning the “It can’t really hurt if I put a little extra water on now can it, is yes it can. It costs you money, it creates a shallow unhealthy root system the reduces the ability of the plant to thrive and produce sugar. And, it can cause the crop to suffer from water stress later in the year because the roots aren’t deep enough to tap into the reserve.