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Fluid Journal : Summer 2014
17 The Fluid Journal Summer 2014 plant). The complications in using these methods are that the soil in these cases is not like the soil in field production. First, there is not an impervious wall around the roots restricting water movement to only up in that field. Second, the soil around the crop plant might have been disturbed using these methods and so it doesn’t mimic the distinct layers of soil stratification that occur after years of rainfall and cultural practices (tillage) in the field. Third, the water lost due to evaporation can only be stopped by some cover, which again may not reflect field conditions (without a cover, the weighing of the container or soil core computes ETo and not Etc). Options So what are the options? The best scenario is to minimize the impact of the plant and cultural practices and yet still measure all the forces acting on soil water. Probe. Using the capacitance probe can come closest to this optimal method, but only when the following criteria are followed: • Installing probe so as not to disturb the native soil structure • Using the correct sampling time appropriate for a soil and crop matrix • Determining the soil texture correctly • Using algorithms to determine which force moved the water from (or into) the location in the soil where the probe is located. Stacked probe. Using a stacked capacitance probe can be configured to measure the soil moisture in 10 cm (4 inch) “slices” in the soil. This is more helpful for water management than just a bulk measurement of the complete soil profile that lysimetry offers. This second new feature that the capacitance probe now offers allows measurement of ion content as well as water content, so that we can now look at basic water and ion content and compare where in the soil profile that consumption is taking place. Is the consumption occurring where the resources needed by the crop are located (i.e., in alignment)? If not, will any benefit be realized if water and nutrient management changes are made to move the locations of the resources? This now defines a new and exciting frontier. Probe ion measurements in the soil contain very broad information, meaning that all ions are measured, not specific ionic species; agriculturally-useful ions (such as calcium and nitrate) as well as non- useful ions (such as chloride or sodium) are measured. We can use complementary methods to extract soil water from different depths in the profile and measure specific ions of interest to get a complete picture of what is available to the crop. Using these complementary techniques, a lot of useful information can be obtained. When fertilizer is applied, the top sensors will show the increase in ions. Over time, the total will decrease through plant consumption and movement into lower layers in the soil. In almost all cases, the ions will increase at lower levels over the crop season. What is moving down? It can be the chlorine from the fertilizer or salty irrigation water, or it can be nitrates. This is where the complementary techniques can determine the element(s) that are moving. Efficiency Plants only take up nutrients in solution (i.e., dissolved salts). As we work toward more production with current resources (or even fewer) this fact must not be overlooked. Preplant and traditional in-season soil tests reflect what could be available for the plant if moisture is sufficient, while the soil water test (using soil samplers) is what is available in the water at that moment for the plant. So the first step in any management plan to increase water use efficiency demands that we must supply the water based on needs of the crop, rather than ETo measurements or general guidance (one inch per week, for example). In order to get nutrients into a crop, the plants need to be taking up water. The first step then, to increased efficiency, is to replace only the water the crop used, and replace it from where the plants got it. This may sound easy, but since we apply water (as irrigation or rain) at a one-source or -depth (mostly close to or at the soil surface), we subsequently can only hope the water distributes the way that gives the greatest benefit to the crop. Without monitoring, it is almost impossible to put back the exact amount that was removed and make sure that it gets to where we want it to go. This leads to the first aspect of inefficiency, since either over- or under-watering causes stress to the crop and stress causes lost production. Lost yield directly reduces input efficiencies, eliminating the opportunity to improve. Measuring The first step is measuring where and how much water is to be replaced, then confirming you achieved that goal. But another problem can be calibrating the scale to know where optimum water levels are and how dry the soil profile can become before replacing water. To achieve the correct volume, we must go back to the crop and measure water consumption. We have learned that as the soil gets drier the effort needed to extract that water gets greater for the crop. This means that as the soil water content goes down, the water uptake rate by the plant is reduced. Getting the most water into the plant will give the greatest potential for yield (and we have determined in crop variety comparisons that the plant that consumes the most water has the best yield). Allowable deficits (the level to which the soil profile can be depleted before yield losses set in) have been measured by universities for a variety of crops, and can be obtained online; these measurements offer the best way to set the lower limit of the water budget. The USDA in Beltsville showed that corn lost 60 bushels as a result of too little (below an allowable deficit) or too much water (above field capacity) for a period of 10 days. This means that waiting too long to irrigate reduces yield and then over-watering reduces more yield--double trouble! This underscores the importance of setting proper budget lines and also scanning the water content of each individual level in the soil, rather than only using the sum of all levels. FUE Now, if all the plant needed was water, we could stop here. It is early in the fertilizer efficiency work we’ve begun, but some data collected so “Measure all forces acting on soil water.”