Missouri Cotton News
MAY 1999

Low soil pH can be easily diagnosed by soil testing and is easily corrected by applications of agricultural limestone(ag lime). Soil test recommendations for ag lime in Missouri are given in units of lbs Effective Neutralizing Material(ENM)/ per ton. The ENM of a given ag lime is dependent on two factors 1) the purity of the material and 2) the particle size or fineness of the grind. These two factors are combined together to estimate the amount of soil acidity that can be neutralized in 3 years time. Most ag lime in this area runs from 400 to 650 ENM/ton.

White lime is relatively pure calcium carbonate(CaCO3) and red lime that is relatively pure dolomite(Ca(54.4%)Mg(45.6%)CO3). The ag lime in this area is neither pure white nor red. The amount of MgCO3 in our red lime usually runs from 10 to 14%.

A frequently asked question is "Based on my soil test should I apply white or red lime?" Some commercial crop consultants have raised concerns over lowering the Calcium/Magnesium ratio (Ca/Mg) by always applying red lime. Given that on a dollar per ENM basis red lime is less expensive than white and it is more economic to use red lime.

By examining the economics, agronomic, and chemistry of this issue I have attempted to shed some light on this question. For this discussion I called upon our local lime dealer and will use his lime prices and ENM values for comparisons.

Economics
As an example a soil test recommendation calls for 1200 ENM/a. We have a choice of white lime with an ENM of 500 for $16.50/ton or red lime with an ENM of 600 for $17.50/ton. Both prices are delivered and spread. To find out how much of each are needed we divide the ENM recommended per acre by the ENM of the lime. To determine the cost per acre we multiply the tons of lime needed per acre by the cost per ton. These calculations have been done for each product below.

White Lime
1200ENM/a divided by 500ENM/ton = 2.4ton/a
2.4ton/a X $16.50/ton = $39.60/a
Red Lime
1200ENM/a divided by 600ENM/ton = 2.0ton/a
2.0ton/a X $17.50/ton = $35.00/a

Clearly the red lime costs less per acre than white lime. But will my yield be the same?

Agronomic
In a two-year study made possible by Cotton Inc. Dr. Gene Stevens (University of Missouri-Delta Center) measured cotton lint yields at specific field locations and compared them to soil test data from the same points. In this data I calculated Ca/Mg ratios and compared them with lint yields. I also calculated Ca/Mg on the basis of their contribution to CEC. Graphs of these comparisons are presented below.

It is clear from these two graphs that Ca/Mg ratio has no effect on cotton lint yields.

Chemistry
Consider a pure red lime (dolomite) which is 54.4% CaCO3 and 45.6% MgCO3. Calcium accounts for 40.0% of CaCO3 while Magnesium accounts for 28.8% of MgCO3. By multiplying the percentage of Calcium in CaCO3 by the percentage of CaCO3 in red lime we find that red lime is 21.8 % Calcium. A similar calculation for Magnesium shows that the same red lime is 13.1% Magnesium in the same red lime. Converting this to a lbs./ton basis a ton of pure red lime contains 436 lbs. Ca and 262 lbs. of Mg.

Now let's look at nutrient removal by crops. I have pulled some numbers from an article by Dr. Paul Tracy of MFA Agri-Services in the Dec 95- Jan '96 issue of Today's Farmer.

          REMOVAL OF PLANT NUTRIENT BY CROPS (LBS/Bu)
          Crop         Portion   Calcium   Magnesium
          Corn         Stover      0.19      0.14
                       Grain       0.02      0.06
                       Total       0.21      0.20
          Cotton       Residue    25.00      8.00
          (500lb bale) Lint +Seed  3.00      4.00
                       Total      28.00     12.00
          Rice         Straw       0.12      0.06
                       Grain       0.04      0.05
                       Total       0.16      0.11
          Sorghum      Stover      0.31      0.15
                       Grain       0.07      0.08
                       Total       0.38      0.23
          Soybeans     Straw       1.50      0.22
                       Grain       0.20      0.23
                       Total       1.70      0.45
          Wheat        Straw       0.21      0.10
                       Grain       0.05      0.14
                       Total       0.26      0.24

When we consider the total amount of Ca and Mg that is removed from the soil by each crop it seems that crops use more Ca than Mg. This makes sense as plants use Ca to maintain rigid stems and leaves. Most calcium is found in the non-grain portion of the growing crop. Magnesium on the other hand tends to be more concentrated in the grain. If we were to look at the total removal of Ca and Mg that the Ca/Mg soil ratio would decrease with each crop year. Now consider that most of the nutrients in the straw, stubble or residue are recycled back to the soil. With this view crop removal in grain would be the better number to use for net crop removal. Now all of the crops listed above remove more Mg than Ca per unit yield.

Given that red lime contains more Ca than Mg and that crops remove more Mg than Ca, I can not make a case for Mg build up in the soil as the result of repeated red lime applications.

On Going Research At The Delta Center

We are investigating Ca/Mg ratios for cotton, rice and soybeans. In a multi year experiment we are treating soil plots with differing amounts of Gypsum (CaSO4) and Epsom salts (MgSO4) in order to produce a wide range of Ca/Mg ratios.

A long term evaluation of red and white lime has been started at the Delta Center. In this project both types of lime will be applied to strips of land at the rates recommended by soil tests. Material costs, crop yields and soil test data will be collected each year for the next ten years. Feel free to stop by our lab and check out our research.

Timing your irrigation on your cotton and knowing how much to apply, commonly referred to as scheduling irrigation, is important. Surveys taken from Bootheel irrigators since 1988 show that yield increase due to irrigation is 176 lbs/acre. However, there are many instances of farmers reporting in the surveys that some irrigated fields made less than their dryland fields! This indicates that irrigation scheduling is off.

Nearly 90 people attended the recent irrigation scheduling workshops held in seven SEMO communities last month. Those growers, on average, estimated that their yield potential was about 14% off of what it could be if they could better nail down their irrigation timing. Most Bootheel irrigators don't have the luxury of learning irrigation from what their father did, who himself had probably honed his skills from what his dad and grandfather did. The average number of years of experience irrigating that the attendees had was only 15. Some people had only been doing it for one or two years.

Irrigation Trigger Points

One of the best skills to learn in irrigating is how many inches to apply at a shot. This ideal application depends on three things: (1) the crop (2) the soil type (3) the method of irrigation

The University of Arkansas has worked on this question for years and have come up with suggestions for the ideal application amount. Another term for application amount is rootzone deficit. Personally, I like to call them trigger points, since they trigger an irrigation. Thus when so much water is used up, you turn on your irrigation system. Table 1 lists trigger points as recommended by the University of Arkansas.

Table 1. Suggested Irrigation Amounts in Inches for Various Crops and Soil Types

Predominant         Soybeans            Cotton                Milo               Corn
Soil           ------------------  ------------------  ------------------  ------------------
               Surface  Sprinkler  Surface  Sprinkler  Surface  Sprinkler  Surface  Sprinkler
---------------------------------------------------------------------------------------------
Clay            2.00      1.50      2.00      1.50      2.50      2.00      1.75     1.25

Silt Loam
w/ pan          1.75      1.25      2.00      1.50      2.00      1.50      1.50     1.00

Silt Loam      
wo/ pan         2.50      2.00      2.50      2.00      3.00      2.50      2.00     1.50

Sandy Loam      2.25      1.75      2.50      2.00      2.50      2.00      1.75     1.25

Sandy           2.00      1.50      2.00      1.50      2.00      1.50      1.50     1.00

The trigger points as shown in Table 1 are not set in stone, and a person may want to change the amount. For example, Table 1 suggests that with cotton on clay using a pivot, ought to have an ideal application amount of 1.5 inches. This may be too much to apply due to run-off problems, and so you then decide to apply 1.25 or 1.00 inches instead.

Special Options on Irrigating Cotton
The jury is still out, but some researchers think it is a good idea to change watering strategies for cotton based on growth stage. In effect, what they say is to water the period from around first bloom to a week or so after peak bloom at a picked-up rate. Early season and late season watering is done less frequently. In this strategy, a pivot owner on a silty loam without a pan problem might use a 2.00 inch deficit for most of the season. However, for the four or five weeks after first bloom, he might go to a 1.00 deficit. Since this is not yet proven in the mid-South (it worked well in an Arizona study) experiment on your own with a small block to see if it is a system that has merit for you.

When to Apply the Trigger Amounts
The next question to answer is when do I trigger these irrigations? There are two main methods Missouri growers can use. One is to use a computer program like the Arkansas Scheduler. This Windows-based program is fairly simple to use. You tell it what trigger amount to use and what crop you planted and when it emerged. After that merely input the daily temperature (only maximum temperature is needed) and rainfall. It will predict when you should irrigate. The program is available for $15 from the University of Arkansas, or my office will provide it free. In both cases, you will need to fill out a registration form so that UA can keep track of users, in the case that changes are made to the program.

Another method is the Woody Woodruff chart method. Dr. Woodruff is a professor emeritus at MU, who has worked there since 1929. He developed a chart method for timing irrigation back in the 1960s. The chart was developed specifically for corn grown on claypan soils in central Missouri. It was recently expanded by putting the logic into a computer, and now customized charts (Fig. 1) can be made for a number of different crops. Once the data is input a personalized chart is printed out. After this only a pencil and a rain gage are needed to schedule your irrigation. Rainfall is plotted on this chart and irrigations are called for by merely looking at the graph and irrigating before the crop gets into the drought zone. The Woodruff methods use historic temperatures, and therefore, will be slightly off on years that are exceedingly too cool or too warm. However, in 90 years out of 100 they will do a great job. I'll be glad to make up personalized Woodies for you, if you send me the data needed. This includes crop (cotton, corn or soybeans), season length in days (e.g., 115 day corn), soil type (indicate if hardpans exist), irrigation method, and planting date. I will use the trigger points as shown in Fig. 1, unless you indicate you wish to use a different amount. You can mail the data to me at the address below.

Monitoring the Soil's Moisture
When using computers and charts for scheduling it is always recommended that you install soil moisture monitoring devices like gypsum blocks and tensiometers. A new generation of sensors are out there that not only allows you to take soil moisture readings, it automatically logs them for you. One such device allows a person to install three sensors that hook into a miniature data-logger. Fig. 2 shows a graph of the output. If you notice carefully, you can even see the morning dew creeping back into the soil. By watching the shape of these water extraction curves, one will know when irrigation is called for.

If you assume that three different locations on a 125-acre pivot are monitored, a 7 year life, 9% interest, and 15 hrs a season (@$10/hr) are used to read the charts, then the annual per acre cost is about $3.50/acre/yr, or about 6 pounds of lint. Can such a device increase irrigated yields by 6 pounds? Probably that and a lot more. I'll be carrying out some studies on these devices this summer and report on them later.

Joe Henggeler, Irrigation Specialist, Commercial Agriculture Program
University of MO Delta Center, PO Box 160,Portageville, MO 63873
or e-mail at: henggelerj@missouri.edu

Fig. 1- A Woodruff chart used to schedule irrigation on corn for the 1997 season.

Fig. 2- Graph from Netafim's soil moisture monitoring and data-logger apparatus. Note that you can even see the increase of moisture in the soil profile every morning due to the morning dew! This sample graph is from a drip-irrigated field. Note the irrigation around Sept. 7. Note the study decline in soil moisture until Sept. 11, and after that the lines are flat. This means there is little available water left in the profile and the person should have irrigated Sept. 11.

Information from graphs like these can make a farmer a better irrigator then he ever hoped to be.

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