Nutrient Sufficiency Approach

Transcription

1 Number 193 May 22, Understanding P and K fertilizer management systems 1 2. How to choose the correct GPS and Guidance System for your operation 5 3. Plant analysis for wheat: Be sure it s done at the proper stage of growth 7 1. Understanding P and K fertilizer management systems When producers send soil samples to K-State for analysis they are asked to specify whether they want Nutrient Sufficiency or Build and Maintain recommendations. If requesting Build and Maintain, they are further asked to specify how quickly they want to build soil test levels. Seems straightforward, but do producers really understand the decision they are making when they check that box? And do they understand the type of risks that each system entails? Let s review the differences between these two systems and the implications for fertilizer management in Kansas. Nutrient Sufficiency Approach First, exactly what is a Nutrient Sufficiency recommendation, and how does that system work? Nutrient Sufficiency Recommendations are commonly referred to as Feed the Crop recommendations. This is the traditional fertilizer recommendation system used in the Great Plains. The concept is that the Agronomy Department in the state s land grant university will conduct a large number of fertilizer response studies and determine exactly how much fertilizer is needed at a specific soil test level, to produce a specific crop. The field response experiments are commonly done to determine two things: * The relationship between soil test level and crop yield when no fertilizers are applied, or Soil Test Correlation; and * How much fertilizer is needed to produce an economically optimum crop at a given soil test level, which is the Soil Test Calibration. Soil Test Correlation. Generally at low soil test levels, yield is reduced if no fertilizer is applied. As soil test levels go up, yields go up. At some soil test level, yields reach a plateau. Eventually 1

2 yields may even go down if the soil test is high enough. The soil test level where maximum yield first occurs is the Critical Level. Actually, the Critical Level is usually assigned as some point slightly below maximum yield, where the maximum economic return to fertilizer is obtained. 110% Corn, Grain Sorghum and Wheat P Sufficiency Kansas State University % Yield 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% Corn Grain Sorghum Wheat 0% Bray P1 Soil Test (ppm) Figure 1. The relationship between Soil Test P Level and crop yield with no fertilizer applied. The purple vertical line is the Critical Level. The purple horizontal line is the point at which yields are slightly below maximum. Since the price of crops and the price of fertilizers change (especially in the past few years!), most agronomists have used 90 to 97% of maximum yield to determine the Critical Level. Figure 1 represents the relationship between soil test levels and crop yield, with no fertilizer applied, in a well-calibrated soil test. This figure contains some of the actual data used to develop the Kansas phosphorus (P) recommendations. The purple vertical line at 20 ppm represents the Critical Level. In this figure, note that most of the points are to the left of the Critical Level. These points represent check plots where no fertilizer was applied, and the relative yield was less than 100%. This suggests that in those plots, if a fertilizer had been applied, a yield increase would have been obtained. Soil Test Calibration. Calibration is the process of determining the rate of nutrient needed at a given soil test level. Theoretically, at very low soil tests, the rate of fertilizer recommended to achieve maximum economic yield is high. As the soil test level increases, the rate of fertilizer recommended decreases, until at the Critical Level, fertilizer rate needed reaches zero. Figure 2 shows an example of a group of soil test calibration response curves. Each curve represents a narrow range of soil test levels and the response to fertilizer additions one would expect. Note that in the curve for P soil test levels of ppm, there is a very small, noneconomic response to the first few pounds of P applied. This 2 to 3% yield increase would normally not cover the cost of the fertilizer required to obtain it. Based on a series of calibration 2

3 curves such as these, agronomists will recommend enough nutrients be added to feed the crop, increase yield to 95 or 97% of maximum, or an estimated economic optimum yield based on traditional fertilizer:crop price ratios. At soil test levels above the Critical Level, the soil can feed the crop and no fertilizer is recommended. Figure 2. Fertilizer P is applied to soils with different soil test levels of P to determine the yield response. In this example, when P fertilizer is applied to a soil with 1-5 ppm P, yields increase dramatically. As P fertilizer is applied to a soil with ppm P, there is little or no yield increase. There is one key point regarding the Nutrient Sufficiency system that needs to be pointed out. If the soil test is below the critical level, you must apply fertilizer to every crop. Let me repeat, you must fertilize every crop (including soybeans), every year, because you are only providing enough nutrients to feed one crop for one year. Unless the soil test levels are very low, the rate of P or potassium (K) recommended will likely be less than that being removed by the crop and the soil test will actually drop. Yes that s right, soil tests can drop by applying recommended fertilizer application rates. Using the K-State Nutrient Sufficiency Recommendations (KSRE Bulletin MF-2586) the recommended rates of P and K are less than crop removal whenever the soil test P level is greater than 10 ppm, or the soil test K level is greater than 80 ppm. This means that over a long period of time, your soil test P levels will equilibrate somewhere in the ppm range if you use the Nutrient Sufficiency approach. Build and Maintain Approach With a Build and Maintain recommendation system, or Feed the Soil system, the objectives are to meet the nutrient needs of the crop and build the soil test level above the Critical Level. Once the soil test level is high enough to supply all the nutrients needed for crop production, then the objective shifts to replacing what crops remove and to maintain this level. 3

4 In Kansas, we consider 20 ppm to be the P Critical Level, and 130 ppm the K Critical Level for all crops. However, the Critical P and K levels will vary due to crop, soil temperature and soil parent material in a particular state. Nebraska has traditionally used 15 ppm as the P Critical Level for corn and soybeans. However, this spring the team in Nebraska adjusted that up to 20 ppm for corn following corn only. They still consider 15 ppm to be adequate for corn following soybeans. In many of the northern states, such as Wisconsin, cool-season crops such as winter wheat or alfalfa also have higher Critical Levels than warm-season crops, to enhance P uptake during late fall and early spring. The logic behind a Build and Maintain approach is that by building P and K levels, you reduce the risk of nutrient deficiency reducing yield, and add management flexibility. For example, once the soil test exceeds the Critical Level, fertilization is not required every year with this approach. Many producers will fertilize just once to cover two or three crops, reducing the number of trips across the field and saving time. Using the Build and Maintain approach, this practice of infrequent fertilizer applications should not hurt yields. With the Nutrient Sufficiency approach, that practice can reduce yields. Is a Build and Maintain system for everyone? No! There can be a considerable investment involved in raising soil test levels. In Kansas, it requires 18 pounds of P 2 O 5 to raise the soil test by 1 ppm. To increase a field from 10 to 20 ppm requires an application of 180 pounds of P 2 O 5 -- above crop removal. At current prices that could easily represent an investment of $100 per acre. So this approach should probably be considered only on land that you own or on which you have a long-term lease arrangement. But having the flexibility of high soil test levels has allowed some producers to reduce or skip P applications as a response to high fertilizer costs the past two years. Just remember that there is no free lunch, and that P which was applied to build soil tests will have to be replaced at some time. Conclusions Both the Nutrient Sufficiency and Build and Maintain approaches to soil tests and fertilizer application work. A number of issues (land tenure, finances, and time savings) will influence which fertilizer management strategy a grower chooses. The approach that is chosen will determine how frequently P and K must be applied, in addition to how much must be applied. The Nutrient Sufficiency approach is used by many producers. However, in some cases they have forgotten this requires nutrient applications each year when soil tests are below 20 ppm P or 130 ppm K. Failing to directly fertilize a crop such as soybeans at low soil test levels, and relying instead on residual fertility from applications made to corn or grain sorghum, will leave bushels on the table. -- Dave Mengel, Soil Fertility Specialist dmengel@ksu.edu -- Dorivar Ruiz-Diaz, Nutrient Management Specialist ruizdiaz@ksu.edu 4

5 2. How to choose the correct GPS and Guidance System for your operation Most producers choose a GPS and guidance system based on price, but a few will spend the time and resources to figure out which system benefits their operation the most. What is the best way to go about making those choices for your operation? Currently, there are three types of GPS units and guidance systems. They vary mainly based on accuracy. These types are: WAAS (wide area augmentation system), Omnistar, and RTK (realtime kinematic). The John Deere systems SFI, II, and RTK -- mimic these systems in both scope and accuracy, except that partial year subscriptions are available when jumping from SFI to SFII. All three systems are the same except that they employ different differential correction techniques (as a stand alone GPS is only rated for 100 ft. or worse resolution). WAAS is a free system supplied by the federal government. Most post-2000 GPS receivers can receive this signal. Typical accuracies are 12 to 15 inches or better, but year-to-year accuracies can vary by as much as 5 ft. Omnistar is a service similar to WAAS, but employs extra techniques to make the system more accurate. Three options are available, ranging from VBS (sub-meter accuracy), XP (2 to 4 inch), and HP (3 to 5 inch) accuracies. These units also have similar year-to-year accuracies). Fees for this service range from $750 to $1500 per year. RTK systems are regarded as the Cadillac of all systems, having accuracies ranging from +/- 1 inch. Actual accuracies may vary as much as 2 to 4 inches when factoring in tractor steering and chassis dynamics limitations. These systems typically cost from $12,000 to $25,000 and need either a base station composed of a privately owned system, correction signal from the internet, or a subscription to a local tower (typically $750 to $1,500 per year). Before you buy, consider this: GPS units with WAAS correction have really closed the gap in guidance, offering pass-to-pass accuracies typically in the range of 12 to 15 inches -- although year-to-year accuracies can still vary by as much as 3 to 5 feet. These systems work well for most general farming purposes, such as general tillage, spraying, and soil sampling, etc. This equipment typically ranges from $1,000 to $5,000 and is affordable by most farmers. Also, many of these systems offer easy upgrades to the Omnistar system for better accuracy by trading in the antennae and inputting a code. Still, many farmers buy the RTK systems because they provide the best all-around guidance and GPS readings for all farming operations, and can be used for most farming operations, such as banding fertilizer as set distances from crops, and so forth. When choosing a system, note the following: - WAAS systems have been reported as working well in some areas and not so well in others. Currently, Kansas is supposed to have the best and most complete coverage, being located in the middle of the country. Some farmers have reported these systems working very well (good enough to plant corn), while others have not. Potential problems are: o Location and time of year the system is used. 5

6 o Manufacturer of system (some are better than others): Ask around to see which one people like in your area. o Sun spots: Sun spots will likely increase in the future up to the year 2012, where they will be at an all time high. These spots increase atmospheric noise and radio signal interference from space. o Jumping of satellites: Two satellites are available for the WAAS transmission. Typically your unit should be fixed on one satellite to prevent jumping when making turns, etc. Check with your equipment manufacturer. Also, these satellites were changed in mid Make sure your unit has been upgraded with the newest software to use these satellites. - Omnistar systems: These systems have been reported as working well, but some people have reported a 2- to 4-inch accuracy offset when changing directions. This may have been caused by power lines and other obstructions (tree lines, etc.). - RTK systems: These systems have been reported as working well in most locations where available. People have reported loss of signal and long re-acquisition times (15 minutes or more) along tree lines, but this depends upon the terrain and location of the unit. Newer units are rumored to have accelerometers and IMU s (inertial mass units) which will continue to guide the vehicle for up to 15 minutes during reacquisition. o Most RTK systems will have a 1 inch or more loss in accuracy after more than one third of the distance from the base station, and higher losses for every two miles after that. o Internet RTK correction, deriving their signals from CORS (continuously operating reference stations) and other RTK base stations, are a new development for getting RTK corrections without the need for a separate tower. These corrections are sent through a wireless internet service provider and can give better accuracies as good as stand-alone base stations. Still, these systems are currently not available in Kansas. They currently only exist in earmarked states such as Iowa and Minnesota. - Most systems cannot control implement drift in contoured crops, hills, or when making turns in fields. Implement guidance is needed these situations. Some equipment exists for this purpose. The following chart can serve as a guideline for which type of guidance system (or GPS) you may choose for a certain farming operation: 6

7 GPS System Farming Operation WAAS Omnistar RTK Soil Sampling General Tillage Spraying Planting Wheat Planting Corn marginal Strip Tillage depends on service Second Pass Cropping marginal Cultivating (2 inch) marginal Second Pass Banding of Fertilizer Note: The actual accuracy and use of a system for particular purpose may still depend somewhat on your location, application, tractor setup, and cropping system. -- Randy Price, Department of Biological and Agricultural Engineering rrprice@ksu.edu 3. Plant analysis for wheat: Be sure it s done at the proper stage of growth Plant analysis is an excellent quality control tool for wheat growers interested in high-yield wheat management, as long as it is done at the proper stage of growth. There are two primary ways plant analysis can be used: as a routine monitoring tool to ensure nutrient levels are adequate, and as a diagnostic tool to help explain some of the variability in wheat growth we see in fields this time of year. Keep in mind, however, that any plant stress (drought, heat, frost, etc.) can have a serious impact on nutrient uptake and plant tissue nutrient concentrations. Sampling under stress conditions for monitoring purposes can give misleading results, and is not advisable. When and what to sample: For monitoring purposes, flag leaves should be collected at random from the field at the late boot to initial heading stage of growth, which is still the case in parts of northwest Kansas. Once the plant pollinates and kernel development begins nutrients will start to flow from the stem and leaves to the developing grain. For this reason, sampling for monitoring purposes is not recommended once the plant begins to shed pollen. The leaves should be allowed to wilt over night to remove excess moisture, placed in a paper bag or mailing envelope, and shipped to a lab for analysis. Do not place the leaves in a plastic bag or other tightly sealed container, as they will begin to rot and decompose during transport, and the sample won't be usable. Interpretation of plant analysis data: The data returned from the lab will be reported as the concentration of nutrient elements, or potentially toxic elements, in the plants. Most labs/agronomists compare plant nutrient concentrations to published sufficiency ranges. A 7

8 sufficiency range is simply the range of concentrations normally found in healthy, productive plants during surveys. It can be thought of as the range of values optimum for plant growth. The medical profession uses a similar range of normal values to evaluate blood work. The sufficiency ranges change with plant age (generally being higher in young plants), vary between plant parts, and can differ between varieties or hybrids. So a value slightly below the sufficiency range does not always mean the plant is deficient in that nutrient, but it is just an indication that the nutrient is relatively low. However, if that nutrient is significantly below the sufficiency range, then one should ask some serious questions about the availability and supply of that nutrient. Levels above sufficiency can also indicate problems. High values might indicate over fertilization and luxury consumption. Plants will also sometimes try to compensate for a shortage of one nutrient by loading up on another. This occurs at times with nutrients such as iron, zinc, and manganese. In some situations very high levels of a required nutrient can lead to toxicity. Manganese is an example of an essential nutrient which can be toxic when present in excess. As a diagnostic tool: Plant analysis is also an excellent diagnostic tool to help understand some of the variation seen in the field. When using plant analysis to diagnose field problems, producers should take comparison samples from both good/normal areas of the field, and problem spots. Collecting soil samples from the same good and bad areas is also a good idea. Don't wait for boot to take diagnostic samples. Early in the season (prior to stem elongation) collect whole plants from different places in your sampling area. Later in the season take the uppermost, fully developed leaves (those with leaf collars visible). Handle the samples the same as those for monitoring. The following table gives broad sufficiency ranges for wheat early in the season, prior to jointing (Feekes 4-6), and later in the season at boot to early heading (Feekes 9-10). Keep in mind that these are the ranges normally found in healthy, productive wheat, and values can vary widely between varieties, with increased maturity and due to soil variability across a field. Growth stage Nutrient Unit Whole plant at tillering-jointing Flag leaf at boot to heading Nitrogen % Phosphorus % Potassium % Calcium % Magnesium % Sulfur %

9 Growth stage Nutrient Unit Tillering-jointing Boot Iron ppm Manganese ppm Zinc ppm Copper ppm Boron ppm Aluminum ppm <200 <200 Plant analysis is an excellent tool to monitor the effectiveness of your fertilizer and lime program, and a very effective diagnostic tool. Producers should consider adding this to their toolbox. -- Dave Mengel, Extension Soil Fertility Specialist dmengel@ksu.edu --Dorizar Ruiz-Diaz, Nutrient Management Specialist ruizdiaz@ksu.edu These e-updates are a regular weekly item from K-State Extension Agronomy and Steve Watson, Agronomy e- Update Editor. All of the Research and Extension faculty in Agronomy will be involved as sources from time to time. If you have any questions or suggestions for topics you'd like to have us address in this weekly update, contact Steve Watson, swatson@ksu.edu, or Jim Shroyer, Research and Extension Crop Production Specialist and State Extension Agronomy Leader jshroyer@ksu.edu 9