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Mar 05

Cracking the Code: Fertilizers in the Right Place, Right Time

Posted on March 5, 2019 at 8:20 AM by Allan Warren

Since we started our “first one’s free” soil testing program for pastures, hay fields, and commercial crops in 2006, we have received a lot of sign-ups, and also a lot of questions. 

Some of the questions have included things such as: 
how to read the soil test reports from the lab? 
how to read the fertilizer labels?
how to determine which fertilizers to apply based on the results?
and when to apply fertilizer?
Since one of the goals of the soil sampling program is to educate landowners so they can interpret future soil test results and make the fertilizer calculations, we always include the information contained in this article with the soil test results.  If needed, we also will sit down with landowners to go through the results together.  

If you are planning to take soil samples and send them to a lab yourself, you may also want to obtain the Oregon State University (OSU) Extension Service’s publications that the District’s farm planners use when making fertilizer recommendations.  They are the Soil Test Interpretation Guide (EC 1478), Pastures: Western Oregon and Western Washington (FG63), and Fertilizer and Lime Materials (FG52).  Copies of these publications can be obtained from our office, or they can be viewed at http://extension.oregonstate.edu/catalog/details.php?sortnum=0134&name=Fertilizer Guides.

Fertilizers in the Right Place, Right Time
A good farm plan bases its fertilizing regime on soil test results and realistic crop yield goals.  This is where a site visit by the District’s farm planners can come in handy.  They can make crop yield estimates based on your soil type and grass health.  Or you can call and have one of them tell you your soil type (based on USDA soil maps) and its yield goal average and potential.  

Calibrate your fertilizer spreader prior to applying fertilizer.  And when you apply it, make sure to put it in the right place.  Avoid spreading fertilizer in areas where water flow may concentrate, in floodplains, or within at least 50 feet of streams, unless buffer requirements for your individual stream are more than 50 feet.  

Apply at the right time is important.  Water can dissolve nitrogen and carry phosphorus attached to soil particles.  This means rain, snow, and floods can transfer nitrates into groundwater and nitrogen and phosphorus over land and into lakes and streams.  Apply fertilizer and manure during the growing season when crops will use the nutrients, not in the winter or on saturated soils.  Split the applications of nitrogen on pastures for maximum plant uptake and minimum loss to runoff.  Generally, it is recommended that 40% of the growing season’s fertilizer be applied in April when the plants start growing, 45% in May (or one month after the first application), and 15% in September.  But only make the fall application if we receive rains before the first hard freeze.  Once it freezes, the plants will go dormant and will not use the fertilizer if it contains nitrogen.  If not taken up by the plants, any nitrate in the soil will leach down and pollute the groundwater.  

Follow the right conservation practices.  Phosphorus is held tightly to soil and is moved mostly by loss of soil due to erosion.  Reduce phosphorus losses using erosion control practices such as grassed waterways, grassed filter strips around livestock holding areas, and streamside buffers.  

Grow legumes to add nitrogen naturally to the soil.  Pasture legumes include clovers and birdsfoot trefoil.  Even grass may add a flush of nitrogen to the soil and up to 20% of the total nitrogen in the organic matter contained in the soil will be released during the growing season.  This is another good reason to apply organic matter in the form of composted manure back onto the fields.  Pastures need less fertilizer than row crops because grazing animals return manure nutrients to the soil.  Rotational grazing will distribute manure more evenly.

Fertilizer Labels: Cracking the Code

The job of fertilizers is to add to nutrients already in the soil.  All fertilizer bags are labeled with a series of numbers that show the percentage of nutrients in a bag.  A label of 27-13-7 means there is 27% nitrogen, 13% phosphate, and 7% potash (potassium) by weight in the bag.  A 100-pound (lb.) bag of 27-13-7 spread over one acre would spread 27 pounds of nitrogen, 13 pounds of phosphate, and 7 pounds of potash (potassium) per acre.  

Organic fertilizers are unprocessed and unrefined fertilizers like blood meal, rock phosphate, and manure.  When organic fertilizers are purchased in bags, they are labeled in the same manner as synthetic fertilizers.  In general, an organic fertilizer releases nutrients over a longer time than synthetic one, although synthetic fertilizers can be purchased that are coated with a material that enables them to release nutrients slowly.  Slow release can be an advantage as nutrients are supplied to a plant throughout a longer period of growth.  A drawback is that the organic fertilizer or coated synthetic fertilizer may not release enough nutrients at a critical growth point.

Soil pH is a measure of soil acidity and affects the availability of nutrients to plants.  A soil with a pH below 7.0 is acidic, while one with a pH above 7.0 is alkaline.  A very acidic soil will release aluminum and manganese and become toxic to plants.  A very alkaline soil will make phosphorus and iron unavailable to plants.  Ideal pH ranges are; 5.5-6.0 for pure grass stands, 6.0-6.5 for legume-grass mixtures and row crops and small grains, and 6.5-7.5 for alfalfa.  Western Washington soils are generally acidic.  If the soil is too acidic for your crop, add lime to raise the pH.  The OSU Fertilizer and Lime Materials (FG52) guide will help you to determine how much lime is needed based on the soil pH and buffer pH shown on your soil test results.  If your soil test shows that magnesium is deficient, apply dolomite lime; magnesium sulfate can also be added, unless sulfate levels are already high in the soil.  Magnesium is important in preventing grass tetany in sheep and cattle that eat grass in the spring.

If you take anything from this article, please remember that it is important to get a soil test to know what nutrients and minerals are already in your soil.   Now that we have expanded the program to include five free soil tests (which also includes one compost test in lieu of two soil tests), there has never been a better time to find out what amendments may help your pasture, hay, or commercial crop to grow to its full potential.  Do not just guess and put out whatever is available at the store. From all the soil testing that the District’s farm planners have done over the last thirteen years, almost all farms that have contained livestock for a number of years have more then enough phosphate and potassium already in the soil.  Generally, the only nutrients needed have been nitrogen, sulfur, magnesium, and calcium. And yes, most soils that we have tested have been acidic and have required lime to improve the pH.

Contact your area farm planner for a free soil test: (253) 845-9770.

Excerpts for this article were taken from: “Fertilizing for Profit”, fact Sheet No. 12, January 1999, from the “Small Acreage Landowner Fact Sheets”, produced by the Tualatin, OR, Soil and Water Conservation District and the Small Acreage Steering Committee.