Bill Jokela, Extension Associate Professor,
Department of Plant and Soil Science, University of Vermont email@example.com
Test Soil To Determine Nutrient Need
Individual fields vary greatly in pH and in their capacity to supply essential nutrients such as P, K, and Mg.
Recent summaries of soil test results from the UVM Agricultural and Environmental Testing lab showed that about a third of field crop samples tested either high or excessive in phosphorus and needed no additional P. However, almost half tested below optimum and received a recommendation for P application.
Reducing or eliminating P application on
the high-testing fields would save money, but cutting out P on the low-testing
fields could cost the farmer by lowering yields. Most farms have some fields
in each category, and soil testing is the only way to tell the difference.
Soil test results can also serve as a guide to allocation of manure to fields where it will give the greatest economic value.
Nitrogen is a different story. Routine N recommendations for corn are not based on a soil test, but are an estimate made from expected yield, previous crop, manure management, and soil drainage. While these recommendations are research-based, they cannot account for variations in N availability in different fields from year-to-year. This is where the Pre-Sidedress Nitrate Test (PSNT) comes in.
It measures nitrate-N in the surface foot of soil when the plants are 8 to 12 inches tall. Research has shown that this is a reliable indicator of N availability for the crop. The PSNT can be a big money-saver.
Results often show that manure and soil
organic matter are providing adequate N and no additional fertilizer N
is needed. But the test can also be valuable in pinpointing situations
where soil or weather conditions have led to significant N loss and additional
N application is needed to prevent yield reductions.
Manage Manure To Maximize The Nitrogen Value To Crops
Manure is a great source of nutrients for crops. When managed well, manure can often eliminate most of the need for fertilizer. Average Vermont dairy manure contains about 23 lbs N/1000 gallons or 10 lbs N/ton. However, about a third to a half of the nitrogen is in the ammonium, or urea, form and can easily be lost as ammonia gas when left on the soil surface. To maximize N for annual crops, manure should be incorporated with tillage as soon as possible. A delay of even six or eight hours can mean a loss of about a quarter of the ammonium-N.
Manure nutrient content can vary two- or
three-fold from farm to farm. Sample the manure on your farm to make a
better estimate of the application rate needed to supply crop needs. Adjust
fertilizer P and K rates too by crediting those nutrients in manure.
Credit Nitrogen From Previous Crop
Plowing down a legume or grass hay crop can provide a substantial portion of the nitrogen needs of a following non-legume crop such as corn. Legumes such as alfalfa or clover fix nitrogen from the atmosphere for use in their own growth, but even a well-managed grass hay crop contains significant N in the roots and above-ground biomass that can be released upon tillage.
The UVM soil test program credits 40 to
120 lb/acre of fertilizer N equivalent for a previous hay crop, depending
on legume content and management. This can translate into dollars saved
on N fertilizer purchases.
Avoid Unnecessary Fertilizer Applications
Soil testing is key to identifying fields for cutting fertilizer rates. Two situations where fertilizer expenses can often be reduced are topdressing phosphorus on legume hay and starter fertilizer on corn.
Common choices for topdressing legume or legume-grass hay are 0-10-30 and 0-10-40. These ratios approximate the nutrient uptake of the crop (assuming there is adequate legume to supply N needs). But if soil test P is high, the soil provides adequate phosphorus for the crop and additional P from fertilizer is unnecessary.
For a soil that has a potash need of 200 lbs/acre, using 0-0-60 instead of 0-10-30 or 0-10-40 could save about $20 per acre.
Starter fertilizer is another place to cut costs. If soil test P and K are high, only low rates are needed - enough to supply about 10 to 20 lbs of P2O5 and K2O per acre plus a small amount of N. For a typical starter analysis, this would usually require 100 lbs/acre or less.
Compared to common starter rates of 200
to 300 lbs/acre, this could result in savings of $10 to $20/acre. If soil
tests are below optimum, a higher starter rate is needed unless most of
the P and K requirement is met with manure or broadcast fertilizer.
Don't Skimp On Fertilizer Where It's Really Needed
While cutting down fertilizer rates can sometimes save money, doing it in the wrong situations can cost, not save.
Corn, for example, is a big user of nitrogen. If the crop doesn't get enough N it shows in reduced yield. Manure can often provide enough N but, whether or not manure is applied, take a PSNT sample to check on N availability and sidedress additional N if needed.
Grass hay also has a large demand for nitrogen.
Several UVM trials in the Champlain Valley have shown a doubling of grass
yields from split applications of fertilizer N or manure. Alfalfa and other
perennial forages remove large amounts of potassium from the soil. Be sure
to provide adequate K from fertilizer or manure to maintain yields and
legume stand. Again, soil testing is key to determining fertilizer need.
For more information about nutrient management and soil testing check the UVM Extension Manure and Nutrient Management site.
For additional information or to obtain soil or manure test kits contact your local UVM Extension office or the UVM Ag Testing Lab at (802) 656-3030 or 800-244-6402.
This article can also be found in AGRIVIEW
This site is maintained by Sid.Bosworth@uvm.edu, Plant & Soil Science Department, University of Vermont.
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