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Mind
your P’s and K’s
Test soil, fertilize based on recommendations to avoid yield-robbing deficiencies later |
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By Vicky
Boyd |
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Higher yielding rice varieties combined with fertilizer cost-cutting
are causing a small but growing number of phosphorus (P) and potassium
(K) deficiency cases. Although Extension specialists say the problem
is far from epidemic, they nonetheless recommend soil sampling and testing
every two to three years and following university fertility recommendations. “We are not trying to act like Chicken Little—it’s not
a truly widespread problem,” says Extension agronomist Dr. Joe
Street. “But we are seeing more and more fertility problems, so
heads up.” Once fields begin to show P and K deficiencies, you can correct them,
but not without possibly losing yield potential. “Why take a risk of losing yield potential simply because you waited until later in the season to see [whether deficiencies appear],” says Pat Bollich, an LSU agronomist specialist in rice plant nutrient. “It’s pay me now or pay me later.” What are P-, K-deficiency symptoms? K deficiencies in many places weren’t seen until farmers began
planting Cocodrie, a high-yielding long grain that now occupies more
than 50 percent of the acreage outside of California. Because of Cocodrie’s
higher grain production, the plants require more K. K symptoms include stunted plant growth with shortened internodes,
cupped or downward curled leaf edges, and chlorotic or brownish leaf
margins and tips. Plants deficient of potassium also are more susceptible
to foliar diseases, and deficient fields won’t green up even after
mid-season nitrogen applications. The problems appear mostly in silt-loam soils or fields that have been recently leveled and are accentuated when both conditions exist. It is not as common on soils with high clay or organic levels. Potassium is recommended for about 30 percent of Arkansas’ rice acreage, Slaton says. Why the deficiencies now? But some producers are trying to cut costs and have reduced or eliminated
P and K in soybeans. A 45-bushel-per-acre irrigated soybean crop mines
about 63 pounds of K20 per acre from the soil, according to the Potash
and Phosphate Institute. A 160-bushel-per-acre rice crop will mine about
28 pounds of K20 per acre from the soil. If you’re in a 1:1 rice/soybean rotation, that adds up to about
90 pounds of K20 removed from the soil per acre over two years. An average rice crop of 160 bushels per acre will remove about 46 pounds of P2O5 per acre while a 45-bushel-per-acre soybean crop will mine 36 pounds per acre. Phosphorus “If you missed tillering, you won’t do a lot of good putting
P out after that time, because you won’t grow any more tillers,”
Bollich says. Research, including recent trials conducted in Cross County, Ark.,
by Slaton, shows that pre-flood phosphorus application is more efficient
than pre-emergent application. Drawing down the ‘K’ bank account “In Mississippi, most of our soybean farmers in the Delta aren’t
putting anything on because our soils are extremely fertile, so we are
constantly reducing it (K),” says Walker, a post-doctoral researcher
at the Delta Research and Extension Center in Stoneville, Miss. “There’s
none being put back to it.” Because K doesn’t generally cause the severe deficiencies that
P does, Bollich says potential yield losses aren’t as acute. But
you should still pay attention to K levels because the nutrient helps
with overall disease resistance. Slaton’s preliminary studies indicate that potassium fertilizer must be applied by mid-season to K-deficient soils to obtain favorable yield responses. Sampling To avoid confusion, know what type of soil analysis is being conducted
on the sample—laboratories in different states conduct different
tests and the results will vary. Critical levels for P tend to vary wider among the tests than those
for K. The University of Arkansas soil lab, for example, performs a modified
Malich-3 test in conjunction with a soil pH test since soil acidity
or alkalinity can affect available phosphorus. Because of the current economics of rice and soybeans, many producers may not want to make large P and K applications to build soil stockpiles. Instead, Walker says, they could make smaller applications to at least maintain soil levels and replace what’s being mined by the crops. Contact Vicky Boyd at (209) 571-0414 or vlboyd@att.net. Does your soil-test P
measure up?
Hotlinks: Fertilization of Louisiana Rice, Louisiana Cooperative Extension Service University of Missouri soil test recommendations for rice:
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