Figure 1. Average U.S. corn, soybean and wheat (all types) yields from 1996 to 2016, as
from USDA-NASS, 2017.
Given yield increases across record acreages, it should be of no surprise that the International
Nutrition Institute (IPNI) has reported steady declines in
The purpose of soil testing in high-yield farming is to determine the relative ability of soil
to supply crop nutrients during a particular growing season.
nutrient levels. Such nutrients include
One of three primary nutrients, phosphorus (P) is essential for plant growth, and a plant must
access it to complete its normal production cycle. Plants absorb P from the soil as primary
and secondary orthophosphates (H2PO4- and HPO42-).
Potassium (K) is one of the essential nutrients and is taken up in significant amounts by crops.
Potassium is vital to photosynthesis, protein synthesis and many other functions in plants.
It's classified as a macronutrient, as are nitrogen (N) and phosphorus (P). Plants take up
K in its ionic form (K+).
Sulfur (S) is a part of every living cell and is important in the formation of proteins. Unlike
the other secondary nutrients like calcium and magnesium (which plants take up as cations),
S is absorbed primarily as the SO42- anion. It can also enter plant leaves from the air as dioxide
This leads to many questions. Are you keenly aware of revised
nutrient removal rates?
A solid fertilizer strategy starts with an understanding of which nutrients are taken from the
soil every time a crop is grown, and incorporating best management practices to replenish
Are you adequately replacing removed nutrients? Will this lead to a soil nutrient
Every five years, IPNI publishes a summary of soil test levels across North America. In its
(2015), several areas show reductions in median soil test levels for P and/or K. Some of the
notable areas of decline over the past 15 years include the eastern Corn Belt, eastern Canada,
Plains, the Mississippi Delta and the southeast United States (IPNI, 2016).
Are We Keeping Up with Soil Nutrient Replenishment?
Each bushel of corn removes about 0.67 lb
Nitrogen (N) is an essential nutrient for plant growth, development and reproduction. Despite
nitrogen being one of the most abundant elements on earth, nitrogen deficiency is probably
the most common nutritional problem affecting plants worldwide – nitrogen from the atmosphere
and earth's crust is not directly available to plants.
(N), 0.35 lb P2O5, 0.25 lb K2O and 0.08 lb S from the
soil. A bushel
of soybeans will remove 3.3 lbs N, 0.73 lb P2O5, 1.2 lbs K2O
and 0.18 lb S (Figure 2).
In a corn-soybean rotation, based on trend-line yields of 180-bushel
corn and 50-bushel soybeans, it would require 191 lbs MAP/ac and 175 lbs MOP/ac to replace the P
and K removed in the harvested grain. As yields approach 210 and 60 bu/ac for corn and soybeans,
respectively, it will require nearly 226 lbs MAP/ac and 208 lbs MOP/ac to replace the nutrients
and avoid soil test reductions. Applying fewer nutrients than are removed in the grain will
soil test levels to decline over time. In many cases, fields with declining soil test levels may
no longer be able to sustain the most profitable yield levels, especially when conditions are
for above-average trend-line yields.