Knowledge of Soil Fertility Helps Maximize Fertilizer Efficiency, Crop Yields

By Crystal McNett, Assistant Editor
Thursday, March 22, 2007 1:27 PM CDT

Proper soil fertility is essential to maximizing crop yields, said Carrie Laboski with the UW-Madison soil science department, at the recent Managing Nutrients on Wisconsin Soils workshop held in Madison.

Soil tests and plant analysis are tools necessary to determine how and when to fertilize.

Soil nutrients are classified into two categories by availability and need. Macronutrients are sorted into structural: carbon, oxygen and hydrogen; primary: nitrogen, phosphorus and potassium; and secondary: calcium, magnesium and sulfur. Micronutrients include boron, manganese, zinc, copper, iron, molybdenum, chlorine and nickel.

Nutrients in the soil solution are in an ionic form at low concentrations and are highly buffered. The soil solution changes and fluctuates with season and moisture conditions, Laboski explained. Exchange sites on clay and organic matter, organic matter decomposition, weathering of soil minerals and rocks, atmosphere, precipitation and organic and inorganic conditions all contribute to the soil solution.

Nutrients move to roots in three different ways: by mass flow, diffusion and root interception.

Root interception is when roots obtain nutrients by physically contacting nutrients in soil solution or on soil surfaces. Roots contact about 1 percent of the soil volume, explained Laboski. Mycorrhizal infection of the root increases root-soil contact. This action can help the supply and absorption of nutrients to the plant, but varies between plant species.

Nutrients exist as positively or negatively charged ions in the soil. Of the major nutrients required by crops, phosphorus and potassium are primarily absorbed by diffusion. "This is why starter fertilizers are so important," Laboski added.

Nitrogen exists in the soil as NO3 and NH4 and may be absorbed by plants in either form, likely via mass flow. These two compounds are different, however, in that NO3 is highly mobile in the soil and therefore leaches easily, Laboski explained. NH4 is held more strongly by the soil and is more available to plants in varying conditions.

Nitrogen deficiency is most commonly known as nitrogen firing. It is characterized in corn by brown curled up leaves. The condition begins at the bottom of the plant, starting at the leaf tip and proceeding up the midrib to the stalk.

Phosphorus is taken up by roots as H2PO4 or HPO4. "It is generally not mobile in soil except under very high concentrations, and therefore does not pose a high leaching risk," Laboski said. Most phosphorus is available between a pH of 5.5 and 7.2.

Purple leaf edges are characteristic of phosphorus deficiency. Laboski added that in very severe cases the entire plant can turn purple.

Potassium is moderately mobile in the soil and is obtained by plants mainly via diffusion. It is held on the cation exchange and is therefore more readily available to the plant. Potassium deficiency is similar to nitrogen firing, but the midrib of the leaf remains green. The brown curled-up effect moves from the outside of the leaf into the center.

Calcium is readily mobile in the soil and moves to the root by mass flow. It does pose a risk for leaching, especially in sandy soils, Laboski said. In addition, "soil with a low pH and extra hydrogen can displace calcium," she said, allowing it to be leached. A calcium deficiency and sometimes be seen in dry soils when water to transport the calcium is scarce.

Magnesium moves to the root via mass flow and diffusion and leaches somewhat more than calcium, Laboski related. "Deficiencies are limited to sandy soils with a low pH," she said. Magnesium deficiency can be identified by interveinal browning of the leaves.

Sulfur is mobile in the soil as SO4 and is absorbed by the plant via mass flow and diffusion. The availability of sulfur is dependant upon the amount of organic matter in the soil. Today, more deficiencies are occurring because of less atmospheric deposition and more pure N-P-K fertilizers. "Cleaner air results in less free sulfur," Laboski explained.

Sulfur deficiency begins in new plant tissue and moves downward. It is characterized by a yellowish coloring of the plant and stunted growth.

Plant analysis

Before symptoms occur, plant analysis can be used to identify nutrient shortages. This tool can be used to determine the nutrient supplying capacity of the soil and the effect of nutrient addition on the nutrient supply in the plant.

Cell sap tests are quick, in-field tests that can give semi-quantitative results, Laboski explained. Total analysis provides a more accurate indicator of plant nutritional status. The analysis is done on either the whole plant or a specific part depending on the stage of growth. Also important to note is that total analysis assumes that nutritional status is related to soil nutrient availability when it could be due to an outside factor like compaction.

Before sampling tissue it is very important to contact the lab that you will be working with, Laboski warned. Questions should be asked to identify what to sample, when to sample, how to handle the sample and how the lab wants to receive the sample.

Tissue test results will identify critical nutrient concentration ranges. However all deficiencies identified may not be feasible to address, Laboski explained. The deficiency may have already caused yield loss that cannot be corrected or the crop may not respond at the growth stage. The crop may also be simply too large to fertilize with available application equipment.

It is also important to note that plant analysis is only one piece of the network that should be used to identify a nutrient deficiency. "You need to collect all the evidence," Laboski said. To be included in the toolkit are nutrient deficiency symptoms, root growth patterns, weather, current field conditions, field history, tissue analysis and soil analysis.

 

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