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Soil testing: Important, effective, underutilized

Mark L. McFarland and Larry Redmon Published on 12 November 2012

Without question, soil testing is one of the most important, most effective and yet most under-utilized best management practices available to agricultural producers.

Assessments in some areas have indicated less than 20 percent of the land receiving fertilizer is soil-tested on a regular basis.

At the same time, significant increases in fertilizer prices over the last decade have made plant nutrition an even greater component of total production costs.

And this is particularly true in the case of forage crops.

Soil testing effectively measures two pools of nutrients in the soil: the native supply and fertilizer carryover.

Native nutrients in a soil are the result of soil mineralogy and organic matter recycling. Typically, heavier soils (loams and clays) have a greater native nutrient supply because they formed from minerals more rich in nutrients like potassium, calcium and magnesium.

Decomposition of organic matter both at the surface (residue) and below ground (roots) also releases nutrients into the soil.

When fertilizer is applied and not taken up by the crop, carryover nutrients can remain in the soil in a plant-available form until the next season.

This is particularly true for nutrients like phosphorus and potassium, which are relatively immobile in the soil.

And while nitrogen can leach or volatilize under certain conditions, if rainfall and crop growth are limited, nitrogen also can be stored in the soil for extended periods.

Research has shown carryover nutrient levels, particularly following a drought or other type of event limiting plant growth (freeze, insect damage or disease) can be substantial and can supply part or all of plant needs when growing conditions improve.

Given the fact many parts of the country recently faced or currently are facing significant drought conditions, the potential for carryover may be substantial where rainfall was insufficient to utilize applied fertilizer.

In areas where soil acidity is a concern, soil testing should be done in late fall so that limestone, if needed, can be applied and allowed time to react and increase pH by the next year.

Most states have regulations requiring quality analysis for limestone products. In states without a limestone law, producers should request quality information so products can be compared for value and proper application rates can be determined.

If low pH is not an issue, soil testing for warm-season forage crops should be done in late winter or early spring to allow sufficient time for results to be received and fertilizer products selected.

Most soil testing laboratories recommend a standard soil sampling depth of zero to six inches. Each “management area” should be sampled separately.

A management area is an individual field or portion of a field that has uniform soil characteristics and has been managed similarly.

Multiple soil cores, typically 12 to 20, are collected within the management area and combined to produce a composite sample for testing.

The results will be specific for that management area and will enable very prescriptive determination of the type and rate of fertilizer needed to optimize production economics based on the crop and yield and/or management goals.

Currently, the primary nutrients (nitrogen, phosphorus and potassium) cost between $0.50 and $0.70 per pound. This represents a 120 to 230 percent increase in cost since 2003, but actually is down considerably from 2008.

The fertilizer market has become much more volatile in recent years, with substantial shifts in product prices over short periods of time.

Increasingly, global markets are influencing pricing, but domestic demand for some products also can dramatically influence cost and availability.

For example, a wet early season limiting anhydrous ammonia application on corn in the Midwest can increase overall demand for urea and UAN and push prices upward.

Comparing products on a cost per pound of nutrient basis is essential to optimize economics. For single-nutrient products such as urea (46-0-0), simply divide the cost per ton by the pounds of nitrogen per ton (2,000 x 0.46 = 920).

For example, at $638 per ton the nitrogen supplied by urea costs $0.68 per pound. With multi-nutrient blends like 19-19-19, assume equal value for all nutrients and simply divide cost per ton by the total pounds of nutrient per ton (3 x 19 = 57 pounds of nutrients per ton), which would be 2,000 x 0.57 = 1,140 pounds.

Higher fertilizer prices have motivated many producers to consider alternative nutrient sources. Options will depend on product availability and can include manures, composts and municipal biosolids.

Unlike commercial fertilizers, manures and composts do not have a guaranteed analysis that provides nutrient concentrations. Manure nutrient levels can vary substantially due to animal species and feed ration.

Nutrient concentrations in composts will depend on the feedstocks used for their production. And the quality of both products can be affected by storage and handling since nitrogen and potassium can be leached by rainfall on exposed stockpiles.

As a result, producers should have products tested to determine economic value and, in combination with a current soil test, the proper application rate. Most commercial and land-grant university laboratories offer organic product testing.

By knowing the pounds of nutrients per ton in a product, organic materials can be compared to each other as well as to inorganic nutrient sources to determine the best option.

For example, a good-quality chicken litter may be worth $80 per ton or more, assuming 50 percent recovery of the nitrogen and 75 percent recovery of phosphorus and potassium.

Depending on the total per-acre cost for the product plus delivery and spreading, organic materials may be more economical than inorganic fertilizer.

When an organic product is used, inorganic sources often are needed to provide the balance of nutrients (especially supplemental nitrogen) required by a crop during a growing season.

Municipal biosolids are the residuals from wastewater treatment. Class B biosolids receive limited treatment and land application requires a permit that includes routine monitoring of the disposal areas.

Many cities now also produce Class A biosolids which are treated to eliminate pathogens (bacteria, viruses, etc.) and tested to ensure heavy metals (such as arsenic, cadmium, zinc, etc.) are below specified levels.

Class A biosolids have a guaranteed analysis (typically 6-3-0), are approved for use on all major crops and, in fact, are approved for use on lawns and gardens.

With high production costs, there also has been an increase in the marketing of what often are referred to as “non-traditional products.”

These are additives or replacements for standard products that claim to reduce input costs or improve production.

Non-traditional product types include soil conditioners, biological stimulants, microbial enhancers, wetting agents and nutrient sources used in an unconventional manner.

Unfortunately, and unlike traditional products, most of these products have not been subjected to appropriate scientific testing to verify product effectiveness and value.

In the case of many products, little if any research has been conducted for the range of crops and growing conditions for which the products are recommended.

Producers always should request research data that have been generated by an unbiased entity such as a university. And the data should be local or regional so that the product has demonstrated value. Manufacturer claims and user testimonials cannot substitute for sound product research.

Profit margins for forage crops are extremely narrow. Producers must use all the management tools and options at their disposal to stay ahead. Soil testing is a decades-old, time-tested and proven management practice that can give producers an added economic advantage.  FG

Dr. Mark L. McFarland is a Regents Fellow and soil fertility specialist in the Department of Soil and Crop Sciences with Texas A&M Agrilife Extension Service. Click here to reach him or 979-845-5366.

Dr. Larry Redmon is a professor and state forage agronomist with the Department of Soil and Crop Sciences with Texas A&M AgriLife Extension Service. Click here to reach him or 979-845-4826.

Soil testing. Photo courtesy of Matthew Walter