How to Understand Soil Fertility

Summary

This presentation explores soil fertility and health, with a focus on small grains such as wheat, barley, and oats, as well as rotational adjustments following alfalfa. It explains the process of developing nutrient recommendations, based on rate response curves and critical nutrient levels, and compares recommendations from Utah State University and commercial labs. The session also discusses the importance of understanding soil test results and adjusting fertilizer applications based on crop types, yield goals, and nitrogen credits from previous crops. Recent research on nitrogen needs following alfalfa is also highlighted, with specific recommendations for forage and grain crops.

Takeaways

• 😀 The script focuses on soil fertility and health, specifically related to small grains like wheat, barley, and oats, as well as forage crops.
• 😀 Recommendations for soil nutrient levels are developed through research, with the goal of achieving maximum crop yields at an economically optimal level.
• 😀 A rate response curve is used to determine critical nutrient levels, where increasing nutrient concentration in the soil results in increased crop yield until it plateaus.
• 😀 Public universities typically recommend nutrient levels closer to the economically optimal range, whereas commercial labs may suggest slightly higher levels to ensure maximum yields.
• 😀 Soil test results vary due to factors like weather, pests, and soil conditions, leading to different recommendations for the same crop across different years.
• 😀 Soil test reports from universities provide a detailed guide to understanding nutrient requirements and offer ranges for nutrient application based on test levels.
• 😀 For wheat, nitrogen recommendations can range from 140 to 170 pounds per acre, depending on the yield goal, with dryland grains needing about one-third to one-half of the nutrient requirement compared to irrigated crops.
• 😀 Following an alfalfa crop in rotation may require nitrogen adjustments, as about 100 units of nitrogen are credited from the previous alfalfa stand, but this needs to be adjusted based on actual soil nitrogen levels.
• 😀 Recent studies show that nitrogen responses vary based on the initial soil nitrate level, with responses generally observed only if nitrate levels are below 17 parts per million.
• 😀 For forage grains following alfalfa, only older stands (greater than 10 years) are responsive to additional nitrogen applications, which aligns with findings from similar studies across the country.

Q & A

What is the main focus of the presentation?

The main focus of the presentation is soil fertility, specifically as it relates to small grains used for both grain and forage crops, along with an overview of fertilizer recommendations and adjustments, especially after alfalfa.

How are fertilizer recommendations developed?

Fertilizer recommendations are developed using a rate response curve, which relates soil nutrient levels to crop yield. The curve helps identify the critical nutrient levels where additional nutrients no longer significantly increase crop yield, and recommendations are based on these critical levels.

What is the difference between university and commercial lab fertilizer recommendations?

University recommendations tend to be more conservative, targeting the economic optimum for cost-effectiveness, while commercial labs often recommend slightly higher nutrient levels to ensure maximum yield, regardless of the cost.

Why do fertilizer recommendations vary based on different years and conditions?

Fertilizer recommendations vary due to factors like weather, pest pressures, soil differences, and other environmental conditions, which can result in slight variations in the rate response curves from year to year.

What is the significance of the critical nutrient levels on the rate response curve?

The critical nutrient levels on the rate response curve represent the point where further increases in nutrient application no longer lead to significant yield improvements, helping to avoid unnecessary fertilization and expenses.

What is the economic optimal level for nutrients, and how is it determined?

The economic optimal level is the amount of a nutrient that maximizes yield without leading to unnecessary costs. It is determined by identifying the point on the rate response curve where nutrient application produces near-maximum yield but does not exceed a cost that outweighs the benefits.

How do soil test results affect fertilizer recommendations?

Soil test results are used to assess the nutrient levels in the soil, which then guides the amount of fertilizer recommended. Based on the soil test levels, a range of nutrient applications is provided, with recommendations adjusting based on whether the nutrient levels are low or high.

Why do commercial laboratories often recommend higher fertilizer amounts than universities?

Commercial laboratories tend to recommend higher fertilizer amounts to ensure that crops receive sufficient nutrients for maximum yield, which may appeal to farmers who want to avoid any risk of under-fertilization, even if it leads to higher costs.

What role does alfalfa play in nitrogen recommendations for rotational crops?

Alfalfa contributes to nitrogen levels in the soil through its nitrogen-fixing ability. Following an alfalfa crop, nitrogen requirements for small grains are often reduced due to residual nitrogen left in the soil, leading to the need for fewer nitrogen applications.

What recent research findings were highlighted regarding nitrogen application after alfalfa?

Recent research shows that nitrogen application is often unnecessary for small grains following alfalfa if soil nitrate levels are higher than 17 parts per million. Only about half of the sites tested showed a response to additional nitrogen, especially if the nitrate levels were lower.