Growing for Health: Fertilizer Recommendations

An approach for giving fertilizer recommendations refers to the way conclusions are drawn based on soil tests. Soil-testing labs and crop consultants may give different recommendations based on the same test results if they use different approaches. This may be confusing for growers. There are four basic fertilizer recommendation approaches:

• Buildup and maintenance
• Sufficiency
• Basic cation saturation ratios
• Quantitative approach

The Buildup and Maintenance Approach

The strategy here is to maintain soil fertility for future years. The goal is to apply more nutrients than the crop removes so the nutrient level in the soil is not limiting the yield. The fertilizer recommendations mean applying enough fertilizers to meet the nutrient requirements of the crop and to build up the nutrient level in the soil to a critical soil test level over a planned time frame. The soil test level is maintained at or above the critical level by applying fertilizer rates that replace the nutrients removed by the crop.

The critical soil test level is the soil test level at which near maximum yield is obtained. It is based on yield response curves, which are a result of years of research and trials and are specific to a particular soil, zone and climatic conditions.

In such a curve, a percentage of the yield’s maximum is drawn versus the soil test level for each nutrient. In the buildup and maintenance approach, nutrient availability in the soil is increased over time for future years. In this approach, more fertilizer is used; therefore, the risk of nutrient deficiencies related to their availability in soil is decreased, while profitability in a given year is decreased. This approach poses the risk of over-fertilizing and could have negative impacts on the environment.

The Sufficiency Approach

In the sufficiency approach, fertilizers are only applied to meet the nutrient requirements of the crop. The goal of this approach is to maximize profitability in a given year while minimizing fertilizer applications and costs.

When soil test levels are low, fertilizer rates higher than the nutrient removal of the crop are recommended. When soil test levels are high, reaching the critical soil test level, the recommendation decreases to almost zero.

Most laboratories and universities use the sufficiency approach for their fertilizer recommendations. This approach, along with the buildup and maintenance approach, is used mainly for giving fertilizer recommendations for phosphorus and potassium.

The Basic Cation Saturation Approach

This approach assumes that a specific ratio of cations (positively charged ions) must exist in the soil in order to achieve maximum yield. It is concerned only with recommendations for calcium, magnesium and potassium.

According to this concept, the proportion of cations should be 65 to 85% calcium, 6 to 12% magnesium and 2 to 5% potassium. This approach is under debate and has many drawbacks, for example, it cannot be used on sandy soils, since sandy soils hold a small amount of cations.

In calcareous soils, the calcium-potassium ratio may be high, resulting in a too-high recommendation for potassium applications. In many field experiments, no correlation was found between the suggested ratios and crop yields.

In many experiments, leaf analysis showed no correlation between the suggested ratios and the level of calcium, magnesium and potassium in the leaves.

The Quantitative Approach

The quantitative approach uses the soil test values as the actual amounts of nutrients available in soil. The amount of nutrients that have to be applied is the difference between the total nutrient requirement of the crop and the amount of the nutrient available in the soil layer that was tested.

This amount is adjusted by dividing it by an efficiency factor (<1), which relates to the efficiency of the fertilizer application and the efficiency of the plant roots in nutrient uptake.

This approach should be used with caution since soil test results are mostly empirical and do not describe the actual amount of available nutrients. However, this approach is easy to understand and implement; therefore, it is commonly used by many crop consultants and agronomists.