ARTICLE SUMMARY: Standard chemical composition soil tests miss key physical and biological factors improving your soil’s productivity and nutrient delivery, especially as you implement carbon farming practices.
Conventional soil testing has been around for many years. Part of its value is the certainty it provides to growers. Through these tests, we know the soil’s exact chemical composition. These tests can precisely measure phosphorus, potassium, calcium and other nutrients in the soil. Using the old “replacement rate” method, growers could simply apply fertilizer at rates sufficient to bring soil tests up to adequate levels. Generally, this method has produced a yield response. But with rising fertilizer prices and increasing awareness of soil health and nutrient availability in soil, old soil tests face significant limitations in usability going forward.
First, rising fertilizer prices continue to put pressure on farm profitability. Continuing the simple “replacement rate” or maintaining chemical availability levels often call for fertilizer application rates that ruin farm budgets. When viewed in light of increasing insights on soil health and the role of soil biological capacity and physical condition to retain, process, and deliver nutrients to plants, looking only at the chemical side is no longer enough.
What exactly do standard soil tests miss? In short, quite a bit. There are physical and biological components that can significantly impact not only nutrient availability, but also water availability and disease suppression. Physical structure controls moisture, which plays a significant role in biological activity to process nutrients. Moreover, excess moisture (saturation) can result in denitrification. Drainage without plants and roots to hold onto nitrogen will result in leaching. Heavy erosion from poor soil structure will remove the most nutritious soil. On the biological side, microbes decompose old plant tissues and make those nutrients available for absorption into soil particles or used by growing plants. They also play a role in making soil nutrients available for plants. In essence, you can have high soil chemical test results, but without biology, very little of that will matter.
Soil tests will dictate our actions, and often our inactions as well. Using only a soil chemical test will guide us to focus solely on fixing soil chemical limitations or imbalances while ignoring the physical and biological limitations. When we consider the role of soil’s physical conditions to deliver nutrients to plants then we begin to value reduced and no-till practices as well as soil tests, such as infiltration and slake tests, to indicate soil structural health. In essence, no-till makes any fertility program more productive. Similarly, when we consider the role of soil biology to retain, process, and deliver nutrients to cash crops then cover crops and soil biology tests become far more important. After all, why bother applying nutrients if they will not be available to your cash crop—or worse, be lost entirely, costing you money and yield? How profitable is that?
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