From May to July of this year, 13 inches of rain fell at the Northwest Research & Outreach Center in Crookston — about 3.5 inches greater than the 30-year average. What started out as a near-normal season for rain took a sudden turn with seven inches of rain in July. Quite literally, when it rained, it poured.
Many factors play into whether heavy rains lead to nutrient loss. Some of these factors are how much it rained, what soil type you have, what crop you grew, how much fertilizer you applied, how quickly it rained after you applied the fertilizer, and whether you have tile (or irrigation). One of the key factors that determines if water will become soil moisture or runoff is rainfall intensity. The soil can only absorb water so fast. This is the soil’s infiltration rate. If rainfall intensity exceeds the soil’s infiltration rate, you get ponding or runoff.
Infiltration rate depends on both the soil’s physical properties (texture, bulk density, etc.) and its moisture content. Courser-textured soils absorb water faster than finer-textured soils. Drier soils absorb water faster than wetter ones. Less compacted soils absorb water faster than more compacted ones. In general, if soil moisture is high and you get a high-intensity rainfall event, then conditions are right for surface runoff.
By my estimation, from May through July rainfall intensity in the Valley likely exceeded infiltration rate on 6 days for sandier soils and up to 21 days on our clayey soils. That means between 6 and 21 chances for ponding and surface runoff. Nutrient loss in surface runoff is more an environmental concern than an agronomic concern. It can damage crops and cause erosion, but it does not carry nutrients away in the same way that leaching does. Surface runoff does not carry much nitrogen with it, but it can carry a lot of sediment-bound phosphorus downstream. Although phosphorus binds tightly to our soil in the field, this can change once it moves into a stream, the pH changes, and it is exposed to different biological drivers.
Once rain makes it into the soil, then nutrient loss from leaching becomes a concern. Rainfall is one of the strongest predictors of nutrient movement into shallow groundwater (or tile drain flow). Rainfall tends to have a stronger influence on nutrient leaching than either fertilizer management or soil texture. This holds true whether you are looking at nitrogen or phosphorus. For nitrogen, the soil moisture that comes with rainfall drives the mineralization process. For phosphorus, we see pulses of phosphorus loss shortly following rainfall. This can temporarily increase nutrient availability for the crop, but nutrients will move along with water as it leaches away.
Although we cannot control the weather, we can try different strategies to improve our chances of keeping nutrients in the field. Managing water with controlled drainage is one option. Testing out cover crops or reduced tillage to see if those help improve soil structure is another strategy. We may not have any silver bullets, but we do have options. That is not to say that it is not important (and economical) to optimize soil fertility. It simply means that when we are looking at soil as a system, fertilizer management is only one aspect keeping nutrients in the field.