Moisture levels
To understand how our current weather patterns can impact on our garden soil, first we need to look at the role that soil moisture might play. After our gardens/lawns end their season (either harvest or dormancy), soil moisture recharge becomes a significant issue as the plants have used soil moisture throughout the growing season. When plant growth ends/subsides, the goal is to have any precipitation go into the soil and fill soil pores.
Fall rains can be very effective for recharging soil moisture. However, as soon as the soil freezes, most additional precipitation (rain or snow) does not enter the soil, and the probability of this moisture running off the soil is high, usually during the late winter and/or spring thaw. In areas that received sufficient rainfall, the amount of precipitation entering the soil has been much greater due to the record-setting winter warmth and minimal frost conditions. In drier regions of the state, these comments only pertain to the potential for soil moisture recharge.
Fertility: microbes and nutrients
How can such a mild winter affect soil fertility? While temperature influences soil physical and chemical properties, its effect is relatively insignificant. However, biological properties, and processes, are greatly affected by temperature. Consider all the microbes that live in our soils and all the work they do in decomposing organic materials (plant residue, compost, manures, leaves, etc.) that are added to the soil. As a rule of thumb, for every 10-degree C (18°F) increase in temperature, microbial activity doubles! As there is essentially no activity in frozen soil, this winter has been beneficial for those microbes in our soils (and composting piles and containers)!
So, the microbes were more active in the soil and decomposed more the past few months than normal. This results in more nutrients being available for plants to take up in the next growing season. Keep in mind that the nutrients of most significance in organic material decomposition from a soil fertility perspective are nitrogen (N), phosphorus (P) and sulfur (S). Therefore, all the nutrients that come from the existing soil organic matter and/or added organic amendments will all have some extra, positive effect this growing season – assuming that they are not lost!
The delicate balance of N
The primary concern with nutrient loss is N, especially when organic amendments or fertilizer is added in the fall. More specifically, as organic N transforms into plant-available N, the form is ammonium N, which is fairly stable in the soil. However, when soil temperatures rise above 50°F, microbes in the soil can easily convert this ammonium N to nitrate N, a form of N that is mobile in the soil and can be lost through leaching with abundant spring rainfall or via denitrification later this season if soil get saturated for an extended period of time. Thus, a warm winter will likely result in somewhat higher quantities of nitrate N in our soils this spring.
This situation can lead to greater pollution potential from soils to our waters. High quantities of nitrate N in the soil always have the potential to be lost depending on the soil and climatic conditions. The presence of elevated nitrates in the spring has greater potential environmental impact as plant uptake is minimal, and spring usually has significant rainfall that can cause leaching from soils.
Turf vs. gardens: different N scenarios
Nitrogen management is more of a concern with turf than with a garden this spring. It is commonly recommended to add fertilizer N in the fall with turf, and with the warmer soils this past season, there was more time for the N to convert to nitrate N by the spring-rain season. The fertilizer inorganic N and the conversion to nitrate N is more rapid with warmer soils. Therefore, significant nitrate will probably be present in the spring – and there is potential to lose this nitrate N. Fortunately, lawns start growing more rapidly and, this spring, will uptake the nitrate N much earlier. For gardens, adding compost, manures and other organic matter in the fall is not as concerning, compared to fertilizer, because these organic N sources must be decomposed first.
While the overall load of nitrate leaching to water systems is relatively low in both scenarios, everyone should manage N to lessen any potential loss of N. While this year’s weather may be increasing nitrate N in the soil, the most prudent management practice we can do on a regular basis is to not overapply N on our lawns or in our gardens. Follow the recommended guidelines.
Spring ahead with caution
With record-high temps at the time of this writing in March, it is tempting to be out planting early this spring. Soil temperatures far above normal might suggest that seeds can be planted with acceptable germination. However, planting seeds and transplants this season should still focus on average last-frost dates rather than soil temperatures. Early planting could result in some early growth, but many of our favorite garden plants are temperature sensitive and can be cut short by frosts later this spring. And we have yet to see what April will bring.
Thanks to Michael Schmitt, Associate Dean of Extension and the College of Food, Agricultural and Natural Resource Sciences and Professor; Carl Rosen, Professor; and Heidi Roop, Assistant Professor and Director of the Minnesota Climate Adaptation Partnership – all Extension Specialists in the Department of Soil, Water, and Climate at the University of Minnesota.