AGRONOMIC NEWS ITEMS
From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335
Summer 1997, No. 5
TOPOGRAPHY AND PLANT NUTRITION
Topography...the lay of the land...influences the nutrition of crop plants in many ways. Farmers have recognized this influence for a long time. Only recently have the tools of the global positioning system (GPS) and geographic information systems (GIS) become available to map and manage topography in a systematic way. To use the tools correctly, we need to understand how topography impacts crop plant nutrition.
Often, topsoils differ from the lower soil horizons in chemical properties. On soils which develop from calcareous parent materials, the lower horizons contain more calcium carbonate (lime) than the topsoil. Thus, as erosion moves topsoil from the hilltop toward the valley, whether by rain, wind or by tillage translocation, the pH of the hilltops may become higher than that of the valleys. The opposite pattern can also be found. In many rolling landscapes, the hilltops and knolls have sandier soil than the lower lying areas. Since sandy soil has less buffering capacity, the pH tends to decline more rapidly with time, owing to the acidifying effects of rain and nitrogen fertilizer applications. So the pH on hills, and the accompanying levels of calcium, magnesium and base saturation, can be either higher or lower than that of the lower land.
Topography influences the way water moves into the soil. Flat areas that catch runoff tend to be ponded with standing water after heavy rains, while knolls shed excess water and are first to become drought stressed in dry periods. The water status of the soil impacts the chemistry of each of the primary plant nutrients:
Even on the fields that appear level to the eye, minute differences in elevation show after a heavy rain. Some areas of the field retain water for a longer time than others. Each year produces different patterns of ponding at different times in the growing season. These variations work together to produce a complex pattern of nutrient availability across the field. In addition, past nutrient and manure applications, previous grazing patterns, old fencerows and various mishaps in the field's management history have added to the spatial variability.
- Nitrogen, a mobile nutrient, becomes more susceptible to loss when the soil becomes wet. Wet soil conditions promote two major loss mechanisms: leaching and denitrification.
- Phosphorus tends to become more soluble when soil is flooded, but quickly becomes less available when the soil dries out. However, when soils are flooded or saturated with water, root growth and activity slows down. Thus, for most crops, phosphorus uptake is reduced by water ponding. Furthermore, flooding can reduce or destroy the mycorrhizal activity in the soil, which can make phosphorus less available for long periods following the flooding.
- Potassium, particularly in reduced tillage soils, can become fixed by clay particles due to the reduction of iron when the soil becomes flooded. This fixation is not readily reversed when the soil dries out again.
Intensive soil sampling often reflects the variability in levels of available nutrients, but it can be difficult to relate variability to the soil type, the crop yield pattern or the topography. If we can map only part of the variability, we will capture only part of the benefit of managing crop plant nutrition on a site-specific basis. To make the successful plant nutrient map, all sources contributing to the variability need to be considered.
For more information, contact Dr. Tom W. Bruulsema, Director, Eastern Canada and Northeast U.S., PPI, 18 Maplewood Drive, Guelph, Ontario N1G 1L8, Canada. Phone: (519) 821-5519; E-mail: firstname.lastname@example.org.
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