Healthy Dose

Phosphorus and soil health

Published online: Jan 01, 2017 Fertilizer Tom Bruulsema, International Plant Nutrition Institute
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This article originally appeared in the International Plant Nutrition Institute’s Plant Nutrition Today newsletter in October 2016. It also appears in the January 2017 issue of Potato Grower

Soil health has elicited interest from many. The idea of looking at the soil as a living system with physical, chemical and biological aspects of its functioning engages imaginations of crop producers and consumers alike. Policymakers and extension educators have particularly locked onto soil health as a key attribute in reduction strategies for phosphorus loss. Are their expectations warranted?

The importance of maintaining good physical structure in soil is well-known. Producers and soil experts have long appreciated the importance of a soil’s capacities to let rainwater enter the profile, to retain that water in a tension range available to plants, and to remain in aggregate forms resistant to erosion. While these properties are not new, they are still important to managing phosphorus loss—and good crop yields. Major practices influencing the physical attributes of the soil include returning adequate amounts of crop residue to the soil, managing tillage to conserve soil organic matter, and avoiding compaction by staying off the land when it is too wet.

The chemical aspect of soil health has also been valued for a long time. Producers sample soils to test for the availability of a wide array of chemical constituents of the soil. A healthy soil needs levels of nutrients that support the full potential of plant growth, since plants are the primary producers of the organic materials that feed the biology of the soil. Soil bacteria, fungi, protozoa, nematodes, arthropods and earthworms all feed on material derived from plants.

The biological aspect is likely the “newest” area of soil health. Tests that measure how rapidly a soil emits carbon dioxide give an indication of how much activity the available organic materials in a soil can support. Such tests, however, need to be interpreted with care. Owing to the very biodegradability being measured by the test, such materials do not last long in the soil, and the test would be expected to give very different results depending on the composition of crop residues and the weather conditions encountered since the last additions of fresh organic matter.

In biological nutrient cycling, one organism dies and another feeds on its contents. A fungus secretes enzymes that break down old plant tissue, bacteria in turn digest the fungus, and higher organisms feed on them both, in turn excreting unneeded nutrients. Each time, carbon dioxide is released, total organic matter declines, and the amounts of each nutrient in the mineral form increase. Membranes and cell walls of microbial organisms are disrupted, and the contents of the cells—including the soluble phosphate stored in the vacuole—are often released into the soil. Each time a nutrient cycles, it goes through release as well as re-absorption. Some of it is prone to loss before another organism can take it up. It can’t be assumed that more nutrient cycling, on its own, means less nutrient loss. Synchrony of release with re-absorption is likely the most important attribute for minimizing nutrient loss.

Soil health is a concept worthy of attention. The practices it encourages—cover crops, conservation tillage, crop rotation and more—go a long way toward preventing soil degradation. These practices are essential to averting the worst cases of phosphorus loss. Some of the latest loading reduction targets, however—particularly those aimed at reducing losses of dissolved phosphate—may not be met through soil health alone, and definitely require attention to nutrient application placement and timing as well.

Many studies, dating back decades, document that in conservation tillage systems—as the top inch or two of soil enriches in soil organic matter, biological activity and nutrients—concentrations and often loads of dissolved phosphorus in runoff increase. Soil conservation should be practiced with care. It can’t be used as an excuse to leave phosphorus fertilizer or manure on the surface of the soil. The optimum combination of tillage and fertilizer placement practices is likely to be specific to soil and landscape, and in many areas, requires continuing research.