Dr. Paul's Research Perspectives
Healthy soils resist typical N and P losses during wet seasons
Chemical fertilizer is lost from conventional fields in moist years, while compost-amended organic soil mitigates leaching and air-borne denitrification.

By Paul Hepperly, PhD, and Christine Ziegler Ulsh

Posted October 12, 2006: Anyone traveling the roads of Berks and Lehigh counties in eastern Pennsylvania this summer saw yellow-splotched corn leaves abounded, particularly in wet spots. These chlorotic (abnormally colored) plants are generally deficient in nitrogen. They usually show yellowing and die back in a V-shaped pattern in the tips of the lower leaves.

A very wet summer with heavy rains can cause nitrogen (N) deficiency in corn. Excess water washes the nitrogen out of the soil in a process called leaching. The overly moist conditions water-log soil bacteria, starving them for oxygen and causing them to scavenge oxygen from soil nitrate. As a side effect, these scavenging bacteria break down the nitrate molecules, releasing nitrogen into the air as nitrogen gas in a process called de-nitrification.

Loss of N to soil leaching and denitrification into the air is not only harmful to the environment, but also causes direct economic losses to farmers by reducing the corn’s yield, protein content and nutritional quality.

In 2004 and 2006, many areas of Pennsylvania experienced very wet conditions. During these years we found that chemical nitrogen fertilizer, applied at the recommended rate, was almost entirely removed from the soils (both by uptake and leaching) in our research fields by mid-season, allowing the corn to develop chlorosis later in the season. However, corn in our organically managed research and production fields, which depends on slower-released compost and plant-based nitrogen sources, remained green and healthy throughout the growing season and developed no chlorosis.

More N not the answer

Although nitrogen deficiency is a vexing problem, it isn’t easily or well solved by increasing nitrogen application. While extra nitrogen generates beautiful blue-green plants and high corn yields under optimized conditions, it can also lead to excessive soil nitrate that, when not used by the crop, leaches into the groundwater supply. Also, the skyrocketing price of chemical N fertilizers (driven by the price of the natural gas that is needed to make it) makes the idea of adding excess nitrogen even less economical or appealing.

Another fertilization mantra is, “the more soluble, the better.” Fast release of nutrients was believed to be the key to optimal growth and high yield. However, with increased fertilizer cost and more emphasis on environmental health, farmers (both conventional and organic) may need to rethink this doctrine.

In The Rodale Institute’s Farming Systems Trial (FST), the organic corn receives fertility through less soluble, slower released compost- or legume-based N. The FST organic corn does not always start with as quick a flush of growth as the conventional corn, but, by mid-season, all growth differences usually vanish. In the end, corn health and yield in the organic systems are equal to or better than they are in the conventional systems (rather like the tortoise and the hare).

The Rodale Institute’s Compost Utilization Trial (CUT) also showed that fields with fertility from manures and compost can equal the crop yields from synthetic commercial fertilizers. Unlike synthetic commercial fertilizer, composts and manures build the level of organic matter, biodiversity and nutrients in soil, and composts also significantly reduce nitrate losses. Soils with higher organic matter content retain and provide nitrogen to corn crops in a slower, more sustained and efficient manner.

Soil scientists have long contended that soil organic matter is unstable unless it is combined with clay colloids. We agree that combining organic matter with clay, a process called soil aggregation, is vital to improve soil texture and better trap nutrients (including minerals), air and water, making them available to plant roots.

We have designed our studies, supported by the Pennsylvania Department of Environmental Protection, to test this aggregation hypothesis. (See Good compost made better for more details on the study.) By accelerating the aggregation process in composting mixtures, we mimic the natural processes that promote soil integrity and improve nutrient and water retention. These soil quality improvements, in turn, increase crop yield and quality.

Our enhanced animal waste bioconversion (composting) process combines humic materials (such as humic acid) with clay colloids from our farm subsoil, using calcium as a “mortaring” agent. Both soil organic matter and clay colloids are negatively-charged ions that repel each other under normal conditions. However, these ions can be bound together by the action of positively-charged cations such as calcium, aluminum or iron. In our trials, we’re using calcium in the form of soluble gypsum to promote soil bonding and aggregation without increasing soil pH.

During the compost process, microbial mucus and gums further bind soil particles together, completing the development of stable soil aggregates. In addition to retaining nutrients, stable soil aggregations stand firm against the destructive dispersing action of water and wind.

Effective research can help farmers and policy makers better understand and manage the “bio-geo-chemical” processes of soil aggregation. Farmers can use this information to accelerate soil development and improve the environment through soil sequestration of carbon and other agricultural nutrients.

We are testing our aggregation hypothesis by comparing the nutrient content and nutrient run-off of: 1) amended animal manure/leaf compost mixtures, 2) standard manure/leaf compost mixtures (non-amended) and 3) aged manure alone, taking readings both on the compost pile and in field application. Two rounds of compost were produced, one based on poultry broiler litter and the other based on dairy manure. Data from field growth and yield of crops fertilized by these composts are also being measured.

NPK ratios for all the compost and manure treatments in 2006 field test, taken in April 2006, shortly before the time of application. (The ratios of the poultry manure fell from the time of its finishing in the fall of 2005, due to uncovered storage over winter.)

Beating E. coli, saving time

Results are very encouraging. In both poultry-litter- and dairy-manure-based composts, we have measured reduction and elimination of the bacterial pathogens E. coli and fecal coliform in both the standard and amended composts, when compared to the aged manure. Both mixed composts converted inorganic nitrogen salts into organic nitrogen forms, a process that eliminates the bacterial pathogens by reducing their preferred food supply (the inorganic nitrogen). Better still, the amended compost completed this conversion about six weeks sooner than the standard mixture, thanks to the aggregation process.

The finished compost mixtures also developed an improved nitrogen (N) to phosphorus (P) ratio (almost 2:1) when compared to the aged manure. This N:P ratio allows a crop’s N requirements to be satisfied by compost without overloading soils with too much P. This reduces the potential for loss of excess P into the environment—a significant problem in livestock-intensive growing regions with insufficient cropland to receive the manure.

At the time of field application, the N:P ratios were varied among the different composts, and the poultry manure compost that had been stored over-winter had lost N. We suspect these N losses would have been reduced if we had covered the compost in storage. However, to ensure the best N:P ratio for the crop, composts should be used as soon as possible after finishing.

Our DEP project corn fields were planted in late May and are growing well. We are seeing differences among the compost plots, with the conventionally fertilized corn standing tallest and most green and some of the compost plots showing stunted growth and nitrogen deficiency. However, we will not get the full picture of the differences (or lack of differences) among these plots until we measure the corn yields, stalk and grain N, and the amount of N and P in the soils and soil water. We will report again as these data come in later this fall.