3 Studies on How Fertility Impacts Vegetable Production
Several intriguing research projects on soil fertility were published this past year. Two were direct research projects, one measuring how 15 years of manure applications affects production, and a second looking at ways to make crop wastes a more-effective source of nutrients. A third study was an extensive analysis of 30 years of research on how fertilizer interacts with weeds, crops, and water.
How Different Crops Respond to Long-Term Manure Applications
Back in 1999, West Virginia University began a long-term organic farming study. A team of researchers published the results of the impact of long-term manure applications in “Long-term Effect of Manure Application in a Certified Organic Production System on Soil Physical and Chemical Parameters and Vegetable Yields”
On 32 16-by-25-foot plots, researchers applied high inputs to half of those plots, and low inputs to the other half. The location of the plots was randomized to help reduce the impact of environmental factors.
Annually since 1999, the high-input plots received 10 tons per acre of dairy manure. They also had a cover crop of rye-vetch (Secale cereale and Vicia villosa) each winter season.
The low-input plots, meanwhile, were given an annual rye-vetch cover crop.
Both test groups were given a mulch of 5 tons per acre of mixed species hay for two of every four years.
Researchers rotated several crops on a four-year rotation, and selected green beans, zucchini, tomato, green pepper, and lettuce to represent the Fabaceae, Cucurbitaceae, Solanaceae, and Asteraceae families.
Here’s what they learned:
Soil organic matter (SOM). Researchers tested the upper 6 inches of soil in both groups. In low-input plots, SOM remained virtually unchanged between 1999 and 2014. SOM started at 4.4% in 1999, increased to 5.2% in 2004, and didn’t change much in 2014, when SOM measured at 5.4%.
Meanwhile, high-put lots saw significant increases during this same time period. Its SOM measured 6.4% by 2004, and continued to increase to 8.7% in 2014.
Bulk density. High-input plots had lower bulk density when compared to low-input plots in 2014. Despite these improvements in soil quality, high-input plots showed very high levels of phosphorus and potassium.
Yields. The most important factor of the study was how inputs affected yield. High-input plots saw an overall yield increase of 22% over low-input plots. But it’s important to note that each crop had a different yield unique:
- Tomato: 9% higher
- Pepper: 25% higher
- Zucchini: 24% higher
- Green bean: 24% higher
“Manure application in addition to green manures and hay mulch incorporation was found to result in significant economic returns,” the researchers said in their report.
Fertilizer as a Form of Weed Control
With the study, “Understanding Crop-Weed-Fertilizer-Water Interactions and Their Implications for Weed Management in Agricultural Systems,” Indian and Australian researchers reviewed existing research on how fertilizers affect and are affected by crops, weeds, and water.
One section of the resulting paper examined an intriguing premise: Can you control weeds with fertilizer? Research indicates that, yes, you can. The methods fell into a few categories:
Timing. A plant’s stage of development affects how much nitrogen it can uptake. So timing nitrogen applications when crops are at an early stage, when they are much more likely to use nitrogen, gives them a boost.
“The timing of fertilizer application may be adjusted to reduce the extraction of nutrients by weeds; thus, minimizing the associated reduction in the crop yields. The time and method of fertilizer application are of utmost importance in managing weeds,” the researchers wrote in their study, published in the journal Science Direct.
Timing fertilizer applications to when it’s most beneficial to crops can give them the boost they need during the critical period of competition between crops and weeds.
Application Methods. In their review of previous studies, the research team found that how fertilizer is applied has “more profound and consistent effects than application timing.”
Annual weeds are more likely to germinate from the soil surface, so surface broadcast fertilizers benefit weeds along with crops. Soil surface fertility impacts how many weeds emerge. In contract, banding fertilizer can reduce weed resources.
Which crops and which weeds are involved have a strong impact, as well, however. Taproot weeds, for example, pull nutrients from a lower soil layer
Soil Types. Both finely textured soils and those with high levels of organic matter can make soil-applied herbicide less available, and so a less effective weed control. Likewise, in less adsorptive soils, leaching losses of applied herbicide is high, which also makes herbicides less effective. Nitrogen can act as an adjuvant to enhance herbicide efficacy, helping herbicide absorption and movement into the leaf tissue.
How Difference Crop Waste Methods Affect Fertility
A research team from Belgium and Denmark took a look at how different methods of processing vegetable crop wastes impact soil carbon and nitrogen.
It may seem counterintuitive, but leaving nitrogen-rich vegetable crop residues in the field after harvest may result nitrogen losses during autumn. A better method is to remove the unharvested crop remains and conserve them in order to reuse in the field. Doing so can improve recycling of nutrients. For that to happen, the researchers found, some form of processing is required.
In their study, “Co-Ensiling, Co-Composting and Anaerobic Co-Digestion of Vegetable Crop Residues: Product Stability and Effect on Soil Carbon and Nitrogen Dynamics,” the team compared three conservation and valorization methods — co-ensiling, co-composting, and anaerobic co-digestion — for fresh crop residues.
The team also measured product quality and stability and the short-term effects of applying these silages, composts, and digestates on soil carbon and nitrogen mineralization and N2O emissions.
Ensiling resulted in highly biodegradable products with a low pH (between 4.2 and 5.2) and more ammonium nitrogen (NH4+N) compared to composts. As a result, there was higher net carbon mineralization, microbial biomass carbon), and a temporary nitrogen immobilization.
Digestates and composts led to lower carbon mineralization rates and microbial biomass carbon. And there was no net nitrogen immobilization nor mineralization. Applying digestates resulted in high mineral nitrogen content and a decrease of the soil pH.
In all three treatments, short-term N2O losses after soil application were very small.
“Growers can choose the most appropriate treatment option and application moment and location, depending on the local soil and crop requirements and the on-farm facilities,” the researchers said in their study.