We are getting prepared for our second year of a three-year project evaluating a warm-season legume cover crop between winter small grain crops. We are conducting the trial in a commercial field on Staten Island in the Delta. We are comparing soil health characteristics, greenhouse gas emissions, and grain yields between the cover crop treatment and the standard dry fallow. While cover cropping, particularly in the warm-season, is not a typical management practice in the annual crop rotations of the Delta, it is a management practice identified in the Healthy Soils Program of the California Department of Food and Agriculture as having the potential to improve soil health, sequester carbon, and reduce greenhouse gas emissions. This article describes the soil results from the first year of cover cropping (2018 season).

Methods: The trial is a randomized complete block design (approximately 4.5 acres) with three replicates of each treatment. The soil type across the trial is a Valdez silt loam. Baseline soil samples were collected in July 2018 following wheat harvest but prior to tillage. Soil was sampled from 0-6, 6-12, 12-24, and 24-36 inch depths. On July 30, 2018, a cowpea cover crop (Vigna unguiculata cv. ‘Red Ripper', Figure 1) was inoculated with Rhizobium and planted after a pre-irrigation. Pre-irrigation was only applied to the cover crop plots. The cover crop was drill-seeded at 7-in row spacing with a planting density of approximately 50 pounds of seed per acre. A second irrigation was applied approximately one month after planting. End-of-season soil sampling (0-6 and 6-12 inch depths) occurred on October 23, 2018, prior to cover crop termination. Soil properties of interest include bulk density, soil moisture, salinity, pH, total nitrogen (N), and total carbon (C). Soil properties were analyzed by the following methods: pH from the soil saturated paste, salinity by the saturated paste extract, and total N and C by combustion method.

Preliminary Results: Soil properties are presented for the baseline condition (Table 1) and for the end of the first cover cropping season (Table 2). Bulk density averaged 1.0 g/cm³ across sample timings, depths, and treatments. Soil moisture (% by volume) was observed to increase from the baseline condition in the cover crop (“CC”) treatment. At baseline sampling, salinity increased with depth from 0.47 to 2.44 dS/m. After one cover cropping season, salinity increased in both treatments, but increased more in the no cover crop (“No CC”) treatment, averaging 1.22 dS/m from 0 to 12 inches. Soil was acidic, which is typical for the region. The pH averaged 5.5 across sample timings, depths, and treatments, but there may be a trend for cover cropping to increase the pH. Total N and C decreased with depth at the baseline sampling. After one cover cropping season, there was little change from the baseline condition in both treatments.

Summary: The Delta is a unique agricultural region with unique environmental challenges. Some soils in the region are subsided due to oxidation of organic matter, and some soils suffer from salinity, having limited ability to leach salts due to low permeability soils and shallow groundwater. Cover cropping is not a typical practice in the annual crop rotations of the region, and summer cover cropping is particularly rare. After the first year of a three-year study, cover cropping had no observed effect on bulk density, Total N, and Total C. Cover cropping may have slightly raised the pH in the top 12 inches, compared to dry fallow. The cover crop treatment, having received two irrigations, had lower salinity in the upper layers of soil compared to dry fallow. We also observed that the 2018-2019 triticale crop that was planted in the field germinated roughly five days earlier in the cover crop plots compared to the fallowed plots. Thus, it appears that summer cover cropping with a legume has the potential to improve soil tilth at a time of year when the field would otherwise be fallowed and dry with no soil cover, and there could be agronomic benefits to subsequent crops. We will continue to monitor these soil properties in 2019 and 2020, and additionally, we will monitor small grain yields and greenhouse gas (CH₄, N₂O) emissions.

We would like to thank Dawit Zeleke and Morgan Johnson (Staten Island), Tom Johnson (Kamprath Seed), and Margaret Smither-Kopperl and Valerie Bullard (USDA-NRCS) for their cooperation on this trial. We would like to acknowledge the California Climate Investments program for funding, and our UC colleagues who are cooperating on this grant in other parts of the state (Jeff Mitchell, Will Horwath, Veronica Romero, Sarah Light, Amber Vinchesi-Vahl, and Scott Stoddard).

Survey: We would also like to alert readers of a cover cropping survey that is being conducted by the Contra Costa County Resource Conservation District. The survey is found here. The purpose of the survey is to learn more about cover cropping practices and barriers to adopting cover cropping on-farm. Even if you farm in another county, please consider filling out the survey, which should take about 10 minutes. The survey is open through the end of June. Your responses will help inform CCC RCD and UCCE programming. Thank you for your participation.

We are now in the second year of a project investigating how to estimate nitrogen (N) mineralization in mineral and organic soils. Nitrogen mineralization is the conversion of organic forms of N, which are not plant-available, to inorganic forms of N which are plant-available, like ammonium and nitrate. Mineralization occurs as a result of microbial decomposition of nutrients. Understanding N mineralization is important because it can help us apply N fertilizers more efficiently, by accounting for soil-available nutrients. Due to high groundwater nitrate concentrations, California growers are facing increasing pressure to improve N use efficiency in an effort to reduce nitrate leaching. To maintain competitive yields, however, growers need accurate estimates of soil-available N so that they can adjust fertilizer application rates with confidence.

In Spring 2016, the project team collected soil samples from 30 fields from Tulelake to Fresno County, including five sites in the Delta (having organic soils) and four other sites in San Joaquin County (having mineral soils). All of the fields were in annual crop rotations and had no recent legume cover crops or manure applications. The Delta soils had organic matter that ranged from about 6 to 23 percent; whereas, the soils from other areas of San Joaquin County had about 1.5 to 2 percent soil organic matter (SOM). The bulk density (i.e. the mass divided by volume) of the Delta soils averaged 0.9 g/cm³ compared to the mineral soils, which averaged 1.2 g/cm³. The reason it is important to measure the bulk density is because when soil nutrients are measured, they need to be converted from concentration to lbs/acre per foot of soil depth using a conversion factor. That factor will change depending on the bulk density.

In general, N mineralization was higher in the organic soils than the mineral soils, but it also varied more across the organic soils (Figure 1). When N mineralization is expressed as a percent of total soil N, however, mineral soils were more variable. This is likely due to the fact that the SOM is more stable in Delta soils than in mineral soils, where SOM is largely derived from recent crop residues. In other words, crop residues influence N mineralization more in mineral soils than in organic soils. 

 

 

 

 

 

 

 

 

Figure 1. Net N mineralization rates of the 30 soils included in the study in 2016.

Preliminary results also show that soil temperature and other soil properties have a strong effect on N mineralization. The soil temperature effect has been modeled to show that as temperature increases, N mineralization increases exponentially. The soil properties which most influenced N mineralization included total soil N, total soil carbon (C), particulate organic C (a measure of the availability of organic matter to microbial decomposition), and pH. These soil variables are more predictive of N mineralization in organic soils than in mineral soils; more work is needed to determine which soil variables best help to predict N mineralization in mineral soils. Soil moisture likely also plays a role in N mineralization, and it will be studied in the future.

We are continuing the study again in 2017 and hope that the results will contribute to a better understanding of N mineralization in both organic and mineral soils, with the ultimate goal of developing an online decision-support tool for growers to help in estimating field-specific N mineralization rates.

A consultant recently brought in some alfalfa plants to get another opinion on why growth was resuming slowly this spring. The field is located in Merced County and is a sandy loam soil. The field is in its fifth year, is glyphosate-tolerant, and has traditionally produced high-quality hay. About 5 acres of a 40-acre field are affected.

There are many reasons why growth may resume slowly this spring. The obvious reason, of course, is that we received a lot of rain this winter. With that rain has come associated problems from cool, anoxic (lack of oxygen) soil conditions. A previous blog article describes things to look for in alfalfa fields where there was standing water this winter, and how to diagnose problems as the weather warms and soil dries out. In this particular example, the consultant assessed soil conditions by augering down about four feet. He did not observe excessive moisture, even down to four feet, and generally, we would presume fairly good drainage from a sandy loam soil. He also sent samples to the lab for chemical analysis. The soil analysis indicated an acceptable pH (7.6), low salinity (EC 0.5-0.8 dS/m), adequate soil phosphorus (12 ppm), and marginal to low potassium (45 ppm). Despite the low potassium, no potassium deficiency symptoms have been observed on the leaves. The grower applied turkey manure to the field in the fall but did not have it analyzed, so we do not know its fertility characteristics. Additionally, the grower applied herbicides in the fall. There has been some concern that the affected acreage is reacting poorly to the herbicides; however, when herbicide damage has occurred, we would generally expect to see the damage over the entire treated acreage, and that is not the case in this field.

In addition to providing me with plant samples, the consultant also provided me with a small container of what he could only assume were little grubs. He wondered if these grubs were feeding on the roots and contributing to the slow spring growth. I took a look at these “grubs” under the dissecting microscope (Fig. 1), and found that they were not grubs, but rather, earthworm cocoons. I am by no means an earthworm expert, but as a graduate student, I investigated earthworms as a biological indicator of soil health in orchard systems. What is seen in Figure 1 are three soft, white (albeit dirty) cocoons. Over time, these would turn brown and have a tougher exterior which resists desiccation. Depending on the species and soil temperature and moisture conditions, earthworms may hatch in as few as three weeks and as many as five months. More information on the earthworm life cycle is available from the UC IPM webpage.

Earthworms do not feed on roots, but rather, ingest the soil and its microorganisms, mixing the soil as they feed. A recent California Agriculture article discusses how soil biodiversity contributes to soil health, defined as soil functionality. Earthworms certainly are a component of soil biodiversity, and their presence often relates to beneficial soil physical and chemical characteristics. While the jury is still out on what might be causing the slow spring growth of this alfalfa field, I think we can rule out these little, white orbs.