Multifunctionality of Cover Crops in South Texas: Looking at multiple benefits of cover cropping on small farms in a subtropical climate

Soil nutrient status

Aim of this study was to assess the effects of 4 different summer cover crops and fallow on the spore density and diversity of mycorrhizal fungi, soil organic carbon and nutrient status in an organic vegetable farm. The study site had high levels of calcium concentration ranging from 18913 ppm to 22511 ppm, high soil pH and very low organic matter content, an environment not conducive for the microbial activity. The soil texture was silty loam with very poor drainage, which is a common characteristic of the Lower Rio Grande Valley soils. The nutrient distribution along our field varied considerably, this could be because of the variation in the water drainage pattern in the field. 

The difference in the soil nutrient status after cover crops are given in Table 2. Our results show that cover crops had mixed results in the soil nutrient status. This is an expected result as all cover crops do not provide the same benefits and farmers should select the cover crops based on their needs and what fits their farms conditions.

Table 2: Change in the soil nutrient status after each cover crop (in ppm). Numbers are calculated by subtracting the pre cover crop soil nutrient concentration from the post cover crop nutrient concentration.

Mycorrhizal fungi spores

Overall the cover crops had a positive effect on the mycorrhizal spore density compared to the control. Total spore density varied significantly among the different cover crop treatments, ranging from 52 to 187 spores per 10 grams of dry soil (Figure 1).  Our results indicate that the mycorrhizal spore density was influenced by the cover crop identity. Highest numbers of spores were found under sunn hemp followed by sudangrass and lablab while the lowest numbers of spores were found under pearl millet followed by control. Among the four cover crops pearl millet had the lowest number of spore count. Low spore density under pearl millet could be because of the high weed density which could have suppressed the mycorrhizal fungi growth.

There was also a difference in the spore size among the different cover crops. Sunn hemp and sudangrass had generally bigger spores while the control and pearl millet generally had smaller (50 spores) with few large spores. 14 different species of mycorrhizal fungi were identified in the study from three different genera: Glomus, Aculospora and Gigaspora. The distribution of these spores varied among the different cover crops. Glomus was most dominant in the sunn hemp and lablab and a high density of Gigaspora occurred in the sudangrass treatment. This could be the result of different soil magnesium concentration under different cover crops. Similar results of higher density of Glomus in soils with high magnesium concentration and high Gigaspora density in low soil magnesium concentration has been reported in previous studies (Schenck & Siqueira 1987; Gryndler et al 1992). The overall diversity of mycorrhizal fungi, however, was similar for the all of the different cover crops and was lower in the control treatment.

Correlation analysis of the number of spores and soil physical and chemical properties provided mixed results. There was a strong positive correlation between the number of spores and soil magnesium (r = 0.96, p=0.0095) and boron concentration (r = 0.92, p=0.025). A strong negative correlation was seen between the number of spores and soil moisture (r = -0.93, p=0.021) indicating that the number of mycorrhizal spores were higher in dry soils compared to wetter soils. There was no significant correlation between the total number of spores and soil organic matter, nitrate or phosphorus concentration.

Soil organic matter is the backbone of sustainable agriculture and when growing vegetables and specialty crops, a soil high in organic matter is very desirable. Cover crops are known to increase soil organic matter and enhance the natural productivity and fertility of soil. In our study all the cover crops increased the soil organic matter compared to the control treatment, fallow. In the control treatment, the total soil organic matter at the end of the experiment was lower compared to the beginning of the experiment. In general grass cover crops are known to contribute more soil carbon than legumes (Hoorman 2009). In our study, sudangrass produced the highest above ground biomass compared to the other cover crops but surprisingly it contributed less organic matter compared to sunn hemp when measured after 4 weeks of incorporating cover crops into the soil. A possible reason for this could be the sudangrass had lignified when tilled and may not have fully decomposed in the soil.

As expected, sunn hemp resulted in highest nitrate concentration in the soil; however contrary to our expectation nitrate lablab caused a decline in the soil nitrate. A possible explanation could be the high density of weeds in the lablab plots. Similarly, sunn hemp followed by lablab and sudangrass had the highest soil phosphorus concentration while the control plot had the lowest phosphorus concentration in soil. While the total calcium concentration in the soil remained fairly stable in all the treatments, cover crops along with the associated mycorrhizal fungi do have the potential to offset the negative effect of high pH on nutrients availability.

Cover crop growth and weed suppresion

Seeding method did not have any significant difference on cover crop performance (data not shown). However, there was a significant difference in the growth and weed suppression potential among the different cover crops in the growth of varying degrees of cover crop and weed growth among the different cover crop treatments. (Table 1). Sudangrass planted 1.5 times the recommended rate produced the highest amount of biomass (2910 kg/ha) followed by sudangrass planted at 3 times the recommended rate, while lablab planted at both rates 1.5 times and 3 times produced the lowest cover crop biomass (94.4kg/ha and 107 kg/ha respectively). As expected, the biomass cover crops was negatively correlated with the weed biomass (r(14) = -0.53, p = .05). The weed to cover crop biomass in the different cover crop plots ranged from 434 kg/ha of weeds in a plot of lablab 1.5 times recommended with a biomass of 944 kg/ha 20 to 220 kg/ha of weeds in a plot of sudangrass 3 times recommended rate with a biomass 1950 kg/ha. The control treatment of 570 kg/ha had the highest above ground biomass of weeds when compared to all of the cover crop treatments at all seeding rates. Weed to cover crop biomass ratio was lowest in the sudangrass treatments at all seeding ratings. The correlation between plant height and the PPFD of the different cover crops show that sudangrass grew vigorously and blocked the most of the light from reaching the soil surface. While there is a possibility that PPFD could have been affected by the weeds our results show a strong negative correlation between the cover crops’ height and the PPFD at the soil surface. A two-way ANOVA of cover crop type and seeding rate on height of the biomass was conducted to compare the effect the cover crop type and seeding rate had on the height of the above ground biomass in lablab, sudangrass, and sunn hemp. A significant main effect of cover crop type on above ground biomass height was found, F(2,56) = 175.071 p=< 0.001. The main effect of seed rate on biomass height was not significant. The cover crop type and seeding rate interaction was significant F(2,56) = 7.084 p=0.002. Of the four different cover crops, sudangrass at both the seeding rates had the highest heights followed by pearl millet. The 1.5 times the seeding rate in sudangrass grew taller than the 3 times the seeding rate sudangrass plots. On the other hand, sunn hemp at 1.5 times the seeding rate had the lowest height growth. Results from this study show that light penetration to the soil surface decreased significantly over time. Among the seven treatments, sudangrass with seeding rate 1.5 times the recommended rate and 3 times the recommended rate resulted in the lowest light penetration along with pearl millet 2 times the recommended rate. (F(3,66) = 3.12 p=0.0319).

Cover crops are a fundamental component of organic farm management. Cover crops have the potential to produce large amounts of biomass and contribute to the maintenance and/or improvement of the physical, chemical and biological characteristics of the soil, including adaptation of effective soil depth through their roots which help in weed suppression and promote soil quality in short period of time. This is the first study documenting the potential for cover crops to suppress weeds during the summer season in the Lower Rio Grande Valley. Without the option to use chemical herbicides, weed management is a challenge for organic growers, and they need to use several biological, chemical, and cultural practices (Liebman and Davis 2000). Our results indicate that, in addition to other soil health benefits (Soti et al 2016), cover crops could serve as a successful weed management tool in organic farms. Cover crops emergence and ground coverage (represented by the light reading) varied significantly among the cover crops selected and their seeding rates. This could be the result of the seed temperature tolerance, field conditions etc. Sunn hemp performs best on well-drained soils with a pH from 5.0-7.5 (USDA NRCS). The soil at this location had a pH of 8.00 and had very high clay content. This may account for the poor performance of sunn hemp. Sudangrass followed by pearl millet had faster growth and biomass accumulation these results are similar to previous studies (Bicksler & Masiunas, 2009; Creamer & Baldwin, 2000; Teasdale, 1993; Ong & Monteith, 1985; Maiti & Bidinger, 1981).

The weed species growing in the control plots parthenium (Parthenium hysterophorus), pigweed (Amaranthus palmeri), and johnsongrass (Sorghum halepense) represent the major problematic weeds in the region. Parthenium is an annual weed that causes major problems in rangelands and cropping systems throughout the southern US and other subtropical regions. It can cost up to $22 million dollars a year in reduced crop production and increased management costs (CRC Weed Management, 2003). Pigweed, an annual weed which causes serious problems for farmers throughout the Southern US, can cost $60-$80 per acre for farmers to manage in their fields (Thompson, 2016). Johnson grass is a perennial weed and is very difficult to control with a single cultural methods or herbicide application (Johnson et al 1997). Johnson grass is one of the most costly weeds that farmers encounter and costs millions of dollars a year in lost crops, poor quality grain, and lower crop yield (Shawnee Co. Weed Department). Our study indicated that the grass species of cover crop, sudangrass and pearl millet, with faster growth and higher biomass accumulation, were more successful in weed suppression and growth compared to the legume species used, lablab and sunn hemp. Potential of grass cover crops to suppress weeds is well documented (Teasdale, 1996; Teasdale et al., 2007; Burgos & Talbert, 1996; Yenish, J. P., Worsham & York 1996.) Cover crops with faster growth rate and above and below ground biomass accumulation rates can suppress weeds by competition for resources such as light, water, and nutrients. Additionally, the reduction of weed biomass could be in part due to the allelopathic effects. Sudangrass is known to have allelopathic compounds and exudes sorgoleone which is very active at very low concentrations and has been shown to suppress weeds and may have been a factor in the low amount of weeds (Scott, 1991). With the growing market demand for organic produce, there is also a growing demand to find practices that reduce the amount of weeds in crop fields in an environmental and economically sound manner.