Soil pH Effects on the Shoot and Root Yield of Crabgrass

Emily B. Aleshire, Graduate Research Assistant, and Chris D. Teutsch, Assistant Professor, Southern Piedmont Agricultural Research and Extension Center, Virginia Polytechnic Institute and State University, Blackstone 23824

Abstract

Recent research indicates that crabgrass (Digitaria species) could provide high-quality summer forage for ruminant livestock in the mid-Atlantic region. Limited data are available documenting the soil pH requirements of crabgrass grown as forage. A greenhouse study was designed to determine the effect of soil pH value on the germination and yield of crabgrass. Treatments consisted of three pH levels: 4.8, 5.5, and 6.3. Pots 8 inches across and 8.5 inches deep containing 10 lb of air-dried soil were seeded at a density of 25 seeds per pot and thinned to 14 plants per pot after germination was complete. Two harvests three to four weeks apart were taken. Within the range of 4.8 to 6.3, soil pH had no effect on crabgrass germination, shoot yields, or root mass in either harvest. These data indicate that crabgrass has potential to produce summer forage on acidic soils commonly found within the southeastern United States.

Introduction

Crabgrass (Digitaria species) is a warm-season annual grass believed to have originated in southern Africa (2). The grass was used as forage in Europe before settlers unintentionally brought it to the United States (6). In 1849, the United States Patent Office (predecessor to the USDA) introduced the use of crabgrass as a forage species (6,11). Crabgrass is commonly found throughout the transition zone between the temperate and subtropical regions of the United States, with its primary production occurring in the summer months (July and August) (6,20).

Crabgrass possesses a prolific growth habit and is well known for its reseeding capabilities (17) (Fig. 1). It is considered to be tolerant of drought and able to establish and grow on soils with low fertility (20). These characteristics and the fact that crabgrass is often an invasive species in lawns, gardens, and crop fields have caused it to be viewed as a weed (15) (Fig. 2). However, crabgrass has nutritive values equal to or higher than other commonly used warm-season grasses. Dalrymple (6) reported that the forage cultivar ‘Red River’ crabgrass had a crude protein (CP) concentration of 15% and in vitro dry matter digestibility (IVDMD) of 73% compared with 15 and 64%, respectively, for ‘Midland’ bermudagrass grown under the same conditions. In addition, many of the characteristics that allow crabgrass to thrive as a weed, such as its prolific seeding and spreading morphology, may make it well adapted as a summer pasture species (2).


Fig. 1. Seed produced by crabgrass at the Southern Piedmont Agricultural Research and Extension Center, Blackstone, VA.		Fig. 2. Crabgrass growing in a sidewalk in its role as an invasive weed.

In the southern United States, soils tend to be acidic in nature (2,4). A survey of hay and pastureland in ten southern states showed that more than half of tested soils had a pH value below 6.0 (2). Warm-season grasses are, in general, tolerant of acidic soil conditions (18). Limited research and anecdotal reports indicate that crabgrass will grow over a wide range of soil pH values, but no research data are available evaluating the soil pH requirement of crabgrass grown as forage (3,6,16). The objective of this study was to determine the effect of varying initial soil pH levels on germination, shoot yield, and root yield of an improved crabgrass cultivar.

Trial with Three Soil pH Values Conducted Twice

A Cecil sandy loam soil (fine, kaolinitic, thermic Typic Kanhapludults) was obtained from a wooded area at the Southern Piedmont Agricultural Research and Extension Center, Blackstone, VA (37.09°N, 77.99°W). This soil had an initial pH value in water of 4.3 [soil:water = 1:1 (wt/wt)] (14). Three hundred sixty lb of soil was divided into three portions (120 lb) for pH adjustment. Pre-experimental tests with small amounts of soil and Ca(OH)₂ indicated that 0.0042 and 0.011 oz Ca(OH)₂/lb soil were required for raising soil pH values to approximately 5.7 and 6.4, respectively. The three 120-lb portions of soil received 0.5, 1.3, and 2.2 oz of Ca(OH)₂, respectively. A rotary cylinder mixer was used to mix all treatments (Fig. 3). In order to stabilize soil pH, all soil mixtures underwent three wetting and drying cycles in which soil was spread into a thin layer, wetted to field capacity using deionized water in sprinkling cans (Fig. 4), allowed to air-dry, and remixed. Total equilibration time was 72 h. Final soil pH values for the three treatments were 4.8, 5.5, and 6.3 (Table 1).


Fig. 3. Soil and amendments being thoroughly mixed in rotary cylinder mixer.		Fig. 4. Deionized water being added to soil during one of the three wetting and drying cycles used to amend soil pH.

Table 1. Soil pH values and nutrient concentrations (lb/acre) before and after the addition of lime and fertilizer.

Treat- ment	pH^a	P^b	K	Ca	Mg
Initial	4.3	4 (Low^c)	32 (Low)	122 (Low-)	26 (Low)
1	4.8	112 (Very High)	114 (Medium)	596 (Low+)	40 (Low)
2	5.5	197 (Very High)	190 (High-)	1233 (Medium+)	51 (Low+)
3	6.3	236 (Very High)	172 (Medium+)	1779(High)	56 (Low+)

^a Soil solution pH [1:1 soil:water (wt/wt)] (14).

^b Mehlich 1 extraction (9).

^c Soil test recommendations for Virginia (8).

A Mehlich 1 extraction of the soil indicated that phosphorus and potassium levels were deficient (Table 1). Recommended fertilizer rates for establishment of summer annual grasses were 75, 120, and 120, lb/acre each of N, P₂O₅, and K₂O, respectively (8,19). Four times the recommended amount of P₂O₅ was applied to account for significant rates of P fixation, and the small soil volumes of the pots. Laboratory grade Ca(NO₃)₂, Ca(H₂PO₄)₂, and KCl were the respective fertilizer sources. All fertilizers were added to the soil during the last mixing. After the first harvest, an additional 75 lb of N per acre was applied as Ca(NO₃)₂ dissolved in 6.8 fluid oz of deionized water per pot.

Plastic pots 8 inches across and 8.5 inches deep were lined with a 1.5 mil thick autoclave bag to prevent loss of water and leaching of nutrients. A polyethylene air tube was placed in each pot to prevent localized anaerobic conditions and allow water to infiltrate through all soil evenly (Fig. 5). Ten lb of air dried soil was placed in each pot and wetted to achieve 80% field capacity using deionized water. Following wetting, 25 ‘Red River’ crabgrass [Digitaria ciliaris (Retz.) Koel] seeds were planted in each pot at a depth of ¼ inch (7). Trial 1 was planted on 18 June 2004, and Trial 2 was planted on 25 June 2004. Within each trial, two pots were seeded for each treatment with four replications for a total of 24 pots per trial.

Fig. 5. Plastic pots lined with polyethylene autoclave bags with an air tube added for aeration prior to filling with soil.

All pots were weighed daily and watered with deionized water to maintain soil moisture at approximately 80% field capacity. Seedling counts were conducted every other day until germination was complete. Shoot yields were determined by clipping plants to a 4-inch residual height when crabgrass had reached the late boot stage. First harvest was on 26 July 2004, and 2 August 2004, for Trials 1 and 2, respectively. Second harvest was on 12 August 2004, and 27 August 2004, for Trials 1 and 2, respectively. After shoots were harvested, one randomly chosen pot for each treatment was selected from each replication for determination of root dry weight. Soil was carefully washed away from the roots (Fig. 6). Shoots and roots were dried in a forced air oven for 5 days at 140°F.

Fig. 6. Roots being washed free of soil after the first harvest.

The experimental design was a randomized complete block with three pH levels as treatments (4.8, 5.5, and 6.3) and four replications, and the study was run twice. Data were analyzed using the general linear model procedure from SAS (SAS Institute Inc., Cary, NC.). No significant trial-by-treatment interactions were found for germination, shoot weight, and root weight, and there were no significant differences in the response of these factors to pH. Standard error values were calculated with the standard error option in the means statement to illustrate differences in shoot and root weights between the first and second harvests.

Impact of Soil pH Value on Germination

Soil pH value at planting had no effect on crabgrass germination at any stand count date (P > 0.30). Mean germination averaged across trials for the last counting date was 19, 19, and 18 plants per pot for pH values of 4.8, 5.5, and 6.3, respectively. These data indicate that a satisfactory stand of seeded crabgrass can be obtained over a wide range of soil pH values. Research is needed to confirm these findings in the field.

Impact of Soil pH Value on Shoot Growth

Soil pH value at planting had no effect on crabgrass shoot yields in either harvest (P > 0.05) (Fig. 7). Buchanan and coworkers (3) also found that crabgrass was tolerant of low levels of soil pH. In contrast to the current study, Pierce et al. (16) reported that shoot dry matter of crabgrass decreased as soil pH increased from 4.8 to 7.8. However, the rate of yield decrease was greatest at pH levels from 6.0 to 7.8, which were primarily outside of the range used in the current study. In the current study, shoot weight at the second harvest was approximately half the DM of the first harvest.

Fig. 7. First and second harvest shoot yields averaged over trials as a function of soil pH value.

Other research has shown that crabgrass yield increased as rate of nitrogen fertilization increased on soils with pH values above 6.0 (1,5). Although data from the current study indicate that crabgrass yield was not impacted by soil pH values, more research is needed to determine if the nitrogen response seen in previous work is similar for soils with more acid pH values, i.e. near 5.0.

Impact of Soil pH Value on Root Growth

Soil pH value at planting had no effect on crabgrass root yields in either harvest (P > 0.50) (Fig. 8). An extensive rooting system is vital for moisture and nutrient acquisition during establishment and production (10,12). These data indicate crabgrass produced an extensive root system over a wide range of soil pH levels under the conditions of this experiment. Root DM increased by approximately 50% from the first to the second harvest.

Fig. 8. First and second harvest root yields averaged over trials as a function of soil pH value.

Conclusion

In the southeastern United States, there is a need for warm-season forages to fill the summer production slump of cool-season grasses. Our data indicate that crabgrass shoot and root growth was insensitive to soil pH values ranging from 4.8 to 6.3. This demonstrates the potential of crabgrass as a forage for low-fertility grasslands commonly found in this region of the United States. These findings could be especially important to producers who have short-term leases on pastureland that prohibit investment in lime applications; and to help prevent over-application of lime which can lead to deficiencies in minerals such as P, K, and Mg and subsequent decreases in yield (13). Results of this greenhouse study must be confirmed in the field.

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