INTRODUCTION

Southeastern Coastal Plain soils are typically sandy textured and have low soil fertility levels and water holding capacities. Frequent periods of drought stress and low crop yields often result in the accumulation of plant nutrients in the soil after crop harvest. Nutrients that remain near the soil surface, such as phosphorus (P), are susceptible to off-site movement in runoff water during rainfall events. The continuous management of these soils using traditional tillage practices (i.e., disking) has contributed to a decline in soil organic carbon (measure of soil organic matter). A decline in soil organic carbon (SOC) lowers nutrient and water retention and contributes to a decline in soil productivity. Therefore, new tillage systems are needed that will improve the ability of Coastal Plain soils to retain nutrients, rebuild SOC contents, and increase nutrient-use efficiency of applied fertilizer. Conservation tillage is one practice that can increase raise the SOC content of soils but it may take several years for significant increases to occur. However, the use of other production practices, such as narrow row widths and cover crops, may accelerate the increase in SOC. One approach to reduce nutrient accumulation in soil is to precision apply nutrients like phosphorus (P) on a site specific basis using GPS technologies. Doing so places P where it is needed and reduces the over-application of fertilizer. Little is known about whether the precision application of P can be used to reduce soil nutrient accumulation and how the cropping system used changes soil nutrient levels. New agricultural management practices are being developed for producers in the region. Information is needed concerning the impact of these new practices on soil chemical and physical properties so that farmer recommendations can be modified accordingly.

OBJECTIVE

To determine how conventional and innovative cropping practices affect long-term changes in soil fertility and SOC contents and to examine how soil type impacts the cropping-system effects. APPROACH Soil samples were collected prior to crop planting in 1998 (soybean), 1999 (corn), 2000 (cotton), and 2001 (corn) from a split landscape study being conducted at the Pee Dee Research and Education Center in Florence. One side of the field is receiving a combination of innovative production practices and the other side a combination of what is considered traditional production practices (Table 1). Soil samples have been collected using two protocols, a 50 x 50 ft grid design and by soil type. In the 50 x 50 ft grid design, soil cores were collected to a depth of 0 to 6 inches. Soil samples collected by soil type were sampled at 0-1 and 1-6 in depths. Soils were collected by depth to examine potential stratification of soil nutrients. Soil samples were analyzed for SOC, pH, and nutrients by the Clemson University Agricultural Service Laboratory. All soil sampling points were indexed using a GPS system. The soybean crop was grown in 1998 to enhance uniformity of soil nitrogen (N) contents across the field. Soil test results from the innovative side indicated that only P could be precision applied. All other nutrients were applied at rates recommended from soil test results conducted on one bulk soil sample (0-6 in depth) taken from each side of the field.

RESULTS AND DISCUSSION

Soil Organic Carbon Contents (SOC)

The SOC contents of soils collected at the 0-1 in. depth were higher with the innovative cropping system than with traditional cropping system (Fig. 1). Little change occurred over time in SOC for soil at the 0-1 in. depth on the traditional side of the field. The SOC content of samples collected at the 1-6 in. depth showed only minor fluctuations over time on both sides of the field.

Soil Phosphorus

Soil P concentrations of samples (0 to 6 in) collected in all years were variable across both sides of the field (Fig. 2). Soil P levels were high in the low yielding areas of the field and areas low in elevation. Residual soil P can be transported off site with runoff and eroded sediments. We have precision applied between 50 to 100 lbs P/ac on the innovative side (Fig. 2). On this side, there appears to be a slow trend towards P levels becoming more uniform across the field and at levels considered medium in fertility. In contrast, P levels are becoming very low in certain areas of the field on the traditional side of the field.

Soil Potassium

Potassium levels were variable across both sides of the field in all years (Fig. 3). Since soil data is available for only one year of each crop, it is too early to make conclusions concerning crop usage and needs under the different cropping systems.

Soil pH

Soil pH is a measure of the soils acidic or basic character. The availability of nutrients, especially micronutrients, is dependent on soil pH . Soil test results each year recommended that no lime be applied, which would explain the reduction in soil pH that occurred (Fig. 4). Cation exchange capacity (CEC) decreased each year probably due to the decline in soil pH (Fig. 5). Cation exchange capacity is an indicator of the soils ability to retain many nutrients and is influenced by soil properties such as soil pH and organic matter.

CONCLUSIONS

After four years, the innovative cropping system centered on conservation tillage increased in SOC content at the 0-1 inch soil depth (near the soil surface). A wide range in soil nutrients levels (P and K) were found across both sides of the field, which is probably typical for most Coastal Plain soils. It appears that soil P levels are slowly becoming more uniform on the innovative side of the field as a result of the precision application of P fertilizer. In contrast, P levels are dropping to very low values in some areas of the traditional side of the field where P fertilizer was broadcast-applied at one rate across the whole field. The preliminary data suggest that it may take 5 or more years for areas high in soil P to decrease to acceptable levels. Although using the innovative cropping system increased SOC (soil organic matter) in the top inch of soil, cation exchange capacity (CEC) of the top 6 inches dropped. Normally, increases in SOC will increase the CEC of the soil. There may have been an increase in CEC in the top inch of soil (which we are examining) which was not detected in the samples taken from the top 6 inches. Also, the decline in soil pH that was found may have negated any potential positive effects the increase in SOC would have had on CEC. In any case, our data indicate that changes in soil fertility and SOC are complex and slow to occur on Coastal Plain soils.

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For further information about this research, please contact: Dr. Jeff Novak 843-669-5203 ext. 110 email