Dr. Trent Roberts, Extension Soil Fertility
Dr. Jarrod Hardke, Rice Extension Agronomist
In an effort to respond to growing interest in the N-STaR program for rice, here are the answers to some frequently asked questions about N-STaR. If you have additional questions, please contact us for more information.
1. What does N-STaR measure?
N-STaR soil analysis is an index of nitrogen availability that measures ammonium and organic-nitrogen in the soil that will feed the plant throughout the growing season. The nitrogen that is quantified is relatively stable and is not prone to the loss mechanisms that are commonly seen with soil nitrate.
2. When is the best time to take N-STaR samples?
Since the nitrogen that is quantified using N-STaR is relatively stable, N-STaR samples can be taken at almost any time of the year. The best time to take soil samples are when the soil is dry and usually occurs during the previous crop harvest or near planting. Samples can be taken when the soil is moist, but the drier the better. The general rule of thumb is that as long as mud isn’t sticking to your boots you can take an N-STaR soil sample.
3. How do I know how deep to sample?
The most important aspect of using the N-STaR program correctly is using the proper sampling depth. The easiest way to do this is to use Web Soil Survey (http://websoilsurvey.sc.egov.usda.gov/App/HomePage.htm) to determine the soil texture/soil series for your individual fields. The most accurate way is to look at your estimated cation exchange capacity (ECEC) from your routine soil test report which is provided by most labs. Soils that have an ECEC <28 cmol/kg should be treated as a silt loam and sampled 0-18 inches deep. Soils that have an ECEC >28 cmol/kg should be treated as a clay and sampled 0-12 inches deep. ***There are some situations west of Crowley’s Ridge that have high soil pH levels that might return ECEC values >28 cmol/kg but are truly silt loams. This is a situation where you need to use all information available to determine the proper depth (field knowledge, Web Soil Survey, and ECEC).
4. What do I do about changes in soil texture within the same field?
When soil texture changes within a single field and is verified either by soil analysis or using a Veris then more of a zone sampling approach should be implemented. Attempt to determine the portion of the field that should be sampled as a silt loam and the portion that should be sampled as a clay and proportion your 10 samples accordingly. Please remember that each sample should be submitted separately and the soil texture for each sample should be indicated on the N-STaR sample submission sheet.
5. How many samples should I take per field?
N-STaR requires a 0-18 inch soil sample for silt loam soils and a 0-12 inch soil sample for clay soils. Due to the depth of these samples, there tends to be very little within field variability, unless there has been significant land leveling in the last 5 years. Current recommendations are to take 10 individual soil samples per field to effectively capture field variability. These samples have to be kept separate and cannot be composited like samples for routine soil analysis. Take at least 10 samples per field and submit at least 10 samples per field.
6. What is the maximum number of acres one N-STaR sample should represent?
Each field requires a minimum of 10 samples to accurately quantify the field variability of the N-STaR value. However, a single soil sample should never represent more than 10 acres. Therefore, fields that are greater than 100 acres will require more than 10 samples per field based on total field acreage.
7. How often should I take an N-STaR sample?
We encourage producers to take N-STaR samples the first two season that rice is planted in a field. This will provide an accurate baseline by which future N-STaR analysis can be compared. Following the first two rice crops, N-STaR samples can be taken every 3rd to 5th rice crop as long as the crop rotation is maintained. If the crop rotation changes, i.e. an extra year of soybean or rice or an alternative crop such as corn is planted, then the producer is encouraged to sample for N-STaR prior to rice planting to ensure that the proper nitrogen credits are accounted for.
8. What equipment do I need to take N-STaR soil samples?
The easiest and most reliable way to sample soils for N-STaR analysis is to use the N-STaR bucket and either a corded drill and small generator or a cordless drill. N-STaR sampling kits are available for purchase from the N-STaR Soil Test Lab which includes all equipment needed except for the drill. N-STaR buckets can also be fabricated using materials often found around the farm shop using the directions outlined in FSA-2168 (http://www.uaex.edu/publications/PDF/FSA-2168.pdf).
9. Can I sample right after putting out lime?
Liming does not influence the types or forms of nitrogen that are quantified using the N-STaR analysis. Therefore N-STaR samples can be collected any time before or after liming.
10. Can I sample right after putting out poultry litter?
Poultry litter contains a combination of both plant available nitrogen (ammonium and nitrate) as well as organic nitrogen (amino acids, urea, etc.). N-STaR measures a combination of ammonium and organic nitrogen which can be found in poultry litter at varying levels. Therefore our current recommendation is to either take N-STaR samples before poultry litter application or wait a minimum of 2 weeks following poultry litter application before taking N-STaR samples. Waiting a minimum of 2 weeks following poultry litter application allows the nitrogen in the poultry litter to stabilize to ensure that the proper nitrogen credits are measured using the N-STaR analysis.
11. Can I sample after putting out Ammonium Sulfate (AMS) or Diammonium Phosphate (DAP) as a starter or preplant?
AMS and DAP contain ammonium which can be quantified using N-STaR. Therefore our current recommendation is to either take N-STaR samples before AMS or DAP application or wait a minimum of 2 weeks following application before taking N-STaR samples. Waiting a minimum of 2 weeks following application allows the nitrogen to stabilize to ensure that the proper nitrogen credits are measured using the N-STaR analysis. Very little of preplant or starter nitrogen is utilized by the rice plant and taking samples immediately after application will lead to lower N-STaR recommendations than are actually required to maximize yield.
12.How long does it take to get the results of my samples back?
With the new additions to the N-STaR lab and a new data processing procedure most results are returned within 10 business days during peak sample season (March-May). During off-peak hours, N-STaR results are usually returned within 5-7 business days.
13.How much does it cost to process each sample?
N-STaR analysis costs $10 per sample and equates to roughly $100 per field. The N-STaR lab charges the break-even cost for sample analysis and does not make any money off of the analysis.
N-STaR Bottom Line
- Sample 0-18 inches on silt loam soils and 0-12 inches on clay soils.
- Take 10 samples per field, but a sample should not account for more than 10 acres.
- Take N-STaR samples the first two times rice is grown in rotation.
- Use a drill and N-STaR bucket to take samples.
- Liming does not affect N-STaR sampling.
- Poultry litter, AMS, and DAP affect N-STaR sampling. Sample either before application, or a minimum of 2 weeks after application of these products.
- Sample results are returned in 5-10 business days.
- Cost is $10 per sample ($100 per field).