Adjusting a Soil Test Recommendation

Specific recommendations must be tailored to the crops you want to grow, as well as other characteristics of the particular soil, climate, and cropping system. Most soil test reports use information that you supply about manure use and previous crops to adapt a general recommendation for your situation. However, once you feel comfortable with interpreting soil tests, you may also want to adjust the recommendations for a particular need. What happens if you decide to apply manure after you sent in the form along with the soil sample? Also, you usually don’t get credit for the nitrogen produced by legume cover crops because most forms don’t even ask about their use. The amount of available nutrients from legume cover crops and from manures is indicated in table 21.5. If you don’t test your soil annually, and the recommendations you receive are only for the current year, you need to figure out what to apply the next year or two, until the soil is tested again.

No single recommendation, based only on the soil test, makes sense for all situations. For example, your gut might tell you that a test is too low (and fertilizer recommendations are too high). Let’s say that although you broadcast 100 pounds N per acre before planting, a high rate of N fertilizer is recommended by the in-season nitrate test (PSNT), even though there wasn’t enough rainfall to leach out nitrate or cause much loss by denitrification. In that case, you might not want to apply the full amount recommended.

Table 21.3: Soil Test Categories for Various Extracting Solutions
A. Modified Morgan’s Solution (Vermont)						C. Mehlich 3 Solution (North Carolina)*
Category	Very Low	Low	Optimum	High	Excessive	Category	Very Low	Low	Optimum	High	Excessive
Probability of response to added nutrient	Very High	High	Low	Very Low		Probability of response to added nutrient	very High	High	Low	Very Low
Available P (ppm)	0-2	2.1-4.0	4.1-7	7.1-20		Available P (ppm)	0-12	13-25	26-50	51-125	>125
K (ppm)	0-50	51-100	101-130	131-160	>160	K (ppm)	0-43	44-87	88-174	>174
Mg (ppm)	0-35	36-50	51-100	>100		Mg (ppm)**	0-25		>25
						*From Hanlon (1998).**Percent of CEC is also a consideration
B. Mehlich 1 Solution (Alabama)*						D. Neutral Ammonium Acetate Solution for K and Mg and Olsen or Bray-1 for P (Nebraska [P and K], Minnesota [Mg])
Category	Very Low	Low	Optimum	High	Excessive	Category	Very Low	Low	Optimum	High	Excessive
Probability of response to added nutrient	Very High	High	Low	Very Low		Probability of response to added nutrient	Very High	High	Low	Very Low
Available P (ppm)	0-6	7-12	13-25	26-50	>50	P (Olson, ppm)	0-3	4-10	11-16	17-20	>20
K (ppm)	0-22	23-45	46-90	>90		P (Bray-1, ppm)	0-5	6-15	16-24	25-30	>30
Mg (ppm)**		0-25	>25			K (ppm)	0-40	41-74	75-124	125-150	>150
Ca for tomatoes (ppm)***	0-150	151-250	>250			Mg (ppm)	0-50		51-100	>101
*From Hanlon (1998)**For corn, legumes, and vegetables on soils with CECs greater than 4.6 me/100g. ***For corn, legumes, and vegetables on soils with CECs from 4.6 to 9.0 me/100g.

Another example: A low potassium level in a soil  test (let’s say around 40 ppm) will certainly mean that you should apply potassium. But how much should you use? When and how should you apply it? The answer to these two questions might be quite different on a low organic matter, sandy soil where high amounts of rainfall normally occur during the growing season (in which case, potassium may leach out if applied the previous fall or early spring) versus a high organic matter, clay loam soil that has a higher CEC and will hold on to potassium added in the fall. This is the type of situation that dictates using labs whose recommendations are developed for soils and cropping systems in your home state or region. It also is an indication that you may need to modify a recommendation for your specific situation.