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Understanding Solubility & Paste Testing

By William McKibben

Paste Testing

Low exchange capacity soils especially have a lot of issues when it comes to holding enough nutrients and maintaining colloidal balance. Sand with low organic matter is basically a soilless media. Since many of the nutri­ents required for crop production are not on the colloid, I find that the paste test is absolutely the best test for this situation. I also use the paste test along with the standard test on the higher exchange capacity soils. Since most of the nutrients taken up by plants are picked up by mass flow or diffusion at the air/water interface near the plant roots, the paste test better correlates to tissue analysis than the standard test.

The paste test is done primarily by taking a large sample, approxi­mately 200-400 grams of a composite soil mixture and saturating it with distilled water (or even better is to use clients’ irrigation water) until it becomes a pancake batter consistency.

Making Paste Analysis Recommendations

Making recommendations from a paste analysis incorporates both the SLAN approach and the BCSR approach. First, it is critical to meet the minimum strategic nutrient levels for growth and reproduc­tion (Figure 16). Secondly, the nutrients in solution should be kept in balance to minimize interference issues. Even if all the strategic levels have been met, if a nutrient such as potassium is two or three times higher than the desired level, other cations such as calcium and magne­sium should be elevated in order to maintain a balance. The type of crop and the level on the standard test would help make that determination. For crops such as corn or alfalfa that require a lot of potassium if the standard test does not show good levels of potassium, I would probably let the level stand and not elevate the calcium and magnesium. When raising levels to just balance out one nutrient, care must be taken not to create a salt issue. If the nutrient in excess is sodium and irrigation is available, flushing the soil may be the best solution. Most of the sodium issues that I see are created by irrigation practices. Irrigating with poor quality water or using poorly designed irrigation systems may result in the accumulation of sodium and/or bicarbonates.

Strategic nutrient level paste guidelines for soil

Figure 16

Discussion of Paste Guidelines

The guidelines for the paste test shown on Figure 16 are just that —guidelines. I cannot emphasize enough that everyone needs to adjust these numbers based on their own crop tissue analysis and the subsequent crop response. There is one unknown factor that we face when using paste numbers and this is what I call the “flow rate into solution.

Solubility Testing

Just what is a soil paste or solubility test and when should you consider running the analysis? Solubility analysis is an attempt to see what is in the soil solution, or that which can readily go into solution off the colloid.

The diagram below is a conceptual picture of a root hair in the soil solution in proximity to a clay particle or soil colloid.

The diagram 1.0 is a conceptual picture of a root hair in the soil solution in proximity to a clay particle or soil colloid. The standard soil analysis measures the dots (nutrients) floating in solution as well as the dots (nutrients) held on the surface of the clay mineral, but not those trapped between the clay layers. In a standard soil test, the minerals attached to the surface of the clay particle are removed for analysis by using an extracting solution. A solubility analysis primarily looks at only the blue dots (nutrients) floating in solution.

soil test diagram

Diagram 1.0

It makes perfect sense to look at a test that measures nutrients only in solution when you study the research work done by Barber and Olsen in 1968 and Dennis in 1971. That research showed the bulk of the soil nutrients taken up by plants was through mass flow and diffusion from the soil solution. Very little nutrition enters the plants by the roots directly intercepting nutrients from the colloid or soil particles. So when should you use a solubility analysis? Those soils with exchange capacities less than 10 should be the first targeted for solubility analysis. This includes all sandy soils even calcareous sands, which tend to get exaggerated TECs on the standard soil test. It is nearly impossible to balance and hold that balance on low exchange capacity soils.

It makes perfect sense to look at a test that measures nutrients only in solution when you study the research work done by Barber and Olsen in 1968 and Dennis in 1971. That research showed the bulk of the soil nutrients taken up by plants was through mass flow and diffusion from the soil solution. Very little nutrition enters the plants by the roots directly intercepting nutrients from the colloid or soil particles. So when should you use a solubility analysis? Those soils with exchange capacities less than 10 should be the first targeted for solubility analysis. This includes all sandy soils even calcareous sands, which tend to get exaggerated TECs on the standard soil test. It is nearly impossible to balance and hold that balance on low exchange capacity soils.

Since the holding capacity of low exchange soils is so small, plants grown on these soils primarily get their nutrition from applied nutrients with variable degrees of solubility. Solubility analysis will take you to a whole new level of understanding the relationship of soil and plant nutrition. When combined with the standard soil test and plant analysis you will get a much clearer picture of what soil nutrition means.

Source: The Art of Balancing Soil Nutrients