The typical depth for collecting surface soil samples is zero to six inches but some crop consultants have divided this sampling depth into smaller increments in order to determine the distribution of immobile nutrients, such as P & K within each layer of soil.
February 24, 2009 By John Harapiak
The typical depth for collecting surface soil samples is zero to six inches but some crop consultants have divided this sampling depth into smaller increments in order to determine the distribution of immobile nutrients, such as P & K within each layer of soil. Their concern was based on the fact that, in no-till/direct seeding, fertilizer is usually applied relatively close to the soil surface and there is insufficient tillage to redistribute these nutrients more uniformly throughout the surface layer of soil.
Stratification of P detected
Some crop consultants suggest that this type of P stratification could limit crop growth compared to P distributed more uniformly throughout the top six inches of soil. At the University of Saskatchewan, Jeff Schoenau decided to determine if this type of nutrient stratification was harming yields in long-term no-till fields. He found a number of no-till fields in which there was evidence of P being concentrated in the upper layers of soil in which to locate their tillage treatments
Periodic tillage often suggested
The question is whether undertaking this type of tillage operation will in fact improve productivity. The findings of Schoenau and his graduate student indicated little or no yield benefit from imposing a series of different tillage treatments on these no-till fields. Schoenau goes on to state that, “Vertical nutrient stratification does not appear to be an agronomic issue, but environmental implications (e.g. P in run-off water) deserves further investigation.”
Tillage alone impacts stratification in Nebraska
Paul Jasa at the University of Nebraska-Lincoln, has been studying the impact of tillage systems on P and K distribution for many years. As illustrated in Table #1, Jasa discovered that nutrient distribution could be significantly impacted by tillage over a period of 20 years, even if no fertilizer was being applied. As was the case with our native prairie soils, in no-till the enrichment of the upper soil with P does exist. This is attributed to the cycling of soil P on to the soil surface by crop residues. It would appear that when crop residues are incorporated into the soil with tillage, soil P levels are effectively reduced.
Tillage alone impacts distribution of broadcast P
Jasa also studied the impact of tillage systems on incorporating 100 lbs/ac of fertilizer P on the distribution of soil P. As illustrated in Table #2, after four years, there were was significantly more soil P present in the zero to two inches layer in no-till than was the case of the other treatments that involved tillage. Despite containing the highest P stratification, Jasa found that the highest yields of corn, soybeans and grain sorghum tended to be associated with the no-till system, thereby suggesting that stratification of P was not a major concern. In the case of potash, the highest leaf tissue analysis in both corn and soybeans was associated with the no-till systems.
Why does tillage of no-till appear to be counter-productive?
It appears tilling of no-till soils to eliminate stratification of P near the surface is not necessarily beneficial and, in fact, could be detrimental. As in the case of comparing band versus broadcast and incorporated applications of P fertilizer, the greater soil contact that is established in the latter results in the soil more rapidly converting available soil or residual fertilizer P into insoluble or relatively unavailable P.
Role of crop residues in no-till
Paul Jasa states that, “We’ve seen that with no-till and good residue cover, there is plenty of moisture and roots near the surface to take up the nutrients. So even with stratification of soil P near the surface, nutrient uptake isn’t a problem. As long as the nutrients, roots and moisture are all present at the same time, stratification of P and K is not an issue. No-till yields are better without tillage as long as moisture and residue cover are maintained.”
Is deeper banding a possible option?
Some crop consultants have suggested that the supposed problem associated with P stratification could be overcome by increasing the depth at which the fertilizer is band applied. However, potential yield increases need to be weighed against increased fuel consumption and higher equipment/ maintenance costs associated with such a practise. Furthermore, as illustrated in Table #3, based on 11 years of Westco data, the benefit of placement of P in deeper bands (i.e. six to seven inches vs. three to four inches) in long-term Alberta barley trials located on a clayey soil appears to be non-existent, since on average, a barley yield loss was associated with deeper placement of fertilizer.
Similar results with deeper P banding in Nebraska
Paul Jasa also evaluated placing P fertilizer five inches deep on fields with a long-term history of no-till with evidence of P being concentrated near the soil surface, to determine if P stratification was indeed a problem that could be corrected by placing fertilizer P in bands at the depth of five inches. It didn’t work out that way since the deep placed P was not as effective as P applied near the seedrow. He states that, “Basically, if stratification of P near the soil surface in no-till fields is a problem, putting some P deeper should increase yields. It didn’t work out that way in my studies.”Are May weather conditions a factor?The Westco ‘depth of banding’ trials were carried out on very P responsive trials located at Ellerslie, Alberta. The data from this 11-year study was sorted according to whether the average May temperatures were wetter, drier, warmer or cooler than normal. As illustrated in the data presented in Table #4, the greatest advantage for shallower band placement occurred in years with cooler than normal May weather. The disadvantage for deeper placement of banded fertilizer was least evident when May temperatures were above normal.
My ‘Take Home’ message
To date, there is no research evidence to suggest that tillage of no-till or direct seeded fields, in order to achieve a more uniform distribution of available P in the top six inches, is warranted. In fact, without even considering the costs of a tillage operation, research data indicates that tillage of these fields can be counter productive in terms of loss of residue cover and soil moisture, reduction of available soil P levels, as well as reduction in cereal crop yields. The potential impact on yields of tap rooted crops, such as canola, is still to be assessed.
|*John Harapiak has more than 40 years of western Canadian based, fertilizer-related experience. He will continue to contribute stories to Top Crop Manager. He can be contacted by e-mail: email@example.com|