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The “new” phosphorus fertilization guidelines for maize by FERTASA. A better scientific alternative or a source of confusion?

By Dr J.J. Bornman: General Manager, Solution Marketing, Omnia Nutriology®. August 2018

Much have been said over the last three months in various popular agricultural magazines regarding the fertilizer norms of the fertilizer industry. The primary reference is to the new eighth edition (2016) of the Fertilizer Handbook of the Fertilizer Association of Southern Africa (FERTASA). Particularly under scrutiny, is the new maize phosphorus fertilization guidelines.

In said publication, tables are published which gives recommended applications of phosphorus to compensate for grain withdrawal and to increase soil phosphorus levels to ensure 95% yield of the potential of any season. The 95% relative yield level is claimed to be near the economic optimum. Applications of up to 246 kg ha-1 phosphorus (P) (approximate current cost equivalent to 3.5 tonnes of white maize) is recommended depending on the yield expectation, application method, soil texture and effective depth of sampling. The guidelines are based on work done by Schmidt, 2003 as published in his PhD thesis and an article published in Plant and Soil (Schmidt, Adriaanse and Du Preez, 2007). This work was done while Scmidt was employed by the Agricultural Research Council (ARC).

This article serves to emphasize the dynamics and the confusion it could create for the novice, related to compiling and interpreting these guidelines and the guidelines in general, especially when economic optima are not considered for final recommendation purposes. Practical examples from the thesis of Schmidt (2003) and similar research work done by the author are used to illustrate some of the concepts that could add to such confusion.

The impact of deductions from research data and the subtleties related to interpretation of such formulated guidelines for final economic and risk aware recommendations, will be discussed under two headings.

Interpretation of response data

The recommended levels for maize grown in sandy soils i.e. soils with a clay plus silt content of ten to twenty percent (Schmidt, 2003) could be criticized if interpreted out of context. Reference is made to a Viljoenskroon site (page 135 of the thesis of Schmidt).

If the response function used to deduce the norm is investigated, it will be noted that the fit of a quadratic response curve of soil phosphorus (kg per ha P vs relative yield percentage) does not explain the variance of data particularly well (an R2 value of 48% is reported). The “economic optimum point” for reaching a 95% relative yield level each season is reported as being 82 kg ha-1 P available (150 mm depth) or a soil P concentration of 36 mg kg-1 (Bray 1).

In Figure 1, the author replicated the data mentioned in the paragraph above. Three response curves are illustrated: the one Schmidt used in his thesis as well as a square root and “broken stick” curve fitted by the author. The R2 of the curves varies between 48% and 59% with the square root fitting being the best and the quadratic indeed the worst.


Figure 1: Data from the PhD thesis of Schmidt, 2003, page 135, depicting relative maize yield vs soil P expressed as kg P per ha. The quadratic response function of Schmidt (R2 = 48%) is depicted in blue. The square root function suggested by the author (Bornman J.J.) is in dark green (R2 = 59%). A broken stick fit is also shown in light green (R2 = 55%). The data were accumulated at a site near Viljoenskroon (Free State).

What is revealing, is the magnitude of varied deductions that can be made from these response curves.

Many publications propose an optimum soil P level to achieve 90% relative yield, while others recommend the use of the 95% relative yield point. Schmidt considered both options in his thesis, but only the soil levels for 95% relative yield level were reported in the Fertilizer Handbook (2016).

Taking these two alternatives into account and using the response curves, the “optimum” soil level could vary between 30 kg P ha-1 and 90 kg P ha-1, depending on the preferred fit and choice of norm. The international preferred fit for phosphorus response curves is usually the square root function, and in this case, it indeed explains 10% more of the variation of relative yield and reflects almost 20 kg ha-1 less soil P than the optimum reported in the thesis and in Table 5.4.4.4 in the Fertilizer Handbook.

The reporting of soil P analyses levels as kg available phosphorus per ha and deriving economic optima from relative yields vs soil P is not new in South Africa. The author has presented and published several research projects on maize, wheat and sugarcane in this regard (Bornman with various co-authors, 1988-2008). In several publications (Bornman, 1991ab&c) maize response curves were also reported amongst others for for light soils in the Northwest rain fed production area (e.g. Wesselsbron). From such a response curve for the Wesselbron locality (R2 of 95%) derived over two years on a 6% clay content soil, the optimum soil P to achieve 95% relative yield was found to be equivalent to 35 kg P ha-1 (150 mm depth). Yields  of between 5 and 6 tonnes of maize seed per ha were indeed achieved. This value coincides fairly well with the recommended value in table 5.4.4.6 of the FERTASA publication of 32 kg P per ha if banded under “low production conditions” on a low soil-P containing soil for an expected yield of 6 tonnes ha-1 of maize grain. In Table 5.4.4.7, where recommendations are made for high production levels, the recommendation however reaches a level of 194 kg ha-1 for a yield potential of 7 tonnes ha-1.

Deriving actual economic optimum recommendation levels

Maintaining soil phosphorus is one consideration or maybe departure point, but the strategy to achieve that may differ from case to case. Questions that could be asked are what the efficiacy of the inherent soil P would be; whether incremental increase of soil P would be considered; if banding, broadcasting or a combination thereof would be applicable; whether it is a P fixing soil; and many more. The key remains the economics.

If economically responsible recommendations are to be made, several parameters need to be considered as mentioned. The guideline only serves as a departure point, but price ratios of the day and the producer’s fixed cost structure dictate the final decision. The producer’s risk aversion would of course be the deciding factor.

As an example, using the best fit square root response functions of both that of Schmidt and the author (with an expected yield of 6 tonnes of maize per ha) and applying a price ratio of 14 (meaning 14 kg of maize pays for one kg of phosphorus) the economic optimum (maximum margin over P cost) application with no soil P present, would be 55 and 48 kg P ha-1 respectively. The margin achieved over P cost would be close to R 9,000 per ha. This translates to a current “economic optimum” sandy soil P level of 23 and 20 mg kg-1 P to 150 mm depth.

What is of absolute importance is that this means that if soil P levels in a sandy soil are above 23 mg/kg (for this example in particular) it is theoretically not economically viable with current price ratios to apply more phosphorus, except for the consideration of replacing phosphorus to be removed by the anticipated or previous yield. Again, only if the producer would consider such an investment viable. At higher soil P levels, much lower application levels may be considered. However, it has been proven repeatedly that some “fresh P” in the band at planting always ensures early plant vigour, especially if cold and wet or windy conditions are experienced and higher yields need to be maintained. For seed maize and irrigated maize it is an absolute necessity. 

To add to the confusion, if the relative response curves are applied to different expected absolute yield levels, the economic optima differ yet again, simply because of the change of slope of the response function. With the square root response curve fitted on the data of Schmidt applied to an absolute yield increase of 6 to 8 tonnes maize seed per ha, the economic rate of application will increase by 7 kg ha-1 of P. This is also reflected in Table 5.4.4.6 where an increase of 8 kg ha-1 is actually recommended for the two-tonne yield increase per hectare. The similarity is however coincidental as the actual increase in the table is on the basis of crop removal and not a physical calculation from a production function.

Lastly, much is said in the press regarding return on investment related to fertilizer input. What is however not expanded upon, is that when return on investment is considered, the total cost structure of a scenario should be taken into consideration. Omnia Nutriology® has invested significantly in developing models to calculate several economic optima when considering an investment in fertilizer. This has been alluded to in a previous publication of the Nutriology® News (Bornman, 2016). When doing such calculations using the square root fit of the model for sandy soils of Schmidt for a yield target of 6 tonnes per ha, the maximum return on investment (ROI) in phosphorus fertilization over an unusually low fixed cost (other production costs besides phosphorus fertilization) of equivalent to 3 tonnes of maize per ha, would be 43% (over total cost) at an application of 50 kg ha-1 of P (if there is no soil P present). If the same calculation is done with a higher, more realistic fixed cost structure of say 5 tonnes per ha, the maximum ROI would, ironically, again be at the point of maximum margin, i.e. 60 kg P per ha. The ROI at this point (if no soil P is present), would be an unfortunate loss of 7%.

With all the above calculations, it is of the essence that inherent soil P is always considered at the current value thereof. In other words, its contribution should be subtracted from economic optima calculated with response curves developed on very low soil P. It is therefore no surprise that modest economic applications (margin on P investment alone) of as little as 3 kg P per ha are reported in the popular press. The dilemma, however, as explained above, is the fact that certain fixed cost structures need to be covered, or in worst cases, that at least the minimum loss in investment is maintained by sober consideration of fertilizer application levels.

In summary, it may be stated that there are such a magnitude of factors impacting on guidelines and eventual recommendations, depending on point of departure as well as eventual economic, lowest perceived risk considerations, that it is no wonder that there are so many conflicting opinions aired in the popular press.

In conclusion, it is highly recommended that a qualified scientific, professionally registered scientist, such as an Omnia Nutriology® agronomist, be consulted for a sound economically based, risk averse fertilizer recommendation.

Primary references

KILIAN, W H & BORNMAN, J J, 1988. Phosphorus fertilization guidelines for wheat in the summer rainfall area. Proceedings of the FSSA Phosphorus Symposium, Pretoria. 164-167.

BORNMAN, J J & VENTER, G C H, 1988. Application of different techniques in the evaluation of P response data in maize. Proceedings of the Phosphorus symposium (international) published by the Fertilizer Society of South Africa and the Dept. of Agriculture and Water supply – Pretoria  195-197

BORNMAN, J J, & VENTER, G C H, 1989. Deriving optimum economic phosphorus guidelines for maize on the Eastern Highveld. Combined congress of the South African Crop Society and the Soil Science Society of South Africa. January, Transkei.

BORNMAN, J J, 1991a. The use of agronomy research data for deriving optimum economic phosphorus recommendations for grain crops in South Africa. IFA Regional Conference for Sub-Saharan Africa. Harare, Zimbabwe.

BORNMAN, J J, 1991b.  Agronomy research data for determining optimum economic phosphorus recommendations on wheat. Better Crops International by PPI. December issue. 16-18

BORNMAN, J J, 1991c. Calculating optimum P rates in South Africa. Fertilizers and Agriculture, April, 3.

BORNMAN, J J & BESTER, J T, 1991. The interpretation of a long-term phosphorus depletion trial. Proceedings of a combined congress of the South African Crop Science Society and Soil Science Society. January, Stellenbosch.

BORNMAN, J J, 1992. Maize fertilization in South Africa. Published in IFA World Fertilizer Use Manual,  Halliday D J, Trenkel M E and Wichman W (editors). International Fertilizer Industry Association, Paris. 63 - 64.

BORNMAN J J & BESTER J T, 1998. Five-year response of Western Cape rainfed wheat to phosphorus levels and sources. Proceedings of the joint meeting of the SA Society of Crop Production and Soil Science Society of SA. 26.

BORNMAN, J J, 1991. The use of agronomy research data for deriving optimum economic phosphorus fertilizer recommendations for grain crops in South Africa. Proceedings of the medal holders function presented by the Fertilizer Society of South Africa. Midrand, Johannesburg. 26-49.

BORNMAN, J.J. 2008. Phosphorus nutrition of sugarcane. Currently a delicate balance between cost and benefit. SASA Sugar Agronomists annual meeting. Mount Edgecombe, October.

BORNMAN, J.J. 2016. Plant nutrition under drought conditions with special reference to grain production in central South Africa. http://www.fertilizer.co.za/knowledge-centre/sustainability/261-plant-nutrition-under-drought-conditions-with-special-reference-to-grain-production-in-central-south-africa. Omnia Nutriology® News, Summer edition.

FERTASA, 2016. Bemestingshandleiding (Fertilizer Handbook). Eighth edition. ISBN 0-909071-87-X

PRINS, A, BORNMAN, J J & MEYER, J H, 1997. Economic fertilizer recommendations for sugarcane in KwaZulu-Natal, incorporating risk quantification using a computer program. Proc. S Afr Sug Technol Ass 38-41.

SCHMIDT, C.J.J., 2003.  Changes in the phosphorus status of soils and the influence on maize yield.  PhD-thesis. University of the Free State, Bloemfontein

SCHMIDT, C.J.J., ADRIAANSE, F.G. & DU PREEZ, C.C., 2007.  Extractable soil phosphorus threshold values for dryland maize on the South African Highveld. S. Afr. J. Plant Soil 24(1),37-46.