Struvite, from human urine, is an alternative phosphorus (P) fertilizer. This study evaluated P release from struvite and its effectiveness for maize in Inanda (Ia), Sepane (Se) and Cartref (Cf) soils. Phosphorus sorption isotherms were determined, an incubation study determined the release pattern of P from struvite, and a pot trial compared struvite with single superphosphate (SSP) on maize P uptake. The amount of P at equilibrium (where Q = 0) in the Cf was 1.39 mg L , respectively. The extractable P in all three soils increased with time, with evidence of declining P release after Day 8. Struvite was most effective as a P source in the Cf followed by Se and Ia. Compared to SSP, there was significant improvement in maize grown on the Cf, while the reverse occurred in the Se. Maize grown in the Ia did not respond to either struvite or SSP. It was concluded that the phosphorus sorption was the main limiting factor that governed struvite effectiveness as a P source.

Nongqwenga -Modi: Phosphorus and potassium quantity/intensity properties of selected South African soils - Abstract There is no general single state function which describes the uptake of phosphorus (P) or potassium (K) by crop or escape in agro-ecosystem as a function external application. It is however proposed that quantity intensity relations have a potential of explaining such a paradigm. Extrapolating parameters from Quantity/Intensity (Q/I) parameters is laborious and technically complex, which makes them not suitable for routine analysis. A study of eleven soil samples was conducted to evaluate parameters which might correlate with Q/I parameters, and further evaluate influential soil properties on the Q/I parameters for both P and K using soils from different bioresource groups in KwaZulu-Natal, South Africa. It was found that pH measured in water could explain 71% variance in P sorption maxima. For K, it was found that EC accounted for 74% variance in K intensity (ARo), and 76% on the amount of K adsorbed (ΔKo). For both P and K no single parameter could uniformly and consistently explain the variance in Q/I parameters. It was concluded that although there is no single parameter which can uniformly explain Q/I parameter for both P and K, there is a potential for their modeling, using routinely determined parameters.

The inconsistent and incoherent approaches by fertilizer recommendations to index crop response has prompted the search for alternative approaches. Some of the problems stem from the overlooking of fundamental soil properties that govern the soil solution, which is where plant roots absorb nutrients for growth. A comparison was made between two contrasting equilibration techniques to evaluate their precision in estimating crop response. Sorption isotherms for phosphorus (P) and potassium (K) were compared to requirement factors. Phosphorus sorption isotherms were determined following the batch equilibration technique. Potassium was developed following equilibration with graded K levels. The requirement factors of both P and K were determined following a 6‐week incubation with four different levels of fertilization. Cowpea (Vigna unguiculata), mustard (Brassica juncea), and maize (Zea mays) were used as test crops. The growth parameters measured included biomass (g), height (cm), and leaf area index. At harvest, yield (g pot−1) and uptake (mg pot−1) were also recorded. Linear correlation studies were carried out to evaluate the association between treatments and the growth parameters of the tested crops. Results showed no significant difference (p < 0.05) in maize growth parameters between the equilibration methods, despite the sorption isotherms estimating higher levels of P and K. The sorption isotherms for P and K were 1.7 and 9.8 times higher than their respective requirement factors. The crop response, although relatively similar in both methods, was weakly correlated with the sorption‐estimated nutrient levels, indicating an overestimation of nutrients. Therefore, the requirement factors were deemed to be a more precise equilibration technique for estimating nutrient levels.

Conventional, extraction based fertilizer recommendations for phosphorus and potassium have been shown to lack mechanistic basis, thus unreliable. This has led to an urgent need for the development and evaluation of accurate and consistent phosphorus and potassium recommendations approaches with mechanistic basis. Also it has been shown that integrating nitrogen mineralization on nitrogen recommendations, has a potential of improving nitrogen recommendations. We established two parallel pot trial studies with the objective of comparing between extraction based fertilizer recommendations with alternative strategies. The first study was to compare the effect of integrating nitrogen mineralization on N recommendations. Second pot experiment in addition to N being recommended after integrating N mineralization; P and K were also recommended with an alternative strategy, which was derived from quantity/intensity relations. No negative impacts were observed on crop growth and nutrient uptake due to the integration of mineralizable nitrogen, despite nitrogen amounts being lower compared to treatments where N was applied without adjusting for mineralizable N. The same was true for the second pot trial, P and N recommended by conventional approach were higher, yet the crop response was not concurrently improved by higher rates. Potassium rates recommended by alternative strategy were higher and this was concurrent with potassium uptake. We therefore concluded, that this NPK recommendation experimental approach (N e P e K e ) is superior to its conventional counterpart (N c P c K c ). Hence, more reliable recommendations can be developed using this approach and this might reduce environmental footprint of agro-ecosystems, and reduce input cost for farmers where warranted.