Potassium application to the cover crop prior to cotton planting as a fertilization strategy in sandy soils

Potassium (K) is a crucial nutrient for cotton, playing a vital role in various physiological processes including stomatal regulation, CO2 fixation, enzyme activation, nutrient transport, stress tolerance, and ultimately, yield and fiber quality. In sandy soils, K's low binding capacity to soil particles due to low organic matter and clay content leads to increased leaching. Cover crops, especially *Urochloa* grasses, can mitigate nutrient losses by enhancing nutrient cycling. This study focuses on investigating whether applying K to a ruzigrass cover crop before planting cotton in sandy soil can provide sufficient K for the cotton crop, eliminating the need for split applications and potentially reducing K fertilizer use. The use of cover crops such as *Urochloa* grasses is a key strategy in no-till systems of tropical agriculture. These grasses have deep, extensive root systems improving soil structure and nutrient acquisition. They also exude organic acids that chelate Al, freeing up cations such as K, Mg, and Ca in the CEC. The deep root systems of these grasses help to establish a network of biopores in the soil, facilitating root growth in the subsequent crop. This study aimed to determine if applying K fertilizer to the cover crop could provide sufficient K for the following cotton crop, improving the fertilizer use efficiency and reducing potential K losses from leaching. This research also explored whether this strategy impacted cotton yield and fiber quality. The potential to reduce K fertilizer application in sandy soils while maintaining cotton yield and fiber quality is important for both economic and environmental sustainability.

Existing research highlights the importance of K for cotton yield and fiber quality. Studies have shown that K uptake in plants occurs through mass flow and diffusion, with diffusion accounting for the majority (72-96%) of plant K demand. However, in sandy tropical soils with low cation exchange capacity (CEC), K can be highly mobile and prone to leaching. Previous work has documented the effectiveness of cover crops in reducing nitrogen (N) leaching, but research on their effects on K dynamics is limited. While studies indicate that K in cover crop residues is readily released upon decomposition, the effectiveness of this approach in K fertilization of subsequent crops, especially in sandy soils, remains unclear. There is some evidence that split K application or slow-release fertilizers are important in enhancing the efficiency of K fertilizers in sandy soils. This suggests that at least partial K fertilizer application to the cover crop before planting might improve fertilizer use efficiency. However, little is known about the effect of this strategy in sandy soils prone to K leaching, particularly concerning cotton fiber quality. Previous studies in soils with higher CEC have demonstrated the benefits of early K fertilization in cotton, however similar effects in sandy soils are not as well documented.

A field experiment was conducted over two seasons (2016/17 and 2017/18) in Presidente Bernardes, São Paulo State, Brazil, on a sandy loam soil (Typic Rhodustult). Two cotton cultivars (FM 913GLT and FM 983GLT) were used. Six K fertilization treatments were implemented in a 2x6 factorial design in a completely randomized block design with five replicates. Treatments included: (1) No K fertilization; (2) 116 kg ha⁻¹ K applied to cotton in splits; (3) No K + ruzigrass cover crop; (4) 116 kg ha⁻¹ K applied to ruzigrass; (5) 58 kg ha⁻¹ K to ruzigrass + 58 kg ha⁻¹ K to cotton; and (6) 116 kg ha⁻¹ K to cotton + ruzigrass cover crop. Ruzigrass was planted in May and desiccated before cotton planting in November. Phosphorus and nitrogen were applied at planting, with additional nitrogen sidedressed later. Cotton was harvested around 132-141 days after emergence. Several parameters were measured, including soil exchangeable K, leaf K concentration, cotton yield components (boll number, boll weight, gin turnout), plant height, number of nodes, fiber quality (strength, length, micronaire, maturity, short fiber content), and apparent K fertilizer use efficiency. Statistical analysis, including ANOVA and Pearson correlation, was used to analyze the data.

The study found that cotton cultivars showed similar responses, and most characteristics did not interact with K management or season. Exchangeable soil K was generally higher in the first season than the second. K fertilization increased soil exchangeable K, although it was lower when K was applied to the grass compared to applications without ruzigrass. Soil exchangeable K was positively correlated with yield, but the correlation coefficient was low (0.42 and 0.53). Cotton grown after K-fertilized ruzigrass generally had higher leaf K concentrations in the first season but not consistently in the second. In the first season, boll weight was unaffected by K management, but in the second season, K fertilization increased it, especially with ruzigrass. Boll number varied yearly; in the first season, K increased boll number only without ruzigrass. Gin turnout was lower without K. Yields were generally higher with K fertilization. Interestingly, in the absence of K, yield was 16% higher with ruzigrass. Plant height responded positively to K application, with the highest height observed when 116 kg ha⁻¹ K was applied without ruzigrass. The percentage of short fibers was lower with higher K availability. Apparent K fertilizer use efficiency was lower when ruzigrass was grown in the season where K had a significant response (2017/18). Micronaire was lowest without K in plots without ruzigrass. Growing ruzigrass increased micronaire as much as applying 116 kg ha⁻¹ K without the grass. Fiber maturity was generally higher when K was applied to the grass or split. Most vegetative and yield components showed positive correlations, although generally weak, except for the relationship of node and boll number with fiber yield. Fiber yield was negatively correlated with most fiber quality parameters, although the correlations were weak.

The results suggest that the application of K to ruzigrass before cotton planting is a viable method to provide sufficient K nutrition to the cotton crop, leading to improved yield and fiber quality. The improved yield in the absence of added K when ruzigrass was used suggests that this approach could potentially reduce the need for K fertilizer applications. This could be due to improved K uptake by ruzigrass and its efficient release after decomposition. The lower apparent use efficiency of K in the presence of ruzigrass suggests that the K fertilizer provided to the ruzigrass was effectively utilized by the subsequent cotton crop, reducing reliance on directly applied K to the cotton crop. The observed impact on fiber quality supports the findings and shows that the presence of the cover crop is synergistic with the effects of K. The variability observed in the results across seasons highlights the importance of rainfall and other environmental factors in K dynamics and uptake.

This study demonstrates that applying K fertilizer to a ruzigrass cover crop before cotton planting in sandy soils is an effective strategy for providing adequate K nutrition to the cotton crop. This approach reduces the need for split K applications and may allow for decreased K fertilizer rates, potentially leading to cost savings and improved environmental sustainability. Further research should investigate optimal K application rates to ruzigrass under varying environmental conditions to refine this fertilization strategy.

The study was conducted over only two years, and the results may not be generalizable to all sandy soils or climatic conditions. The interaction between rainfall patterns and K cycling needs more investigation. Further research is warranted to assess the long-term effects of this approach on soil K levels and potential K depletion.