Ruzigrass as cover crop improves the potassium partial balance and use by subsequent cotton

In soil, potassium (K) is present as structural K, non-exchangeable K (Kne) and exchangeable K (Ke). Exchangeable K is considered available to plants, but exudates from plant roots and microbes can dissolve Kne and make it available. Some grasses used as cover crops are particularly efficient in releasing Kne. In this work, we assessed the effects of early K application to ruzigrass in a sandy soil on soil K forms, K uptake by cotton and soil K balance. Potassium was applied (i) to ruzigrass; (ii) half to ruzigrass and half to cotton; (iii) on cotton grown on ruzigrass; and (iv) on cotton in the absence of ruzigrass, or K was not applied (v) in the presence of ruzigrass and (vi) in absence or ruzigrass. The accumulation of K by ruzigrass ranged from 40 to 250 kg ha−1, and about half of it was washed out of the plant residues by 20 days after desiccation. Ke was lowest throughout the soil profile in the absence of K fertilization regardless of ruzigrass. By contrast, in the 0–10 cm soil layer, Kne was 88% higher with unfertilized ruzigrass than in the absence of both K fertilization and ruzigrass. K fertilization of cotton increased Ke to a depth of 40–60 cm when grown on ruzigrass. The K balance was most negative in the absence of K fertilization and ruzigrass (0 K), but a similar negative balance of 116 kg ha−1 of K was observed when ruzigrass was grown without K fertilization. Conversely, K application to ruzigrass resulted in a positive Ke balance. The release of K from ruzigrass residues delayed the decrease in soil Kne and increased soil Ke 40 days after desiccation. In conclusion, applying K to ruzigrass grown before cotton is an effective strategy for ensuring the release of K to cotton, improving the soil K balance, and avoiding K depletion.