Peatlands have several constraints that need to be taken more seriously, such as low soil fertility and high acidity levels caused by organic acids. The low productivity of peatlands is also a problem that needs to be addressed promptly, as it is related to the low availability of nutrients for plant cultivation. One action that can be taken to improve the availability of nutrients is by applying organic materials such as chicken manure and cow manure. This study aims to determine the effect of applying chicken manure and cow manure on the availability of N, P, K nutrients, and how they affect the uptake of N by mustard plants. The method used in this study is a completely randomized design with one factor and a 2-week incubation period. There are 8 fertilizer treatments applied to peat soil, which are as follows: 1) control (no treatment); 2) 10 tha^-1 of chicken manure; 3) 5 tha^-1 of chicken manure; 4) 10 tha^-1 of cow manure; 5) 5 tha^-1 of cow manure; 6) 5 tha^-1 of chicken manure + 5 tha^-1 of cow manure; 7) 2.5 tha^-1 of chicken manure + 7.5 tha^-1 of cow manure; 8) 7.5 tha^-1 of chicken manure + 2.5 tha^-1 of cow manure. These 8 treatments are repeated 3 times, resulting in 24 experimental units. From the research results, it is known that the application of various formulas of chicken manure and cow manure is capable of increasing pH, the availability of N and P nutrients, dry weight, and N uptake by plants. However, the availability of K nutrient did not significantly affect the given treatments.